lib/t_cose/src/t_cose_util.c

/*
 *  t_cose_util.c
 *
 * Copyright 2019, Laurence Lundblade
 *
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * See BSD-3-Clause license in README.mdE.
 */

#include "t_cose_util.h"
#include "qcbor.h"
#include "t_cose_defines.h"
#include "t_cose_common.h"
#include "t_cose_crypto.h"


/**
 * \file t_cose_util.c
 *
 * \brief Implementation of t_cose utility functions.
 *
 */


/*
 * Public function. See t_cose_util.h
 */
int32_t hash_alg_id_from_sig_alg_id(int32_t cose_sig_alg_id)
{
    /* if other hashes, particularly those that output bigger hashes
     * are added here, various other parts of this code have to be
     * changed to have larger buffers.
     */
    switch(cose_sig_alg_id) {

        case COSE_ALGORITHM_ES256:
            return COSE_ALG_SHA256_PROPRIETARY;

        default:
            return INT32_MAX;
    }
}


/*
 * Format of to-be-signed bytes used by create_tbs_hash().
 * This is defined in COSE (RFC 8152). It is the input
 * to the hash.
 *
 * Sig_structure = [
 *    context : "Signature" / "Signature1" / "CounterSignature",
 *    body_protected : empty_or_serialized_map,
 *    ? sign_protected : empty_or_serialized_map,
 *    external_aad : bstr,
 *    payload : bstr
 * ]
 */


/**
 * This is the size of the first part of the CBOR encoded TBS
 * bytes. It is around 20 bytes. See create_tbs_hash().
 */
#define T_COSE_SIZE_OF_TBS \
    1 + /* For opening the array */ \
    sizeof(COSE_SIG_CONTEXT_STRING_SIGNATURE1) + /* "Signature1" */ \
    2 + /* Overhead for encoding string */ \
    T_COSE_SIGN1_MAX_PROT_HEADER + /* entire protected headers */ \
    3 * ( /* 3 NULL bstrs for fields not used */ \
        1 /* size of a NULL bstr */  \
    )


/*
 * Public function. See t_cose_util.h
 */
enum t_cose_err_t create_tbs_hash(int32_t cose_alg_id,
                                  struct q_useful_buf buffer_for_hash,
                                  struct q_useful_buf_c *hash,
                                  struct q_useful_buf_c protected_headers,
                                  struct q_useful_buf_c payload)
{
    /* approximate stack use on 32-bit machine:
     * local use: 320
     * with calls: 360
     */
    enum t_cose_err_t           return_value;
    QCBOREncodeContext          cbor_encode_ctx;
    UsefulBuf_MAKE_STACK_UB(    buffer_for_TBS_first_part, T_COSE_SIZE_OF_TBS);
    struct q_useful_buf_c       tbs_first_part;
    QCBORError                  qcbor_result;
    struct t_cose_crypto_hash   hash_ctx;
    int32_t                     hash_alg_id;

    /* This builds the CBOR-format to-be-signed bytes */
    QCBOREncode_Init(&cbor_encode_ctx, buffer_for_TBS_first_part);
    QCBOREncode_OpenArray(&cbor_encode_ctx);
    /* context */
    QCBOREncode_AddSZString(&cbor_encode_ctx,
                            COSE_SIG_CONTEXT_STRING_SIGNATURE1);
    /* body_protected */
    QCBOREncode_AddBytes(&cbor_encode_ctx,
                         protected_headers);
    /* sign_protected */
    QCBOREncode_AddBytes(&cbor_encode_ctx, NULL_Q_USEFUL_BUF_C);
    /* external_aad */
    QCBOREncode_AddBytes(&cbor_encode_ctx, NULL_Q_USEFUL_BUF_C);
    /* fake payload */
    QCBOREncode_AddBytes(&cbor_encode_ctx, NULL_Q_USEFUL_BUF_C);
    QCBOREncode_CloseArray(&cbor_encode_ctx);

    /* get the result and convert it to struct q_useful_buf_c representation */
    qcbor_result = QCBOREncode_Finish(&cbor_encode_ctx, &tbs_first_part);
    if(qcbor_result) {
        /* Mainly means that the protected_headers were too big
         (which should never happen) */
        return_value = T_COSE_ERR_SIG_STRUCT;
        goto Done;
    }

    /* Start the hashing */
    hash_alg_id = hash_alg_id_from_sig_alg_id(cose_alg_id);
    /* Don't check hash_alg_id for failure. t_cose_crypto_hash_start()
     will handle it properly
     */
    return_value = t_cose_crypto_hash_start(&hash_ctx, hash_alg_id);
    if(return_value) {
        goto Done;
    }

    /* Hash the first part of the TBS. Take all but the last two
     * bytes. The last two bytes are the fake payload from above. It
     * is replaced by the real payload which is hashed next. The fake
     * payload is needed so the array count is right. This is one of
     * the main things that make it possible to implement with one
     * buffer for the whole cose sign1.
     */
    t_cose_crypto_hash_update(&hash_ctx,
                              q_useful_buf_head(tbs_first_part,
                                                tbs_first_part.len - 2));

    /* Hash the payload */
    t_cose_crypto_hash_update(&hash_ctx, payload);

    /* Finish the hash and set up to return it */
    return_value = t_cose_crypto_hash_finish(&hash_ctx,
                                             buffer_for_hash,
                                             hash);

Done:
    return return_value;
}


/*
 * Public function. See t_cose_util.h
 */
enum t_cose_err_t
get_short_circuit_kid(struct q_useful_buf buffer_for_kid,
                      struct q_useful_buf_c *kid)
{
    /* This is a random hard coded key ID that is used to indicate
     * short-circuit signing. It is OK to hard code this as the
     * probability of collision with this ID is very low and the same
     * as for collision between any two key IDs of any sort.
     */
    uint8_t defined_short_circuit_kid[] = {
        0xef, 0x95, 0x4b, 0x4b, 0xd9, 0xbd, 0xf6, 0x70,
        0xd0, 0x33, 0x60, 0x82, 0xf5, 0xef, 0x15, 0x2a,
        0xf8, 0xf3, 0x5b, 0x6a, 0x6c, 0x00, 0xef, 0xa6,
        0xa9, 0xa7, 0x1f, 0x49, 0x51, 0x7e, 0x18, 0xc6};

    /* Prevent a dumb error where the size constant in the header is
     * wrong.This check will be evaluated at compile time and optimize
     * out when all is correct.
     */
    if(sizeof(defined_short_circuit_kid) != T_COSE_SHORT_CIRCUIT_KID_SIZE) {
        return T_COSE_ERR_BAD_SHORT_CIRCUIT_KID;
    }

    *kid = q_useful_buf_copy(buffer_for_kid,
                             Q_USEFUL_BUF_FROM_BYTE_ARRAY_LITERAL(
                                 defined_short_circuit_kid));

    return q_useful_buf_c_is_null(*kid) ?
              T_COSE_ERR_KEY_BUFFER_SIZE :
              T_COSE_SUCCESS;
}