In pull request #4328 the seeding of the DRBG via RAND_add()/RAND_seed()
was implemented by buffering the data in a random pool where it is
picked up later by the rand_drbg_get_entropy() callback. This buffer
was limited to the size of 4096 bytes.
When a larger input was added via RAND_add() or RAND_seed() to the DRBG,
the reseeding failed, but the error returned by the DRBG was ignored
by the two calling functions, which both don't return an error code.
As a consequence, the data provided by the application was effectively
ignored.
This commit fixes the problem by a more efficient implementation which
does not copy the data in memory and by raising the buffer the size limit
to INT32_MAX (2 gigabytes). This is less than the NIST limit of 2^35 bits
but it was chosen intentionally to avoid platform dependent problems
like integer sizes and/or signed/unsigned conversion.
Additionally, the DRBG is now less permissive on errors: In addition to
pushing a message to the openssl error stack, it enters the error state,
which forces a reinstantiation on next call.
Thanks go to Dr. Falko Strenzke for reporting this issue to the
openssl-security mailing list. After internal discussion the issue
has been categorized as not being security relevant, because the DRBG
reseeds automatically and is fully functional even without additional
randomness provided by the application.
Fixes #7381
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7382)
(cherry picked from commit
3064b55134434a0b2850f07eff57120f35bb269a)
*)
+ Changes between 1.1.1 and 1.1.1a [xx XXX xxxx]
+
+ *) Fixed the issue that RAND_add()/RAND_seed() silently discards random input
+ if its length exceeds 4096 bytes. The limit has been raised to a buffer size
+ of two gigabytes and the error handling improved.
+
+ This issue was reported to OpenSSL by Dr. Falko Strenzke. It has been
+ categorized as a normal bug, not a security issue, because the DRBG reseeds
+ automatically and is fully functional even without additional randomness
+ provided by the application.
+
Changes between 1.1.0i and 1.1.1 [11 Sep 2018]
*) Add a new ClientHello callback. Provides a callback interface that gives
*) A minor bug in ssl/s3_clnt.c where there would always be 4 0
bytes sent in the client random.
[Edward Bishop <ebishop@spyglass.com>]
-
RAND_F_RAND_POOL_ADD:103:rand_pool_add
RAND_F_RAND_POOL_ADD_BEGIN:113:rand_pool_add_begin
RAND_F_RAND_POOL_ADD_END:114:rand_pool_add_end
+RAND_F_RAND_POOL_ATTACH:124:rand_pool_attach
RAND_F_RAND_POOL_BYTES_NEEDED:115:rand_pool_bytes_needed
RAND_F_RAND_POOL_NEW:116:rand_pool_new
RAND_F_RAND_WRITE_FILE:112:RAND_write_file
* RAND_POOL functions
*/
RAND_POOL *rand_pool_new(int entropy_requested, size_t min_len, size_t max_len);
+RAND_POOL *rand_pool_attach(const unsigned char *buffer, size_t len,
+ size_t entropy);
void rand_pool_free(RAND_POOL *pool);
const unsigned char *rand_pool_buffer(RAND_POOL *pool);
return 0;
drbg->min_entropylen = ctr->keylen;
- drbg->max_entropylen = DRBG_MINMAX_FACTOR * drbg->min_entropylen;
+ drbg->max_entropylen = DRBG_MAX_LENGTH;
drbg->min_noncelen = drbg->min_entropylen / 2;
- drbg->max_noncelen = DRBG_MINMAX_FACTOR * drbg->min_noncelen;
+ drbg->max_noncelen = DRBG_MAX_LENGTH;
drbg->max_perslen = DRBG_MAX_LENGTH;
drbg->max_adinlen = DRBG_MAX_LENGTH;
} else {
if (drbg->pool != NULL) {
RANDerr(RAND_F_RAND_DRBG_RESTART, ERR_R_INTERNAL_ERROR);
+ drbg->state = DRBG_ERROR;
rand_pool_free(drbg->pool);
drbg->pool = NULL;
+ return 0;
}
if (buffer != NULL) {
if (drbg->max_entropylen < len) {
RANDerr(RAND_F_RAND_DRBG_RESTART,
RAND_R_ENTROPY_INPUT_TOO_LONG);
+ drbg->state = DRBG_ERROR;
return 0;
}
if (entropy > 8 * len) {
RANDerr(RAND_F_RAND_DRBG_RESTART, RAND_R_ENTROPY_OUT_OF_RANGE);
+ drbg->state = DRBG_ERROR;
return 0;
}
/* will be picked up by the rand_drbg_get_entropy() callback */
- drbg->pool = rand_pool_new(entropy, len, len);
+ drbg->pool = rand_pool_attach(buffer, len, entropy);
if (drbg->pool == NULL)
return 0;
-
- rand_pool_add(drbg->pool, buffer, len, entropy);
} else {
if (drbg->max_adinlen < len) {
RANDerr(RAND_F_RAND_DRBG_RESTART,
RAND_R_ADDITIONAL_INPUT_TOO_LONG);
+ drbg->state = DRBG_ERROR;
return 0;
}
adin = buffer;
if (num < 0 || randomness < 0.0)
return 0;
- if (randomness > (double)drbg->max_entropylen) {
+ if (randomness > (double)RAND_DRBG_STRENGTH) {
/*
* The purpose of this check is to bound |randomness| by a
* relatively small value in order to prevent an integer
* overflow when multiplying by 8 in the rand_drbg_restart()
- * call below.
+ * call below. Note that randomness is measured in bytes,
+ * not bits, so this value corresponds to eight times the
+ * security strength.
*/
- return 0;
+ randomness = (double)RAND_DRBG_STRENGTH;
}
rand_drbg_lock(drbg);
{ERR_PACK(ERR_LIB_RAND, RAND_F_RAND_POOL_ADD_BEGIN, 0),
"rand_pool_add_begin"},
{ERR_PACK(ERR_LIB_RAND, RAND_F_RAND_POOL_ADD_END, 0), "rand_pool_add_end"},
+ {ERR_PACK(ERR_LIB_RAND, RAND_F_RAND_POOL_ATTACH, 0), "rand_pool_attach"},
{ERR_PACK(ERR_LIB_RAND, RAND_F_RAND_POOL_BYTES_NEEDED, 0),
"rand_pool_bytes_needed"},
{ERR_PACK(ERR_LIB_RAND, RAND_F_RAND_POOL_NEW, 0), "rand_pool_new"},
-/* Max size of additional input and personalization string. */
-# define DRBG_MAX_LENGTH 4096
+/*
+ * Maximum input size for the DRBG (entropy, nonce, personalization string)
+ *
+ * NIST SP800 90Ar1 allows a maximum of (1 << 35) bits i.e., (1 << 32) bytes.
+ *
+ * We lower it to 'only' INT32_MAX bytes, which is equivalent to 2 gigabytes.
+ */
+# define DRBG_MAX_LENGTH INT32_MAX
+
+
/*
- * The quotient between max_{entropy,nonce}len and min_{entropy,nonce}len
+ * Maximum allocation size for RANDOM_POOL buffers
*
- * The current factor is large enough that the RAND_POOL can store a
- * random input which has a lousy entropy rate of 0.0625 bits per byte.
- * This input will be sent through the derivation function which 'compresses'
- * the low quality input into a high quality output.
+ * The max_len value for the buffer provided to the rand_drbg_get_entropy()
+ * callback is currently 2^31 bytes (2 gigabytes), if a derivation function
+ * is used. Since this is much too large to be allocated, the rand_pool_new()
+ * function chooses more modest values as default pool length, bounded
+ * by RAND_POOL_MIN_LENGTH and RAND_POOL_MAX_LENGTH
+ *
+ * The choice of the RAND_POOL_FACTOR is large enough such that the
+ * RAND_POOL can store a random input which has a lousy entropy rate of
+ * 8/256 (= 0.03125) bits per byte. This input will be sent through the
+ * derivation function which 'compresses' the low quality input into a
+ * high quality output.
+ *
+ * The factor 1.5 below is the pessimistic estimate for the extra amount
+ * of entropy required when no get_nonce() callback is defined.
+ */
+# define RAND_POOL_FACTOR 256
+# define RAND_POOL_MAX_LENGTH (RAND_POOL_FACTOR * \
+ 3 * (RAND_DRBG_STRENGTH / 16))
+/*
+ * = (RAND_POOL_FACTOR * \
+ * 1.5 * (RAND_DRBG_STRENGTH / 8))
*/
-# define DRBG_MINMAX_FACTOR 128
/* DRBG status values */
unsigned char *buffer; /* points to the beginning of the random pool */
size_t len; /* current number of random bytes contained in the pool */
+ int attached; /* true pool was attached to existing buffer */
+
size_t min_len; /* minimum number of random bytes requested */
size_t max_len; /* maximum number of random bytes (allocated buffer size) */
size_t entropy; /* current entropy count in bits */
- size_t requested_entropy; /* requested entropy count in bits */
+ size_t entropy_requested; /* requested entropy count in bits */
};
/*
unsigned short flags; /* various external flags */
/*
- * The random pool is used by RAND_add()/drbg_add() to attach random
+ * The random_data is used by RAND_add()/drbg_add() to attach random
* data to the global drbg, such that the rand_drbg_get_entropy() callback
* can pull it during instantiation and reseeding. This is necessary to
* reconcile the different philosophies of the RAND and the RAND_DRBG
return 0;
}
- pool = rand_pool_new(entropy, min_len, max_len);
- if (pool == NULL)
- return 0;
-
- if (drbg->pool) {
- rand_pool_add(pool,
- rand_pool_buffer(drbg->pool),
- rand_pool_length(drbg->pool),
- rand_pool_entropy(drbg->pool));
- rand_pool_free(drbg->pool);
- drbg->pool = NULL;
+ if (drbg->pool != NULL) {
+ pool = drbg->pool;
+ pool->entropy_requested = entropy;
+ } else {
+ pool = rand_pool_new(entropy, min_len, max_len);
}
if (drbg->parent) {
void rand_drbg_cleanup_entropy(RAND_DRBG *drbg,
unsigned char *out, size_t outlen)
{
- OPENSSL_secure_clear_free(out, outlen);
+ if (drbg->pool == NULL)
+ OPENSSL_secure_clear_free(out, outlen);
+ else
+ drbg->pool = NULL;
}
/* fill random pool and seed the current legacy RNG */
pool = rand_pool_new(RAND_DRBG_STRENGTH,
RAND_DRBG_STRENGTH / 8,
- DRBG_MINMAX_FACTOR * (RAND_DRBG_STRENGTH / 8));
+ RAND_POOL_MAX_LENGTH);
if (pool == NULL)
return 0;
* Allocate memory and initialize a new random pool
*/
-RAND_POOL *rand_pool_new(int entropy, size_t min_len, size_t max_len)
+RAND_POOL *rand_pool_new(int entropy_requested, size_t min_len, size_t max_len)
{
RAND_POOL *pool = OPENSSL_zalloc(sizeof(*pool));
if (pool == NULL) {
RANDerr(RAND_F_RAND_POOL_NEW, ERR_R_MALLOC_FAILURE);
- goto err;
+ return NULL;
}
pool->min_len = min_len;
- pool->max_len = max_len;
+ pool->max_len = (max_len > RAND_POOL_MAX_LENGTH) ?
+ RAND_POOL_MAX_LENGTH : max_len;
pool->buffer = OPENSSL_secure_zalloc(pool->max_len);
if (pool->buffer == NULL) {
goto err;
}
- pool->requested_entropy = entropy;
+ pool->entropy_requested = entropy_requested;
return pool;
return NULL;
}
+/*
+ * Attach new random pool to the given buffer
+ *
+ * This function is intended to be used only for feeding random data
+ * provided by RAND_add() and RAND_seed() into the <master> DRBG.
+ */
+RAND_POOL *rand_pool_attach(const unsigned char *buffer, size_t len,
+ size_t entropy)
+{
+ RAND_POOL *pool = OPENSSL_zalloc(sizeof(*pool));
+
+ if (pool == NULL) {
+ RANDerr(RAND_F_RAND_POOL_ATTACH, ERR_R_MALLOC_FAILURE);
+ return NULL;
+ }
+
+ /*
+ * The const needs to be cast away, but attached buffers will not be
+ * modified (in contrary to allocated buffers which are zeroed and
+ * freed in the end).
+ */
+ pool->buffer = (unsigned char *) buffer;
+ pool->len = len;
+
+ pool->attached = 1;
+
+ pool->min_len = pool->max_len = pool->len;
+ pool->entropy = entropy;
+
+ return pool;
+}
+
/*
* Free |pool|, securely erasing its buffer.
*/
if (pool == NULL)
return;
- OPENSSL_secure_clear_free(pool->buffer, pool->max_len);
+ /*
+ * Although it would be advisable from a cryptographical viewpoint,
+ * we are not allowed to clear attached buffers, since they are passed
+ * to rand_pool_attach() as `const unsigned char*`.
+ * (see corresponding comment in rand_pool_attach()).
+ */
+ if (!pool->attached)
+ OPENSSL_secure_clear_free(pool->buffer, pool->max_len);
OPENSSL_free(pool);
}
*/
size_t rand_pool_entropy_available(RAND_POOL *pool)
{
- if (pool->entropy < pool->requested_entropy)
+ if (pool->entropy < pool->entropy_requested)
return 0;
if (pool->len < pool->min_len)
size_t rand_pool_entropy_needed(RAND_POOL *pool)
{
- if (pool->entropy < pool->requested_entropy)
- return pool->requested_entropy - pool->entropy;
+ if (pool->entropy < pool->entropy_requested)
+ return pool->entropy_requested - pool->entropy;
return 0;
}
* Note: 'buflen' equals the size of the buffer which is used by the
* get_entropy() callback of the RAND_DRBG. It is roughly bounded by
*
- * 2 * DRBG_MINMAX_FACTOR * (RAND_DRBG_STRENGTH / 8) = 2^13
+ * 2 * RAND_POOL_FACTOR * (RAND_DRBG_STRENGTH / 8) = 2^14
*
* which is way below the OSSL_SSIZE_MAX limit. Therefore sign conversion
* between size_t and ssize_t is safe even without a range check.
# define RAND_F_RAND_POOL_ADD 103
# define RAND_F_RAND_POOL_ADD_BEGIN 113
# define RAND_F_RAND_POOL_ADD_END 114
+# define RAND_F_RAND_POOL_ATTACH 124
# define RAND_F_RAND_POOL_BYTES_NEEDED 115
# define RAND_F_RAND_POOL_NEW 116
# define RAND_F_RAND_WRITE_FILE 112
*/
static void cleanup_pool_entropy(RAND_DRBG *drbg, unsigned char *out, size_t outlen)
{
- OPENSSL_secure_clear_free(drbg->pool->buffer, drbg->pool->max_len);
OPENSSL_free(drbg->pool);
drbg->pool = NULL;
}
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