src/transport/CryptoContext.cpp

/*
 *
 *    Copyright (c) 2020-2021 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 CHIP Secure Session object.
 *
 */

#include <crypto/CHIPCryptoPAL.h>
#include <lib/core/CHIPEncoding.h>
#include <lib/support/BufferWriter.h>
#include <lib/support/CodeUtils.h>
#include <transport/CryptoContext.h>
#include <transport/raw/MessageHeader.h>

#include <lib/support/BytesToHex.h>

#include <string.h>

namespace chip {

namespace {

constexpr size_t kMaxAADLen = 128;

/* Session Establish Key Info */
constexpr uint8_t SEKeysInfo[] = { 0x53, 0x65, 0x73, 0x73, 0x69, 0x6f, 0x6e, 0x4b, 0x65, 0x79, 0x73 };

/* Session Resumption Key Info */
constexpr uint8_t RSEKeysInfo[] = { 0x53, 0x65, 0x73, 0x73, 0x69, 0x6f, 0x6e, 0x52, 0x65, 0x73, 0x75,
                                    0x6d, 0x70, 0x74, 0x69, 0x6f, 0x6e, 0x4b, 0x65, 0x79, 0x73 };

} // namespace

using namespace Crypto;

#ifdef ENABLE_HSM_HKDF
using HKDF_sha_crypto = HKDF_shaHSM;
#else
using HKDF_sha_crypto = HKDF_sha;
#endif

CryptoContext::CryptoContext() : mKeyAvailable(false) {}

CryptoContext::~CryptoContext()
{
    for (auto & key : mKeys)
    {
        ClearSecretData(key, sizeof(CryptoKey));
    }
    mKeyContext = nullptr;
}

CHIP_ERROR CryptoContext::InitFromSecret(const ByteSpan & secret, const ByteSpan & salt, SessionInfoType infoType, SessionRole role)
{
    HKDF_sha_crypto mHKDF;
    VerifyOrReturnError(mKeyAvailable == false, CHIP_ERROR_INCORRECT_STATE);
    VerifyOrReturnError(secret.data() != nullptr, CHIP_ERROR_INVALID_ARGUMENT);
    VerifyOrReturnError(secret.size() > 0, CHIP_ERROR_INVALID_ARGUMENT);
    VerifyOrReturnError((salt.size() == 0) || (salt.data() != nullptr), CHIP_ERROR_INVALID_ARGUMENT);

    const uint8_t * info = SEKeysInfo;
    size_t infoLen       = sizeof(SEKeysInfo);

    if (infoType == SessionInfoType::kSessionResumption)
    {
        info    = RSEKeysInfo;
        infoLen = sizeof(RSEKeysInfo);
    }

#if CHIP_CONFIG_SECURITY_TEST_MODE

    // If enabled, override the generated session key with a known key pair
    // to allow man-in-the-middle session key recovery for testing purposes.

#define TEST_SECRET_SIZE 32
    constexpr uint8_t kTestSharedSecret[TEST_SECRET_SIZE] = CHIP_CONFIG_TEST_SHARED_SECRET_VALUE;
    static_assert(sizeof(CHIP_CONFIG_TEST_SHARED_SECRET_VALUE) == TEST_SECRET_SIZE,
                  "CHIP_CONFIG_TEST_SHARED_SECRET_VALUE must be 32 bytes");
    const ByteSpan & testSalt = ByteSpan(nullptr, 0);
    (void) info;
    (void) infoLen;

#warning                                                                                                                           \
    "Warning: CONFIG_SECURITY_TEST_MODE=1 bypassing key negotiation... All sessions will use known, fixed test key, and NodeID=0 in NONCE. Node can only communicate with other nodes built with this flag set. Requires build flag 'treat_warnings_as_errors=false'."
    ChipLogError(SecureChannel,
                 "Warning: CONFIG_SECURITY_TEST_MODE=1 bypassing key negotiation... All sessions will use known, fixed test key, "
                 "and NodeID=0 in NONCE. "
                 "Node can only communicate with other nodes built with this flag set.");

    ReturnErrorOnFailure(mHKDF.HKDF_SHA256(kTestSharedSecret, TEST_SECRET_SIZE, testSalt.data(), testSalt.size(), SEKeysInfo,
                                           sizeof(SEKeysInfo), &mKeys[0][0], sizeof(mKeys)));
#else

    ReturnErrorOnFailure(
        mHKDF.HKDF_SHA256(secret.data(), secret.size(), salt.data(), salt.size(), info, infoLen, &mKeys[0][0], sizeof(mKeys)));

#endif

    mKeyAvailable = true;
    mSessionRole  = role;

    return CHIP_NO_ERROR;
}

CHIP_ERROR CryptoContext::InitFromKeyPair(const Crypto::P256Keypair & local_keypair,
                                          const Crypto::P256PublicKey & remote_public_key, const ByteSpan & salt,
                                          SessionInfoType infoType, SessionRole role)
{

    VerifyOrReturnError(mKeyAvailable == false, CHIP_ERROR_INCORRECT_STATE);

    P256ECDHDerivedSecret secret;
    ReturnErrorOnFailure(local_keypair.ECDH_derive_secret(remote_public_key, secret));

    return InitFromSecret(ByteSpan(secret, secret.Length()), salt, infoType, role);
}

CHIP_ERROR CryptoContext::BuildNonce(NonceView nonce, uint8_t securityFlags, uint32_t messageCounter, NodeId nodeId)
{
    Encoding::LittleEndian::BufferWriter bbuf(nonce.data(), nonce.size());

    bbuf.Put8(securityFlags);
    bbuf.Put32(messageCounter);
#if CHIP_CONFIG_SECURITY_TEST_MODE
    bbuf.Put64(0); // Simplifies decryption of CASE sessions when in TEST_MODE.
#else
    bbuf.Put64(nodeId);
#endif

    return bbuf.Fit() ? CHIP_NO_ERROR : CHIP_ERROR_NO_MEMORY;
}

CHIP_ERROR CryptoContext::GetAdditionalAuthData(const PacketHeader & header, uint8_t * aad, uint16_t & len)
{
    VerifyOrReturnError(len >= header.EncodeSizeBytes(), CHIP_ERROR_INVALID_ARGUMENT);

    // Use unencrypted part of header as AAD. This will help
    // integrity protect the whole message
    uint16_t actualEncodedHeaderSize;

    ReturnErrorOnFailure(header.Encode(aad, len, &actualEncodedHeaderSize));
    VerifyOrReturnError(len >= actualEncodedHeaderSize, CHIP_ERROR_INVALID_ARGUMENT);

    len = actualEncodedHeaderSize;

    return CHIP_NO_ERROR;
}

CHIP_ERROR CryptoContext::Encrypt(const uint8_t * input, size_t input_length, uint8_t * output, ConstNonceView nonce,
                                  PacketHeader & header, MessageAuthenticationCode & mac) const
{

    const size_t taglen = header.MICTagLength();

    VerifyOrDie(taglen <= kMaxTagLen);

    VerifyOrReturnError(input != nullptr, CHIP_ERROR_INVALID_ARGUMENT);
    VerifyOrReturnError(input_length > 0, CHIP_ERROR_INVALID_ARGUMENT);
    VerifyOrReturnError(output != nullptr, CHIP_ERROR_INVALID_ARGUMENT);

    uint8_t AAD[kMaxAADLen];
    uint16_t aadLen = sizeof(AAD);
    uint8_t tag[kMaxTagLen];

    ReturnErrorOnFailure(GetAdditionalAuthData(header, AAD, aadLen));

    if (mKeyContext)
    {
        ByteSpan plaintext(input, input_length);
        MutableByteSpan ciphertext(output, input_length);
        MutableByteSpan mic(tag, taglen);

        ReturnErrorOnFailure(mKeyContext->EncryptMessage(plaintext, ByteSpan(AAD, aadLen), nonce, mic, ciphertext));
    }
    else
    {
        VerifyOrReturnError(mKeyAvailable, CHIP_ERROR_INVALID_USE_OF_SESSION_KEY);
        KeyUsage usage = kR2IKey;

        // Message is encrypted before sending. If the secure session was created by session
        // initiator, we'll use I2R key to encrypt the message that's being transmitted.
        // Otherwise, we'll use R2I key, as the responder is sending the message.
        if (mSessionRole == SessionRole::kInitiator)
        {
            usage = kI2RKey;
        }

        ReturnErrorOnFailure(AES_CCM_encrypt(input, input_length, AAD, aadLen, mKeys[usage], Crypto::kAES_CCM128_Key_Length,
                                             nonce.data(), nonce.size(), output, tag, taglen));
    }

    mac.SetTag(&header, tag, taglen);

    return CHIP_NO_ERROR;
}

CHIP_ERROR CryptoContext::Decrypt(const uint8_t * input, size_t input_length, uint8_t * output, ConstNonceView nonce,
                                  const PacketHeader & header, const MessageAuthenticationCode & mac) const
{
    const size_t taglen = header.MICTagLength();
    const uint8_t * tag = mac.GetTag();
    uint8_t AAD[kMaxAADLen];
    uint16_t aadLen = sizeof(AAD);

    VerifyOrReturnError(input != nullptr, CHIP_ERROR_INVALID_ARGUMENT);
    VerifyOrReturnError(input_length > 0, CHIP_ERROR_INVALID_ARGUMENT);
    VerifyOrReturnError(output != nullptr, CHIP_ERROR_INVALID_ARGUMENT);

    ReturnErrorOnFailure(GetAdditionalAuthData(header, AAD, aadLen));

    if (nullptr != mKeyContext)
    {
        ByteSpan ciphertext(input, input_length);
        MutableByteSpan plaintext(output, input_length);
        ByteSpan mic(tag, taglen);

        CHIP_ERROR err = mKeyContext->DecryptMessage(ciphertext, ByteSpan(AAD, aadLen), nonce, mic, plaintext);
        ReturnErrorOnFailure(err);
    }
    else
    {
        VerifyOrReturnError(mKeyAvailable, CHIP_ERROR_INVALID_USE_OF_SESSION_KEY);
        KeyUsage usage = kI2RKey;

        // Message is decrypted on receive. If the secure session was created by session
        // initiator, we'll use R2I key to decrypt the message (as it was sent by responder).
        // Otherwise, we'll use I2R key, as the responder is sending the message.
        if (mSessionRole == SessionRole::kInitiator)
        {
            usage = kR2IKey;
        }

        ReturnErrorOnFailure(AES_CCM_decrypt(input, input_length, AAD, aadLen, tag, taglen, mKeys[usage],
                                             Crypto::kAES_CCM128_Key_Length, nonce.data(), nonce.size(), output));
    }
    return CHIP_NO_ERROR;
}

} // namespace chip