trezor-firmware/core/embed/prodtest/optiga_prodtest.c

/*
 * This file is part of the Trezor project, https://trezor.io/
 *
 * Copyright (c) SatoshiLabs
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <string.h>

#include "aes/aes.h"
#include "buffer.h"
#include "der.h"
#include "ecdsa.h"
#include "memzero.h"
#include "nist256p1.h"
#include "optiga_commands.h"
#include "optiga_prodtest.h"
#include "optiga_transport.h"
#include "prodtest_common.h"
#include "rand.h"
#include "secret.h"
#include "sha2.h"

typedef enum {
  OPTIGA_PAIRING_UNPAIRED = 0,
  OPTIGA_PAIRING_PAIRED,
  OPTIGA_PAIRING_ERR_RNG,
  OPTIGA_PAIRING_ERR_READ,
  OPTIGA_PAIRING_ERR_HANDSHAKE,
} optiga_pairing;

static optiga_pairing optiga_pairing_state = OPTIGA_PAIRING_UNPAIRED;

// Data object access conditions.
static const optiga_metadata_item ACCESS_PAIRED =
    OPTIGA_ACCESS_CONDITION(OPTIGA_ACCESS_COND_CONF, OID_KEY_PAIRING);
static const optiga_metadata_item KEY_USE_SIGN = {
    (const uint8_t[]){OPTIGA_KEY_USAGE_SIGN}, 1};
static const optiga_metadata_item TYPE_PTFBIND = {
    (const uint8_t[]){OPTIGA_DATA_TYPE_PTFBIND}, 1};

static bool optiga_paired(void) {
  const char *details = "";

  switch (optiga_pairing_state) {
    case OPTIGA_PAIRING_PAIRED:
      return true;
    case OPTIGA_PAIRING_ERR_RNG:
      details = "optiga_get_random error";
      break;
    case OPTIGA_PAIRING_ERR_READ:
      details = "failed to read pairing secret";
      break;
    case OPTIGA_PAIRING_ERR_HANDSHAKE:
      details = "optiga_sec_chan_handshake";
      break;
    default:
      break;
  }

  vcp_println("ERROR Optiga not paired (%s).", details);
  return false;
}

static bool set_metadata(uint16_t oid, const optiga_metadata *metadata) {
  uint8_t serialized[OPTIGA_MAX_METADATA_SIZE] = {0};
  size_t size = 0;
  optiga_result ret = optiga_serialize_metadata(metadata, serialized,
                                                sizeof(serialized), &size);
  if (OPTIGA_SUCCESS != ret) {
    vcp_println("ERROR optiga_serialize_metadata error %d for OID 0x%04x.", ret,
                oid);
    return false;
  }

  optiga_set_data_object(oid, true, serialized, size);

  ret =
      optiga_get_data_object(oid, true, serialized, sizeof(serialized), &size);
  if (OPTIGA_SUCCESS != ret) {
    vcp_println("ERROR optiga_get_metadata error %d for OID 0x%04x.", ret, oid);
    return false;
  }

  optiga_metadata metadata_stored = {0};
  ret = optiga_parse_metadata(serialized, size, &metadata_stored);
  if (OPTIGA_SUCCESS != ret) {
    vcp_println("ERROR optiga_parse_metadata error %d.", ret);
    return false;
  }

  if (!optiga_compare_metadata(metadata, &metadata_stored)) {
    vcp_println("ERROR optiga_compare_metadata failed.");
    return false;
  }

  return true;
}

void pair_optiga(void) {
  // The pairing key may already be written and locked. The success of the
  // pairing procedure is determined by optiga_sec_chan_handshake(). Therefore
  // it is OK for some of the intermediate operations to fail.

  // Enable writing the pairing secret to OPTIGA.
  optiga_metadata metadata = {0};
  metadata.change = OPTIGA_META_ACCESS_ALWAYS;
  metadata.execute = OPTIGA_META_ACCESS_ALWAYS;
  metadata.data_type = TYPE_PTFBIND;
  set_metadata(OID_KEY_PAIRING, &metadata);  // Ignore result.

  // Generate pairing secret.
  uint8_t secret[SECRET_OPTIGA_KEY_LEN] = {0};
  optiga_result ret = optiga_get_random(secret, sizeof(secret));
  if (OPTIGA_SUCCESS != ret) {
    optiga_pairing_state = OPTIGA_PAIRING_ERR_RNG;
    return;
  }

  // Store pairing secret.
  ret = optiga_set_data_object(OID_KEY_PAIRING, false, secret, sizeof(secret));
  if (OPTIGA_SUCCESS == ret) {
    secret_erase();
    secret_write_header();
    secret_write(secret, SECRET_OPTIGA_KEY_OFFSET, SECRET_OPTIGA_KEY_LEN);
  }

  // Verify whether the secret was stored correctly in flash and OPTIGA.
  memzero(secret, sizeof(secret));
  if (secret_read(secret, SECRET_OPTIGA_KEY_OFFSET, SECRET_OPTIGA_KEY_LEN) !=
      sectrue) {
    optiga_pairing_state = OPTIGA_PAIRING_ERR_READ;
    return;
  }

  ret = optiga_sec_chan_handshake(secret, sizeof(secret));
  memzero(secret, sizeof(secret));
  if (OPTIGA_SUCCESS != ret) {
    optiga_pairing_state = OPTIGA_PAIRING_ERR_HANDSHAKE;
    return;
  }

  optiga_pairing_state = OPTIGA_PAIRING_PAIRED;
  return;
}

void optiga_lock(void) {
  if (!optiga_paired()) return;

  // Delete trust anchor.
  optiga_result ret =
      optiga_set_data_object(OID_TRUST_ANCHOR, false, (const uint8_t *)"\0", 1);
  if (OPTIGA_SUCCESS != ret) {
    vcp_println("ERROR optiga_set_data error %d for 0x%04x.", ret,
                OID_TRUST_ANCHOR);
    return;
  }

  // Set data object metadata.
  optiga_metadata metadata = {0};

  // Set metadata for device certificate.
  memzero(&metadata, sizeof(metadata));
  metadata.lcso = OPTIGA_META_LCS_OPERATIONAL;
  metadata.change = OPTIGA_META_ACCESS_NEVER;
  metadata.read = OPTIGA_META_ACCESS_ALWAYS;
  metadata.execute = OPTIGA_META_ACCESS_ALWAYS;
  if (!set_metadata(OID_CERT_DEV, &metadata)) {
    return;
  }

  // Set metadata for FIDO attestation certificate.
  memzero(&metadata, sizeof(metadata));
  metadata.lcso = OPTIGA_META_LCS_OPERATIONAL;
  metadata.change = OPTIGA_META_ACCESS_NEVER;
  metadata.read = OPTIGA_META_ACCESS_ALWAYS;
  metadata.execute = OPTIGA_META_ACCESS_ALWAYS;
  if (!set_metadata(OID_CERT_FIDO, &metadata)) {
    return;
  }

  // Set metadata for device private key.
  memzero(&metadata, sizeof(metadata));
  metadata.lcso = OPTIGA_META_LCS_OPERATIONAL;
  metadata.change = OPTIGA_META_ACCESS_NEVER;
  metadata.read = OPTIGA_META_ACCESS_NEVER;
  metadata.execute = ACCESS_PAIRED;
  metadata.key_usage = KEY_USE_SIGN;
  if (!set_metadata(OID_KEY_DEV, &metadata)) {
    return;
  }

  // Set metadata for FIDO attestation private key.
  memzero(&metadata, sizeof(metadata));
  metadata.lcso = OPTIGA_META_LCS_OPERATIONAL;
  metadata.change = OPTIGA_META_ACCESS_NEVER;
  metadata.read = OPTIGA_META_ACCESS_NEVER;
  metadata.execute = ACCESS_PAIRED;
  metadata.key_usage = KEY_USE_SIGN;
  if (!set_metadata(OID_KEY_FIDO, &metadata)) {
    return;
  }

  // Set metadata for pairing key.
  memzero(&metadata, sizeof(metadata));
  metadata.lcso = OPTIGA_META_LCS_OPERATIONAL;
  metadata.change = OPTIGA_META_ACCESS_NEVER;
  metadata.read = OPTIGA_META_ACCESS_NEVER;
  metadata.execute = OPTIGA_META_ACCESS_ALWAYS;
  metadata.data_type = TYPE_PTFBIND;
  if (!set_metadata(OID_KEY_PAIRING, &metadata)) {
    return;
  }

  vcp_println("OK");
}

optiga_locked_status get_optiga_locked_status(void) {
  if (!optiga_paired()) return OPTIGA_LOCKED_ERROR;

  const uint16_t oids[] = {OID_CERT_DEV, OID_CERT_FIDO, OID_KEY_DEV,
                           OID_KEY_FIDO, OID_KEY_PAIRING};

  optiga_metadata locked_metadata = {0};
  locked_metadata.lcso = OPTIGA_META_LCS_OPERATIONAL;
  for (size_t i = 0; i < sizeof(oids) / sizeof(oids[0]); ++i) {
    uint8_t metadata_buffer[OPTIGA_MAX_METADATA_SIZE] = {0};
    size_t metadata_size = 0;
    optiga_result ret =
        optiga_get_data_object(oids[i], true, metadata_buffer,
                               sizeof(metadata_buffer), &metadata_size);
    if (OPTIGA_SUCCESS != ret) {
      vcp_println("ERROR optiga_get_metadata error %d for OID 0x%04x.", ret,
                  oids[i]);
      return OPTIGA_LOCKED_ERROR;
    }

    optiga_metadata stored_metadata = {0};
    ret =
        optiga_parse_metadata(metadata_buffer, metadata_size, &stored_metadata);
    if (OPTIGA_SUCCESS != ret) {
      vcp_println("ERROR optiga_parse_metadata error %d.", ret);
      return OPTIGA_LOCKED_ERROR;
    }

    if (!optiga_compare_metadata(&locked_metadata, &stored_metadata)) {
      return OPTIGA_LOCKED_FALSE;
    }
  }

  return OPTIGA_LOCKED_TRUE;
}

void check_locked(void) {
  switch (get_optiga_locked_status()) {
    case OPTIGA_LOCKED_TRUE:
      vcp_println("OK YES");
      break;
    case OPTIGA_LOCKED_FALSE:
      vcp_println("OK NO");
      break;
    default:
      // Error reported by get_optiga_locked_status().
      break;
  }
}

void optigaid_read(void) {
  if (!optiga_paired()) return;

  uint8_t optiga_id[27] = {0};
  size_t optiga_id_size = 0;

  optiga_result ret =
      optiga_get_data_object(OPTIGA_OID_COPROC_UID, false, optiga_id,
                             sizeof(optiga_id), &optiga_id_size);
  if (OPTIGA_SUCCESS != ret) {
    vcp_println("ERROR optiga_get_data_object error %d for 0x%04x.", ret,
                OPTIGA_OID_COPROC_UID);
    return;
  }

  vcp_print("OK ");
  vcp_println_hex(optiga_id, optiga_id_size);
}

void cert_read(uint16_t oid) {
  if (!optiga_paired()) return;

  static uint8_t cert[OPTIGA_MAX_CERT_SIZE] = {0};
  size_t cert_size = 0;
  optiga_result ret =
      optiga_get_data_object(oid, false, cert, sizeof(cert), &cert_size);
  if (OPTIGA_SUCCESS != ret) {
    vcp_println("ERROR optiga_get_data_object error %d for 0x%04x.", ret, oid);
    return;
  }

  size_t offset = 0;
  if (cert[0] == 0xC0) {
    // TLS identity certificate chain.
    size_t tls_identity_size = (cert[1] << 8) + cert[2];
    size_t cert_chain_size = (cert[3] << 16) + (cert[4] << 8) + cert[5];
    size_t first_cert_size = (cert[6] << 16) + (cert[7] << 8) + cert[8];
    if (tls_identity_size + 3 > cert_size ||
        cert_chain_size + 3 > tls_identity_size ||
        first_cert_size > cert_chain_size) {
      vcp_println("ERROR invalid TLS identity in 0x%04x.", oid);
      return;
    }
    offset = 9;
    cert_size = first_cert_size;
  }

  if (cert_size == 0) {
    vcp_println("ERROR no certificate in 0x%04x.", oid);
    return;
  }

  vcp_print("OK ");
  vcp_println_hex(cert + offset, cert_size);
}

void cert_write(uint16_t oid, char *data) {
  if (!optiga_paired()) return;

  // Enable writing to the certificate slot.
  optiga_metadata metadata = {0};
  metadata.change = OPTIGA_META_ACCESS_ALWAYS;
  set_metadata(oid, &metadata);  // Ignore result.

  uint8_t data_bytes[OPTIGA_MAX_CERT_SIZE];

  int len = get_from_hex(data_bytes, sizeof(data_bytes), data);
  if (len < 0) {
    vcp_println("ERROR Hexadecimal decoding error %d.", len);
    return;
  }

  optiga_result ret = optiga_set_data_object(oid, false, data_bytes, len);
  if (OPTIGA_SUCCESS != ret) {
    vcp_println("ERROR optiga_set_data error %d for 0x%04x.", ret, oid);
    return;
  }

  // Verify that the certificate was written correctly.
  static uint8_t cert[OPTIGA_MAX_CERT_SIZE] = {0};
  size_t cert_size = 0;
  ret = optiga_get_data_object(oid, false, cert, sizeof(cert), &cert_size);
  if (OPTIGA_SUCCESS != ret || cert_size != len ||
      memcmp(data_bytes, cert, len) != 0) {
    vcp_println("ERROR optiga_get_data_object error %d for 0x%04x.", ret, oid);
    return;
  }

  if (oid == OID_CERT_DEV && !check_device_cert_chain(cert, cert_size)) {
    // Error returned by check_device_cert_chain().
    return;
  }

  vcp_println("OK");
}

void pubkey_read(uint16_t oid) {
  if (!optiga_paired()) return;

  // Enable key agreement usage.

  optiga_metadata metadata = {0};
  metadata.key_usage = OPTIGA_META_KEY_USE_KEYAGREE;
  metadata.execute = OPTIGA_META_ACCESS_ALWAYS;

  if (!set_metadata(oid, &metadata)) {
    return;
  }

  // Execute ECDH with base point to get the x-coordinate of the public key.
  static const uint8_t BASE_POINT[] = {
      0x03, 0x42, 0x00, 0x04, 0x6b, 0x17, 0xd1, 0xf2, 0xe1, 0x2c, 0x42, 0x47,
      0xf8, 0xbc, 0xe6, 0xe5, 0x63, 0xa4, 0x40, 0xf2, 0x77, 0x03, 0x7d, 0x81,
      0x2d, 0xeb, 0x33, 0xa0, 0xf4, 0xa1, 0x39, 0x45, 0xd8, 0x98, 0xc2, 0x96,
      0x4f, 0xe3, 0x42, 0xe2, 0xfe, 0x1a, 0x7f, 0x9b, 0x8e, 0xe7, 0xeb, 0x4a,
      0x7c, 0x0f, 0x9e, 0x16, 0x2b, 0xce, 0x33, 0x57, 0x6b, 0x31, 0x5e, 0xce,
      0xcb, 0xb6, 0x40, 0x68, 0x37, 0xbf, 0x51, 0xf5};
  uint8_t public_key[32] = {0};
  size_t public_key_size = 0;
  optiga_result ret =
      optiga_calc_ssec(OPTIGA_CURVE_P256, oid, BASE_POINT, sizeof(BASE_POINT),
                       public_key, sizeof(public_key), &public_key_size);
  if (OPTIGA_SUCCESS != ret) {
    vcp_println("ERROR optiga_calc_ssec error %d.", ret);
    return;
  }

  vcp_print("OK ");
  vcp_println_hex(public_key, public_key_size);
}

void keyfido_write(char *data) {
  if (!optiga_paired()) return;

  const size_t EPH_PUB_KEY_SIZE = 33;
  const size_t PAYLOAD_SIZE = 32;
  const size_t CIPHERTEXT_OFFSET = EPH_PUB_KEY_SIZE;
  const size_t EXPECTED_SIZE = EPH_PUB_KEY_SIZE + PAYLOAD_SIZE;

  // Enable key agreement usage for device key.

  optiga_metadata metadata = {0};
  metadata.key_usage = OPTIGA_META_KEY_USE_KEYAGREE;
  metadata.execute = OPTIGA_META_ACCESS_ALWAYS;

  if (!set_metadata(OID_KEY_DEV, &metadata)) {
    return;
  }

  // Read encrypted FIDO attestation private key.

  uint8_t data_bytes[EXPECTED_SIZE];
  int len = get_from_hex(data_bytes, sizeof(data_bytes), data);
  if (len < 0) {
    vcp_println("ERROR Hexadecimal decoding error %d.", len);
    return;
  }

  if (len != EXPECTED_SIZE) {
    vcp_println("ERROR Unexpected input length.");
    return;
  }

  // Expand sender's ephemeral public key.
  uint8_t public_key[3 + 65] = {0x03, 0x42, 0x00};
  if (ecdsa_uncompress_pubkey(&nist256p1, data_bytes, &public_key[3]) != 1) {
    vcp_println("ERROR Failed to decode public key.");
    return;
  }

  // Execute ECDH with device private key.
  uint8_t secret[32] = {0};
  size_t secret_size = 0;
  optiga_result ret = optiga_calc_ssec(OPTIGA_CURVE_P256, OID_KEY_DEV,
                                       public_key, sizeof(public_key), secret,
                                       sizeof(secret), &secret_size);
  if (OPTIGA_SUCCESS != ret) {
    memzero(secret, sizeof(secret));
    vcp_println("ERROR optiga_calc_ssec error %d.", ret);
    return;
  }

  // Hash the shared secret. Use the result as the decryption key.
  sha256_Raw(secret, secret_size, secret);
  aes_decrypt_ctx ctx = {0};
  AES_RETURN aes_ret = aes_decrypt_key256(secret, &ctx);
  if (EXIT_SUCCESS != aes_ret) {
    vcp_println("ERROR aes_decrypt_key256 error.");
    memzero(&ctx, sizeof(ctx));
    memzero(secret, sizeof(secret));
    return;
  }

  // Decrypt the FIDO attestation key.
  uint8_t fido_key[PAYLOAD_SIZE];

  // The IV is intentionally all-zero, which is not a problem, because the
  // encryption key is unique for each ciphertext.
  uint8_t iv[AES_BLOCK_SIZE] = {0};
  aes_ret = aes_cbc_decrypt(&data_bytes[CIPHERTEXT_OFFSET], fido_key,
                            sizeof(fido_key), iv, &ctx);
  memzero(&ctx, sizeof(ctx));
  memzero(secret, sizeof(secret));
  if (EXIT_SUCCESS != aes_ret) {
    memzero(fido_key, sizeof(fido_key));
    vcp_println("ERROR aes_cbc_decrypt error.");
    return;
  }

  // Write trust anchor certificate to OID 0xE0E8
  ret = optiga_set_trust_anchor();
  if (OPTIGA_SUCCESS != ret) {
    memzero(fido_key, sizeof(fido_key));
    vcp_println("ERROR optiga_set_trust_anchor error %d.", ret);
    return;
  }

  // Set change access condition for the FIDO key to Int(0xE0E8), so that we
  // can write the FIDO key using the trust anchor in OID 0xE0E8.
  memzero(&metadata, sizeof(metadata));
  metadata.change.ptr = (const uint8_t *)"\x21\xe0\xe8";
  metadata.change.len = 3;
  if (!set_metadata(OID_KEY_FIDO, &metadata)) {
    return;
  }

  // Store the FIDO attestation key.
  ret = optiga_set_priv_key(OID_KEY_FIDO, fido_key);
  memzero(fido_key, sizeof(fido_key));
  if (OPTIGA_SUCCESS != ret) {
    vcp_println("ERROR optiga_set_priv_key error %d.", ret);
    return;
  }

  vcp_println("OK");
}

void sec_read(void) {
  if (!optiga_paired()) return;

  uint8_t sec = 0;
  size_t size = 0;

  optiga_result ret =
      optiga_get_data_object(OPTIGA_OID_SEC, false, &sec, sizeof(sec), &size);
  if (OPTIGA_SUCCESS != ret || sizeof(sec) != size) {
    vcp_println("ERROR optiga_get_data_object error %d for 0x%04x.", ret,
                OPTIGA_OID_SEC);
    return;
  }

  vcp_print("OK ");
  vcp_println_hex(&sec, sizeof(sec));
}

// clang-format off
static const uint8_t ECDSA_WITH_SHA256[] = {
  0x30, 0x0a, // a sequence of 10 bytes
    0x06, 0x08, // an OID of 8 bytes
      0x2a, 0x86, 0x48, 0xce, 0x3d, 0x04, 0x03, 0x02,
};
// clang-format on

static const uint8_t ROOT_PUBLIC_KEYS[][65] = {
    {
        // Production root public key.
        0x04, 0xca, 0x97, 0x48, 0x0a, 0xc0, 0xd7, 0xb1, 0xe6, 0xef, 0xaf,
        0xe5, 0x18, 0xcd, 0x43, 0x3c, 0xec, 0x2b, 0xf8, 0xab, 0x98, 0x22,
        0xd7, 0x6e, 0xaf, 0xd3, 0x43, 0x63, 0xb5, 0x5d, 0x63, 0xe6, 0x03,
        0x80, 0xbf, 0xf2, 0x0a, 0xcc, 0x75, 0xcd, 0xe0, 0x3c, 0xff, 0xcb,
        0x50, 0xab, 0x6f, 0x8c, 0xe7, 0x0c, 0x87, 0x8e, 0x37, 0xeb, 0xc5,
        0x8f, 0xf7, 0xcc, 0xa0, 0xa8, 0x3b, 0x16, 0xb1, 0x5f, 0xa5,
    },
    {
        // Development root public key.
        0x04, 0x7f, 0x77, 0x36, 0x8d, 0xea, 0x2d, 0x4d, 0x61, 0xe9, 0x89,
        0xf4, 0x74, 0xa5, 0x67, 0x23, 0xc3, 0x21, 0x2d, 0xac, 0xf8, 0xa8,
        0x08, 0xd8, 0x79, 0x55, 0x95, 0xef, 0x38, 0x44, 0x14, 0x27, 0xc4,
        0x38, 0x9b, 0xc4, 0x54, 0xf0, 0x20, 0x89, 0xd7, 0xf0, 0x8b, 0x87,
        0x30, 0x05, 0xe4, 0xc2, 0x8d, 0x43, 0x24, 0x68, 0x99, 0x78, 0x71,
        0xc0, 0xbf, 0x28, 0x6f, 0xd3, 0x86, 0x1e, 0x21, 0xe9, 0x6a,
    },
};

bool check_device_cert_chain(const uint8_t *chain, size_t chain_size) {
  // Checks the integrity of the device certificate chain to ensure that the
  // certificate data was not corrupted in transport and that the device
  // certificate belongs to this device. THIS IS NOT A FULL VERIFICATION OF THE
  // CERTIFICATE CHAIN.

  // Enable signing with the device private key.
  optiga_metadata metadata = {0};
  metadata.key_usage = KEY_USE_SIGN;
  metadata.execute = OPTIGA_META_ACCESS_ALWAYS;
  if (!set_metadata(OID_KEY_DEV, &metadata)) {
    vcp_println("ERROR check_device_cert_chain, set_metadata.");
    return false;
  }

  // Generate a P-256 signature using the device private key.
  uint8_t digest[SHA256_DIGEST_LENGTH] = {1};
  uint8_t der_sig[72] = {DER_SEQUENCE};
  size_t der_sig_size = 0;
  if (optiga_calc_sign(OID_KEY_DEV, digest, sizeof(digest), &der_sig[2],
                       sizeof(der_sig) - 2, &der_sig_size) != OPTIGA_SUCCESS) {
    vcp_println("ERROR check_device_cert_chain, optiga_calc_sign.");
    return false;
  }
  der_sig[1] = der_sig_size;

  uint8_t sig[64] = {0};
  if (ecdsa_sig_from_der(der_sig, der_sig_size + 2, sig) != 0) {
    vcp_println("ERROR check_device_cert_chain, ecdsa_sig_from_der.");
    return false;
  }

  BUFFER_READER chain_reader = {0};
  buffer_reader_init(&chain_reader, chain, chain_size);
  int cert_count = 0;
  while (buffer_remaining(&chain_reader) > 0) {
    // Read the next certificate in the chain.
    cert_count += 1;
    DER_ITEM cert = {0};
    if (!der_read_item(&chain_reader, &cert) || cert.id != DER_SEQUENCE) {
      vcp_println("ERROR check_device_cert_chain, der_read_item 1, cert %d.",
                  cert_count);
      return false;
    }

    // Read the tbsCertificate.
    DER_ITEM tbs_cert = {0};
    if (!der_read_item(&cert.buf, &tbs_cert)) {
      vcp_println("ERROR check_device_cert_chain, der_read_item 2, cert %d.",
                  cert_count);
      return false;
    }

    // Read the Subject Public Key Info.
    DER_ITEM pub_key_info = {0};
    for (int i = 0; i < 7; ++i) {
      if (!der_read_item(&tbs_cert.buf, &pub_key_info)) {
        vcp_println("ERROR check_device_cert_chain, der_read_item 3, cert %d.",
                    cert_count);
        return false;
      }
    }

    // Read the public key.
    DER_ITEM pub_key = {0};
    uint8_t unused_bits = 0;
    const uint8_t *pub_key_bytes = NULL;
    for (int i = 0; i < 2; ++i) {
      if (!der_read_item(&pub_key_info.buf, &pub_key)) {
        vcp_println("ERROR check_device_cert_chain, der_read_item 4, cert %d.",
                    cert_count);
        return false;
      }
    }

    if (!buffer_get(&pub_key.buf, &unused_bits) ||
        buffer_remaining(&pub_key.buf) != 65 ||
        !buffer_ptr(&pub_key.buf, &pub_key_bytes)) {
      vcp_println("ERROR check_device_cert_chain, reading public key, cert %d.",
                  cert_count);
      return false;
    }

    // Verify the previous signature.
    if (ecdsa_verify_digest(&nist256p1, pub_key_bytes, sig, digest) != 0) {
      vcp_println(
          "ERROR check_device_cert_chain, ecdsa_verify_digest, cert %d.",
          cert_count);
      return false;
    }

    // Prepare the hash of tbsCertificate for the next signature verification.
    sha256_Raw(tbs_cert.buf.data, tbs_cert.buf.size, digest);

    // Read the signatureAlgorithm and ensure it matches ECDSA_WITH_SHA256.
    DER_ITEM sig_alg = {0};
    if (!der_read_item(&cert.buf, &sig_alg) ||
        sig_alg.buf.size != sizeof(ECDSA_WITH_SHA256) ||
        memcmp(ECDSA_WITH_SHA256, sig_alg.buf.data,
               sizeof(ECDSA_WITH_SHA256)) != 0) {
      vcp_println(
          "ERROR check_device_cert_chain, checking signatureAlgorithm, cert "
          "%d.",
          cert_count);
      return false;
    }

    // Read the signatureValue.
    DER_ITEM sig_val = {0};
    if (!der_read_item(&cert.buf, &sig_val) || sig_val.id != DER_BIT_STRING ||
        !buffer_get(&sig_val.buf, &unused_bits) || unused_bits != 0) {
      vcp_println(
          "ERROR check_device_cert_chain, reading signatureValue, cert %d.",
          cert_count);
      return false;
    }

    // Extract the signature for the next signature verification.
    const uint8_t *sig_bytes = NULL;
    if (!buffer_ptr(&sig_val.buf, &sig_bytes) ||
        ecdsa_sig_from_der(sig_bytes, buffer_remaining(&sig_val.buf), sig) !=
            0) {
      vcp_println("ERROR check_device_cert_chain, ecdsa_sig_from_der, cert %d.",
                  cert_count);
      return false;
    }
  }

  // Verify that the last certificate in the chain is valid for one of the known
  // root public keys.
  for (int i = 0; i < sizeof(ROOT_PUBLIC_KEYS) / sizeof(ROOT_PUBLIC_KEYS[0]);
       ++i) {
    if (ecdsa_verify_digest(&nist256p1, ROOT_PUBLIC_KEYS[i], sig, digest) ==
        0) {
      return true;
    }
  }

  vcp_println("ERROR check_device_cert_chain, ecdsa_verify_digest root.");
  return false;
}