/*
 *
 *    Copyright (c) 2021-2022 Project CHIP Authors
 *    All rights reserved.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *        http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */

/**
 *    @file
 *      This file implements the the CHIP CASE Session object that provides
 *      APIs for constructing a secure session using a certificate from the device's
 *      operational credentials.
 *
 */
#include <protocols/secure_channel/CASESession.h>

#include <atomic>
#include <inttypes.h>
#include <memory>
#include <string.h>

#include <lib/core/CHIPEncoding.h>
#include <lib/core/CHIPSafeCasts.h>
#include <lib/support/CHIPMem.h>
#include <lib/support/CodeUtils.h>
#include <lib/support/SafeInt.h>
#include <lib/support/ScopedBuffer.h>
#include <lib/support/TypeTraits.h>
#include <messaging/SessionParameters.h>
#include <platform/PlatformManager.h>
#include <protocols/Protocols.h>
#include <protocols/secure_channel/CASEDestinationId.h>
#include <protocols/secure_channel/PairingSession.h>
#include <protocols/secure_channel/SessionResumptionStorage.h>
#include <protocols/secure_channel/StatusReport.h>
#include <system/SystemClock.h>
#include <system/TLVPacketBufferBackingStore.h>
#include <tracing/macros.h>
#include <transport/SessionManager.h>

namespace {

enum
{
    kTag_TBEData_SenderNOC    = 1,
    kTag_TBEData_SenderICAC   = 2,
    kTag_TBEData_Signature    = 3,
    kTag_TBEData_ResumptionID = 4,
};

enum
{
    kTag_TBSData_SenderNOC      = 1,
    kTag_TBSData_SenderICAC     = 2,
    kTag_TBSData_SenderPubKey   = 3,
    kTag_TBSData_ReceiverPubKey = 4,
};

enum
{
    kTag_Sigma1_InitiatorRandom    = 1,
    kTag_Sigma1_InitiatorSessionId = 2,
    kTag_Sigma1_DestinationId      = 3,
    kTag_Sigma1_InitiatorEphPubKey = 4,
    kTag_Sigma1_InitiatorMRPParams = 5,
    kTag_Sigma1_ResumptionID       = 6,
    kTag_Sigma1_InitiatorResumeMIC = 7,
};

enum
{
    kTag_Sigma2_ResponderRandom    = 1,
    kTag_Sigma2_ResponderSessionId = 2,
    kTag_Sigma2_ResponderEphPubKey = 3,
    kTag_Sigma2_Encrypted2         = 4,
    kTag_Sigma2_ResponderMRPParams = 5,
};

enum
{
    kTag_Sigma3_Encrypted3 = 1,
};

} // namespace

namespace chip {

using namespace Crypto;
using namespace Credentials;
using namespace Messaging;
using namespace Encoding;
using namespace Protocols::SecureChannel;

constexpr uint8_t kKDFSR2Info[] = { 0x53, 0x69, 0x67, 0x6d, 0x61, 0x32 };
constexpr uint8_t kKDFSR3Info[] = { 0x53, 0x69, 0x67, 0x6d, 0x61, 0x33 };

constexpr uint8_t kKDFS1RKeyInfo[] = { 0x53, 0x69, 0x67, 0x6d, 0x61, 0x31, 0x5f, 0x52, 0x65, 0x73, 0x75, 0x6d, 0x65 };
constexpr uint8_t kKDFS2RKeyInfo[] = { 0x53, 0x69, 0x67, 0x6d, 0x61, 0x32, 0x5f, 0x52, 0x65, 0x73, 0x75, 0x6d, 0x65 };

constexpr uint8_t kResume1MIC_Nonce[] =
    /* "NCASE_SigmaS1" */ { 0x4e, 0x43, 0x41, 0x53, 0x45, 0x5f, 0x53, 0x69, 0x67, 0x6d, 0x61, 0x53, 0x31 };
constexpr uint8_t kResume2MIC_Nonce[] =
    /* "NCASE_SigmaS2" */ { 0x4e, 0x43, 0x41, 0x53, 0x45, 0x5f, 0x53, 0x69, 0x67, 0x6d, 0x61, 0x53, 0x32 };
constexpr uint8_t kTBEData2_Nonce[] =
    /* "NCASE_Sigma2N" */ { 0x4e, 0x43, 0x41, 0x53, 0x45, 0x5f, 0x53, 0x69, 0x67, 0x6d, 0x61, 0x32, 0x4e };
constexpr uint8_t kTBEData3_Nonce[] =
    /* "NCASE_Sigma3N" */ { 0x4e, 0x43, 0x41, 0x53, 0x45, 0x5f, 0x53, 0x69, 0x67, 0x6d, 0x61, 0x33, 0x4e };
constexpr size_t kTBEDataNonceLength = sizeof(kTBEData2_Nonce);
static_assert(sizeof(kTBEData2_Nonce) == sizeof(kTBEData3_Nonce), "TBEData2_Nonce and TBEData3_Nonce must be same size");

// Amounts of time to allow for server-side processing of messages.
//
// These timeout values only allow for the server-side processing and assume that any transport-specific
// latency will be added to them.
//
// The session establishment fails if the response is not received within the resulting timeout window,
// which accounts for both transport latency and the server-side latency.
static constexpr ExchangeContext::Timeout kExpectedLowProcessingTime    = System::Clock::Seconds16(2);
static constexpr ExchangeContext::Timeout kExpectedSigma1ProcessingTime = kExpectedLowProcessingTime;
static constexpr ExchangeContext::Timeout kExpectedHighProcessingTime   = System::Clock::Seconds16(30);

// Helper for managing a session's outstanding work.
// Holds work data which is provided to a scheduled work callback (standalone),
// then (if not canceled) to a scheduled after work callback (on the session).
template <class DATA>
class CASESession::WorkHelper
{
public:
    // Work callback, processed in the background via `PlatformManager::ScheduleBackgroundWork`.
    // This is a non-member function which does not use the associated session.
    // The return value is passed to the after work callback (called afterward).
    // Set `cancel` to true if calling the after work callback is not necessary.
    typedef CHIP_ERROR (*WorkCallback)(DATA & data, bool & cancel);

    // After work callback, processed in the main Matter task via `PlatformManager::ScheduleWork`.
    // This is a member function to be called on the associated session after the work callback.
    // The `status` value is the result of the work callback (called beforehand), or the status of
    // queueing the after work callback back to the Matter thread, if the work callback succeeds
    // but queueing fails.
    //
    // When this callback is called asynchronously (i.e. via ScheduleWork), the helper guarantees
    // that it will keep itself (and hence `data`) alive until the callback completes.
    typedef CHIP_ERROR (CASESession::*AfterWorkCallback)(DATA & data, CHIP_ERROR status);

public:
    // Create a work helper using the specified session, work callback, after work callback, and data (template arg).
    // Lifetime is managed by sharing between the caller (typically the session) and the helper itself (while work is scheduled).
    static Platform::SharedPtr<WorkHelper> Create(CASESession & session, WorkCallback workCallback,
                                                  AfterWorkCallback afterWorkCallback)
    {
        struct EnableShared : public WorkHelper
        {
            EnableShared(CASESession & session, WorkCallback workCallback, AfterWorkCallback afterWorkCallback) :
                WorkHelper(session, workCallback, afterWorkCallback)
            {}
        };
        auto ptr = Platform::MakeShared<EnableShared>(session, workCallback, afterWorkCallback);
        if (ptr)
        {
            ptr->mWeakPtr = ptr; // used by `ScheduleWork`
        }
        return ptr;
    }

    // Do the work immediately.
    // No scheduling, no outstanding work, no shared lifetime management.
    //
    // The caller must guarantee that it keeps the helper alive across this call, most likely by
    // holding a reference to it on the stack.
    CHIP_ERROR DoWork()
    {
        // Ensure that this function is being called from main Matter thread
        assertChipStackLockedByCurrentThread();

        VerifyOrReturnError(mSession && mWorkCallback && mAfterWorkCallback, CHIP_ERROR_INCORRECT_STATE);
        auto * helper   = this;
        bool cancel     = false;
        helper->mStatus = helper->mWorkCallback(helper->mData, cancel);
        if (!cancel)
        {
            helper->mStatus = (helper->mSession->*(helper->mAfterWorkCallback))(helper->mData, helper->mStatus);
        }
        return helper->mStatus;
    }

    // Schedule the work for later execution.
    // If lifetime is managed, the helper shares management while work is outstanding.
    CHIP_ERROR ScheduleWork()
    {
        VerifyOrReturnError(mSession && mWorkCallback && mAfterWorkCallback, CHIP_ERROR_INCORRECT_STATE);
        // Hold strong ptr while work is outstanding
        mStrongPtr  = mWeakPtr.lock(); // set in `Create`
        auto status = DeviceLayer::PlatformMgr().ScheduleBackgroundWork(WorkHandler, reinterpret_cast<intptr_t>(this));
        if (status != CHIP_NO_ERROR)
        {
            // Release strong ptr since scheduling failed.
            mStrongPtr.reset();
        }
        return status;
    }

    // Cancel the work, by clearing the associated session.
    void CancelWork() { mSession.store(nullptr); }

    bool IsCancelled() const { return mSession.load() == nullptr; }

    // This API returns true when background thread fails to schedule the AfterWorkCallback
    bool UnableToScheduleAfterWorkCallback() { return mScheduleAfterWorkFailed.load(); }

    // Do after work immediately.
    // No scheduling, no outstanding work, no shared lifetime management.
    void DoAfterWork()
    {
        VerifyOrDie(UnableToScheduleAfterWorkCallback());
        AfterWorkHandler(reinterpret_cast<intptr_t>(this));
    }

private:
    // Create a work helper using the specified session, work callback, after work callback, and data (template arg).
    // Lifetime is not managed, see `Create` for that option.
    WorkHelper(CASESession & session, WorkCallback workCallback, AfterWorkCallback afterWorkCallback) :
        mSession(&session), mWorkCallback(workCallback), mAfterWorkCallback(afterWorkCallback)
    {}

    // Handler for the work callback.
    static void WorkHandler(intptr_t arg)
    {
        auto * helper = reinterpret_cast<WorkHelper *>(arg);
        // Hold strong ptr while work is handled
        auto strongPtr(std::move(helper->mStrongPtr));
        VerifyOrReturn(!helper->IsCancelled());
        bool cancel = false;
        // Execute callback in background thread; data must be OK with this
        helper->mStatus = helper->mWorkCallback(helper->mData, cancel);
        VerifyOrReturn(!cancel && !helper->IsCancelled());
        // Hold strong ptr to ourselves while work is outstanding
        helper->mStrongPtr.swap(strongPtr);
        auto status = DeviceLayer::PlatformMgr().ScheduleWork(AfterWorkHandler, reinterpret_cast<intptr_t>(helper));
        if (status != CHIP_NO_ERROR)
        {
            ChipLogError(SecureChannel, "Failed to Schedule the AfterWorkCallback on foreground thread: %" CHIP_ERROR_FORMAT,
                         status.Format());

            // We failed to schedule after work callback, so setting mScheduleAfterWorkFailed flag to true
            // This can be checked from foreground thread and after work callback can be retried
            helper->mStatus = status;

            // Release strong ptr to self since scheduling failed, because nothing guarantees
            // that AfterWorkHandler will get called at this point to release the reference,
            // and we don't want to leak.  That said, we want to ensure that "helper" stays
            // alive through the end of this function (so we can set mScheduleAfterWorkFailed
            // on it), but also want to avoid racing on the single SharedPtr instance in
            // helper->mStrongPtr.  That means we need to not touch helper->mStrongPtr after
            // writing to mScheduleAfterWorkFailed.
            //
            // The simplest way to do this is to move the reference in helper->mStrongPtr to
            // our stack, where it outlives all our accesses to "helper".
            strongPtr.swap(helper->mStrongPtr);

            // helper and any of its state should not be touched after storing mScheduleAfterWorkFailed.
            helper->mScheduleAfterWorkFailed.store(true);
        }
    }

    // Handler for the after work callback.
    static void AfterWorkHandler(intptr_t arg)
    {
        // Ensure that this function is being called from main Matter thread
        assertChipStackLockedByCurrentThread();

        auto * helper = reinterpret_cast<WorkHelper *>(arg);
        // Hold strong ptr while work is handled, and ensure that helper->mStrongPtr does not keep
        // holding a reference.
        auto strongPtr(std::move(helper->mStrongPtr));
        if (!strongPtr)
        {
            // This can happen if scheduling AfterWorkHandler failed.  Just grab a strong ref
            // to handler directly, to fulfill our API contract of holding a strong reference
            // across the after-work callback.  At this point, we are guaranteed that the
            // background thread is not touching the helper anymore.
            strongPtr = helper->mWeakPtr.lock();
        }
        if (auto * session = helper->mSession.load())
        {
            // Execute callback in Matter thread; session should be OK with this
            (session->*(helper->mAfterWorkCallback))(helper->mData, helper->mStatus);
        }
    }

private:
    // Lifetime management: `ScheduleWork` sets `mStrongPtr` from `mWeakPtr`.
    Platform::WeakPtr<WorkHelper> mWeakPtr;

    // Lifetime management: `ScheduleWork` sets `mStrongPtr` from `mWeakPtr`.
    Platform::SharedPtr<WorkHelper> mStrongPtr;

    // Associated session, cleared by `CancelWork`.
    std::atomic<CASESession *> mSession;

    // Work callback, called by `WorkHandler`.
    WorkCallback mWorkCallback;

    // After work callback, called by `AfterWorkHandler`.
    AfterWorkCallback mAfterWorkCallback;

    // Return value of `mWorkCallback`, passed to `mAfterWorkCallback`.
    CHIP_ERROR mStatus;

    // If background thread fails to schedule AfterWorkCallback then this flag is set to true
    // and CASEServer then can check this one and run the AfterWorkCallback for us.
    //
    // When this happens, the write to this boolean _must_ be the last code that touches this
    // object on the background thread.  After that, the Matter thread owns the object.
    std::atomic<bool> mScheduleAfterWorkFailed{ false };

public:
    // Data passed to `mWorkCallback` and `mAfterWorkCallback`.
    DATA mData;
};

struct CASESession::SendSigma3Data
{
    FabricIndex fabricIndex;

    // Use one or the other
    const FabricTable * fabricTable;
    const Crypto::OperationalKeystore * keystore;

    chip::Platform::ScopedMemoryBuffer<uint8_t> msg_R3_Signed;
    size_t msg_r3_signed_len;

    chip::Platform::ScopedMemoryBuffer<uint8_t> msg_R3_Encrypted;
    size_t msg_r3_encrypted_len;

    chip::Platform::ScopedMemoryBuffer<uint8_t> icacBuf;
    MutableByteSpan icaCert;

    chip::Platform::ScopedMemoryBuffer<uint8_t> nocBuf;
    MutableByteSpan nocCert;

    P256ECDSASignature tbsData3Signature;
};

struct CASESession::HandleSigma3Data
{
    chip::Platform::ScopedMemoryBuffer<uint8_t> msg_R3_Signed;
    size_t msg_r3_signed_len;

    ByteSpan initiatorNOC;
    ByteSpan initiatorICAC;

    uint8_t rootCertBuf[kMaxCHIPCertLength];
    ByteSpan fabricRCAC;

    P256ECDSASignature tbsData3Signature;

    FabricId fabricId;
    NodeId initiatorNodeId;

    ValidationContext validContext;
};

CASESession::~CASESession()
{
    // Let's clear out any security state stored in the object, before destroying it.
    Clear();
}

void CASESession::OnSessionReleased()
{
    // Call into our super-class before we clear our state.
    PairingSession::OnSessionReleased();
    Clear();
}

void CASESession::Clear()
{
    MATTER_TRACE_SCOPE("Clear", "CASESession");
    // Cancel any outstanding work.
    if (mSendSigma3Helper)
    {
        mSendSigma3Helper->CancelWork();
        mSendSigma3Helper.reset();
    }
    if (mHandleSigma3Helper)
    {
        mHandleSigma3Helper->CancelWork();
        mHandleSigma3Helper.reset();
    }

    // This function zeroes out and resets the memory used by the object.
    // It's done so that no security related information will be leaked.
    mCommissioningHash.Clear();
    PairingSession::Clear();

    mState = State::kInitialized;
    Crypto::ClearSecretData(mIPK);

    if (mFabricsTable != nullptr)
    {
        mFabricsTable->RemoveFabricDelegate(this);

        mFabricsTable->ReleaseEphemeralKeypair(mEphemeralKey);
        mEphemeralKey = nullptr;
    }

    mLocalNodeId  = kUndefinedNodeId;
    mPeerNodeId   = kUndefinedNodeId;
    mFabricsTable = nullptr;
    mFabricIndex  = kUndefinedFabricIndex;
}

void CASESession::InvalidateIfPendingEstablishmentOnFabric(FabricIndex fabricIndex)
{
    if (mFabricIndex != fabricIndex)
    {
        return;
    }
    if (!IsSessionEstablishmentInProgress())
    {
        return;
    }
    AbortPendingEstablish(CHIP_ERROR_CANCELLED);
}

CHIP_ERROR CASESession::Init(SessionManager & sessionManager, Credentials::CertificateValidityPolicy * policy,
                             SessionEstablishmentDelegate * delegate, const ScopedNodeId & sessionEvictionHint)
{
    MATTER_TRACE_SCOPE("Init", "CASESession");
    VerifyOrReturnError(delegate != nullptr, CHIP_ERROR_INVALID_ARGUMENT);
    VerifyOrReturnError(mGroupDataProvider != nullptr, CHIP_ERROR_INVALID_ARGUMENT);
    VerifyOrReturnError(sessionManager.GetSessionKeystore() != nullptr, CHIP_ERROR_INVALID_ARGUMENT);

    Clear();

    ReturnErrorOnFailure(mCommissioningHash.Begin());

    mDelegate = delegate;
    ReturnErrorOnFailure(AllocateSecureSession(sessionManager, sessionEvictionHint));

    mValidContext.Reset();
    mValidContext.mRequiredKeyUsages.Set(KeyUsageFlags::kDigitalSignature);
    mValidContext.mRequiredKeyPurposes.Set(KeyPurposeFlags::kServerAuth);
    mValidContext.mValidityPolicy = policy;

    return CHIP_NO_ERROR;
}

CHIP_ERROR
CASESession::PrepareForSessionEstablishment(SessionManager & sessionManager, FabricTable * fabricTable,
                                            SessionResumptionStorage * sessionResumptionStorage,
                                            Credentials::CertificateValidityPolicy * policy,
                                            SessionEstablishmentDelegate * delegate, const ScopedNodeId & previouslyEstablishedPeer,
                                            Optional<ReliableMessageProtocolConfig> mrpLocalConfig)
{
    MATTER_TRACE_SCOPE("PrepareForSessionEstablishment", "CASESession");
    // Below VerifyOrReturnError is not SuccessOrExit since we only want to goto `exit:` after
    // Init has been successfully called.
    VerifyOrReturnError(fabricTable != nullptr, CHIP_ERROR_INVALID_ARGUMENT);
    ReturnErrorOnFailure(Init(sessionManager, policy, delegate, previouslyEstablishedPeer));

    CHIP_ERROR err = CHIP_NO_ERROR;

    SuccessOrExit(err = fabricTable->AddFabricDelegate(this));

    mFabricsTable             = fabricTable;
    mRole                     = CryptoContext::SessionRole::kResponder;
    mSessionResumptionStorage = sessionResumptionStorage;
    mLocalMRPConfig           = mrpLocalConfig.ValueOr(GetDefaultMRPConfig());

    ChipLogDetail(SecureChannel, "Allocated SecureSession (%p) - waiting for Sigma1 msg",
                  mSecureSessionHolder.Get().Value()->AsSecureSession());

exit:
    if (err != CHIP_NO_ERROR)
    {
        Clear();
    }
    return err;
}

CHIP_ERROR CASESession::EstablishSession(SessionManager & sessionManager, FabricTable * fabricTable, ScopedNodeId peerScopedNodeId,
                                         ExchangeContext * exchangeCtxt, SessionResumptionStorage * sessionResumptionStorage,
                                         Credentials::CertificateValidityPolicy * policy, SessionEstablishmentDelegate * delegate,
                                         Optional<ReliableMessageProtocolConfig> mrpLocalConfig)
{
    MATTER_TRACE_SCOPE("EstablishSession", "CASESession");
    CHIP_ERROR err = CHIP_NO_ERROR;

    // Return early on error here, as we have not initialized any state yet
    ReturnErrorCodeIf(exchangeCtxt == nullptr, CHIP_ERROR_INVALID_ARGUMENT);
    ReturnErrorCodeIf(fabricTable == nullptr, CHIP_ERROR_INVALID_ARGUMENT);

    // Use FabricTable directly to avoid situation of dangling index from stale FabricInfo
    // until we factor-out any FabricInfo direct usage.
    ReturnErrorCodeIf(peerScopedNodeId.GetFabricIndex() == kUndefinedFabricIndex, CHIP_ERROR_INVALID_ARGUMENT);
    const auto * fabricInfo = fabricTable->FindFabricWithIndex(peerScopedNodeId.GetFabricIndex());
    ReturnErrorCodeIf(fabricInfo == nullptr, CHIP_ERROR_INVALID_ARGUMENT);

    err = Init(sessionManager, policy, delegate, peerScopedNodeId);

    mRole = CryptoContext::SessionRole::kInitiator;

    // We are setting the exchange context specifically before checking for error.
    // This is to make sure the exchange will get closed if Init() returned an error.
    mExchangeCtxt.Emplace(*exchangeCtxt);

    // From here onwards, let's go to exit on error, as some state might have already
    // been initialized
    SuccessOrExit(err);

    SuccessOrExit(err = fabricTable->AddFabricDelegate(this));

    mFabricsTable             = fabricTable;
    mFabricIndex              = fabricInfo->GetFabricIndex();
    mSessionResumptionStorage = sessionResumptionStorage;
    mLocalMRPConfig           = mrpLocalConfig.ValueOr(GetDefaultMRPConfig());

    mExchangeCtxt.Value()->UseSuggestedResponseTimeout(kExpectedSigma1ProcessingTime);
    mPeerNodeId  = peerScopedNodeId.GetNodeId();
    mLocalNodeId = fabricInfo->GetNodeId();

    ChipLogProgress(SecureChannel, "Initiating session on local FabricIndex %u from 0x" ChipLogFormatX64 " -> 0x" ChipLogFormatX64,
                    static_cast<unsigned>(mFabricIndex), ChipLogValueX64(mLocalNodeId), ChipLogValueX64(mPeerNodeId));

    err = SendSigma1();
    SuccessOrExit(err);

exit:
    if (err != CHIP_NO_ERROR)
    {
        Clear();
    }
    return err;
}

void CASESession::OnResponseTimeout(ExchangeContext * ec)
{
    MATTER_TRACE_SCOPE("OnResponseTimeout", "CASESession");
    VerifyOrReturn(ec != nullptr, ChipLogError(SecureChannel, "CASESession::OnResponseTimeout was called by null exchange"));
    VerifyOrReturn(mExchangeCtxt.HasValue() && (&mExchangeCtxt.Value().Get() == ec),
                   ChipLogError(SecureChannel, "CASESession::OnResponseTimeout exchange doesn't match"));
    ChipLogError(SecureChannel,
                 "CASESession timed out while waiting for a response from peer " ChipLogFormatScopedNodeId ". Current state was %u",
                 ChipLogValueScopedNodeId(ScopedNodeId(mPeerNodeId, mFabricIndex)), to_underlying(mState));
    MATTER_TRACE_COUNTER("CASETimeout");
    // Discard the exchange so that Clear() doesn't try aborting it.  The
    // exchange will handle that.
    DiscardExchange();
    AbortPendingEstablish(CHIP_ERROR_TIMEOUT);
}

void CASESession::AbortPendingEstablish(CHIP_ERROR err)
{
    MATTER_TRACE_SCOPE("AbortPendingEstablish", "CASESession");
    // This needs to come before Clear() which will reset mState.
    SessionEstablishmentStage state = MapCASEStateToSessionEstablishmentStage(mState);
    Clear();
    // Do this last in case the delegate frees us.
    NotifySessionEstablishmentError(err, state);
}

CHIP_ERROR CASESession::DeriveSecureSession(CryptoContext & session)
{
    switch (mState)
    {
    case State::kFinished: {
        std::array<uint8_t, sizeof(mIPK) + kSHA256_Hash_Length> msg_salt;

        {
            Encoding::LittleEndian::BufferWriter bbuf(msg_salt);
            bbuf.Put(mIPK, sizeof(mIPK));
            bbuf.Put(mMessageDigest, sizeof(mMessageDigest));

            VerifyOrReturnError(bbuf.Fit(), CHIP_ERROR_BUFFER_TOO_SMALL);
        }

        ReturnErrorOnFailure(session.InitFromSecret(*mSessionManager->GetSessionKeystore(), mSharedSecret.Span(),
                                                    ByteSpan(msg_salt), CryptoContext::SessionInfoType::kSessionEstablishment,
                                                    mRole));

        return CHIP_NO_ERROR;
    }
    case State::kFinishedViaResume: {
        std::array<uint8_t, sizeof(mInitiatorRandom) + decltype(mResumeResumptionId)().size()> msg_salt;

        {
            Encoding::LittleEndian::BufferWriter bbuf(msg_salt);
            bbuf.Put(mInitiatorRandom, sizeof(mInitiatorRandom));
            bbuf.Put(mResumeResumptionId.data(), mResumeResumptionId.size());

            VerifyOrReturnError(bbuf.Fit(), CHIP_ERROR_BUFFER_TOO_SMALL);
        }

        ReturnErrorOnFailure(session.InitFromSecret(*mSessionManager->GetSessionKeystore(), mSharedSecret.Span(),
                                                    ByteSpan(msg_salt), CryptoContext::SessionInfoType::kSessionResumption, mRole));

        return CHIP_NO_ERROR;
    }
    default:
        return CHIP_ERROR_INCORRECT_STATE;
    }
}

CHIP_ERROR CASESession::RecoverInitiatorIpk()
{
    Credentials::GroupDataProvider::KeySet ipkKeySet;

    CHIP_ERROR err = mGroupDataProvider->GetIpkKeySet(mFabricIndex, ipkKeySet);

    if (err != CHIP_NO_ERROR)
    {
        ChipLogError(SecureChannel, "Failed to obtain IPK for initiating: %" CHIP_ERROR_FORMAT, err.Format());
        return err;
    }
    if ((ipkKeySet.num_keys_used == 0) || (ipkKeySet.num_keys_used > Credentials::GroupDataProvider::KeySet::kEpochKeysMax))
    {
        ChipLogError(SecureChannel, "Found invalid IPK keyset for initiator.");
        return CHIP_ERROR_INTERNAL;
    }

    // For the generation of the Destination Identifier,
    // the originator SHALL use the operational group key with the second oldest
    // EpochStartTime, if one exists, otherwise it SHALL use the single operational
    // group key available. The EpochStartTime are already ordered
    size_t ipkIndex = (ipkKeySet.num_keys_used > 1) ? ((ipkKeySet.num_keys_used - 1) - 1) : 0;
    memcpy(&mIPK[0], ipkKeySet.epoch_keys[ipkIndex].key, sizeof(mIPK));

    // Leaving this logging code for debug, but this cannot be enabled at runtime
    // since it leaks private security material.
#if 0
    ChipLogProgress(SecureChannel, "RecoverInitiatorIpk: GroupDataProvider %p, Got IPK for FabricIndex %u", mGroupDataProvider,
                    static_cast<unsigned>(mFabricIndex));
    ChipLogByteSpan(SecureChannel, ByteSpan(mIPK));
#endif

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::SendSigma1()
{
    MATTER_TRACE_SCOPE("SendSigma1", "CASESession");
    size_t data_len = TLV::EstimateStructOverhead(kSigmaParamRandomNumberSize,          // initiatorRandom
                                                  sizeof(uint16_t),                     // initiatorSessionId,
                                                  kSHA256_Hash_Length,                  // destinationId
                                                  kP256_PublicKey_Length,               // InitiatorEphPubKey,
                                                  SessionParameters::kEstimatedTLVSize, // initiatorSessionParams
                                                  SessionResumptionStorage::kResumptionIdSize, CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES);

    System::PacketBufferTLVWriter tlvWriter;
    System::PacketBufferHandle msg_R1;
    TLV::TLVType outerContainerType                    = TLV::kTLVType_NotSpecified;
    uint8_t destinationIdentifier[kSHA256_Hash_Length] = { 0 };

    // Lookup fabric info.
    const auto * fabricInfo = mFabricsTable->FindFabricWithIndex(mFabricIndex);
    ReturnErrorCodeIf(fabricInfo == nullptr, CHIP_ERROR_INCORRECT_STATE);

    // Validate that we have a session ID allocated.
    VerifyOrReturnError(GetLocalSessionId().HasValue(), CHIP_ERROR_INCORRECT_STATE);

    // Generate an ephemeral keypair
    mEphemeralKey = mFabricsTable->AllocateEphemeralKeypairForCASE();
    VerifyOrReturnError(mEphemeralKey != nullptr, CHIP_ERROR_NO_MEMORY);
    ReturnErrorOnFailure(mEphemeralKey->Initialize(ECPKeyTarget::ECDH));

    // Fill in the random value
    ReturnErrorOnFailure(DRBG_get_bytes(mInitiatorRandom, sizeof(mInitiatorRandom)));

    // Construct Sigma1 Msg
    msg_R1 = System::PacketBufferHandle::New(data_len);
    VerifyOrReturnError(!msg_R1.IsNull(), CHIP_ERROR_NO_MEMORY);

    tlvWriter.Init(std::move(msg_R1));
    ReturnErrorOnFailure(tlvWriter.StartContainer(TLV::AnonymousTag(), TLV::kTLVType_Structure, outerContainerType));
    ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(1), ByteSpan(mInitiatorRandom)));
    // Retrieve Session Identifier
    ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(2), GetLocalSessionId().Value()));

    // Generate a Destination Identifier based on the node we are attempting to reach
    {
        // Obtain originator IPK matching the fabric where we are trying to open a session. mIPK
        // will be properly set thereafter.
        ReturnErrorOnFailure(RecoverInitiatorIpk());

        FabricId fabricId = fabricInfo->GetFabricId();
        Crypto::P256PublicKey rootPubKey;
        ReturnErrorOnFailure(mFabricsTable->FetchRootPubkey(mFabricIndex, rootPubKey));
        Credentials::P256PublicKeySpan rootPubKeySpan{ rootPubKey.ConstBytes() };

        MutableByteSpan destinationIdSpan(destinationIdentifier);
        ReturnErrorOnFailure(GenerateCaseDestinationId(ByteSpan(mIPK), ByteSpan(mInitiatorRandom), rootPubKeySpan, fabricId,
                                                       mPeerNodeId, destinationIdSpan));
    }
    ReturnErrorOnFailure(tlvWriter.PutBytes(TLV::ContextTag(3), destinationIdentifier, sizeof(destinationIdentifier)));

    ReturnErrorOnFailure(
        tlvWriter.PutBytes(TLV::ContextTag(4), mEphemeralKey->Pubkey(), static_cast<uint32_t>(mEphemeralKey->Pubkey().Length())));

    ReturnErrorOnFailure(EncodeSessionParameters(TLV::ContextTag(5), mLocalMRPConfig, tlvWriter));

    // Try to find persistent session, and resume it.
    bool resuming = false;
    if (mSessionResumptionStorage != nullptr)
    {
        CHIP_ERROR err = mSessionResumptionStorage->FindByScopedNodeId(fabricInfo->GetScopedNodeIdForNode(mPeerNodeId),
                                                                       mResumeResumptionId, mSharedSecret, mPeerCATs);
        if (err == CHIP_NO_ERROR)
        {
            // Found valid resumption state, try to resume the session.
            ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(6), mResumeResumptionId));

            uint8_t initiatorResume1MIC[CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES];
            MutableByteSpan resumeMICSpan(initiatorResume1MIC);
            ReturnErrorOnFailure(GenerateSigmaResumeMIC(ByteSpan(mInitiatorRandom), ByteSpan(mResumeResumptionId),
                                                        ByteSpan(kKDFS1RKeyInfo), ByteSpan(kResume1MIC_Nonce), resumeMICSpan));

            ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(7), resumeMICSpan));
            resuming = true;
        }
    }

    ReturnErrorOnFailure(tlvWriter.EndContainer(outerContainerType));
    ReturnErrorOnFailure(tlvWriter.Finalize(&msg_R1));

    ReturnErrorOnFailure(mCommissioningHash.AddData(ByteSpan{ msg_R1->Start(), msg_R1->DataLength() }));

    // Call delegate to send the msg to peer
    ReturnErrorOnFailure(mExchangeCtxt.Value()->SendMessage(Protocols::SecureChannel::MsgType::CASE_Sigma1, std::move(msg_R1),
                                                            SendFlags(SendMessageFlags::kExpectResponse)));

    mState = resuming ? State::kSentSigma1Resume : State::kSentSigma1;

    ChipLogProgress(SecureChannel, "Sent Sigma1 msg");

    mDelegate->OnSessionEstablishmentStarted();

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::HandleSigma1_and_SendSigma2(System::PacketBufferHandle && msg)
{
    MATTER_TRACE_SCOPE("HandleSigma1_and_SendSigma2", "CASESession");
    ReturnErrorOnFailure(HandleSigma1(std::move(msg)));

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::FindLocalNodeFromDestinationId(const ByteSpan & destinationId, const ByteSpan & initiatorRandom)
{
    MATTER_TRACE_SCOPE("FindLocalNodeFromDestinationId", "CASESession");
    VerifyOrReturnError(mFabricsTable != nullptr, CHIP_ERROR_INCORRECT_STATE);

    bool found = false;
    for (const FabricInfo & fabricInfo : *mFabricsTable)
    {
        // Basic data for candidate fabric, used to compute candidate destination identifiers
        FabricId fabricId = fabricInfo.GetFabricId();
        NodeId nodeId     = fabricInfo.GetNodeId();
        Crypto::P256PublicKey rootPubKey;
        ReturnErrorOnFailure(mFabricsTable->FetchRootPubkey(fabricInfo.GetFabricIndex(), rootPubKey));
        Credentials::P256PublicKeySpan rootPubKeySpan{ rootPubKey.ConstBytes() };

        // Get IPK operational group key set for current candidate fabric
        GroupDataProvider::KeySet ipkKeySet;
        CHIP_ERROR err = mGroupDataProvider->GetIpkKeySet(fabricInfo.GetFabricIndex(), ipkKeySet);
        if ((err != CHIP_NO_ERROR) ||
            ((ipkKeySet.num_keys_used == 0) || (ipkKeySet.num_keys_used > Credentials::GroupDataProvider::KeySet::kEpochKeysMax)))
        {
            continue;
        }

        // Try every IPK candidate we have for a match
        for (size_t keyIdx = 0; keyIdx < ipkKeySet.num_keys_used; ++keyIdx)
        {
            uint8_t candidateDestinationId[kSHA256_Hash_Length];
            MutableByteSpan candidateDestinationIdSpan(candidateDestinationId);
            ByteSpan candidateIpkSpan(ipkKeySet.epoch_keys[keyIdx].key);

            err = GenerateCaseDestinationId(ByteSpan(candidateIpkSpan), ByteSpan(initiatorRandom), rootPubKeySpan, fabricId, nodeId,
                                            candidateDestinationIdSpan);
            if ((err == CHIP_NO_ERROR) && (candidateDestinationIdSpan.data_equal(destinationId)))
            {
                // Found a match, stop working, cache IPK, update local fabric context
                found = true;
                MutableByteSpan ipkSpan(mIPK);
                CopySpanToMutableSpan(candidateIpkSpan, ipkSpan);
                mFabricIndex = fabricInfo.GetFabricIndex();
                mLocalNodeId = nodeId;
                break;
            }
        }

        if (found)
        {
            break;
        }
    }

    return found ? CHIP_NO_ERROR : CHIP_ERROR_KEY_NOT_FOUND;
}

CHIP_ERROR CASESession::TryResumeSession(SessionResumptionStorage::ConstResumptionIdView resumptionId, ByteSpan resume1MIC,
                                         ByteSpan initiatorRandom)
{
    MATTER_TRACE_SCOPE("TryResumeSession", "CASESession");
    VerifyOrReturnError(mSessionResumptionStorage != nullptr, CHIP_ERROR_INCORRECT_STATE);
    VerifyOrReturnError(mFabricsTable != nullptr, CHIP_ERROR_INCORRECT_STATE);

    SessionResumptionStorage::ConstResumptionIdView resumptionIdSpan(resumptionId);
    ScopedNodeId node;
    ReturnErrorOnFailure(mSessionResumptionStorage->FindByResumptionId(resumptionIdSpan, node, mSharedSecret, mPeerCATs));

    // Cross check resume1MIC with the shared secret
    ReturnErrorOnFailure(
        ValidateSigmaResumeMIC(resume1MIC, initiatorRandom, resumptionId, ByteSpan(kKDFS1RKeyInfo), ByteSpan(kResume1MIC_Nonce)));

    const auto * fabricInfo = mFabricsTable->FindFabricWithIndex(node.GetFabricIndex());
    VerifyOrReturnError(fabricInfo != nullptr, CHIP_ERROR_INCORRECT_STATE);

    mFabricIndex = node.GetFabricIndex();
    mPeerNodeId  = node.GetNodeId();
    mLocalNodeId = fabricInfo->GetNodeId();

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::HandleSigma1(System::PacketBufferHandle && msg)
{
    MATTER_TRACE_SCOPE("HandleSigma1", "CASESession");
    CHIP_ERROR err = CHIP_NO_ERROR;
    System::PacketBufferTLVReader tlvReader;

    uint16_t initiatorSessionId;
    ByteSpan destinationIdentifier;
    ByteSpan initiatorRandom;

    ChipLogProgress(SecureChannel, "Received Sigma1 msg");
    MATTER_TRACE_COUNTER("Sigma1");

    bool sessionResumptionRequested = false;
    ByteSpan resumptionId;
    ByteSpan resume1MIC;
    ByteSpan initiatorPubKey;

    SuccessOrExit(err = mCommissioningHash.AddData(ByteSpan{ msg->Start(), msg->DataLength() }));

    tlvReader.Init(std::move(msg));
    SuccessOrExit(err = ParseSigma1(tlvReader, initiatorRandom, initiatorSessionId, destinationIdentifier, initiatorPubKey,
                                    sessionResumptionRequested, resumptionId, resume1MIC));

    ChipLogDetail(SecureChannel, "Peer assigned session key ID %d", initiatorSessionId);
    SetPeerSessionId(initiatorSessionId);

    VerifyOrExit(mFabricsTable != nullptr, err = CHIP_ERROR_INCORRECT_STATE);

    if (sessionResumptionRequested && resumptionId.size() == SessionResumptionStorage::kResumptionIdSize &&
        CHIP_NO_ERROR ==
            TryResumeSession(SessionResumptionStorage::ConstResumptionIdView(resumptionId.data()), resume1MIC, initiatorRandom))
    {
        std::copy(initiatorRandom.begin(), initiatorRandom.end(), mInitiatorRandom);
        std::copy(resumptionId.begin(), resumptionId.end(), mResumeResumptionId.begin());

        // Send Sigma2Resume message to the initiator
        SuccessOrExit(err = SendSigma2Resume());

        mDelegate->OnSessionEstablishmentStarted();

        // Early returning here, since we have sent Sigma2Resume, and no further processing is needed for the Sigma1 message
        return CHIP_NO_ERROR;
    }

    // Attempt to match the initiator's desired destination based on local fabric table.
    err = FindLocalNodeFromDestinationId(destinationIdentifier, initiatorRandom);
    if (err == CHIP_NO_ERROR)
    {
        ChipLogProgress(SecureChannel, "CASE matched destination ID: fabricIndex %u, NodeID 0x" ChipLogFormatX64,
                        static_cast<unsigned>(mFabricIndex), ChipLogValueX64(mLocalNodeId));

        // Side-effect of FindLocalNodeFromDestinationId success was that mFabricIndex/mLocalNodeId are now
        // set to the local fabric and associated NodeId that was targeted by the initiator.
    }
    else
    {
        ChipLogError(SecureChannel, "CASE failed to match destination ID with local fabrics");
        ChipLogByteSpan(SecureChannel, destinationIdentifier);
    }
    SuccessOrExit(err);

    // ParseSigma1 ensures that:
    // mRemotePubKey.Length() == initiatorPubKey.size() == kP256_PublicKey_Length.
    memcpy(mRemotePubKey.Bytes(), initiatorPubKey.data(), mRemotePubKey.Length());

    SuccessOrExit(err = SendSigma2());

    mDelegate->OnSessionEstablishmentStarted();

exit:

    if (err == CHIP_ERROR_KEY_NOT_FOUND)
    {
        SendStatusReport(mExchangeCtxt, kProtocolCodeNoSharedRoot);
        mState = State::kInitialized;
    }
    else if (err != CHIP_NO_ERROR)
    {
        SendStatusReport(mExchangeCtxt, kProtocolCodeInvalidParam);
        mState = State::kInitialized;
    }
    return err;
}

CHIP_ERROR CASESession::SendSigma2Resume()
{
    MATTER_TRACE_SCOPE("SendSigma2Resume", "CASESession");
    size_t max_sigma2_resume_data_len =
        TLV::EstimateStructOverhead(SessionResumptionStorage::kResumptionIdSize, CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES,
                                    sizeof(uint16_t), SessionParameters::kEstimatedTLVSize);

    System::PacketBufferTLVWriter tlvWriter;
    System::PacketBufferHandle msg_R2_resume;
    TLV::TLVType outerContainerType = TLV::kTLVType_NotSpecified;

    // Validate that we have a session ID allocated.
    VerifyOrReturnError(GetLocalSessionId().HasValue(), CHIP_ERROR_INCORRECT_STATE);

    msg_R2_resume = System::PacketBufferHandle::New(max_sigma2_resume_data_len);
    VerifyOrReturnError(!msg_R2_resume.IsNull(), CHIP_ERROR_NO_MEMORY);

    tlvWriter.Init(std::move(msg_R2_resume));

    // Generate a new resumption ID
    ReturnErrorOnFailure(DRBG_get_bytes(mNewResumptionId.data(), mNewResumptionId.size()));

    ReturnErrorOnFailure(tlvWriter.StartContainer(TLV::AnonymousTag(), TLV::kTLVType_Structure, outerContainerType));
    ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(1), mNewResumptionId));

    uint8_t sigma2ResumeMIC[CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES];
    MutableByteSpan resumeMICSpan(sigma2ResumeMIC);
    ReturnErrorOnFailure(GenerateSigmaResumeMIC(ByteSpan(mInitiatorRandom), mNewResumptionId, ByteSpan(kKDFS2RKeyInfo),
                                                ByteSpan(kResume2MIC_Nonce), resumeMICSpan));

    ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(2), resumeMICSpan));

    ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(3), GetLocalSessionId().Value()));

    ReturnErrorOnFailure(EncodeSessionParameters(TLV::ContextTag(4), mLocalMRPConfig, tlvWriter));

    ReturnErrorOnFailure(tlvWriter.EndContainer(outerContainerType));
    ReturnErrorOnFailure(tlvWriter.Finalize(&msg_R2_resume));

    // Call delegate to send the msg to peer
    ReturnErrorOnFailure(mExchangeCtxt.Value()->SendMessage(Protocols::SecureChannel::MsgType::CASE_Sigma2Resume,
                                                            std::move(msg_R2_resume),
                                                            SendFlags(SendMessageFlags::kExpectResponse)));

    mState = State::kSentSigma2Resume;

    ChipLogDetail(SecureChannel, "Sent Sigma2Resume msg");

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::SendSigma2()
{
    MATTER_TRACE_SCOPE("SendSigma2", "CASESession");

    VerifyOrReturnError(GetLocalSessionId().HasValue(), CHIP_ERROR_INCORRECT_STATE);
    VerifyOrReturnError(mFabricsTable != nullptr, CHIP_ERROR_INCORRECT_STATE);

    chip::Platform::ScopedMemoryBuffer<uint8_t> icacBuf;
    VerifyOrReturnError(icacBuf.Alloc(kMaxCHIPCertLength), CHIP_ERROR_NO_MEMORY);

    chip::Platform::ScopedMemoryBuffer<uint8_t> nocBuf;
    VerifyOrReturnError(nocBuf.Alloc(kMaxCHIPCertLength), CHIP_ERROR_NO_MEMORY);

    MutableByteSpan icaCert{ icacBuf.Get(), kMaxCHIPCertLength };
    ReturnErrorOnFailure(mFabricsTable->FetchICACert(mFabricIndex, icaCert));

    MutableByteSpan nocCert{ nocBuf.Get(), kMaxCHIPCertLength };
    ReturnErrorOnFailure(mFabricsTable->FetchNOCCert(mFabricIndex, nocCert));

    // Fill in the random value
    uint8_t msg_rand[kSigmaParamRandomNumberSize];
    ReturnErrorOnFailure(DRBG_get_bytes(&msg_rand[0], sizeof(msg_rand)));

    // Generate an ephemeral keypair
    mEphemeralKey = mFabricsTable->AllocateEphemeralKeypairForCASE();
    VerifyOrReturnError(mEphemeralKey != nullptr, CHIP_ERROR_NO_MEMORY);
    ReturnErrorOnFailure(mEphemeralKey->Initialize(ECPKeyTarget::ECDH));

    // Generate a Shared Secret
    ReturnErrorOnFailure(mEphemeralKey->ECDH_derive_secret(mRemotePubKey, mSharedSecret));

    uint8_t msg_salt[kIPKSize + kSigmaParamRandomNumberSize + kP256_PublicKey_Length + kSHA256_Hash_Length];

    MutableByteSpan saltSpan(msg_salt);
    ReturnErrorOnFailure(ConstructSaltSigma2(ByteSpan(msg_rand), mEphemeralKey->Pubkey(), ByteSpan(mIPK), saltSpan));

    AutoReleaseSessionKey sr2k(*mSessionManager->GetSessionKeystore());
    ReturnErrorOnFailure(DeriveSigmaKey(saltSpan, ByteSpan(kKDFSR2Info), sr2k));

    // Construct Sigma2 TBS Data
    size_t msg_r2_signed_len =
        TLV::EstimateStructOverhead(kMaxCHIPCertLength, kMaxCHIPCertLength, kP256_PublicKey_Length, kP256_PublicKey_Length);

    chip::Platform::ScopedMemoryBuffer<uint8_t> msg_R2_Signed;
    VerifyOrReturnError(msg_R2_Signed.Alloc(msg_r2_signed_len), CHIP_ERROR_NO_MEMORY);

    ReturnErrorOnFailure(ConstructTBSData(nocCert, icaCert, ByteSpan(mEphemeralKey->Pubkey(), mEphemeralKey->Pubkey().Length()),
                                          ByteSpan(mRemotePubKey, mRemotePubKey.Length()), msg_R2_Signed.Get(), msg_r2_signed_len));

    // Generate a Signature
    P256ECDSASignature tbsData2Signature;
    ReturnErrorOnFailure(
        mFabricsTable->SignWithOpKeypair(mFabricIndex, ByteSpan{ msg_R2_Signed.Get(), msg_r2_signed_len }, tbsData2Signature));
    msg_R2_Signed.Free();

    // Construct Sigma2 TBE Data
    size_t msg_r2_signed_enc_len = TLV::EstimateStructOverhead(nocCert.size(), icaCert.size(), tbsData2Signature.Length(),
                                                               SessionResumptionStorage::kResumptionIdSize);

    chip::Platform::ScopedMemoryBuffer<uint8_t> msg_R2_Encrypted;
    VerifyOrReturnError(msg_R2_Encrypted.Alloc(msg_r2_signed_enc_len + CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES), CHIP_ERROR_NO_MEMORY);

    TLV::TLVWriter tlvWriter;
    TLV::TLVType outerContainerType = TLV::kTLVType_NotSpecified;

    tlvWriter.Init(msg_R2_Encrypted.Get(), msg_r2_signed_enc_len);
    ReturnErrorOnFailure(tlvWriter.StartContainer(TLV::AnonymousTag(), TLV::kTLVType_Structure, outerContainerType));
    ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(kTag_TBEData_SenderNOC), nocCert));
    if (!icaCert.empty())
    {
        ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(kTag_TBEData_SenderICAC), icaCert));
    }

    // We are now done with ICAC and NOC certs so we can release the memory.
    {
        icacBuf.Free();
        icaCert = MutableByteSpan{};

        nocBuf.Free();
        nocCert = MutableByteSpan{};
    }

    ReturnErrorOnFailure(tlvWriter.PutBytes(TLV::ContextTag(kTag_TBEData_Signature), tbsData2Signature.ConstBytes(),
                                            static_cast<uint32_t>(tbsData2Signature.Length())));

    // Generate a new resumption ID
    ReturnErrorOnFailure(DRBG_get_bytes(mNewResumptionId.data(), mNewResumptionId.size()));
    ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(kTag_TBEData_ResumptionID), mNewResumptionId));

    ReturnErrorOnFailure(tlvWriter.EndContainer(outerContainerType));
    ReturnErrorOnFailure(tlvWriter.Finalize());
    msg_r2_signed_enc_len = static_cast<size_t>(tlvWriter.GetLengthWritten());

    // Generate the encrypted data blob
    ReturnErrorOnFailure(AES_CCM_encrypt(msg_R2_Encrypted.Get(), msg_r2_signed_enc_len, nullptr, 0, sr2k.KeyHandle(),
                                         kTBEData2_Nonce, kTBEDataNonceLength, msg_R2_Encrypted.Get(),
                                         msg_R2_Encrypted.Get() + msg_r2_signed_enc_len, CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES));

    // Construct Sigma2 Msg
    size_t size_of_local_session_id = sizeof(uint16_t);
    size_t data_len =
        TLV::EstimateStructOverhead(kSigmaParamRandomNumberSize, size_of_local_session_id, kP256_PublicKey_Length,
                                    msg_r2_signed_enc_len, CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES, SessionParameters::kEstimatedTLVSize);

    System::PacketBufferHandle msg_R2 = System::PacketBufferHandle::New(data_len);
    VerifyOrReturnError(!msg_R2.IsNull(), CHIP_ERROR_NO_MEMORY);

    System::PacketBufferTLVWriter tlvWriterMsg2;
    outerContainerType = TLV::kTLVType_NotSpecified;

    tlvWriterMsg2.Init(std::move(msg_R2));
    ReturnErrorOnFailure(tlvWriterMsg2.StartContainer(TLV::AnonymousTag(), TLV::kTLVType_Structure, outerContainerType));
    ReturnErrorOnFailure(tlvWriterMsg2.PutBytes(TLV::ContextTag(1), &msg_rand[0], sizeof(msg_rand)));
    ReturnErrorOnFailure(tlvWriterMsg2.Put(TLV::ContextTag(2), GetLocalSessionId().Value()));
    ReturnErrorOnFailure(tlvWriterMsg2.PutBytes(TLV::ContextTag(3), mEphemeralKey->Pubkey(),
                                                static_cast<uint32_t>(mEphemeralKey->Pubkey().Length())));
    ReturnErrorOnFailure(tlvWriterMsg2.PutBytes(TLV::ContextTag(4), msg_R2_Encrypted.Get(),
                                                static_cast<uint32_t>(msg_r2_signed_enc_len + CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES)));

    ReturnErrorOnFailure(EncodeSessionParameters(TLV::ContextTag(5), mLocalMRPConfig, tlvWriterMsg2));

    ReturnErrorOnFailure(tlvWriterMsg2.EndContainer(outerContainerType));
    ReturnErrorOnFailure(tlvWriterMsg2.Finalize(&msg_R2));

    ReturnErrorOnFailure(mCommissioningHash.AddData(ByteSpan{ msg_R2->Start(), msg_R2->DataLength() }));

    // Call delegate to send the msg to peer
    ReturnErrorOnFailure(mExchangeCtxt.Value()->SendMessage(Protocols::SecureChannel::MsgType::CASE_Sigma2, std::move(msg_R2),
                                                            SendFlags(SendMessageFlags::kExpectResponse)));

    mState = State::kSentSigma2;

    ChipLogProgress(SecureChannel, "Sent Sigma2 msg");
    MATTER_TRACE_COUNTER("Sigma2");

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::HandleSigma2Resume(System::PacketBufferHandle && msg)
{
    MATTER_TRACE_SCOPE("HandleSigma2Resume", "CASESession");
    CHIP_ERROR err = CHIP_NO_ERROR;
    System::PacketBufferTLVReader tlvReader;
    TLV::TLVType containerType = TLV::kTLVType_Structure;

    uint16_t responderSessionId;

    uint32_t decodeTagIdSeq = 0;

    ChipLogDetail(SecureChannel, "Received Sigma2Resume msg");
    MATTER_TRACE_COUNTER("Sigma2Resume");

    uint8_t sigma2ResumeMIC[CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES];

    tlvReader.Init(std::move(msg));
    SuccessOrExit(err = tlvReader.Next(containerType, TLV::AnonymousTag()));
    SuccessOrExit(err = tlvReader.EnterContainer(containerType));

    SuccessOrExit(err = tlvReader.Next());
    VerifyOrExit(TLV::TagNumFromTag(tlvReader.GetTag()) == ++decodeTagIdSeq, err = CHIP_ERROR_INVALID_TLV_TAG);
    SessionResumptionStorage::ResumptionIdStorage resumptionId;
    VerifyOrExit(tlvReader.GetLength() == resumptionId.size(), err = CHIP_ERROR_INVALID_TLV_ELEMENT);
    SuccessOrExit(err = tlvReader.GetBytes(resumptionId.data(), resumptionId.size()));

    SuccessOrExit(err = tlvReader.Next());
    VerifyOrExit(TLV::TagNumFromTag(tlvReader.GetTag()) == ++decodeTagIdSeq, err = CHIP_ERROR_INVALID_TLV_TAG);
    VerifyOrExit(tlvReader.GetLength() == CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES, err = CHIP_ERROR_INVALID_TLV_ELEMENT);
    SuccessOrExit(err = tlvReader.GetBytes(sigma2ResumeMIC, CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES));

    SuccessOrExit(err = ValidateSigmaResumeMIC(ByteSpan(sigma2ResumeMIC), ByteSpan(mInitiatorRandom), resumptionId,
                                               ByteSpan(kKDFS2RKeyInfo), ByteSpan(kResume2MIC_Nonce)));

    SuccessOrExit(err = tlvReader.Next());
    VerifyOrExit(TLV::TagNumFromTag(tlvReader.GetTag()) == ++decodeTagIdSeq, err = CHIP_ERROR_INVALID_TLV_TAG);
    SuccessOrExit(err = tlvReader.Get(responderSessionId));

    if (tlvReader.Next() != CHIP_END_OF_TLV)
    {
        SuccessOrExit(err = DecodeMRPParametersIfPresent(TLV::ContextTag(4), tlvReader));
        mExchangeCtxt.Value()->GetSessionHandle()->AsUnauthenticatedSession()->SetRemoteSessionParameters(
            GetRemoteSessionParameters());
    }

    ChipLogDetail(SecureChannel, "Peer assigned session session ID %d", responderSessionId);
    SetPeerSessionId(responderSessionId);

    if (mSessionResumptionStorage != nullptr)
    {
        CHIP_ERROR err2 = mSessionResumptionStorage->Save(GetPeer(), resumptionId, mSharedSecret, mPeerCATs);
        if (err2 != CHIP_NO_ERROR)
            ChipLogError(SecureChannel, "Unable to save session resumption state: %" CHIP_ERROR_FORMAT, err2.Format());
    }

    SendStatusReport(mExchangeCtxt, kProtocolCodeSuccess);

    mState = State::kFinishedViaResume;
    Finish();

exit:
    if (err != CHIP_NO_ERROR)
    {
        SendStatusReport(mExchangeCtxt, kProtocolCodeInvalidParam);
    }
    return err;
}

CHIP_ERROR CASESession::HandleSigma2_and_SendSigma3(System::PacketBufferHandle && msg)
{
    MATTER_TRACE_SCOPE("HandleSigma2_and_SendSigma3", "CASESession");
    ReturnErrorOnFailure(HandleSigma2(std::move(msg)));
    ReturnErrorOnFailure(SendSigma3a());

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::HandleSigma2(System::PacketBufferHandle && msg)
{
    MATTER_TRACE_SCOPE("HandleSigma2", "CASESession");
    CHIP_ERROR err = CHIP_NO_ERROR;
    System::PacketBufferTLVReader tlvReader;
    TLV::TLVReader decryptedDataTlvReader;
    TLV::TLVType containerType = TLV::kTLVType_Structure;

    const uint8_t * buf = msg->Start();
    size_t buflen       = msg->DataLength();

    uint8_t msg_salt[kIPKSize + kSigmaParamRandomNumberSize + kP256_PublicKey_Length + kSHA256_Hash_Length];

    chip::Platform::ScopedMemoryBuffer<uint8_t> msg_R2_Encrypted;
    size_t msg_r2_encrypted_len          = 0;
    size_t msg_r2_encrypted_len_with_tag = 0;

    chip::Platform::ScopedMemoryBuffer<uint8_t> msg_R2_Signed;
    size_t msg_r2_signed_len;
    size_t max_msg_r2_signed_enc_len;
    constexpr size_t kCaseOverheadForFutureTbeData = 128;

    AutoReleaseSessionKey sr2k(*mSessionManager->GetSessionKeystore());

    P256ECDSASignature tbsData2Signature;

    NodeId responderNodeId;
    P256PublicKey responderPublicKey;

    uint8_t responderRandom[kSigmaParamRandomNumberSize];
    ByteSpan responderNOC;
    ByteSpan responderICAC;

    uint16_t responderSessionId;

    ChipLogProgress(SecureChannel, "Received Sigma2 msg");

    FabricId fabricId = kUndefinedFabricId;
    {
        VerifyOrExit(mFabricsTable != nullptr, err = CHIP_ERROR_INCORRECT_STATE);
        const auto * fabricInfo = mFabricsTable->FindFabricWithIndex(mFabricIndex);
        VerifyOrExit(fabricInfo != nullptr, err = CHIP_ERROR_INCORRECT_STATE);
        fabricId = fabricInfo->GetFabricId();
    }

    VerifyOrExit(mEphemeralKey != nullptr, err = CHIP_ERROR_INTERNAL);
    VerifyOrExit(buf != nullptr, err = CHIP_ERROR_MESSAGE_INCOMPLETE);

    tlvReader.Init(std::move(msg));
    SuccessOrExit(err = tlvReader.Next(containerType, TLV::AnonymousTag()));
    SuccessOrExit(err = tlvReader.EnterContainer(containerType));

    // Retrieve Responder's Random value
    SuccessOrExit(err = tlvReader.Next(TLV::kTLVType_ByteString, TLV::ContextTag(kTag_Sigma2_ResponderRandom)));
    SuccessOrExit(err = tlvReader.GetBytes(responderRandom, sizeof(responderRandom)));

    // Assign Session ID
    SuccessOrExit(err = tlvReader.Next(TLV::kTLVType_UnsignedInteger, TLV::ContextTag(kTag_Sigma2_ResponderSessionId)));
    SuccessOrExit(err = tlvReader.Get(responderSessionId));

    ChipLogDetail(SecureChannel, "Peer assigned session session ID %d", responderSessionId);
    SetPeerSessionId(responderSessionId);

    // Retrieve Responder's Ephemeral Pubkey
    SuccessOrExit(err = tlvReader.Next(TLV::kTLVType_ByteString, TLV::ContextTag(kTag_Sigma2_ResponderEphPubKey)));
    SuccessOrExit(err = tlvReader.GetBytes(mRemotePubKey, static_cast<uint32_t>(mRemotePubKey.Length())));

    // Generate a Shared Secret
    SuccessOrExit(err = mEphemeralKey->ECDH_derive_secret(mRemotePubKey, mSharedSecret));

    // Generate the S2K key
    {
        MutableByteSpan saltSpan(msg_salt);
        SuccessOrExit(err = ConstructSaltSigma2(ByteSpan(responderRandom), mRemotePubKey, ByteSpan(mIPK), saltSpan));
        SuccessOrExit(err = DeriveSigmaKey(saltSpan, ByteSpan(kKDFSR2Info), sr2k));
    }

    SuccessOrExit(err = mCommissioningHash.AddData(ByteSpan{ buf, buflen }));

    // Generate decrypted data
    SuccessOrExit(err = tlvReader.Next(TLV::kTLVType_ByteString, TLV::ContextTag(kTag_Sigma2_Encrypted2)));

    max_msg_r2_signed_enc_len =
        TLV::EstimateStructOverhead(Credentials::kMaxCHIPCertLength, Credentials::kMaxCHIPCertLength, tbsData2Signature.Length(),
                                    SessionResumptionStorage::kResumptionIdSize, kCaseOverheadForFutureTbeData);
    msg_r2_encrypted_len_with_tag = tlvReader.GetLength();

    // Validate we did not receive a buffer larger than legal
    VerifyOrExit(msg_r2_encrypted_len_with_tag <= max_msg_r2_signed_enc_len, err = CHIP_ERROR_INVALID_TLV_ELEMENT);
    VerifyOrExit(msg_r2_encrypted_len_with_tag > CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES, err = CHIP_ERROR_INVALID_TLV_ELEMENT);
    VerifyOrExit(msg_R2_Encrypted.Alloc(msg_r2_encrypted_len_with_tag), err = CHIP_ERROR_NO_MEMORY);

    SuccessOrExit(err = tlvReader.GetBytes(msg_R2_Encrypted.Get(), static_cast<uint32_t>(msg_r2_encrypted_len_with_tag)));
    msg_r2_encrypted_len = msg_r2_encrypted_len_with_tag - CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES;

    SuccessOrExit(err = AES_CCM_decrypt(msg_R2_Encrypted.Get(), msg_r2_encrypted_len, nullptr, 0,
                                        msg_R2_Encrypted.Get() + msg_r2_encrypted_len, CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES,
                                        sr2k.KeyHandle(), kTBEData2_Nonce, kTBEDataNonceLength, msg_R2_Encrypted.Get()));

    decryptedDataTlvReader.Init(msg_R2_Encrypted.Get(), msg_r2_encrypted_len);
    containerType = TLV::kTLVType_Structure;
    SuccessOrExit(err = decryptedDataTlvReader.Next(containerType, TLV::AnonymousTag()));
    SuccessOrExit(err = decryptedDataTlvReader.EnterContainer(containerType));

    SuccessOrExit(err = decryptedDataTlvReader.Next(TLV::kTLVType_ByteString, TLV::ContextTag(kTag_TBEData_SenderNOC)));
    SuccessOrExit(err = decryptedDataTlvReader.Get(responderNOC));

    SuccessOrExit(err = decryptedDataTlvReader.Next());
    if (TLV::TagNumFromTag(decryptedDataTlvReader.GetTag()) == kTag_TBEData_SenderICAC)
    {
        VerifyOrExit(decryptedDataTlvReader.GetType() == TLV::kTLVType_ByteString, err = CHIP_ERROR_WRONG_TLV_TYPE);
        SuccessOrExit(err = decryptedDataTlvReader.Get(responderICAC));
        SuccessOrExit(err = decryptedDataTlvReader.Next(TLV::kTLVType_ByteString, TLV::ContextTag(kTag_TBEData_Signature)));
    }

    // Validate responder identity located in msg_r2_encrypted
    // Constructing responder identity
    {
        CompressedFabricId unused;
        FabricId responderFabricId;
        SuccessOrExit(err = SetEffectiveTime());
        SuccessOrExit(err = mFabricsTable->VerifyCredentials(mFabricIndex, responderNOC, responderICAC, mValidContext, unused,
                                                             responderFabricId, responderNodeId, responderPublicKey));
        VerifyOrExit(fabricId == responderFabricId, err = CHIP_ERROR_INVALID_CASE_PARAMETER);
        // Verify that responderNodeId (from responderNOC) matches one that was included
        // in the computation of the Destination Identifier when generating Sigma1.
        VerifyOrExit(mPeerNodeId == responderNodeId, err = CHIP_ERROR_INVALID_CASE_PARAMETER);
    }

    // Construct msg_R2_Signed and validate the signature in msg_r2_encrypted
    msg_r2_signed_len = TLV::EstimateStructOverhead(sizeof(uint16_t), responderNOC.size(), responderICAC.size(),
                                                    kP256_PublicKey_Length, kP256_PublicKey_Length);

    VerifyOrExit(msg_R2_Signed.Alloc(msg_r2_signed_len), err = CHIP_ERROR_NO_MEMORY);

    SuccessOrExit(err = ConstructTBSData(responderNOC, responderICAC, ByteSpan(mRemotePubKey, mRemotePubKey.Length()),
                                         ByteSpan(mEphemeralKey->Pubkey(), mEphemeralKey->Pubkey().Length()), msg_R2_Signed.Get(),
                                         msg_r2_signed_len));

    VerifyOrExit(TLV::TagNumFromTag(decryptedDataTlvReader.GetTag()) == kTag_TBEData_Signature, err = CHIP_ERROR_INVALID_TLV_TAG);
    VerifyOrExit(tbsData2Signature.Capacity() >= decryptedDataTlvReader.GetLength(), err = CHIP_ERROR_INVALID_TLV_ELEMENT);
    tbsData2Signature.SetLength(decryptedDataTlvReader.GetLength());
    SuccessOrExit(err = decryptedDataTlvReader.GetBytes(tbsData2Signature.Bytes(), tbsData2Signature.Length()));

    // Validate signature
    SuccessOrExit(err = responderPublicKey.ECDSA_validate_msg_signature(msg_R2_Signed.Get(), msg_r2_signed_len, tbsData2Signature));

    // Retrieve session resumption ID
    SuccessOrExit(err = decryptedDataTlvReader.Next(TLV::kTLVType_ByteString, TLV::ContextTag(kTag_TBEData_ResumptionID)));
    SuccessOrExit(err = decryptedDataTlvReader.GetBytes(mNewResumptionId.data(), mNewResumptionId.size()));

    // Retrieve peer CASE Authenticated Tags (CATs) from peer's NOC.
    SuccessOrExit(err = ExtractCATsFromOpCert(responderNOC, mPeerCATs));

    // Retrieve responderMRPParams if present
    if (tlvReader.Next() != CHIP_END_OF_TLV)
    {
        SuccessOrExit(err = DecodeMRPParametersIfPresent(TLV::ContextTag(kTag_Sigma2_ResponderMRPParams), tlvReader));
        mExchangeCtxt.Value()->GetSessionHandle()->AsUnauthenticatedSession()->SetRemoteSessionParameters(
            GetRemoteSessionParameters());
    }

exit:
    if (err != CHIP_NO_ERROR)
    {
        SendStatusReport(mExchangeCtxt, kProtocolCodeInvalidParam);
    }
    return err;
}

CHIP_ERROR CASESession::SendSigma3a()
{
    MATTER_TRACE_SCOPE("SendSigma3", "CASESession");
    CHIP_ERROR err = CHIP_NO_ERROR;

    ChipLogDetail(SecureChannel, "Sending Sigma3");

    auto helper = WorkHelper<SendSigma3Data>::Create(*this, &SendSigma3b, &CASESession::SendSigma3c);
    VerifyOrExit(helper, err = CHIP_ERROR_NO_MEMORY);
    {
        auto & data = helper->mData;

        VerifyOrExit(mFabricsTable != nullptr, err = CHIP_ERROR_INCORRECT_STATE);
        data.fabricIndex = mFabricIndex;
        data.fabricTable = nullptr;
        data.keystore    = nullptr;

        {
            const FabricInfo * fabricInfo = mFabricsTable->FindFabricWithIndex(mFabricIndex);
            VerifyOrExit(fabricInfo != nullptr, err = CHIP_ERROR_KEY_NOT_FOUND);
            auto * keystore = mFabricsTable->GetOperationalKeystore();
            if (!fabricInfo->HasOperationalKey() && keystore != nullptr && keystore->SupportsSignWithOpKeypairInBackground())
            {
                // NOTE: used to sign in background.
                data.keystore = keystore;
            }
            else
            {
                // NOTE: used to sign in foreground.
                data.fabricTable = mFabricsTable;
            }
        }

        VerifyOrExit(mEphemeralKey != nullptr, err = CHIP_ERROR_INTERNAL);

        VerifyOrExit(data.icacBuf.Alloc(kMaxCHIPCertLength), err = CHIP_ERROR_NO_MEMORY);
        data.icaCert = MutableByteSpan{ data.icacBuf.Get(), kMaxCHIPCertLength };

        VerifyOrExit(data.nocBuf.Alloc(kMaxCHIPCertLength), err = CHIP_ERROR_NO_MEMORY);
        data.nocCert = MutableByteSpan{ data.nocBuf.Get(), kMaxCHIPCertLength };

        SuccessOrExit(err = mFabricsTable->FetchICACert(mFabricIndex, data.icaCert));
        SuccessOrExit(err = mFabricsTable->FetchNOCCert(mFabricIndex, data.nocCert));

        // Prepare Sigma3 TBS Data Blob
        data.msg_r3_signed_len =
            TLV::EstimateStructOverhead(data.icaCert.size(), data.nocCert.size(), kP256_PublicKey_Length, kP256_PublicKey_Length);

        VerifyOrExit(data.msg_R3_Signed.Alloc(data.msg_r3_signed_len), err = CHIP_ERROR_NO_MEMORY);

        SuccessOrExit(err = ConstructTBSData(
                          data.nocCert, data.icaCert, ByteSpan(mEphemeralKey->Pubkey(), mEphemeralKey->Pubkey().Length()),
                          ByteSpan(mRemotePubKey, mRemotePubKey.Length()), data.msg_R3_Signed.Get(), data.msg_r3_signed_len));

        if (data.keystore != nullptr)
        {
            SuccessOrExit(err = helper->ScheduleWork());
            mSendSigma3Helper = helper;
            mExchangeCtxt.Value()->WillSendMessage();
            mState = State::kSendSigma3Pending;
        }
        else
        {
            SuccessOrExit(err = helper->DoWork());
        }
    }

exit:
    if (err != CHIP_NO_ERROR)
    {
        SendStatusReport(mExchangeCtxt, kProtocolCodeInvalidParam);
        mState = State::kInitialized;
    }

    return err;
}

CHIP_ERROR CASESession::SendSigma3b(SendSigma3Data & data, bool & cancel)
{
    // Generate a signature
    if (data.keystore != nullptr)
    {
        // Recommended case: delegate to operational keystore
        ReturnErrorOnFailure(data.keystore->SignWithOpKeypair(
            data.fabricIndex, ByteSpan{ data.msg_R3_Signed.Get(), data.msg_r3_signed_len }, data.tbsData3Signature));
    }
    else
    {
        // Legacy case: delegate to fabric table fabric info
        ReturnErrorOnFailure(data.fabricTable->SignWithOpKeypair(
            data.fabricIndex, ByteSpan{ data.msg_R3_Signed.Get(), data.msg_r3_signed_len }, data.tbsData3Signature));
    }

    // Prepare Sigma3 TBE Data Blob
    data.msg_r3_encrypted_len =
        TLV::EstimateStructOverhead(data.nocCert.size(), data.icaCert.size(), data.tbsData3Signature.Length());

    VerifyOrReturnError(data.msg_R3_Encrypted.Alloc(data.msg_r3_encrypted_len + CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES),
                        CHIP_ERROR_NO_MEMORY);

    {
        TLV::TLVWriter tlvWriter;
        TLV::TLVType outerContainerType = TLV::kTLVType_NotSpecified;

        tlvWriter.Init(data.msg_R3_Encrypted.Get(), data.msg_r3_encrypted_len);
        ReturnErrorOnFailure(tlvWriter.StartContainer(TLV::AnonymousTag(), TLV::kTLVType_Structure, outerContainerType));
        ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(kTag_TBEData_SenderNOC), data.nocCert));
        if (!data.icaCert.empty())
        {
            ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(kTag_TBEData_SenderICAC), data.icaCert));
        }

        // We are now done with ICAC and NOC certs so we can release the memory.
        {
            data.icacBuf.Free();
            data.icaCert = MutableByteSpan{};

            data.nocBuf.Free();
            data.nocCert = MutableByteSpan{};
        }

        ReturnErrorOnFailure(tlvWriter.PutBytes(TLV::ContextTag(kTag_TBEData_Signature), data.tbsData3Signature.ConstBytes(),
                                                static_cast<uint32_t>(data.tbsData3Signature.Length())));
        ReturnErrorOnFailure(tlvWriter.EndContainer(outerContainerType));
        ReturnErrorOnFailure(tlvWriter.Finalize());
        data.msg_r3_encrypted_len = static_cast<size_t>(tlvWriter.GetLengthWritten());
    }

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::SendSigma3c(SendSigma3Data & data, CHIP_ERROR status)
{
    CHIP_ERROR err = CHIP_NO_ERROR;

    System::PacketBufferHandle msg_R3;
    size_t data_len;

    uint8_t msg_salt[kIPKSize + kSHA256_Hash_Length];

    AutoReleaseSessionKey sr3k(*mSessionManager->GetSessionKeystore());

    VerifyOrDieWithMsg(data.keystore == nullptr || mState == State::kSendSigma3Pending, SecureChannel, "Bad internal state.");

    SuccessOrExit(err = status);

    // Generate S3K key
    {
        MutableByteSpan saltSpan(msg_salt);
        SuccessOrExit(err = ConstructSaltSigma3(ByteSpan(mIPK), saltSpan));
        SuccessOrExit(err = DeriveSigmaKey(saltSpan, ByteSpan(kKDFSR3Info), sr3k));
    }

    // Generated Encrypted data blob
    SuccessOrExit(err =
                      AES_CCM_encrypt(data.msg_R3_Encrypted.Get(), data.msg_r3_encrypted_len, nullptr, 0, sr3k.KeyHandle(),
                                      kTBEData3_Nonce, kTBEDataNonceLength, data.msg_R3_Encrypted.Get(),
                                      data.msg_R3_Encrypted.Get() + data.msg_r3_encrypted_len, CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES));

    // Generate Sigma3 Msg
    data_len = TLV::EstimateStructOverhead(CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES, data.msg_r3_encrypted_len);

    msg_R3 = System::PacketBufferHandle::New(data_len);
    VerifyOrExit(!msg_R3.IsNull(), err = CHIP_ERROR_NO_MEMORY);

    {
        System::PacketBufferTLVWriter tlvWriter;
        TLV::TLVType outerContainerType = TLV::kTLVType_NotSpecified;

        tlvWriter.Init(std::move(msg_R3));
        err = tlvWriter.StartContainer(TLV::AnonymousTag(), TLV::kTLVType_Structure, outerContainerType);
        SuccessOrExit(err);
        err = tlvWriter.PutBytes(TLV::ContextTag(1), data.msg_R3_Encrypted.Get(),
                                 static_cast<uint32_t>(data.msg_r3_encrypted_len + CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES));
        SuccessOrExit(err);
        err = tlvWriter.EndContainer(outerContainerType);
        SuccessOrExit(err);
        err = tlvWriter.Finalize(&msg_R3);
        SuccessOrExit(err);
    }

    err = mCommissioningHash.AddData(ByteSpan{ msg_R3->Start(), msg_R3->DataLength() });
    SuccessOrExit(err);

    // Call delegate to send the Msg3 to peer
    err = mExchangeCtxt.Value()->SendMessage(Protocols::SecureChannel::MsgType::CASE_Sigma3, std::move(msg_R3),
                                             SendFlags(SendMessageFlags::kExpectResponse));
    SuccessOrExit(err);

    ChipLogProgress(SecureChannel, "Sent Sigma3 msg");

    {
        MutableByteSpan messageDigestSpan(mMessageDigest);
        SuccessOrExit(err = mCommissioningHash.Finish(messageDigestSpan));
    }

    mState = State::kSentSigma3;

exit:
    mSendSigma3Helper.reset();

    // If data.keystore is set, processing occurred in the background, so if an error occurred,
    // need to send status report (normally occurs in SendSigma3a), and discard exchange and
    // abort pending establish (normally occurs in OnMessageReceived).
    if (data.keystore != nullptr && err != CHIP_NO_ERROR)
    {
        SendStatusReport(mExchangeCtxt, kProtocolCodeInvalidParam);
        DiscardExchange();
        AbortPendingEstablish(err);
    }

    return err;
}

CHIP_ERROR CASESession::HandleSigma3a(System::PacketBufferHandle && msg)
{
    MATTER_TRACE_SCOPE("HandleSigma3", "CASESession");
    CHIP_ERROR err = CHIP_NO_ERROR;
    System::PacketBufferTLVReader tlvReader;
    TLV::TLVReader decryptedDataTlvReader;
    TLV::TLVType containerType = TLV::kTLVType_Structure;

    const uint8_t * buf   = msg->Start();
    const uint16_t bufLen = msg->DataLength();

    constexpr size_t kCaseOverheadForFutureTbeData = 128;

    chip::Platform::ScopedMemoryBuffer<uint8_t> msg_R3_Encrypted;
    size_t msg_r3_encrypted_len          = 0;
    size_t msg_r3_encrypted_len_with_tag = 0;
    size_t max_msg_r3_signed_enc_len;

    AutoReleaseSessionKey sr3k(*mSessionManager->GetSessionKeystore());

    uint8_t msg_salt[kIPKSize + kSHA256_Hash_Length];

    ChipLogProgress(SecureChannel, "Received Sigma3 msg");
    MATTER_TRACE_COUNTER("Sigma3");

    auto helper = WorkHelper<HandleSigma3Data>::Create(*this, &HandleSigma3b, &CASESession::HandleSigma3c);
    VerifyOrExit(helper, err = CHIP_ERROR_NO_MEMORY);
    {
        auto & data = helper->mData;

        {
            VerifyOrExit(mFabricsTable != nullptr, err = CHIP_ERROR_INCORRECT_STATE);
            const auto * fabricInfo = mFabricsTable->FindFabricWithIndex(mFabricIndex);
            VerifyOrExit(fabricInfo != nullptr, err = CHIP_ERROR_INCORRECT_STATE);
            data.fabricId = fabricInfo->GetFabricId();
        }

        VerifyOrExit(mEphemeralKey != nullptr, err = CHIP_ERROR_INTERNAL);

        tlvReader.Init(std::move(msg));
        SuccessOrExit(err = tlvReader.Next(containerType, TLV::AnonymousTag()));
        SuccessOrExit(err = tlvReader.EnterContainer(containerType));

        // Fetch encrypted data
        max_msg_r3_signed_enc_len = TLV::EstimateStructOverhead(Credentials::kMaxCHIPCertLength, Credentials::kMaxCHIPCertLength,
                                                                data.tbsData3Signature.Length(), kCaseOverheadForFutureTbeData);

        SuccessOrExit(err = tlvReader.Next(TLV::kTLVType_ByteString, TLV::ContextTag(kTag_Sigma3_Encrypted3)));

        msg_r3_encrypted_len_with_tag = tlvReader.GetLength();

        // Validate we did not receive a buffer larger than legal
        VerifyOrExit(msg_r3_encrypted_len_with_tag <= max_msg_r3_signed_enc_len, err = CHIP_ERROR_INVALID_TLV_ELEMENT);
        VerifyOrExit(msg_r3_encrypted_len_with_tag > CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES, err = CHIP_ERROR_INVALID_TLV_ELEMENT);

        VerifyOrExit(msg_R3_Encrypted.Alloc(msg_r3_encrypted_len_with_tag), err = CHIP_ERROR_NO_MEMORY);
        SuccessOrExit(err = tlvReader.GetBytes(msg_R3_Encrypted.Get(), static_cast<uint32_t>(msg_r3_encrypted_len_with_tag)));
        msg_r3_encrypted_len = msg_r3_encrypted_len_with_tag - CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES;

        // Step 1
        {
            MutableByteSpan saltSpan(msg_salt);
            SuccessOrExit(err = ConstructSaltSigma3(ByteSpan(mIPK), saltSpan));
            SuccessOrExit(err = DeriveSigmaKey(saltSpan, ByteSpan(kKDFSR3Info), sr3k));
        }

        SuccessOrExit(err = mCommissioningHash.AddData(ByteSpan{ buf, bufLen }));

        // Step 2 - Decrypt data blob
        SuccessOrExit(err = AES_CCM_decrypt(msg_R3_Encrypted.Get(), msg_r3_encrypted_len, nullptr, 0,
                                            msg_R3_Encrypted.Get() + msg_r3_encrypted_len, CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES,
                                            sr3k.KeyHandle(), kTBEData3_Nonce, kTBEDataNonceLength, msg_R3_Encrypted.Get()));

        decryptedDataTlvReader.Init(msg_R3_Encrypted.Get(), msg_r3_encrypted_len);
        containerType = TLV::kTLVType_Structure;
        SuccessOrExit(err = decryptedDataTlvReader.Next(containerType, TLV::AnonymousTag()));
        SuccessOrExit(err = decryptedDataTlvReader.EnterContainer(containerType));

        SuccessOrExit(err = decryptedDataTlvReader.Next(TLV::kTLVType_ByteString, TLV::ContextTag(kTag_TBEData_SenderNOC)));
        SuccessOrExit(err = decryptedDataTlvReader.Get(data.initiatorNOC));

        SuccessOrExit(err = decryptedDataTlvReader.Next());
        if (TLV::TagNumFromTag(decryptedDataTlvReader.GetTag()) == kTag_TBEData_SenderICAC)
        {
            VerifyOrExit(decryptedDataTlvReader.GetType() == TLV::kTLVType_ByteString, err = CHIP_ERROR_WRONG_TLV_TYPE);
            SuccessOrExit(err = decryptedDataTlvReader.Get(data.initiatorICAC));
            SuccessOrExit(err = decryptedDataTlvReader.Next(TLV::kTLVType_ByteString, TLV::ContextTag(kTag_TBEData_Signature)));
        }

        // Step 4 - Construct Sigma3 TBS Data
        data.msg_r3_signed_len = TLV::EstimateStructOverhead(sizeof(uint16_t), data.initiatorNOC.size(), data.initiatorICAC.size(),
                                                             kP256_PublicKey_Length, kP256_PublicKey_Length);

        VerifyOrExit(data.msg_R3_Signed.Alloc(data.msg_r3_signed_len), err = CHIP_ERROR_NO_MEMORY);

        SuccessOrExit(err = ConstructTBSData(data.initiatorNOC, data.initiatorICAC, ByteSpan(mRemotePubKey, mRemotePubKey.Length()),
                                             ByteSpan(mEphemeralKey->Pubkey(), mEphemeralKey->Pubkey().Length()),
                                             data.msg_R3_Signed.Get(), data.msg_r3_signed_len));

        VerifyOrExit(TLV::TagNumFromTag(decryptedDataTlvReader.GetTag()) == kTag_TBEData_Signature,
                     err = CHIP_ERROR_INVALID_TLV_TAG);
        VerifyOrExit(data.tbsData3Signature.Capacity() >= decryptedDataTlvReader.GetLength(), err = CHIP_ERROR_INVALID_TLV_ELEMENT);
        data.tbsData3Signature.SetLength(decryptedDataTlvReader.GetLength());
        SuccessOrExit(err = decryptedDataTlvReader.GetBytes(data.tbsData3Signature.Bytes(), data.tbsData3Signature.Length()));

        // Prepare for Step 5/6
        {
            MutableByteSpan fabricRCAC{ data.rootCertBuf };
            SuccessOrExit(err = mFabricsTable->FetchRootCert(mFabricIndex, fabricRCAC));
            data.fabricRCAC = fabricRCAC;
            // TODO probably should make SetEffectiveTime static and call closer to VerifyCredentials
            SuccessOrExit(err = SetEffectiveTime());
        }

        // Copy remaining needed data into work structure
        {
            data.validContext = mValidContext;

            // initiatorNOC and initiatorICAC are spans into msg_R3_Encrypted
            // which is going away, so to save memory, redirect them to their
            // copies in msg_R3_signed, which is staying around
            TLV::TLVReader signedDataTlvReader;
            signedDataTlvReader.Init(data.msg_R3_Signed.Get(), data.msg_r3_signed_len);
            SuccessOrExit(err = signedDataTlvReader.Next(TLV::kTLVType_Structure, TLV::AnonymousTag()));
            SuccessOrExit(err = signedDataTlvReader.EnterContainer(containerType));

            SuccessOrExit(err = signedDataTlvReader.Next(TLV::kTLVType_ByteString, TLV::ContextTag(kTag_TBSData_SenderNOC)));
            SuccessOrExit(err = signedDataTlvReader.Get(data.initiatorNOC));

            if (!data.initiatorICAC.empty())
            {
                SuccessOrExit(err = signedDataTlvReader.Next(TLV::kTLVType_ByteString, TLV::ContextTag(kTag_TBSData_SenderICAC)));
                SuccessOrExit(err = signedDataTlvReader.Get(data.initiatorICAC));
            }
        }

        SuccessOrExit(err = helper->ScheduleWork());
        mHandleSigma3Helper = helper;
        mExchangeCtxt.Value()->WillSendMessage();
        mState = State::kHandleSigma3Pending;
    }

exit:
    if (err != CHIP_NO_ERROR)
    {
        SendStatusReport(mExchangeCtxt, kProtocolCodeInvalidParam);
    }

    return err;
}

CHIP_ERROR CASESession::HandleSigma3b(HandleSigma3Data & data, bool & cancel)
{
    // Step 5/6
    // Validate initiator identity located in msg->Start()
    // Constructing responder identity
    CompressedFabricId unused;
    FabricId initiatorFabricId;
    P256PublicKey initiatorPublicKey;
    ReturnErrorOnFailure(FabricTable::VerifyCredentials(data.initiatorNOC, data.initiatorICAC, data.fabricRCAC, data.validContext,
                                                        unused, initiatorFabricId, data.initiatorNodeId, initiatorPublicKey));
    VerifyOrReturnError(data.fabricId == initiatorFabricId, CHIP_ERROR_INVALID_CASE_PARAMETER);

    // TODO - Validate message signature prior to validating the received operational credentials.
    //        The op cert check requires traversal of cert chain, that is a more expensive operation.
    //        If message signature check fails, the cert chain check will be unnecessary, but with the
    //        current flow of code, a malicious node can trigger a DoS style attack on the device.
    //        The same change should be made in Sigma2 processing.
    // Step 7 - Validate Signature
    ReturnErrorOnFailure(
        initiatorPublicKey.ECDSA_validate_msg_signature(data.msg_R3_Signed.Get(), data.msg_r3_signed_len, data.tbsData3Signature));

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::HandleSigma3c(HandleSigma3Data & data, CHIP_ERROR status)
{
    CHIP_ERROR err = CHIP_NO_ERROR;

    VerifyOrExit(mState == State::kHandleSigma3Pending, err = CHIP_ERROR_INCORRECT_STATE);

    SuccessOrExit(err = status);

    mPeerNodeId = data.initiatorNodeId;

    {
        MutableByteSpan messageDigestSpan(mMessageDigest);
        SuccessOrExit(err = mCommissioningHash.Finish(messageDigestSpan));
    }

    // Retrieve peer CASE Authenticated Tags (CATs) from peer's NOC.
    {
        SuccessOrExit(err = ExtractCATsFromOpCert(data.initiatorNOC, mPeerCATs));
    }

    if (mSessionResumptionStorage != nullptr)
    {
        CHIP_ERROR err2 = mSessionResumptionStorage->Save(GetPeer(), mNewResumptionId, mSharedSecret, mPeerCATs);
        if (err2 != CHIP_NO_ERROR)
        {
            ChipLogError(SecureChannel, "Unable to save session resumption state: %" CHIP_ERROR_FORMAT, err2.Format());
        }
    }

    SendStatusReport(mExchangeCtxt, kProtocolCodeSuccess);

    mState = State::kFinished;
    Finish();

exit:
    mHandleSigma3Helper.reset();

    if (err != CHIP_NO_ERROR)
    {
        SendStatusReport(mExchangeCtxt, kProtocolCodeInvalidParam);
        // Abort the pending establish, which is normally done by CASESession::OnMessageReceived,
        // but in the background processing case must be done here.
        DiscardExchange();
        AbortPendingEstablish(err);
    }

    return err;
}

CHIP_ERROR CASESession::DeriveSigmaKey(const ByteSpan & salt, const ByteSpan & info, AutoReleaseSessionKey & key) const
{
    return mSessionManager->GetSessionKeystore()->DeriveKey(mSharedSecret, salt, info, key.KeyHandle());
}

CHIP_ERROR CASESession::ConstructSaltSigma2(const ByteSpan & rand, const Crypto::P256PublicKey & pubkey, const ByteSpan & ipk,
                                            MutableByteSpan & salt)
{
    uint8_t md[kSHA256_Hash_Length];
    memset(salt.data(), 0, salt.size());
    Encoding::LittleEndian::BufferWriter bbuf(salt.data(), salt.size());

    bbuf.Put(ipk.data(), ipk.size());
    bbuf.Put(rand.data(), kSigmaParamRandomNumberSize);
    bbuf.Put(pubkey, pubkey.Length());
    MutableByteSpan messageDigestSpan(md);
    ReturnErrorOnFailure(mCommissioningHash.GetDigest(messageDigestSpan));
    bbuf.Put(messageDigestSpan.data(), messageDigestSpan.size());

    size_t saltWritten = 0;
    VerifyOrReturnError(bbuf.Fit(saltWritten), CHIP_ERROR_BUFFER_TOO_SMALL);
    salt = salt.SubSpan(0, saltWritten);

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::ConstructSaltSigma3(const ByteSpan & ipk, MutableByteSpan & salt)
{
    uint8_t md[kSHA256_Hash_Length];
    memset(salt.data(), 0, salt.size());
    Encoding::LittleEndian::BufferWriter bbuf(salt.data(), salt.size());

    bbuf.Put(ipk.data(), ipk.size());
    MutableByteSpan messageDigestSpan(md);
    ReturnErrorOnFailure(mCommissioningHash.GetDigest(messageDigestSpan));
    bbuf.Put(messageDigestSpan.data(), messageDigestSpan.size());

    size_t saltWritten = 0;
    VerifyOrReturnError(bbuf.Fit(saltWritten), CHIP_ERROR_BUFFER_TOO_SMALL);
    salt = salt.SubSpan(0, saltWritten);

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::ConstructSigmaResumeKey(const ByteSpan & initiatorRandom, const ByteSpan & resumptionID,
                                                const ByteSpan & skInfo, const ByteSpan & nonce, AutoReleaseSessionKey & resumeKey)
{
    constexpr size_t saltSize = kSigmaParamRandomNumberSize + SessionResumptionStorage::kResumptionIdSize;
    uint8_t salt[saltSize];

    memset(salt, 0, saltSize);
    Encoding::LittleEndian::BufferWriter bbuf(salt, saltSize);

    bbuf.Put(initiatorRandom.data(), initiatorRandom.size());
    bbuf.Put(resumptionID.data(), resumptionID.size());

    size_t saltWritten = 0;
    VerifyOrReturnError(bbuf.Fit(saltWritten), CHIP_ERROR_BUFFER_TOO_SMALL);

    return DeriveSigmaKey(ByteSpan(salt, saltWritten), skInfo, resumeKey);
}

CHIP_ERROR CASESession::GenerateSigmaResumeMIC(const ByteSpan & initiatorRandom, const ByteSpan & resumptionID,
                                               const ByteSpan & skInfo, const ByteSpan & nonce, MutableByteSpan & resumeMIC)
{
    VerifyOrReturnError(resumeMIC.size() >= CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES, CHIP_ERROR_BUFFER_TOO_SMALL);

    AutoReleaseSessionKey srk(*mSessionManager->GetSessionKeystore());
    ReturnErrorOnFailure(ConstructSigmaResumeKey(initiatorRandom, resumptionID, skInfo, nonce, srk));
    ReturnErrorOnFailure(AES_CCM_encrypt(nullptr, 0, nullptr, 0, srk.KeyHandle(), nonce.data(), nonce.size(), nullptr,
                                         resumeMIC.data(), CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES));
    resumeMIC.reduce_size(CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES);

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::ValidateSigmaResumeMIC(const ByteSpan & resumeMIC, const ByteSpan & initiatorRandom,
                                               const ByteSpan & resumptionID, const ByteSpan & skInfo, const ByteSpan & nonce)
{
    VerifyOrReturnError(resumeMIC.size() == CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES, CHIP_ERROR_BUFFER_TOO_SMALL);

    AutoReleaseSessionKey srk(*mSessionManager->GetSessionKeystore());
    ReturnErrorOnFailure(ConstructSigmaResumeKey(initiatorRandom, resumptionID, skInfo, nonce, srk));
    ReturnErrorOnFailure(AES_CCM_decrypt(nullptr, 0, nullptr, 0, resumeMIC.data(), resumeMIC.size(), srk.KeyHandle(), nonce.data(),
                                         nonce.size(), nullptr));

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::ConstructTBSData(const ByteSpan & senderNOC, const ByteSpan & senderICAC, const ByteSpan & senderPubKey,
                                         const ByteSpan & receiverPubKey, uint8_t * tbsData, size_t & tbsDataLen)
{
    TLV::TLVWriter tlvWriter;
    TLV::TLVType outerContainerType = TLV::kTLVType_NotSpecified;

    tlvWriter.Init(tbsData, tbsDataLen);
    ReturnErrorOnFailure(tlvWriter.StartContainer(TLV::AnonymousTag(), TLV::kTLVType_Structure, outerContainerType));
    ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(kTag_TBSData_SenderNOC), senderNOC));
    if (!senderICAC.empty())
    {
        ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(kTag_TBSData_SenderICAC), senderICAC));
    }
    ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(kTag_TBSData_SenderPubKey), senderPubKey));
    ReturnErrorOnFailure(tlvWriter.Put(TLV::ContextTag(kTag_TBSData_ReceiverPubKey), receiverPubKey));
    ReturnErrorOnFailure(tlvWriter.EndContainer(outerContainerType));
    ReturnErrorOnFailure(tlvWriter.Finalize());
    tbsDataLen = static_cast<size_t>(tlvWriter.GetLengthWritten());

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::SetEffectiveTime()
{
    System::Clock::Milliseconds64 currentUnixTimeMS;
    CHIP_ERROR err = System::SystemClock().GetClock_RealTimeMS(currentUnixTimeMS);

    if (err == CHIP_NO_ERROR)
    {
        // If the system has given us a wall clock time, we must use it or
        // fail.  Conversion failures here are therefore always an error.
        System::Clock::Seconds32 currentUnixTime = std::chrono::duration_cast<System::Clock::Seconds32>(currentUnixTimeMS);
        ReturnErrorOnFailure(mValidContext.SetEffectiveTimeFromUnixTime<CurrentChipEpochTime>(currentUnixTime));
    }
    else
    {
        // If we don't have wall clock time, the spec dictates that we should
        // fall back to Last Known Good Time.  Ultimately, the calling application's
        // validity policy will determine whether this is permissible.
        System::Clock::Seconds32 lastKnownGoodChipEpochTime;
        ChipLogError(SecureChannel,
                     "The device does not support GetClock_RealTimeMS() API: %" CHIP_ERROR_FORMAT
                     ".  Falling back to Last Known Good UTC Time",
                     err.Format());
        VerifyOrReturnError(mFabricsTable != nullptr, CHIP_ERROR_INCORRECT_STATE);
        err = mFabricsTable->GetLastKnownGoodChipEpochTime(lastKnownGoodChipEpochTime);
        if (err != CHIP_NO_ERROR)
        {
            // If we have no time available, the Validity Policy will
            // determine what to do.
            ChipLogError(SecureChannel, "Failed to retrieve Last Known Good UTC Time");
        }
        else
        {
            mValidContext.SetEffectiveTime<LastKnownGoodChipEpochTime>(lastKnownGoodChipEpochTime);
        }
    }
    return CHIP_NO_ERROR;
}

void CASESession::OnSuccessStatusReport()
{
    ChipLogProgress(SecureChannel, "Success status report received. Session was established");

    if (mSessionResumptionStorage != nullptr)
    {
        CHIP_ERROR err2 = mSessionResumptionStorage->Save(GetPeer(), mNewResumptionId, mSharedSecret, mPeerCATs);
        if (err2 != CHIP_NO_ERROR)
            ChipLogError(SecureChannel, "Unable to save session resumption state: %" CHIP_ERROR_FORMAT, err2.Format());
    }

    switch (mState)
    {
    case State::kSentSigma3:
        mState = State::kFinished;
        break;
    case State::kSentSigma2Resume:
        mState = State::kFinishedViaResume;
        break;
    default:
        VerifyOrDie(false && "Reached invalid internal state keeping in CASE session");
        break;
    }

    Finish();
}

CHIP_ERROR CASESession::OnFailureStatusReport(Protocols::SecureChannel::GeneralStatusCode generalCode, uint16_t protocolCode,
                                              Optional<uintptr_t> protocolData)
{
    CHIP_ERROR err = CHIP_NO_ERROR;
    switch (protocolCode)
    {
    case kProtocolCodeInvalidParam:
        err = CHIP_ERROR_INVALID_CASE_PARAMETER;
        break;

    case kProtocolCodeNoSharedRoot:
        err = CHIP_ERROR_NO_SHARED_TRUSTED_ROOT;
        break;

    case kProtocolCodeBusy:
        err = CHIP_ERROR_BUSY;
        if (protocolData.HasValue())
        {
            mDelegate->OnResponderBusy(System::Clock::Milliseconds16(static_cast<uint16_t>(protocolData.Value())));
        }
        break;

    default:
        err = CHIP_ERROR_INTERNAL;
        break;
    };
    mState = State::kInitialized;
    ChipLogError(SecureChannel, "Received error (protocol code %d) during pairing process: %" CHIP_ERROR_FORMAT, protocolCode,
                 err.Format());
    return err;
}

CHIP_ERROR CASESession::ParseSigma1(TLV::ContiguousBufferTLVReader & tlvReader, ByteSpan & initiatorRandom,
                                    uint16_t & initiatorSessionId, ByteSpan & destinationId, ByteSpan & initiatorEphPubKey,
                                    bool & resumptionRequested, ByteSpan & resumptionId, ByteSpan & initiatorResumeMIC)
{
    using namespace TLV;

    constexpr uint8_t kInitiatorRandomTag    = 1;
    constexpr uint8_t kInitiatorSessionIdTag = 2;
    constexpr uint8_t kDestinationIdTag      = 3;
    constexpr uint8_t kInitiatorPubKeyTag    = 4;
    constexpr uint8_t kInitiatorMRPParamsTag = 5;
    constexpr uint8_t kResumptionIDTag       = 6;
    constexpr uint8_t kResume1MICTag         = 7;

    TLVType containerType = kTLVType_Structure;
    ReturnErrorOnFailure(tlvReader.Next(containerType, AnonymousTag()));
    ReturnErrorOnFailure(tlvReader.EnterContainer(containerType));

    ReturnErrorOnFailure(tlvReader.Next(ContextTag(kInitiatorRandomTag)));
    ReturnErrorOnFailure(tlvReader.GetByteView(initiatorRandom));
    VerifyOrReturnError(initiatorRandom.size() == kSigmaParamRandomNumberSize, CHIP_ERROR_INVALID_CASE_PARAMETER);

    ReturnErrorOnFailure(tlvReader.Next(ContextTag(kInitiatorSessionIdTag)));
    ReturnErrorOnFailure(tlvReader.Get(initiatorSessionId));

    ReturnErrorOnFailure(tlvReader.Next(ContextTag(kDestinationIdTag)));
    ReturnErrorOnFailure(tlvReader.GetByteView(destinationId));
    VerifyOrReturnError(destinationId.size() == kSHA256_Hash_Length, CHIP_ERROR_INVALID_CASE_PARAMETER);

    ReturnErrorOnFailure(tlvReader.Next(ContextTag(kInitiatorPubKeyTag)));
    ReturnErrorOnFailure(tlvReader.GetByteView(initiatorEphPubKey));
    VerifyOrReturnError(initiatorEphPubKey.size() == kP256_PublicKey_Length, CHIP_ERROR_INVALID_CASE_PARAMETER);

    // Optional members start here.
    CHIP_ERROR err = tlvReader.Next();
    if (err == CHIP_NO_ERROR && tlvReader.GetTag() == ContextTag(kInitiatorMRPParamsTag))
    {
        ReturnErrorOnFailure(DecodeMRPParametersIfPresent(TLV::ContextTag(kInitiatorMRPParamsTag), tlvReader));
        mExchangeCtxt.Value()->GetSessionHandle()->AsUnauthenticatedSession()->SetRemoteSessionParameters(
            GetRemoteSessionParameters());
        err = tlvReader.Next();
    }

    bool resumptionIDTagFound = false;
    bool resume1MICTagFound   = false;

    if (err == CHIP_NO_ERROR && tlvReader.GetTag() == ContextTag(kResumptionIDTag))
    {
        resumptionIDTagFound = true;
        ReturnErrorOnFailure(tlvReader.GetByteView(resumptionId));
        VerifyOrReturnError(resumptionId.size() == SessionResumptionStorage::kResumptionIdSize, CHIP_ERROR_INVALID_CASE_PARAMETER);
        err = tlvReader.Next();
    }

    if (err == CHIP_NO_ERROR && tlvReader.GetTag() == ContextTag(kResume1MICTag))
    {
        resume1MICTagFound = true;
        ReturnErrorOnFailure(tlvReader.GetByteView(initiatorResumeMIC));
        VerifyOrReturnError(initiatorResumeMIC.size() == CHIP_CRYPTO_AEAD_MIC_LENGTH_BYTES, CHIP_ERROR_INVALID_CASE_PARAMETER);
        err = tlvReader.Next();
    }

    if (err == CHIP_END_OF_TLV)
    {
        // We ran out of struct members, but that's OK, because they were optional.
        err = CHIP_NO_ERROR;
    }

    ReturnErrorOnFailure(err);
    ReturnErrorOnFailure(tlvReader.ExitContainer(containerType));

    if (resumptionIDTagFound && resume1MICTagFound)
    {
        resumptionRequested = true;
    }
    else if (!resumptionIDTagFound && !resume1MICTagFound)
    {
        resumptionRequested = false;
    }
    else
    {
        return CHIP_ERROR_UNEXPECTED_TLV_ELEMENT;
    }

    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::ValidateReceivedMessage(ExchangeContext * ec, const PayloadHeader & payloadHeader,
                                                const System::PacketBufferHandle & msg)
{
    VerifyOrReturnError(ec != nullptr, CHIP_ERROR_INVALID_ARGUMENT);

    // mExchangeCtxt can be nullptr if this is the first message (CASE_Sigma1) received by CASESession
    // via UnsolicitedMessageHandler. The exchange context is allocated by exchange manager and provided
    // to the handler (CASESession object).
    if (mExchangeCtxt.HasValue())
    {
        if (&mExchangeCtxt.Value().Get() != ec)
        {
            ReturnErrorOnFailure(CHIP_ERROR_INVALID_ARGUMENT);
        }
    }
    else
    {
        mExchangeCtxt.Emplace(*ec);
    }
    mExchangeCtxt.Value()->UseSuggestedResponseTimeout(kExpectedHighProcessingTime);

    VerifyOrReturnError(!msg.IsNull(), CHIP_ERROR_INVALID_ARGUMENT);
    return CHIP_NO_ERROR;
}

CHIP_ERROR CASESession::OnMessageReceived(ExchangeContext * ec, const PayloadHeader & payloadHeader,
                                          System::PacketBufferHandle && msg)
{
    MATTER_TRACE_SCOPE("OnMessageReceived", "CASESession");
    CHIP_ERROR err                            = ValidateReceivedMessage(ec, payloadHeader, msg);
    Protocols::SecureChannel::MsgType msgType = static_cast<Protocols::SecureChannel::MsgType>(payloadHeader.GetMessageType());
    SuccessOrExit(err);

#if CONFIG_BUILD_FOR_HOST_UNIT_TEST
    if (mStopHandshakeAtState.HasValue() && mState == mStopHandshakeAtState.Value())
    {
        mStopHandshakeAtState = Optional<State>::Missing();
        // For testing purposes we are trying to stop a successful CASESession from happening by dropping part of the
        // handshake in the middle. We are trying to keep both sides of the CASESession establishment in an active
        // pending state. In order to keep this side open we have to tell the exchange context that we will send an
        // async message.
        //
        // Should you need to resume the CASESession, you could theoretically pass along the msg to a callback that gets
        // registered when setting mStopHandshakeAtState.
        mExchangeCtxt.Value()->WillSendMessage();
        return CHIP_NO_ERROR;
    }
#endif // CONFIG_BUILD_FOR_HOST_UNIT_TEST

#if CHIP_CONFIG_SLOW_CRYPTO
    if (msgType == Protocols::SecureChannel::MsgType::CASE_Sigma1 || msgType == Protocols::SecureChannel::MsgType::CASE_Sigma2 ||
        msgType == Protocols::SecureChannel::MsgType::CASE_Sigma2Resume ||
        msgType == Protocols::SecureChannel::MsgType::CASE_Sigma3)
    {
        SuccessOrExit(err = mExchangeCtxt.Value()->FlushAcks());
    }
#endif // CHIP_CONFIG_SLOW_CRYPTO

    // By default, CHIP_ERROR_INVALID_MESSAGE_TYPE is returned if in the current state
    // a message handler is not defined for the received message type.
    err = CHIP_ERROR_INVALID_MESSAGE_TYPE;

    switch (mState)
    {
    case State::kInitialized:
        if (msgType == Protocols::SecureChannel::MsgType::CASE_Sigma1)
        {
            err = HandleSigma1_and_SendSigma2(std::move(msg));
        }
        break;
    case State::kSentSigma1:
        switch (static_cast<Protocols::SecureChannel::MsgType>(payloadHeader.GetMessageType()))
        {
        case Protocols::SecureChannel::MsgType::CASE_Sigma2:
            err = HandleSigma2_and_SendSigma3(std::move(msg));
            break;

        case MsgType::StatusReport:
            err = HandleStatusReport(std::move(msg), /* successExpected*/ false);
            break;

        default:
            // Return the default error that was set above
            break;
        };
        break;
    case State::kSentSigma1Resume:
        switch (static_cast<Protocols::SecureChannel::MsgType>(payloadHeader.GetMessageType()))
        {
        case Protocols::SecureChannel::MsgType::CASE_Sigma2:
            err = HandleSigma2_and_SendSigma3(std::move(msg));
            break;

        case Protocols::SecureChannel::MsgType::CASE_Sigma2Resume:
            err = HandleSigma2Resume(std::move(msg));
            break;

        case MsgType::StatusReport:
            err = HandleStatusReport(std::move(msg), /* successExpected*/ false);
            break;

        default:
            // Return the default error that was set above
            break;
        };
        break;
    case State::kSentSigma2:
        switch (static_cast<Protocols::SecureChannel::MsgType>(payloadHeader.GetMessageType()))
        {
        case Protocols::SecureChannel::MsgType::CASE_Sigma3:
            err = HandleSigma3a(std::move(msg));
            break;

        case MsgType::StatusReport:
            err = HandleStatusReport(std::move(msg), /* successExpected*/ false);
            break;

        default:
            // Return the default error that was set above
            break;
        };
        break;
    case State::kSentSigma3:
    case State::kSentSigma2Resume:
        if (msgType == Protocols::SecureChannel::MsgType::StatusReport)
        {
            err = HandleStatusReport(std::move(msg), /* successExpected*/ true);
        }
        break;
    default:
        // Return the default error that was set above
        break;
    };

exit:

    if (err == CHIP_ERROR_INVALID_MESSAGE_TYPE)
    {
        ChipLogError(SecureChannel, "Received message (type %d) cannot be handled in %d state.", to_underlying(msgType),
                     to_underlying(mState));
    }

    // Call delegate to indicate session establishment failure.
    if (err != CHIP_NO_ERROR)
    {
        // Discard the exchange so that Clear() doesn't try aborting it.  The
        // exchange will handle that.
        DiscardExchange();
        AbortPendingEstablish(err);
    }
    return err;
}

System::Clock::Timeout CASESession::ComputeSigma1ResponseTimeout(const ReliableMessageProtocolConfig & remoteMrpConfig)
{
    return GetRetransmissionTimeout(remoteMrpConfig.mActiveRetransTimeout, remoteMrpConfig.mIdleRetransTimeout,
                                    // Assume peer is idle, since that's what we
                                    // will assume for our initial message.
                                    System::Clock::kZero, remoteMrpConfig.mActiveThresholdTime) +
        kExpectedSigma1ProcessingTime;
}

System::Clock::Timeout CASESession::ComputeSigma2ResponseTimeout(const ReliableMessageProtocolConfig & remoteMrpConfig)
{
    return GetRetransmissionTimeout(remoteMrpConfig.mActiveRetransTimeout, remoteMrpConfig.mIdleRetransTimeout,
                                    // Assume peer is idle, as a worst-case assumption.
                                    System::Clock::kZero, remoteMrpConfig.mActiveThresholdTime) +
        kExpectedHighProcessingTime;
}

bool CASESession::InvokeBackgroundWorkWatchdog()
{
    bool watchdogFired = false;

    if (mSendSigma3Helper && mSendSigma3Helper->UnableToScheduleAfterWorkCallback())
    {
        ChipLogError(SecureChannel, "SendSigma3Helper was unable to schedule the AfterWorkCallback");
        mSendSigma3Helper->DoAfterWork();
        watchdogFired = true;
    }

    if (mHandleSigma3Helper && mHandleSigma3Helper->UnableToScheduleAfterWorkCallback())
    {
        ChipLogError(SecureChannel, "HandleSigma3Helper was unable to schedule the AfterWorkCallback");
        mHandleSigma3Helper->DoAfterWork();
        watchdogFired = true;
    }

    return watchdogFired;
}

// Helper function to map CASESession::State to SessionEstablishmentStage
SessionEstablishmentStage CASESession::MapCASEStateToSessionEstablishmentStage(State caseState)
{
    switch (caseState)
    {
    case State::kInitialized:
        return SessionEstablishmentStage::kNotInKeyExchange;
    case State::kSentSigma1:
    case State::kSentSigma1Resume:
        return SessionEstablishmentStage::kSentSigma1;
    case State::kSentSigma2:
    case State::kSentSigma2Resume:
        return SessionEstablishmentStage::kSentSigma2;
    case State::kSendSigma3Pending:
        return SessionEstablishmentStage::kReceivedSigma2;
    case State::kSentSigma3:
        return SessionEstablishmentStage::kSentSigma3;
    case State::kHandleSigma3Pending:
        return SessionEstablishmentStage::kReceivedSigma3;
    // Add more mappings here for other states
    default:
        return SessionEstablishmentStage::kUnknown; // Default mapping
    }
}

} // namespace chip