tests/suites/test_suite_ssl_decrypt.function

/* BEGIN_HEADER */
/* Testing of mbedtls_ssl_decrypt_buf() specifically, focusing on negative
 * testing (using malformed inputs). */

#include <mbedtls/ssl.h>
#include <test/ssl_helpers.h>

/* END_HEADER */

/* BEGIN_DEPENDENCIES
 * depends_on:MBEDTLS_SSL_TLS_C
 * END_DEPENDENCIES
 */

/* BEGIN_CASE depends_on:MBEDTLS_CIPHER_MODE_CBC:MBEDTLS_AES_C:MBEDTLS_SSL_PROTO_TLS1_2 */
void ssl_decrypt_non_etm_cbc(int cipher_type, int hash_id, int trunc_hmac,
                             int length_selector)
{
    /*
     * Test record decryption for CBC without EtM, focused on the verification
     * of padding and MAC.
     *
     * Actually depends on TLS >= 1.0 (SSL 3.0 computes the MAC differently),
     * and either AES, ARIA, Camellia or DES, but since the test framework
     * doesn't support alternation in dependency statements, just depend on
     * TLS 1.2 and AES.
     *
     * The length_selector argument is interpreted as follows:
     * - if it's -1, the plaintext length is 0 and minimal padding is applied
     * - if it's -2, the plaintext length is 0 and maximal padding is applied
     * - otherwise it must be in [0, 255] and is padding_length from RFC 5246:
     *   it's the length of the rest of the padding, that is, excluding the
     *   byte that encodes the length. The minimal non-zero plaintext length
     *   that gives this padding_length is automatically selected.
     */
    mbedtls_ssl_context ssl; /* ONLY for debugging */
    mbedtls_ssl_transform t0, t1;
    mbedtls_record rec, rec_save;
    unsigned char *buf = NULL, *buf_save = NULL;
    size_t buflen, olen = 0;
    size_t plaintext_len, block_size, i;
    unsigned char padlen; /* excluding the padding_length byte */
    int exp_ret;
    const unsigned char pad_max_len = 255; /* Per the standard */

    mbedtls_ssl_init(&ssl);
    mbedtls_ssl_transform_init(&t0);
    mbedtls_ssl_transform_init(&t1);
    USE_PSA_INIT();

    /* Set up transforms with dummy keys */
    TEST_ASSERT(mbedtls_test_ssl_build_transforms(&t0, &t1, cipher_type, hash_id,
                                                  0, trunc_hmac,
                                                  MBEDTLS_SSL_MINOR_VERSION_3,
                                                  0, 0) == 0);

    /* Determine padding/plaintext length */
    TEST_ASSERT(length_selector >= -2 && length_selector <= 255);
    block_size = t0.ivlen;
    if (length_selector < 0) {
        plaintext_len = 0;

        /* Minimal padding
         * The +1 is for the padding_length byte, not counted in padlen. */
        padlen = block_size - (t0.maclen + 1) % block_size;

        /* Maximal padding? */
        if (length_selector == -2) {
            padlen += block_size * ((pad_max_len - padlen) / block_size);
        }
    } else {
        padlen = length_selector;

        /* Minimal non-zero plaintext_length giving desired padding.
         * The +1 is for the padding_length byte, not counted in padlen. */
        plaintext_len = block_size - (padlen + t0.maclen + 1) % block_size;
    }

    /* Prepare a buffer for record data */
    buflen = block_size
             + plaintext_len
             + t0.maclen
             + padlen + 1;
    TEST_CALLOC(buf, buflen);
    TEST_CALLOC(buf_save, buflen);

    /* Prepare a dummy record header */
    memset(rec.ctr, 0, sizeof(rec.ctr));
    rec.type    = MBEDTLS_SSL_MSG_APPLICATION_DATA;
    rec.ver[0]  = MBEDTLS_SSL_MAJOR_VERSION_3;
    rec.ver[1]  = MBEDTLS_SSL_MINOR_VERSION_3;
#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
    rec.cid_len = 0;
#endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */

    /* Prepare dummy record content */
    rec.buf     = buf;
    rec.buf_len = buflen;
    rec.data_offset = block_size;
    rec.data_len = plaintext_len;
    memset(rec.buf + rec.data_offset, 42, rec.data_len);

    /* Set dummy IV */
    memset(t0.iv_enc, 0x55, t0.ivlen);
    memcpy(rec.buf, t0.iv_enc, t0.ivlen);

    /*
     * Prepare a pre-encryption record (with MAC and padding), and save it.
     */
    TEST_EQUAL(0, mbedtls_test_ssl_prepare_record_mac(&rec, &t0));

    /* Pad */
    memset(rec.buf + rec.data_offset + rec.data_len, padlen, padlen + 1);
    rec.data_len += padlen + 1;

    /* Save correct pre-encryption record */
    rec_save = rec;
    rec_save.buf = buf_save;
    memcpy(buf_save, buf, buflen);

    /*
     * Encrypt and decrypt the correct record, expecting success
     */
    TEST_EQUAL(0, mbedtls_cipher_crypt(&t0.cipher_ctx_enc,
                                       t0.iv_enc, t0.ivlen,
                                       rec.buf + rec.data_offset, rec.data_len,
                                       rec.buf + rec.data_offset, &olen));
    rec.data_offset -= t0.ivlen;
    rec.data_len    += t0.ivlen;

    TEST_EQUAL(0, mbedtls_ssl_decrypt_buf(&ssl, &t1, &rec));

    /*
     * Modify each byte of the pre-encryption record before encrypting and
     * decrypting it, expecting failure every time.
     */
    for (i = block_size; i < buflen; i++) {
        mbedtls_test_set_step(i);

        /* Restore correct pre-encryption record */
        rec = rec_save;
        rec.buf = buf;
        memcpy(buf, buf_save, buflen);

        /* Corrupt one byte of the data (could be plaintext, MAC or padding) */
        rec.buf[i] ^= 0x01;

        /* Encrypt */
        TEST_EQUAL(0, mbedtls_cipher_crypt(&t0.cipher_ctx_enc,
                                           t0.iv_enc, t0.ivlen,
                                           rec.buf + rec.data_offset, rec.data_len,
                                           rec.buf + rec.data_offset, &olen));
        rec.data_offset -= t0.ivlen;
        rec.data_len    += t0.ivlen;

        /* Decrypt and expect failure */
        TEST_EQUAL(MBEDTLS_ERR_SSL_INVALID_MAC,
                   mbedtls_ssl_decrypt_buf(&ssl, &t1, &rec));
    }

    /*
     * Use larger values of the padding bytes - with small buffers, this tests
     * the case where the announced padlen would be larger than the buffer
     * (and before that, than the buffer minus the size of the MAC), to make
     * sure our padding checking code does not perform any out-of-bounds reads
     * in this case. (With larger buffers, ie when the plaintext is long or
     * maximal length padding is used, this is less relevant but still doesn't
     * hurt to test.)
     *
     * (Start the loop with correct padding, just to double-check that record
     * saving did work, and that we're overwriting the correct bytes.)
     */
    for (i = padlen; i <= pad_max_len; i++) {
        mbedtls_test_set_step(i);

        /* Restore correct pre-encryption record */
        rec = rec_save;
        rec.buf = buf;
        memcpy(buf, buf_save, buflen);

        /* Set padding bytes to new value */
        memset(buf + buflen - padlen - 1, i, padlen + 1);

        /* Encrypt */
        TEST_EQUAL(0, mbedtls_cipher_crypt(&t0.cipher_ctx_enc,
                                           t0.iv_enc, t0.ivlen,
                                           rec.buf + rec.data_offset, rec.data_len,
                                           rec.buf + rec.data_offset, &olen));
        rec.data_offset -= t0.ivlen;
        rec.data_len    += t0.ivlen;

        /* Decrypt and expect failure except the first time */
        exp_ret = (i == padlen) ? 0 : MBEDTLS_ERR_SSL_INVALID_MAC;
        TEST_EQUAL(exp_ret, mbedtls_ssl_decrypt_buf(&ssl, &t1, &rec));
    }

exit:
    mbedtls_ssl_free(&ssl);
    mbedtls_ssl_transform_free(&t0);
    mbedtls_ssl_transform_free(&t1);
    mbedtls_free(buf);
    mbedtls_free(buf_save);
    USE_PSA_DONE();
}
/* END_CASE */