library/ssl_cookie.c

/*
 *  DTLS cookie callbacks implementation
 *
 *  Copyright The Mbed TLS Contributors
 *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
 */
/*
 * These session callbacks use a simple chained list
 * to store and retrieve the session information.
 */

#include "common.h"

#if defined(MBEDTLS_SSL_COOKIE_C)

#include "mbedtls/platform.h"

#include "mbedtls/ssl_cookie.h"
#include "mbedtls/ssl_internal.h"
#include "mbedtls/error.h"
#include "mbedtls/platform_util.h"
#include "mbedtls/constant_time.h"

#include <string.h>

/*
 * If DTLS is in use, then at least one of SHA-1, SHA-256, SHA-512 is
 * available. Try SHA-256 first, 512 wastes resources since we need to stay
 * with max 32 bytes of cookie for DTLS 1.0
 */
#if defined(MBEDTLS_SHA256_C)
#define COOKIE_MD           MBEDTLS_MD_SHA224
#define COOKIE_MD_OUTLEN    32
#define COOKIE_HMAC_LEN     28
#elif defined(MBEDTLS_SHA512_C)
#define COOKIE_MD           MBEDTLS_MD_SHA384
#define COOKIE_MD_OUTLEN    48
#define COOKIE_HMAC_LEN     28
#elif defined(MBEDTLS_SHA1_C)
#define COOKIE_MD           MBEDTLS_MD_SHA1
#define COOKIE_MD_OUTLEN    20
#define COOKIE_HMAC_LEN     20
#else
#error "DTLS hello verify needs SHA-1 or SHA-2"
#endif

/*
 * Cookies are formed of a 4-bytes timestamp (or serial number) and
 * an HMAC of timestamp and client ID.
 */
#define COOKIE_LEN      (4 + COOKIE_HMAC_LEN)

void mbedtls_ssl_cookie_init(mbedtls_ssl_cookie_ctx *ctx)
{
    mbedtls_md_init(&ctx->hmac_ctx);
#if !defined(MBEDTLS_HAVE_TIME)
    ctx->serial = 0;
#endif
    ctx->timeout = MBEDTLS_SSL_COOKIE_TIMEOUT;

#if defined(MBEDTLS_THREADING_C)
    mbedtls_mutex_init(&ctx->mutex);
#endif
}

void mbedtls_ssl_cookie_set_timeout(mbedtls_ssl_cookie_ctx *ctx, unsigned long delay)
{
    ctx->timeout = delay;
}

void mbedtls_ssl_cookie_free(mbedtls_ssl_cookie_ctx *ctx)
{
    mbedtls_md_free(&ctx->hmac_ctx);

#if defined(MBEDTLS_THREADING_C)
    mbedtls_mutex_free(&ctx->mutex);
#endif

    mbedtls_platform_zeroize(ctx, sizeof(mbedtls_ssl_cookie_ctx));
}

int mbedtls_ssl_cookie_setup(mbedtls_ssl_cookie_ctx *ctx,
                             int (*f_rng)(void *, unsigned char *, size_t),
                             void *p_rng)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    unsigned char key[COOKIE_MD_OUTLEN];

    if ((ret = f_rng(p_rng, key, sizeof(key))) != 0) {
        return ret;
    }

    ret = mbedtls_md_setup(&ctx->hmac_ctx, mbedtls_md_info_from_type(COOKIE_MD), 1);
    if (ret != 0) {
        return ret;
    }

    ret = mbedtls_md_hmac_starts(&ctx->hmac_ctx, key, sizeof(key));
    if (ret != 0) {
        return ret;
    }

    mbedtls_platform_zeroize(key, sizeof(key));

    return 0;
}

/*
 * Generate the HMAC part of a cookie
 */
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_cookie_hmac(mbedtls_md_context_t *hmac_ctx,
                           const unsigned char time[4],
                           unsigned char **p, unsigned char *end,
                           const unsigned char *cli_id, size_t cli_id_len)
{
    unsigned char hmac_out[COOKIE_MD_OUTLEN];

    MBEDTLS_SSL_CHK_BUF_PTR(*p, end, COOKIE_HMAC_LEN);

    if (mbedtls_md_hmac_reset(hmac_ctx) != 0 ||
        mbedtls_md_hmac_update(hmac_ctx, time, 4) != 0 ||
        mbedtls_md_hmac_update(hmac_ctx, cli_id, cli_id_len) != 0 ||
        mbedtls_md_hmac_finish(hmac_ctx, hmac_out) != 0) {
        return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
    }

    memcpy(*p, hmac_out, COOKIE_HMAC_LEN);
    *p += COOKIE_HMAC_LEN;

    return 0;
}

/*
 * Generate cookie for DTLS ClientHello verification
 */
int mbedtls_ssl_cookie_write(void *p_ctx,
                             unsigned char **p, unsigned char *end,
                             const unsigned char *cli_id, size_t cli_id_len)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    mbedtls_ssl_cookie_ctx *ctx = (mbedtls_ssl_cookie_ctx *) p_ctx;
    unsigned long t;

    if (ctx == NULL || cli_id == NULL) {
        return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
    }

    MBEDTLS_SSL_CHK_BUF_PTR(*p, end, COOKIE_LEN);

#if defined(MBEDTLS_HAVE_TIME)
    t = (unsigned long) mbedtls_time(NULL);
#else
    t = ctx->serial++;
#endif

    MBEDTLS_PUT_UINT32_BE(t, *p, 0);
    *p += 4;

#if defined(MBEDTLS_THREADING_C)
    if ((ret = mbedtls_mutex_lock(&ctx->mutex)) != 0) {
        return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_SSL_INTERNAL_ERROR, ret);
    }
#endif

    ret = ssl_cookie_hmac(&ctx->hmac_ctx, *p - 4,
                          p, end, cli_id, cli_id_len);

#if defined(MBEDTLS_THREADING_C)
    if (mbedtls_mutex_unlock(&ctx->mutex) != 0) {
        return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_SSL_INTERNAL_ERROR,
                                 MBEDTLS_ERR_THREADING_MUTEX_ERROR);
    }
#endif

    return ret;
}

/*
 * Check a cookie
 */
int mbedtls_ssl_cookie_check(void *p_ctx,
                             const unsigned char *cookie, size_t cookie_len,
                             const unsigned char *cli_id, size_t cli_id_len)
{
    unsigned char ref_hmac[COOKIE_HMAC_LEN];
    int ret = 0;
    unsigned char *p = ref_hmac;
    mbedtls_ssl_cookie_ctx *ctx = (mbedtls_ssl_cookie_ctx *) p_ctx;
    unsigned long cur_time, cookie_time;

    if (ctx == NULL || cli_id == NULL) {
        return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
    }

    if (cookie_len != COOKIE_LEN) {
        return -1;
    }

#if defined(MBEDTLS_THREADING_C)
    if ((ret = mbedtls_mutex_lock(&ctx->mutex)) != 0) {
        return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_SSL_INTERNAL_ERROR, ret);
    }
#endif

    if (ssl_cookie_hmac(&ctx->hmac_ctx, cookie,
                        &p, p + sizeof(ref_hmac),
                        cli_id, cli_id_len) != 0) {
        ret = -1;
    }

#if defined(MBEDTLS_THREADING_C)
    if (mbedtls_mutex_unlock(&ctx->mutex) != 0) {
        ret = MBEDTLS_ERROR_ADD(MBEDTLS_ERR_SSL_INTERNAL_ERROR,
                                MBEDTLS_ERR_THREADING_MUTEX_ERROR);
    }
#endif

    if (ret != 0) {
        goto exit;
    }

    if (mbedtls_ct_memcmp(cookie + 4, ref_hmac, sizeof(ref_hmac)) != 0) {
        ret = -1;
        goto exit;
    }

#if defined(MBEDTLS_HAVE_TIME)
    cur_time = (unsigned long) mbedtls_time(NULL);
#else
    cur_time = ctx->serial;
#endif

    cookie_time = ((unsigned long) cookie[0] << 24) |
                  ((unsigned long) cookie[1] << 16) |
                  ((unsigned long) cookie[2] <<  8) |
                  ((unsigned long) cookie[3]);

    if (ctx->timeout != 0 && cur_time - cookie_time > ctx->timeout) {
        ret = -1;
        goto exit;
    }

exit:
    mbedtls_platform_zeroize(ref_hmac, sizeof(ref_hmac));
    return ret;
}
#endif /* MBEDTLS_SSL_COOKIE_C */