boot/bootutil/src/image_ec256.c

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.
 */

#include "syscfg/syscfg.h"

#if MYNEWT_VAL(BOOTUTIL_SIGN_EC256)
#include "bootutil/sign_key.h"

#include "mbedtls/oid.h"
#include "mbedtls/asn1.h"

#include "tinycrypt/ecc_dsa.h"
#include "bootutil_priv.h"

/*
 * Declaring these like this adds NULL termination.
 */
static const uint8_t ec_pubkey_oid[] = MBEDTLS_OID_EC_ALG_UNRESTRICTED;
static const uint8_t ec_secp256r1_oid[] = MBEDTLS_OID_EC_GRP_SECP256R1;

/*
 * Parse the public key used for signing.
 */
static int
tinycrypt_import_key(EccPoint *pubkey, uint8_t *cp, uint8_t *end)
{
    size_t len;
    mbedtls_asn1_buf alg;
    mbedtls_asn1_buf param;

    if (mbedtls_asn1_get_tag(&cp, end, &len,
        MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE)) {
        return -1;
    }
    end = cp + len;

    if (mbedtls_asn1_get_alg(&cp, end, &alg, &param)) {
        return -2;
    }
    if (alg.len != sizeof(ec_pubkey_oid) - 1 ||
        memcmp(alg.p, ec_pubkey_oid, sizeof(ec_pubkey_oid) - 1)) {
        return -3;
    }
    if (param.len != sizeof(ec_secp256r1_oid) - 1 ||
        memcmp(param.p, ec_secp256r1_oid, sizeof(ec_secp256r1_oid) - 1)) {
        return -4;
    }
    if (mbedtls_asn1_get_bitstring_null(&cp, end, &len)) {
        return -6;
    }
    if (cp + len != end) {
        return -7;
    }

    if (len != 2 * NUM_ECC_BYTES + 1) {
        return -8;
    }
    if (cp[0] != 0x04) {
        return -9;
    }

    ecc_bytes2native(pubkey->x, cp + 1);
    ecc_bytes2native(pubkey->y, cp + 1 + NUM_ECC_BYTES);

    return 0;
}

/*
 * cp points to ASN1 string containing an integer.
 * Verify the tag, and that the length is 32 bytes.
 */
static int
tinycrypt_read_bigint(uint32_t i[NUM_ECC_DIGITS], uint8_t **cp, uint8_t *end)
{
    size_t len;

    if (mbedtls_asn1_get_tag(cp, end, &len, MBEDTLS_ASN1_INTEGER)) {
        return -3;
    }

    for (; *cp < end; *cp = *cp + 1, len--) {
        if (**cp != 0) {
            break;
        }
    }
    if (len != NUM_ECC_BYTES) {
        return -1;
    }
    ecc_bytes2native(i, *cp);
    *cp += len;
    return 0;
}

/*
 * Read in signature. Signature has r and s encoded as integers.
 */
static int
tinycrypt_decode_sig(uint32_t r[NUM_ECC_DIGITS], uint32_t s[NUM_ECC_DIGITS],
                     uint8_t *cp, uint8_t *end)
{
    int rc;
    size_t len;

    rc = mbedtls_asn1_get_tag(&cp, end, &len,
                              MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE);
    if (rc) {
        return -1;
    }
    if (cp + len != end) {
        return -2;
    }
    rc = tinycrypt_read_bigint(r, &cp, end);
    if (rc) {
        return -3;
    }
    rc = tinycrypt_read_bigint(s, &cp, end);
    if (rc) {
        return -4;
    }
    return 0;
}

int
bootutil_verify_sig(uint8_t *hash, uint32_t hlen, uint8_t *sig, int slen,
  uint8_t key_id)
{
    int rc;
    uint8_t *cp;
    uint8_t *end;
    EccPoint ctx;
    uint32_t r[NUM_ECC_DIGITS];
    uint32_t s[NUM_ECC_DIGITS];
    uint32_t hash_t[NUM_ECC_DIGITS];

    cp = (uint8_t *)bootutil_keys[key_id].key;
    end = cp + *bootutil_keys[key_id].len;

    rc = tinycrypt_import_key(&ctx, cp, end);
    if (rc) {
        return -1;
    }

    while (sig[slen - 1] == '\0') {
        slen--;
    }

    rc = tinycrypt_decode_sig(r, s, sig, sig + slen);
    if (rc) {
        return -1;
    }

    /*
     * This is simplified, as the hash length is also 32 bytes.
     */
    if (hlen != NUM_ECC_BYTES) {
        return -1;
    }

    ecc_bytes2native(hash_t, hash);
    rc = ecdsa_verify(&ctx, hash_t, r, s);
    if (rc == 1) {
        return 0;
    } else {
        return -2;
    }
}
#endif /* MYNEWT_VAL(BOOTUTIL_SIGN_EC256) */