path: root/src/cbor_cert_op.c

// 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 using the ED25519-SHA512 signature scheme.

#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"

#if DICE_PUBLIC_KEY_SIZE != 32
#error "Only Ed25519 is supported; 32 bytes needed to store the public key."
#endif
#if DICE_SIGNATURE_SIZE != 64
#error "Only Ed25519 is supported; 64 bytes needed to store the signature."
#endif

// Max size of COSE_Sign1 including payload.
static const size_t kMaxCertificateSize = 2048;
// Max size of COSE_Key encoding.
static const size_t kMaxPublicKeySize = 64;
// Max size of the COSE_Sign1 protected attributes.
static const size_t kMaxProtectedAttributesSize = 16;

DiceResult DiceCoseEncodePublicKey(
    void* context_not_used, const uint8_t public_key[DICE_PUBLIC_KEY_SIZE],
    size_t buffer_size, uint8_t* buffer, size_t* encoded_size) {
  (void)context_not_used;

  // Constants per RFC 8152.
  const int64_t kCoseKeyKtyLabel = 1;
  const int64_t kCoseKeyAlgLabel = 3;
  const int64_t kCoseKeyOpsLabel = 4;
  const int64_t kCoseOkpCrvLabel = -1;
  const int64_t kCoseOkpXLabel = -2;
  const int64_t kCoseKeyTypeOkp = 1;
  const int64_t kCoseAlgEdDSA = -8;
  const int64_t kCoseKeyOpsVerify = 2;
  const int64_t kCoseCrvEd25519 = 6;

  struct CborOut out;
  CborOutInit(buffer, buffer_size, &out);
  CborWriteMap(/*num_pairs=*/5, &out);
  // Add the key type.
  CborWriteInt(kCoseKeyKtyLabel, &out);
  CborWriteInt(kCoseKeyTypeOkp, &out);
  // Add the algorithm.
  CborWriteInt(kCoseKeyAlgLabel, &out);
  CborWriteInt(kCoseAlgEdDSA, &out);
  // Add the KeyOps.
  CborWriteInt(kCoseKeyOpsLabel, &out);
  CborWriteArray(/*num_elements=*/1, &out);
  CborWriteInt(kCoseKeyOpsVerify, &out);
  // Add the curve.
  CborWriteInt(kCoseOkpCrvLabel, &out);
  CborWriteInt(kCoseCrvEd25519, &out);
  // Add the public key.
  CborWriteInt(kCoseOkpXLabel, &out);
  CborWriteBstr(/*data_size=*/DICE_PUBLIC_KEY_SIZE, public_key, &out);
  if (CborOutOverflowed(&out)) {
    return kDiceResultBufferTooSmall;
  }
  *encoded_size = CborOutSize(&out);
  return kDiceResultOk;
}

static DiceResult EncodeProtectedAttributes(size_t buffer_size, uint8_t* buffer,
                                            size_t* encoded_size) {
  // Constants per RFC 8152.
  const int64_t kCoseHeaderAlgLabel = 1;
  const int64_t kCoseAlgEdDSA = -8;

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

static DiceResult EncodeCoseTbs(const uint8_t* protected_attributes,
                                size_t protected_attributes_size,
                                const uint8_t* payload, size_t payload_size,
                                const uint8_t* aad, size_t aad_size,
                                size_t buffer_size, uint8_t* buffer,
                                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);
  // Payload from COSE_Sign1.
  CborWriteBstr(payload_size, payload, &out);
  if (CborOutOverflowed(&out)) {
    return kDiceResultBufferTooSmall;
  }
  *encoded_size = CborOutSize(&out);
  return kDiceResultOk;
}

static DiceResult EncodeCoseSign1(const uint8_t* protected_attributes,
                                  size_t protected_attributes_size,
                                  const uint8_t* payload, size_t payload_size,
                                  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.
  CborWriteBstr(payload_size, payload, &out);
  // Signature.
  CborWriteBstr(/*num_elements=*/DICE_SIGNATURE_SIZE, signature, &out);
  if (CborOutOverflowed(&out)) {
    return kDiceResultBufferTooSmall;
  }
  *encoded_size = CborOutSize(&out);
  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[kMaxProtectedAttributesSize];
  size_t protected_attributes_size = 0;
  result = EncodeProtectedAttributes(sizeof(protected_attributes),
                                     protected_attributes,
                                     &protected_attributes_size);
  if (result != kDiceResultOk) {
    return result;
  }

  // Construct a To-Be-Signed (TBS) structure based on the relevant fields of
  // the COSE_Sign1.
  result = EncodeCoseTbs(protected_attributes, protected_attributes_size,
                         payload, payload_size, aad, aad_size, buffer_size,
                         buffer, encoded_size);
  if (result != kDiceResultOk) {
    return result;
  }

  // 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, 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;
  // 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;
  }

  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];
    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);
  if (CborOutOverflowed(&out)) {
    return kDiceResultBufferTooSmall;
  }
  *encoded_size = CborOutSize(&out);
  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];

  // These are 'variably modified' types so need to be declared upfront.
  uint8_t encoded_public_key[kMaxPublicKeySize];
  uint8_t payload[kMaxCertificateSize];

  // Derive keys and IDs from the private key seeds.
  uint8_t subject_public_key[DICE_PUBLIC_KEY_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_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_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_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.
  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) {
    goto out;
  }

  // The CWT is the payload in both the TBS and the final COSE_Sign1 structure.
  size_t payload_size = 0;
  result = EncodeCwt(context, input_values, authority_id_hex, subject_id_hex,
                     encoded_public_key, encoded_public_key_size,
                     sizeof(payload), payload, &payload_size);
  if (result != kDiceResultOk) {
    goto out;
  }

  result = DiceCoseSignAndEncodeSign1(
      context, payload, payload_size, /*aad=*/NULL, /*aad_size=*/0,
      authority_private_key, 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;
}