src/cbor_cert_op.c - open-dice - Git at Google

 // Copyright 2021 Google LLC
 //
 // 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
 //
 //     https://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.

 // This is a DiceGenerateCertificate implementation that generates a CWT-style
 // CBOR certificate. The function DiceCoseEncodePublicKey depends on the
 // signature algorithm type, and must be implemented elsewhere.

 #include <stddef.h>
 #include <stdint.h>
 #include <string.h>

 #include "dice/cbor_writer.h"
 #include "dice/dice.h"
 #include "dice/ops.h"
 #include "dice/ops/trait/cose.h"
 #include "dice/utils.h"

 // Max size of COSE_Key encoding.
 #define DICE_MAX_PUBLIC_KEY_SIZE (DICE_PUBLIC_KEY_BUFFER_SIZE + 32)
 // Max size of the COSE_Sign1 protected attributes.
 #define DICE_MAX_PROTECTED_ATTRIBUTES_SIZE 16

 static DiceResult EncodeProtectedAttributes(size_t buffer_size, uint8_t* buffer,
                                             size_t* encoded_size) {
   // Constants per RFC 8152.
   const int64_t kCoseHeaderAlgLabel = 1;

   struct CborOut out;
   CborOutInit(buffer, buffer_size, &out);
   CborWriteMap(/*num_elements=*/1, &out);
   // Add the algorithm.
   CborWriteInt(kCoseHeaderAlgLabel, &out);
   CborWriteInt(DICE_COSE_KEY_ALG_VALUE, &out);
   *encoded_size = CborOutSize(&out);
   if (CborOutOverflowed(&out)) {
     return kDiceResultBufferTooSmall;
   }
   return kDiceResultOk;
 }

 static DiceResult EncodeCoseTbs(const uint8_t* protected_attributes,
                                 size_t protected_attributes_size,
                                 size_t payload_size, const uint8_t* aad,
                                 size_t aad_size, size_t buffer_size,
                                 uint8_t* buffer, uint8_t** payload,
                                 size_t* encoded_size) {
   struct CborOut out;
   CborOutInit(buffer, buffer_size, &out);
   // TBS is an array of four elements.
   CborWriteArray(/*num_elements=*/4, &out);
   // Context string field.
   CborWriteTstr("Signature1", &out);
   // Protected attributes from COSE_Sign1.
   CborWriteBstr(protected_attributes_size, protected_attributes, &out);
   // Additional authenticated data.
   CborWriteBstr(aad_size, aad, &out);
   // Space for the payload, to be filled in by the caller.
   *payload = CborAllocBstr(payload_size, &out);
   *encoded_size = CborOutSize(&out);
   if (CborOutOverflowed(&out)) {
     return kDiceResultBufferTooSmall;
   }
   return kDiceResultOk;
 }

 static DiceResult EncodeCoseSign1(const uint8_t* protected_attributes,
                                   size_t protected_attributes_size,
                                   const uint8_t* payload, size_t payload_size,
                                   bool move_payload,
                                   const uint8_t signature[DICE_SIGNATURE_SIZE],
                                   size_t buffer_size, uint8_t* buffer,
                                   size_t* encoded_size) {
   struct CborOut out;
   CborOutInit(buffer, buffer_size, &out);
   // COSE_Sign1 is an array of four elements.
   CborWriteArray(/*num_elements=*/4, &out);
   // Protected attributes.
   CborWriteBstr(protected_attributes_size, protected_attributes, &out);
   // Empty map for unprotected attributes.
   CborWriteMap(/*num_pairs=*/0, &out);
   // Payload.
   if (move_payload) {
     // The payload is already present in the buffer, so we can move it into
     // place.
     uint8_t* payload_alloc = CborAllocBstr(payload_size, &out);
     if (payload_alloc) {
       // We're assuming what we've written above is small enough that it doesn't
       // overwrite the payload. Check in case that stops being true.
       if (payload < payload_alloc) {
         return kDiceResultPlatformError;
       }
       memmove(payload_alloc, payload, payload_size);
     }
   } else {
     CborWriteBstr(payload_size, payload, &out);
   }
   // Signature.
   CborWriteBstr(/*num_elements=*/DICE_SIGNATURE_SIZE, signature, &out);
   *encoded_size = CborOutSize(&out);
   if (CborOutOverflowed(&out)) {
     return kDiceResultBufferTooSmall;
   }
   return kDiceResultOk;
 }

 DiceResult DiceCoseSignAndEncodeSign1(
     void* context, const uint8_t* payload, size_t payload_size,
     const uint8_t* aad, size_t aad_size,
     const uint8_t private_key[DICE_PRIVATE_KEY_SIZE], size_t buffer_size,
     uint8_t* buffer, size_t* encoded_size) {
   DiceResult result;

   *encoded_size = 0;

   // The encoded protected attributes are used in the TBS and the final
   // COSE_Sign1 structure.
   uint8_t protected_attributes[DICE_MAX_PROTECTED_ATTRIBUTES_SIZE];
   size_t protected_attributes_size = 0;
   result = EncodeProtectedAttributes(sizeof(protected_attributes),
                                      protected_attributes,
                                      &protected_attributes_size);
   if (result != kDiceResultOk) {
     return kDiceResultPlatformError;
   }

   // Construct a To-Be-Signed (TBS) structure based on the relevant fields of
   // the COSE_Sign1.
   uint8_t* payload_buffer;
   result = EncodeCoseTbs(protected_attributes, protected_attributes_size,
                          payload_size, aad, aad_size, buffer_size, buffer,
                          &payload_buffer, encoded_size);
   if (result != kDiceResultOk) {
     // Check how big the buffer needs to be in total.
     size_t final_encoded_size = 0;
     EncodeCoseSign1(protected_attributes, protected_attributes_size, payload,
                     payload_size, /*move_payload=*/false, /*signature=*/NULL,
                     /*buffer_size=*/0, /*buffer=*/NULL, &final_encoded_size);
     if (*encoded_size < final_encoded_size) {
       *encoded_size = final_encoded_size;
     }
     return result;
   }
   memcpy(payload_buffer, payload, payload_size);

   // Sign the TBS with the authority key.
   uint8_t signature[DICE_SIGNATURE_SIZE];
   result = DiceSign(context, buffer, *encoded_size, private_key, signature);
   if (result != kDiceResultOk) {
     return result;
   }

   // The final certificate is an untagged COSE_Sign1 structure.
   return EncodeCoseSign1(protected_attributes, protected_attributes_size,
                          payload, payload_size, /*move_payload=*/false,
                          signature, buffer_size, buffer, encoded_size);
 }

 // Encodes a CBOR Web Token (CWT) with an issuer, subject, and additional
 // fields.
 static DiceResult EncodeCwt(void* context, const DiceInputValues* input_values,
                             const char* authority_id_hex,
                             const char* subject_id_hex,
                             const uint8_t* encoded_public_key,
                             size_t encoded_public_key_size, size_t buffer_size,
                             uint8_t* buffer, size_t* encoded_size) {
   // Constants per RFC 8392.
   const int64_t kCwtIssuerLabel = 1;
   const int64_t kCwtSubjectLabel = 2;
   // Constants per the Open Profile for DICE specification.
   const int64_t kCodeHashLabel = -4670545;
   const int64_t kCodeDescriptorLabel = -4670546;
   const int64_t kConfigHashLabel = -4670547;
   const int64_t kConfigDescriptorLabel = -4670548;
   const int64_t kAuthorityHashLabel = -4670549;
   const int64_t kAuthorityDescriptorLabel = -4670550;
   const int64_t kModeLabel = -4670551;
   const int64_t kSubjectPublicKeyLabel = -4670552;
   const int64_t kKeyUsageLabel = -4670553;
   const int64_t kProfileNameLabel = -4670554;
   // Key usage constant per RFC 5280.
   const uint8_t kKeyUsageCertSign = 32;

   // Count the number of entries.
   uint32_t map_pairs = 7;
   if (input_values->code_descriptor_size > 0) {
     map_pairs += 1;
   }
   if (input_values->config_type == kDiceConfigTypeDescriptor) {
     map_pairs += 2;
   } else {
     map_pairs += 1;
   }
   if (input_values->authority_descriptor_size > 0) {
     map_pairs += 1;
   }
   if (DICE_PROFILE_NAME) {
     map_pairs += 1;
   }

   struct CborOut out;
   CborOutInit(buffer, buffer_size, &out);
   CborWriteMap(map_pairs, &out);
   // Add the issuer.
   CborWriteInt(kCwtIssuerLabel, &out);
   CborWriteTstr(authority_id_hex, &out);
   // Add the subject.
   CborWriteInt(kCwtSubjectLabel, &out);
   CborWriteTstr(subject_id_hex, &out);
   // Add the code hash.
   CborWriteInt(kCodeHashLabel, &out);
   CborWriteBstr(DICE_HASH_SIZE, input_values->code_hash, &out);
   // Add the code descriptor, if provided.
   if (input_values->code_descriptor_size > 0) {
     CborWriteInt(kCodeDescriptorLabel, &out);
     CborWriteBstr(input_values->code_descriptor_size,
                   input_values->code_descriptor, &out);
   }
   // Add the config inputs.
   if (input_values->config_type == kDiceConfigTypeDescriptor) {
     uint8_t config_descriptor_hash[DICE_HASH_SIZE];
     // Skip hashing if we're not going to use the answer.
     if (!CborOutOverflowed(&out)) {
       DiceResult result = DiceHash(context, input_values->config_descriptor,
                                    input_values->config_descriptor_size,
                                    config_descriptor_hash);
       if (result != kDiceResultOk) {
         return result;
       }
     }
     // Add the config descriptor.
     CborWriteInt(kConfigDescriptorLabel, &out);
     CborWriteBstr(input_values->config_descriptor_size,
                   input_values->config_descriptor, &out);
     // Add the Config hash.
     CborWriteInt(kConfigHashLabel, &out);
     CborWriteBstr(DICE_HASH_SIZE, config_descriptor_hash, &out);
   } else if (input_values->config_type == kDiceConfigTypeInline) {
     // Add the inline config.
     CborWriteInt(kConfigDescriptorLabel, &out);
     CborWriteBstr(DICE_INLINE_CONFIG_SIZE, input_values->config_value, &out);
   }
   // Add the authority inputs.
   CborWriteInt(kAuthorityHashLabel, &out);
   CborWriteBstr(DICE_HASH_SIZE, input_values->authority_hash, &out);
   if (input_values->authority_descriptor_size > 0) {
     CborWriteInt(kAuthorityDescriptorLabel, &out);
     CborWriteBstr(input_values->authority_descriptor_size,
                   input_values->authority_descriptor, &out);
   }
   uint8_t mode_byte = input_values->mode;
   uint8_t key_usage = kKeyUsageCertSign;
   // Add the mode input.
   CborWriteInt(kModeLabel, &out);
   CborWriteBstr(/*data_sisze=*/1, &mode_byte, &out);
   // Add the subject public key.
   CborWriteInt(kSubjectPublicKeyLabel, &out);
   CborWriteBstr(encoded_public_key_size, encoded_public_key, &out);
   // Add the key usage.
   CborWriteInt(kKeyUsageLabel, &out);
   CborWriteBstr(/*data_size=*/1, &key_usage, &out);
   // Add the profile name
   if (DICE_PROFILE_NAME) {
     CborWriteInt(kProfileNameLabel, &out);
     CborWriteTstr(DICE_PROFILE_NAME, &out);
   }
   *encoded_size = CborOutSize(&out);
   if (CborOutOverflowed(&out)) {
     return kDiceResultBufferTooSmall;
   }
   return kDiceResultOk;
 }

 DiceResult DiceGenerateCertificate(
     void* context,
     const uint8_t subject_private_key_seed[DICE_PRIVATE_KEY_SEED_SIZE],
     const uint8_t authority_private_key_seed[DICE_PRIVATE_KEY_SEED_SIZE],
     const DiceInputValues* input_values, size_t certificate_buffer_size,
     uint8_t* certificate, size_t* certificate_actual_size) {
   DiceResult result = kDiceResultOk;

   *certificate_actual_size = 0;
   if (input_values->config_type != kDiceConfigTypeDescriptor &&
       input_values->config_type != kDiceConfigTypeInline) {
     return kDiceResultInvalidInput;
   }

   // Declare buffers which are cleared on 'goto out'.
   uint8_t subject_private_key[DICE_PRIVATE_KEY_SIZE];
   uint8_t authority_private_key[DICE_PRIVATE_KEY_SIZE];

   // Derive keys and IDs from the private key seeds.
   uint8_t subject_public_key[DICE_PUBLIC_KEY_BUFFER_SIZE];
   result = DiceKeypairFromSeed(context, subject_private_key_seed,
                                subject_public_key, subject_private_key);
   if (result != kDiceResultOk) {
     goto out;
   }

   uint8_t subject_id[DICE_ID_SIZE];
   result = DiceDeriveCdiCertificateId(context, subject_public_key,
                                       DICE_PUBLIC_KEY_BUFFER_SIZE, subject_id);
   if (result != kDiceResultOk) {
     goto out;
   }
   char subject_id_hex[41];
   DiceHexEncode(subject_id, sizeof(subject_id), subject_id_hex,
                 sizeof(subject_id_hex));
   subject_id_hex[sizeof(subject_id_hex) - 1] = '\0';

   uint8_t authority_public_key[DICE_PUBLIC_KEY_BUFFER_SIZE];
   result = DiceKeypairFromSeed(context, authority_private_key_seed,
                                authority_public_key, authority_private_key);
   if (result != kDiceResultOk) {
     goto out;
   }

   uint8_t authority_id[DICE_ID_SIZE];
   result = DiceDeriveCdiCertificateId(
       context, authority_public_key, DICE_PUBLIC_KEY_BUFFER_SIZE, authority_id);
   if (result != kDiceResultOk) {
     goto out;
   }
   char authority_id_hex[41];
   DiceHexEncode(authority_id, sizeof(authority_id), authority_id_hex,
                 sizeof(authority_id_hex));
   authority_id_hex[sizeof(authority_id_hex) - 1] = '\0';

   // The public key encoded as a COSE_Key structure is embedded in the CWT.
   uint8_t encoded_public_key[DICE_MAX_PUBLIC_KEY_SIZE];
   size_t encoded_public_key_size = 0;
   result = DiceCoseEncodePublicKey(
       context, subject_public_key, sizeof(encoded_public_key),
       encoded_public_key, &encoded_public_key_size);
   if (result != kDiceResultOk) {
     result = kDiceResultPlatformError;
     goto out;
   }

   // The encoded protected attributes are used in the TBS and the final
   // COSE_Sign1 structure.
   uint8_t protected_attributes[DICE_MAX_PROTECTED_ATTRIBUTES_SIZE];
   size_t protected_attributes_size = 0;
   result = EncodeProtectedAttributes(sizeof(protected_attributes),
                                      protected_attributes,
                                      &protected_attributes_size);
   if (result != kDiceResultOk) {
     result = kDiceResultPlatformError;
     goto out;
   }

   // Find out how big the CWT will be.
   size_t cwt_size;
   EncodeCwt(context, input_values, authority_id_hex, subject_id_hex,
             encoded_public_key, encoded_public_key_size, /*buffer_size=*/0,
             /*buffer=*/NULL, &cwt_size);

   // We need space to assemble the TBS. The size of the buffer needed depends on
   // the size of the CWT, which is outside our control (e.g. it might have a
   // very large config descriptor). So we use the certificate buffer as
   // temporary storage; if we run out of space we will make sure the caller
   // knows the size we actually need for this.
   // Encode the TBS, leaving space for the final payload (the CWT).
   uint8_t* cwt_ptr;
   size_t tbs_size;
   result =
       EncodeCoseTbs(protected_attributes, protected_attributes_size, cwt_size,
                     /*aad=*/NULL, /*aad_size=*/0, certificate_buffer_size,
                     certificate, &cwt_ptr, &tbs_size);

   if (result != kDiceResultOk) {
     // There wasn't enough space to put together the TBS. The total buffer size
     // we need is either the amount needed for the TBS, or the amount needed for
     // encoded payload and signature.
     size_t final_encoded_size = 0;
     EncodeCoseSign1(protected_attributes, protected_attributes_size, cwt_ptr,
                     cwt_size, /*move_payload=*/false, /*signature=*/NULL,
                     /*buffer_size=*/0, /*buffer=*/NULL, &final_encoded_size);
     *certificate_actual_size =
         final_encoded_size > tbs_size ? final_encoded_size : tbs_size;
     result = kDiceResultBufferTooSmall;
     goto out;
   }

   // Now we can encode the payload directly into the allocated BSTR in the TBS.
   size_t final_cwt_size;
   result = EncodeCwt(context, input_values, authority_id_hex, subject_id_hex,
                      encoded_public_key, encoded_public_key_size, cwt_size,
                      cwt_ptr, &final_cwt_size);
   if (result == kDiceResultBufferTooSmall || final_cwt_size != cwt_size) {
     result = kDiceResultPlatformError;
   }
   if (result != kDiceResultOk) {
     goto out;
   }

   // Sign the now-complete TBS.
   uint8_t signature[DICE_SIGNATURE_SIZE];
   result = DiceSign(context, certificate, tbs_size, authority_private_key,
                     signature);
   if (result != kDiceResultOk) {
     goto out;
   }

   // And now we can produce the complete CoseSign1, including the signature, and
   // moving the payload into place as we do it.
   result = EncodeCoseSign1(protected_attributes, protected_attributes_size,
                            cwt_ptr, cwt_size, /*move_payload=*/true, signature,
                            certificate_buffer_size, certificate,
                            certificate_actual_size);

 out:
   DiceClearMemory(context, sizeof(subject_private_key), subject_private_key);
   DiceClearMemory(context, sizeof(authority_private_key),
                   authority_private_key);

   return result;
 }
	// Copyright 2021 Google LLC
	//
	// 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
	//
	// https://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.

	// This is a DiceGenerateCertificate implementation that generates a CWT-style
	// CBOR certificate. The function DiceCoseEncodePublicKey depends on the
	// signature algorithm type, and must be implemented elsewhere.

	#include <stddef.h>
	#include <stdint.h>
	#include <string.h>

	#include "dice/cbor_writer.h"
	#include "dice/dice.h"
	#include "dice/ops.h"
	#include "dice/ops/trait/cose.h"
	#include "dice/utils.h"

	// Max size of COSE_Key encoding.
	#define DICE_MAX_PUBLIC_KEY_SIZE (DICE_PUBLIC_KEY_BUFFER_SIZE + 32)
	// Max size of the COSE_Sign1 protected attributes.
	#define DICE_MAX_PROTECTED_ATTRIBUTES_SIZE 16

	static DiceResult EncodeProtectedAttributes(size_t buffer_size, uint8_t* buffer,
	size_t* encoded_size) {
	// Constants per RFC 8152.
	const int64_t kCoseHeaderAlgLabel = 1;

	struct CborOut out;
	CborOutInit(buffer, buffer_size, &out);
	CborWriteMap(/num_elements=/1, &out);
	// Add the algorithm.
	CborWriteInt(kCoseHeaderAlgLabel, &out);
	CborWriteInt(DICE_COSE_KEY_ALG_VALUE, &out);
	*encoded_size = CborOutSize(&out);
	if (CborOutOverflowed(&out)) {
	return kDiceResultBufferTooSmall;
	}
	return kDiceResultOk;
	}

	static DiceResult EncodeCoseTbs(const uint8_t* protected_attributes,
	size_t protected_attributes_size,
	size_t payload_size, const uint8_t* aad,
	size_t aad_size, size_t buffer_size,
	uint8_t* buffer, uint8_t** payload,
	size_t* encoded_size) {
	struct CborOut out;
	CborOutInit(buffer, buffer_size, &out);
	// TBS is an array of four elements.
	CborWriteArray(/num_elements=/4, &out);
	// Context string field.
	CborWriteTstr("Signature1", &out);
	// Protected attributes from COSE_Sign1.
	CborWriteBstr(protected_attributes_size, protected_attributes, &out);
	// Additional authenticated data.
	CborWriteBstr(aad_size, aad, &out);
	// Space for the payload, to be filled in by the caller.
	*payload = CborAllocBstr(payload_size, &out);
	*encoded_size = CborOutSize(&out);
	if (CborOutOverflowed(&out)) {
	return kDiceResultBufferTooSmall;
	}
	return kDiceResultOk;
	}

	static DiceResult EncodeCoseSign1(const uint8_t* protected_attributes,
	size_t protected_attributes_size,
	const uint8_t* payload, size_t payload_size,
	bool move_payload,
	const uint8_t signature[DICE_SIGNATURE_SIZE],
	size_t buffer_size, uint8_t* buffer,
	size_t* encoded_size) {
	struct CborOut out;
	CborOutInit(buffer, buffer_size, &out);
	// COSE_Sign1 is an array of four elements.
	CborWriteArray(/num_elements=/4, &out);
	// Protected attributes.
	CborWriteBstr(protected_attributes_size, protected_attributes, &out);
	// Empty map for unprotected attributes.
	CborWriteMap(/num_pairs=/0, &out);
	// Payload.
	if (move_payload) {
	// The payload is already present in the buffer, so we can move it into
	// place.
	uint8_t* payload_alloc = CborAllocBstr(payload_size, &out);
	if (payload_alloc) {
	// We're assuming what we've written above is small enough that it doesn't
	// overwrite the payload. Check in case that stops being true.
	if (payload < payload_alloc) {
	return kDiceResultPlatformError;
	}
	memmove(payload_alloc, payload, payload_size);
	}
	} else {
	CborWriteBstr(payload_size, payload, &out);
	}
	// Signature.
	CborWriteBstr(/num_elements=/DICE_SIGNATURE_SIZE, signature, &out);
	*encoded_size = CborOutSize(&out);
	if (CborOutOverflowed(&out)) {
	return kDiceResultBufferTooSmall;
	}
	return kDiceResultOk;
	}

	DiceResult DiceCoseSignAndEncodeSign1(
	void* context, const uint8_t* payload, size_t payload_size,
	const uint8_t* aad, size_t aad_size,
	const uint8_t private_key[DICE_PRIVATE_KEY_SIZE], size_t buffer_size,
	uint8_t* buffer, size_t* encoded_size) {
	DiceResult result;

	*encoded_size = 0;

	// The encoded protected attributes are used in the TBS and the final
	// COSE_Sign1 structure.
	uint8_t protected_attributes[DICE_MAX_PROTECTED_ATTRIBUTES_SIZE];
	size_t protected_attributes_size = 0;
	result = EncodeProtectedAttributes(sizeof(protected_attributes),
	protected_attributes,
	&protected_attributes_size);
	if (result != kDiceResultOk) {
	return kDiceResultPlatformError;
	}

	// Construct a To-Be-Signed (TBS) structure based on the relevant fields of
	// the COSE_Sign1.
	uint8_t* payload_buffer;
	result = EncodeCoseTbs(protected_attributes, protected_attributes_size,
	payload_size, aad, aad_size, buffer_size, buffer,
	&payload_buffer, encoded_size);
	if (result != kDiceResultOk) {
	// Check how big the buffer needs to be in total.
	size_t final_encoded_size = 0;
	EncodeCoseSign1(protected_attributes, protected_attributes_size, payload,
	payload_size, /move_payload=/false, /signature=/NULL,
	/buffer_size=/0, /buffer=/NULL, &final_encoded_size);
	if (*encoded_size < final_encoded_size) {
	*encoded_size = final_encoded_size;
	}
	return result;
	}
	memcpy(payload_buffer, payload, payload_size);

	// Sign the TBS with the authority key.
	uint8_t signature[DICE_SIGNATURE_SIZE];
	result = DiceSign(context, buffer, *encoded_size, private_key, signature);
	if (result != kDiceResultOk) {
	return result;
	}

	// The final certificate is an untagged COSE_Sign1 structure.
	return EncodeCoseSign1(protected_attributes, protected_attributes_size,
	payload, payload_size, /move_payload=/false,
	signature, buffer_size, buffer, encoded_size);
	}

	// Encodes a CBOR Web Token (CWT) with an issuer, subject, and additional
	// fields.
	static DiceResult EncodeCwt(void* context, const DiceInputValues* input_values,
	const char* authority_id_hex,
	const char* subject_id_hex,
	const uint8_t* encoded_public_key,
	size_t encoded_public_key_size, size_t buffer_size,
	uint8_t* buffer, size_t* encoded_size) {
	// Constants per RFC 8392.
	const int64_t kCwtIssuerLabel = 1;
	const int64_t kCwtSubjectLabel = 2;
	// Constants per the Open Profile for DICE specification.
	const int64_t kCodeHashLabel = -4670545;
	const int64_t kCodeDescriptorLabel = -4670546;
	const int64_t kConfigHashLabel = -4670547;
	const int64_t kConfigDescriptorLabel = -4670548;
	const int64_t kAuthorityHashLabel = -4670549;
	const int64_t kAuthorityDescriptorLabel = -4670550;
	const int64_t kModeLabel = -4670551;
	const int64_t kSubjectPublicKeyLabel = -4670552;
	const int64_t kKeyUsageLabel = -4670553;
	const int64_t kProfileNameLabel = -4670554;
	// Key usage constant per RFC 5280.
	const uint8_t kKeyUsageCertSign = 32;

	// Count the number of entries.
	uint32_t map_pairs = 7;
	if (input_values->code_descriptor_size > 0) {
	map_pairs += 1;
	}
	if (input_values->config_type == kDiceConfigTypeDescriptor) {
	map_pairs += 2;
	} else {
	map_pairs += 1;
	}
	if (input_values->authority_descriptor_size > 0) {
	map_pairs += 1;
	}
	if (DICE_PROFILE_NAME) {
	map_pairs += 1;
	}

	struct CborOut out;
	CborOutInit(buffer, buffer_size, &out);
	CborWriteMap(map_pairs, &out);
	// Add the issuer.
	CborWriteInt(kCwtIssuerLabel, &out);
	CborWriteTstr(authority_id_hex, &out);
	// Add the subject.
	CborWriteInt(kCwtSubjectLabel, &out);
	CborWriteTstr(subject_id_hex, &out);
	// Add the code hash.
	CborWriteInt(kCodeHashLabel, &out);
	CborWriteBstr(DICE_HASH_SIZE, input_values->code_hash, &out);
	// Add the code descriptor, if provided.
	if (input_values->code_descriptor_size > 0) {
	CborWriteInt(kCodeDescriptorLabel, &out);
	CborWriteBstr(input_values->code_descriptor_size,
	input_values->code_descriptor, &out);
	}
	// Add the config inputs.
	if (input_values->config_type == kDiceConfigTypeDescriptor) {
	uint8_t config_descriptor_hash[DICE_HASH_SIZE];
	// Skip hashing if we're not going to use the answer.
	if (!CborOutOverflowed(&out)) {
	DiceResult result = DiceHash(context, input_values->config_descriptor,
	input_values->config_descriptor_size,
	config_descriptor_hash);
	if (result != kDiceResultOk) {
	return result;
	}
	}
	// Add the config descriptor.
	CborWriteInt(kConfigDescriptorLabel, &out);
	CborWriteBstr(input_values->config_descriptor_size,
	input_values->config_descriptor, &out);
	// Add the Config hash.
	CborWriteInt(kConfigHashLabel, &out);
	CborWriteBstr(DICE_HASH_SIZE, config_descriptor_hash, &out);
	} else if (input_values->config_type == kDiceConfigTypeInline) {
	// Add the inline config.
	CborWriteInt(kConfigDescriptorLabel, &out);
	CborWriteBstr(DICE_INLINE_CONFIG_SIZE, input_values->config_value, &out);
	}
	// Add the authority inputs.
	CborWriteInt(kAuthorityHashLabel, &out);
	CborWriteBstr(DICE_HASH_SIZE, input_values->authority_hash, &out);
	if (input_values->authority_descriptor_size > 0) {
	CborWriteInt(kAuthorityDescriptorLabel, &out);
	CborWriteBstr(input_values->authority_descriptor_size,
	input_values->authority_descriptor, &out);
	}
	uint8_t mode_byte = input_values->mode;
	uint8_t key_usage = kKeyUsageCertSign;
	// Add the mode input.
	CborWriteInt(kModeLabel, &out);
	CborWriteBstr(/data_sisze=/1, &mode_byte, &out);
	// Add the subject public key.
	CborWriteInt(kSubjectPublicKeyLabel, &out);
	CborWriteBstr(encoded_public_key_size, encoded_public_key, &out);
	// Add the key usage.
	CborWriteInt(kKeyUsageLabel, &out);
	CborWriteBstr(/data_size=/1, &key_usage, &out);
	// Add the profile name
	if (DICE_PROFILE_NAME) {
	CborWriteInt(kProfileNameLabel, &out);
	CborWriteTstr(DICE_PROFILE_NAME, &out);
	}
	*encoded_size = CborOutSize(&out);
	if (CborOutOverflowed(&out)) {
	return kDiceResultBufferTooSmall;
	}
	return kDiceResultOk;
	}

	DiceResult DiceGenerateCertificate(
	void* context,
	const uint8_t subject_private_key_seed[DICE_PRIVATE_KEY_SEED_SIZE],
	const uint8_t authority_private_key_seed[DICE_PRIVATE_KEY_SEED_SIZE],
	const DiceInputValues* input_values, size_t certificate_buffer_size,
	uint8_t* certificate, size_t* certificate_actual_size) {
	DiceResult result = kDiceResultOk;

	*certificate_actual_size = 0;
	if (input_values->config_type != kDiceConfigTypeDescriptor &&
	input_values->config_type != kDiceConfigTypeInline) {
	return kDiceResultInvalidInput;
	}

	// Declare buffers which are cleared on 'goto out'.
	uint8_t subject_private_key[DICE_PRIVATE_KEY_SIZE];
	uint8_t authority_private_key[DICE_PRIVATE_KEY_SIZE];

	// Derive keys and IDs from the private key seeds.
	uint8_t subject_public_key[DICE_PUBLIC_KEY_BUFFER_SIZE];
	result = DiceKeypairFromSeed(context, subject_private_key_seed,
	subject_public_key, subject_private_key);
	if (result != kDiceResultOk) {
	goto out;
	}

	uint8_t subject_id[DICE_ID_SIZE];
	result = DiceDeriveCdiCertificateId(context, subject_public_key,
	DICE_PUBLIC_KEY_BUFFER_SIZE, subject_id);
	if (result != kDiceResultOk) {
	goto out;
	}
	char subject_id_hex[41];
	DiceHexEncode(subject_id, sizeof(subject_id), subject_id_hex,
	sizeof(subject_id_hex));
	subject_id_hex[sizeof(subject_id_hex) - 1] = '\0';

	uint8_t authority_public_key[DICE_PUBLIC_KEY_BUFFER_SIZE];
	result = DiceKeypairFromSeed(context, authority_private_key_seed,
	authority_public_key, authority_private_key);
	if (result != kDiceResultOk) {
	goto out;
	}

	uint8_t authority_id[DICE_ID_SIZE];
	result = DiceDeriveCdiCertificateId(
	context, authority_public_key, DICE_PUBLIC_KEY_BUFFER_SIZE, authority_id);
	if (result != kDiceResultOk) {
	goto out;
	}
	char authority_id_hex[41];
	DiceHexEncode(authority_id, sizeof(authority_id), authority_id_hex,
	sizeof(authority_id_hex));
	authority_id_hex[sizeof(authority_id_hex) - 1] = '\0';

	// The public key encoded as a COSE_Key structure is embedded in the CWT.
	uint8_t encoded_public_key[DICE_MAX_PUBLIC_KEY_SIZE];
	size_t encoded_public_key_size = 0;
	result = DiceCoseEncodePublicKey(
	context, subject_public_key, sizeof(encoded_public_key),
	encoded_public_key, &encoded_public_key_size);
	if (result != kDiceResultOk) {
	result = kDiceResultPlatformError;
	goto out;
	}

	// The encoded protected attributes are used in the TBS and the final
	// COSE_Sign1 structure.
	uint8_t protected_attributes[DICE_MAX_PROTECTED_ATTRIBUTES_SIZE];
	size_t protected_attributes_size = 0;
	result = EncodeProtectedAttributes(sizeof(protected_attributes),
	protected_attributes,
	&protected_attributes_size);
	if (result != kDiceResultOk) {
	result = kDiceResultPlatformError;
	goto out;
	}

	// Find out how big the CWT will be.
	size_t cwt_size;
	EncodeCwt(context, input_values, authority_id_hex, subject_id_hex,
	encoded_public_key, encoded_public_key_size, /buffer_size=/0,
	/buffer=/NULL, &cwt_size);

	// We need space to assemble the TBS. The size of the buffer needed depends on
	// the size of the CWT, which is outside our control (e.g. it might have a
	// very large config descriptor). So we use the certificate buffer as
	// temporary storage; if we run out of space we will make sure the caller
	// knows the size we actually need for this.
	// Encode the TBS, leaving space for the final payload (the CWT).
	uint8_t* cwt_ptr;
	size_t tbs_size;
	result =
	EncodeCoseTbs(protected_attributes, protected_attributes_size, cwt_size,
	/aad=/NULL, /aad_size=/0, certificate_buffer_size,
	certificate, &cwt_ptr, &tbs_size);

	if (result != kDiceResultOk) {
	// There wasn't enough space to put together the TBS. The total buffer size
	// we need is either the amount needed for the TBS, or the amount needed for
	// encoded payload and signature.
	size_t final_encoded_size = 0;
	EncodeCoseSign1(protected_attributes, protected_attributes_size, cwt_ptr,
	cwt_size, /move_payload=/false, /signature=/NULL,
	/buffer_size=/0, /buffer=/NULL, &final_encoded_size);
	*certificate_actual_size =
	final_encoded_size > tbs_size ? final_encoded_size : tbs_size;
	result = kDiceResultBufferTooSmall;
	goto out;
	}

	// Now we can encode the payload directly into the allocated BSTR in the TBS.
	size_t final_cwt_size;
	result = EncodeCwt(context, input_values, authority_id_hex, subject_id_hex,
	encoded_public_key, encoded_public_key_size, cwt_size,
	cwt_ptr, &final_cwt_size);
	if (result == kDiceResultBufferTooSmall \|\| final_cwt_size != cwt_size) {
	result = kDiceResultPlatformError;
	}
	if (result != kDiceResultOk) {
	goto out;
	}

	// Sign the now-complete TBS.
	uint8_t signature[DICE_SIGNATURE_SIZE];
	result = DiceSign(context, certificate, tbs_size, authority_private_key,
	signature);
	if (result != kDiceResultOk) {
	goto out;
	}

	// And now we can produce the complete CoseSign1, including the signature, and
	// moving the payload into place as we do it.
	result = EncodeCoseSign1(protected_attributes, protected_attributes_size,
	cwt_ptr, cwt_size, /move_payload=/true, signature,
	certificate_buffer_size, certificate,
	certificate_actual_size);

	out:
	DiceClearMemory(context, sizeof(subject_private_key), subject_private_key);
	DiceClearMemory(context, sizeof(authority_private_key),
	authority_private_key);

	return result;
	}