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Implement a fast aes-ctr prng

Browse Source 
This module is currently implemented to use the same technique as
libottery (later used by the bsds' arc4random replacement), using
AES-CTR-256 as its underlying stream cipher.  It's backtracking-
resistant immediately after each call, and prediction-resistant
after a while.

Here's how it works:

We generate psuedorandom bytes using AES-CTR-256.  We generate BUFLEN bytes
at a time.  When we do this, we keep the first SEED_LEN bytes as the key
and the IV for our next invocation of AES_CTR, and yield the remaining
BUFLEN - SEED_LEN bytes to the user as they invoke the PRNG.  As we yield
bytes to the user, we clear them from the buffer.

Every RESEED_AFTER times we refill the buffer, we mix in an additional
SEED_LEN bytes from our strong PRNG into the seed.

If the user ever asks for a huge number of bytes at once, we pull SEED_LEN
bytes from the PRNG and use them with our stream cipher to fill the user's
request.
This commit is contained in:
Nick Mathewson 2019-02-05 12:49:04 -05:00
parent 3d3578ab41
commit f3cbd6426c
5 changed files with 459 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

32
src/lib/crypt_ops/crypto_rand.h
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@ -16,6 +16,7 @@
#include "lib/cc/compat_compiler.h"
#include "lib/cc/torint.h"
#include "lib/testsupport/testsupport.h"
#include "lib/malloc/malloc.h"
/* random numbers */
int crypto_seed_rng(void) ATTR_WUR;
@ -24,6 +25,7 @@ void crypto_rand_unmocked(char *to, size_t n);
void crypto_strongest_rand(uint8_t *out, size_t out_len);
MOCK_DECL(void,crypto_strongest_rand_,(uint8_t *out, size_t out_len));
int crypto_rand_int(unsigned int max);
unsigned crypto_rand_uint(unsigned limit);
int crypto_rand_int_range(unsigned int min, unsigned int max);
uint64_t crypto_rand_uint64_range(uint64_t min, uint64_t max);
time_t crypto_rand_time_range(time_t min, time_t max);
@ -41,6 +43,36 @@ void *smartlist_choose(const struct smartlist_t *sl);
void smartlist_shuffle(struct smartlist_t *sl);
int crypto_force_rand_ssleay(void);
/**
* A fast PRNG, for use when the PRNG provided by our crypto library isn't
* fast enough. This one _should_ be cryptographically strong, but
* has seen less auditing than the PRNGs in OpenSSL and NSS. Use with
* caution.
*
* Note that this object is NOT thread-safe. If you need a thread-safe
* prng, use crypto_rand(), or wrap this in a mutex.
**/
typedef struct crypto_fast_rng_t crypto_fast_rng_t;
/**
* Number of bytes used to seed a crypto_rand_fast_t.
**/
crypto_fast_rng_t *crypto_fast_rng_new(void);
#define CRYPTO_FAST_RNG_SEED_LEN 48
crypto_fast_rng_t *crypto_fast_rng_new_from_seed(const uint8_t *seed);
void crypto_fast_rng_getbytes(crypto_fast_rng_t *rng, uint8_t *out, size_t n);
void crypto_fast_rng_free_(crypto_fast_rng_t *);
#define crypto_fast_rng_free(c) \
FREE_AND_NULL(crypto_fast_rng_t, crypto_fast_rng_free_, (c))
unsigned crypto_fast_rng_get_uint(crypto_fast_rng_t *rng, unsigned limit);
uint64_t crypto_fast_rng_get_uint64(crypto_fast_rng_t *rng, uint64_t limit);
double crypto_fast_rng_get_double(crypto_fast_rng_t *rng);
#if defined(TOR_UNIT_TESTS)
/* Used for white-box testing */
size_t crypto_fast_rng_get_bytes_used_per_stream(void);
#endif
#ifdef CRYPTO_RAND_PRIVATE
STATIC int crypto_strongest_rand_raw(uint8_t *out, size_t out_len);

263
src/lib/crypt_ops/crypto_rand_fast.c Normal file
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@ -0,0 +1,263 @@
/* Copyright (c) 2001, Matej Pfajfar.
* Copyright (c) 2001-2004, Roger Dingledine.
* Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
* Copyright (c) 2007-2019, The Tor Project, Inc. */
/* See LICENSE for licensing information */
/**
* \file crypto_rand_fast.c
*
* \brief A fast strong PRNG for use when our underlying cryptographic
* library's PRNG isn't fast enough.
**/
/* This library is currently implemented to use the same implementation
* technique as libottery, using AES-CTR-256 as our underlying stream cipher.
* It's backtracking-resistant immediately, and prediction-resistant after
* a while.
*
* Here's how it works:
*
* We generate pseudorandom bytes using AES-CTR-256. We generate BUFLEN bytes
* at a time. When we do this, we keep the first SEED_LEN bytes as the key
* and the IV for our next invocation of AES_CTR, and yield the remaining
* BUFLEN - SEED_LEN bytes to the user as they invoke the PRNG. As we yield
* bytes to the user, we clear them from the buffer.
*
* After we have refilled the buffer RESEED_AFTER times, we mix in an
* additional SEED_LEN bytes from our strong PRNG into the seed.
*
* If the user ever asks for a huge number of bytes at once, we pull SEED_LEN
* bytes from the PRNG and use them with our stream cipher to fill the user's
* request.
*/
#define CRYPTO_RAND_FAST_PRIVATE
#include "lib/crypt_ops/crypto_rand.h"
#include "lib/crypt_ops/crypto_cipher.h"
#include "lib/crypt_ops/crypto_digest.h"
#include "lib/crypt_ops/crypto_util.h"
#include "lib/intmath/cmp.h"
#include "lib/cc/ctassert.h"
#include "lib/malloc/map_anon.h"
#include "lib/log/util_bug.h"
#include <string.h>
/* Alias for CRYPTO_FAST_RNG_SEED_LEN to make our code shorter.
*/
#define SEED_LEN (CRYPTO_FAST_RNG_SEED_LEN)
/* The amount of space that we mmap for a crypto_fast_rng_t.
*/
#define MAPLEN 4096
/* The number of random bytes that we can yield to the user after each
* time we fill a crypto_fast_rng_t's buffer.
*/
#define BUFLEN (MAPLEN - 2*sizeof(uint16_t) - SEED_LEN)
/* The number of buffer refills after which we should fetch more
* entropy from crypto_strongest_rand().
*/
#define RESEED_AFTER 16
/* The length of the stream cipher key we will use for the PRNG, in bytes.
*/
#define KEY_LEN (CRYPTO_FAST_RNG_SEED_LEN - CIPHER_IV_LEN)
/* The length of the stream cipher key we will use for the PRNG, in bits.
*/
#define KEY_BITS (KEY_LEN * 8)
/* Make sure that we have a key length we can actually use with AES. */
CTASSERT(KEY_BITS == 128 || KEY_BITS == 192 || KEY_BITS == 256);
struct crypto_fast_rng_t {
/** How many more fills does this buffer have before we should mix
* in the output of crypto_rand()? */
uint16_t n_till_reseed;
/** How many bytes are remaining in cbuf.bytes? */
uint16_t bytes_left;
struct cbuf {
/** The seed (key and IV) that we will use the next time that we refill
* cbuf. */
uint8_t seed[SEED_LEN];
/**
* Bytes that we are yielding to the user. The next byte to be
* yielded is at bytes[BUFLEN-bytes_left]; all other bytes in this
* array are set to zero.
*/
uint8_t bytes[BUFLEN];
} buf;
};
/* alignof(uint8_t) should be 1, so there shouldn't be any padding in cbuf.
*/
CTASSERT(sizeof(struct cbuf) == BUFLEN+SEED_LEN);
/* We're trying to fit all of the RNG state into a nice mmapable chunk.
*/
CTASSERT(sizeof(crypto_fast_rng_t) <= MAPLEN);
/**
* Initialize and return a new fast PRNG, using a strong random seed.
*
* Note that this object is NOT thread-safe. If you need a thread-safe
* prng, use crypto_rand(), or wrap this in a mutex.
**/
crypto_fast_rng_t *
crypto_fast_rng_new(void)
{
uint8_t seed[SEED_LEN];
crypto_strongest_rand(seed, sizeof(seed));
crypto_fast_rng_t *result = crypto_fast_rng_new_from_seed(seed);
memwipe(seed, 0, sizeof(seed));
return result;
}
/**
* Initialize and return a new fast PRNG, using a seed value specified
* in <b>seed</b>. This value must be CRYPTO_FAST_RNG_SEED_LEN bytes
* long.
*
* Note that this object is NOT thread-safe. If you need a thread-safe
* prng, use crypto_rand(), or wrap this in a mutex.
**/
crypto_fast_rng_t *
crypto_fast_rng_new_from_seed(const uint8_t *seed)
{
/* We try to allocate this object as securely as we can, to avoid
* having it get dumped, swapped, or shared after fork.
*/
crypto_fast_rng_t *result = tor_mmap_anonymous(sizeof(*result),
ANONMAP_PRIVATE | ANONMAP_NOINHERIT);
memcpy(result->buf.seed, seed, SEED_LEN);
/* Causes an immediate refill once the user asks for data. */
result->bytes_left = 0;
result->n_till_reseed = RESEED_AFTER;
return result;
}
/**
* Helper: create a crypto_cipher_t object from SEED_LEN bytes of
* input. The first KEY_LEN bytes are used as the stream cipher's key,
* and the remaining CIPHER_IV_LEN bytes are used as its IV.
**/
static inline crypto_cipher_t *
cipher_from_seed(const uint8_t *seed)
{
return crypto_cipher_new_with_iv_and_bits(seed, seed+KEY_LEN, KEY_BITS);
}
/**
* Helper: refill the seed bytes and output buffer of <b>rng</b>, using
* the input seed bytes as input (key and IV) for the stream cipher.
*
* If the n_till_reseed counter has reached zero, mix more random bytes into
* the seed before refilling the buffer.
**/
static void
crypto_fast_rng_refill(crypto_fast_rng_t *rng)
{
if (rng->n_till_reseed-- == 0) {
/* It's time to reseed the RNG. We'll do this by using our XOF to mix the
* old value for the seed with some additional bytes from
* crypto_strongest_rand(). */
crypto_xof_t *xof = crypto_xof_new();
crypto_xof_add_bytes(xof, rng->buf.seed, SEED_LEN);
{
uint8_t seedbuf[SEED_LEN];
crypto_strongest_rand(seedbuf, SEED_LEN);
crypto_xof_add_bytes(xof, seedbuf, SEED_LEN);
memwipe(seedbuf, 0, SEED_LEN);
}
crypto_xof_squeeze_bytes(xof, rng->buf.seed, SEED_LEN);
crypto_xof_free(xof);
rng->n_till_reseed = RESEED_AFTER;
}
/* Now fill rng->buf with output from our stream cipher, initialized from
* that seed value. */
crypto_cipher_t *c = cipher_from_seed(rng->buf.seed);
memset(&rng->buf, 0, sizeof(rng->buf));
crypto_cipher_crypt_inplace(c, (char*)&rng->buf, sizeof(rng->buf));
crypto_cipher_free(c);
rng->bytes_left = sizeof(rng->buf.bytes);
}
/**
* Release all storage held by <b>rng</b>.
**/
void
crypto_fast_rng_free_(crypto_fast_rng_t *rng)
{
if (!rng)
return;
memwipe(rng, 0, sizeof(*rng));
tor_munmap_anonymous(rng, sizeof(*rng));
}
/**
* Helper: extract bytes from the PRNG, refilling it as necessary. Does not
* optimize the case when the user has asked for a huge output.
**/
static void
crypto_fast_rng_getbytes_impl(crypto_fast_rng_t *rng, uint8_t *out,
const size_t n)
{
size_t bytes_to_yield = n;
while (bytes_to_yield) {
if (rng->bytes_left == 0)
crypto_fast_rng_refill(rng);
const size_t to_copy = MIN(rng->bytes_left, bytes_to_yield);
tor_assert(sizeof(rng->buf.bytes) >= rng->bytes_left);
uint8_t *copy_from = rng->buf.bytes +
(sizeof(rng->buf.bytes) - rng->bytes_left);
memcpy(out, copy_from, to_copy);
memset(copy_from, 0, to_copy);
out += to_copy;
bytes_to_yield -= to_copy;
rng->bytes_left -= to_copy;
}
}
/**
* Extract <b>n</b> bytes from <b>rng</b> into the buffer at <b>out</b>.
**/
void
crypto_fast_rng_getbytes(crypto_fast_rng_t *rng, uint8_t *out, size_t n)
{
if (PREDICT_UNLIKELY(n > BUFLEN)) {
/* The user has asked for a lot of output; generate it from a stream
* cipher seeded by the PRNG rather than by pulling it out of the PRNG
* directly.
*/
uint8_t seed[SEED_LEN];
crypto_fast_rng_getbytes_impl(rng, seed, SEED_LEN);
crypto_cipher_t *c = cipher_from_seed(seed);
memset(out, 0, n);
crypto_cipher_crypt_inplace(c, (char*)out, n);
crypto_cipher_free(c);
memwipe(seed, 0, sizeof(seed));
return;
}
crypto_fast_rng_getbytes_impl(rng, out, n);
}
#if defined(TOR_UNIT_TESTS)
/** for white-box testing: return the number of bytes that are returned from
* the user for each invocation of the stream cipher in this RNG. */
size_t
crypto_fast_rng_get_bytes_used_per_stream(void)
{
return BUFLEN;
}
#endif

56
src/lib/crypt_ops/crypto_rand_numeric.c
View File

@ -31,9 +31,11 @@
} \
} while (0)
/** Helper: Return a pseudorandom integer chosen uniformly from the
* values between 0 and limit-1 inclusive. */
static unsigned
/**
* Return a pseudorandom integer chosen uniformly from the values between 0
* and <b>limit</b>-1 inclusive. limit must be strictly between 0 and
* UINT_MAX. */
unsigned
crypto_rand_uint(unsigned limit)
{
tor_assert(limit < UINT_MAX);
@ -108,6 +110,14 @@ crypto_rand_uint64(uint64_t max)
crypto_rand((char*)&val, sizeof(val)));
}
#if SIZEOF_INT == 4
#define UINT_MAX_AS_DOUBLE 4294967296.0
#elif SIZEOF_INT == 8
#define UINT_MAX_AS_DOUBLE 1.8446744073709552e+19
#else
#error SIZEOF_INT is neither 4 nor 8
#endif /* SIZEOF_INT == 4 || ... */
/**
* Return a pseudorandom double d, chosen uniformly from the range
* 0.0 <= d < 1.0.
@ -119,12 +129,38 @@ crypto_rand_double(void)
* more than 32 bits of resolution */
unsigned int u;
crypto_rand((char*)&u, sizeof(u));
#if SIZEOF_INT == 4
#define UINT_MAX_AS_DOUBLE 4294967296.0
#elif SIZEOF_INT == 8
#define UINT_MAX_AS_DOUBLE 1.8446744073709552e+19
#else
#error SIZEOF_INT is neither 4 nor 8
#endif /* SIZEOF_INT == 4 || ... */
return ((double)u) / UINT_MAX_AS_DOUBLE;
}
/**
* As crypto_rand_uint, but extract the result from a crypto_fast_rng_t
*/
unsigned
crypto_fast_rng_get_uint(crypto_fast_rng_t *rng, unsigned limit)
{
tor_assert(limit < UINT_MAX);
IMPLEMENT_RAND_UNSIGNED(unsigned, UINT_MAX, limit,
crypto_fast_rng_getbytes(rng, (void*)&val, sizeof(val)));
}
/**
* As crypto_rand_uint64, but extract the result from a crypto_fast_rng_t.
*/
uint64_t
crypto_fast_rng_get_uint64(crypto_fast_rng_t *rng, uint64_t limit)
{
tor_assert(limit < UINT64_MAX);
IMPLEMENT_RAND_UNSIGNED(uint64_t, UINT64_MAX, limit,
crypto_fast_rng_getbytes(rng, (void*)&val, sizeof(val)));
}
/**
* As crypto_rand_, but extract the result from a crypto_fast_rng_t.
*/
double
crypto_fast_rng_get_double(crypto_fast_rng_t *rng)
{
unsigned int u;
crypto_fast_rng_getbytes(rng, (void*)&u, sizeof(u));
return ((double)u) / UINT_MAX_AS_DOUBLE;
}

1
src/lib/crypt_ops/include.am
View File

@ -17,6 +17,7 @@ src_lib_libtor_crypt_ops_a_SOURCES = \
src/lib/crypt_ops/crypto_ope.c \
src/lib/crypt_ops/crypto_pwbox.c \
src/lib/crypt_ops/crypto_rand.c \
src/lib/crypt_ops/crypto_rand_fast.c \
src/lib/crypt_ops/crypto_rand_numeric.c \
src/lib/crypt_ops/crypto_rsa.c \
src/lib/crypt_ops/crypto_s2k.c \

117
src/test/test_crypto_rng.c
View File

@ -173,6 +173,121 @@ test_crypto_rng_strongest(void *arg)
#undef N
}
static void
test_crypto_rng_fast(void *arg)
{
(void)arg;
crypto_fast_rng_t *rng = crypto_fast_rng_new();
tt_assert(rng);
/* Rudimentary black-block test to make sure that our prng outputs
* have all bits sometimes on and all bits sometimes off. */
uint64_t m1 = 0, m2 = ~(uint64_t)0;
const int N = 128;
for (int i=0; i < N; ++i) {
uint64_t v;
crypto_fast_rng_getbytes(rng, (void*)&v, sizeof(v));
m1 |= v;
m2 &= v;
}
tt_u64_op(m1, OP_EQ, ~(uint64_t)0);
tt_u64_op(m2, OP_EQ, 0);
/* Check range functions. */
int counts[5];
memset(counts, 0, sizeof(counts));
for (int i=0; i < N; ++i) {
unsigned u = crypto_fast_rng_get_uint(rng, 5);
tt_int_op(u, OP_GE, 0);
tt_int_op(u, OP_LT, 5);
counts[u]++;
uint64_t u64 = crypto_fast_rng_get_uint64(rng, UINT64_C(1)<<40);
tt_u64_op(u64, OP_GE, 0);
tt_u64_op(u64, OP_LT, UINT64_C(1)<<40);
}
/* All values should have come up once. */
for (int i=0; i<5; ++i) {
tt_int_op(counts[i], OP_GT, 0);
}
done:
crypto_fast_rng_free(rng);
}
static void
test_crypto_rng_fast_whitebox(void *arg)
{
(void)arg;
const size_t buflen = crypto_fast_rng_get_bytes_used_per_stream();
char *buf = tor_malloc_zero(buflen);
char *buf2 = tor_malloc_zero(buflen);
crypto_cipher_t *cipher = NULL;
uint8_t seed[CRYPTO_FAST_RNG_SEED_LEN];
memset(seed, 0, sizeof(seed));
/* Start with a prng with zero key and zero IV. */
crypto_fast_rng_t *rng = crypto_fast_rng_new_from_seed(seed);
tt_assert(rng);
/* We'll use a stream cipher to keep in sync */
cipher = crypto_cipher_new_with_iv_and_bits(seed, seed+32, 256);
/* The first 48 bytes are used for the next seed -- let's make sure we have
* them.
*/
memset(seed, 0, sizeof(seed));
crypto_cipher_crypt_inplace(cipher, (char*)seed, sizeof(seed));
/* if we get 128 bytes, they should match the bytes from the aes256-counter
* stream, starting at position 48.
*/
crypto_fast_rng_getbytes(rng, (uint8_t*)buf, 128);
memset(buf2, 0, 128);
crypto_cipher_crypt_inplace(cipher, buf2, 128);
tt_mem_op(buf, OP_EQ, buf2, 128);
/* Try that again, with an odd number of bytes. */
crypto_fast_rng_getbytes(rng, (uint8_t*)buf, 199);
memset(buf2, 0, 199);
crypto_cipher_crypt_inplace(cipher, buf2, 199);
tt_mem_op(buf, OP_EQ, buf2, 199);
/* Make sure that refilling works as expected: skip all but the last 5 bytes
* of this steam. */
size_t skip = buflen - (199+128) - 5;
crypto_fast_rng_getbytes(rng, (uint8_t*)buf, skip);
crypto_cipher_crypt_inplace(cipher, buf2, skip);
/* Now get the next 128 bytes. The first 5 will come from this stream, and
* the next 5 will come from the stream keyed by the new value of 'seed'. */
crypto_fast_rng_getbytes(rng, (uint8_t*)buf, 128);
memset(buf2, 0, 128);
crypto_cipher_crypt_inplace(cipher, buf2, 5);
crypto_cipher_free(cipher);
cipher = crypto_cipher_new_with_iv_and_bits(seed, seed+32, 256);
memset(seed, 0, sizeof(seed));
crypto_cipher_crypt_inplace(cipher, (char*)seed, sizeof(seed));
crypto_cipher_crypt_inplace(cipher, buf2+5, 128-5);
tt_mem_op(buf, OP_EQ, buf2, 128);
/* And check the next 7 bytes to make sure we didn't discard anything. */
crypto_fast_rng_getbytes(rng, (uint8_t*)buf, 7);
memset(buf2, 0, 7);
crypto_cipher_crypt_inplace(cipher, buf2, 7);
tt_mem_op(buf, OP_EQ, buf2, 7);
done:
crypto_fast_rng_free(rng);
crypto_cipher_free(cipher);
tor_free(buf);
tor_free(buf2);
}
struct testcase_t crypto_rng_tests[] = {
{ "rng", test_crypto_rng, 0, NULL, NULL },
{ "rng_range", test_crypto_rng_range, 0, NULL, NULL },
@ -183,5 +298,7 @@ struct testcase_t crypto_rng_tests[] = {
&passthrough_setup, (void*)"nofallback" },
{ "rng_strongest_broken", test_crypto_rng_strongest, TT_FORK,
&passthrough_setup, (void*)"broken" },
{ "fast", test_crypto_rng_fast, 0, NULL, NULL },
{ "fast_whitebox", test_crypto_rng_fast_whitebox, 0, NULL, NULL },
END_OF_TESTCASES
};