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Diffstat (limited to 'src/lib/crypt_ops/crypto_rand_fast.c')
| -rw-r--r-- | src/lib/crypt_ops/crypto_rand_fast.c | 438 |
1 files changed, 438 insertions, 0 deletions
diff --git a/src/lib/crypt_ops/crypto_rand_fast.c b/src/lib/crypt_ops/crypto_rand_fast.c new file mode 100644 index 0000000000..172ea48bdb --- /dev/null +++ b/src/lib/crypt_ops/crypto_rand_fast.c @@ -0,0 +1,438 @@ +/* Copyright (c) 2001, Matej Pfajfar. + * Copyright (c) 2001-2004, Roger Dingledine. + * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson. + * Copyright (c) 2007-2020, 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_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/thread/threads.h" + +#include "lib/log/util_bug.h" + +#ifdef HAVE_SYS_TYPES_H +#include <sys/types.h> +#endif +#ifdef HAVE_UNISTD_H +#include <unistd.h> +#endif + +#include <string.h> + +#ifdef NOINHERIT_CAN_FAIL +#define CHECK_PID +#endif + +#ifdef CHECK_PID +#define PID_FIELD_LEN sizeof(pid_t) +#else +#define PID_FIELD_LEN 0 +#endif + +/* 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 - PID_FIELD_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_strongest_rand()? + * + * This value may be negative if unit tests are enabled. If so, it + * indicates that we should never mix in extra data from + * crypto_strongest_rand(). + */ + int16_t n_till_reseed; + /** How many bytes are remaining in cbuf_t.bytes? */ + uint16_t bytes_left; +#ifdef CHECK_PID + /** Which process owns this fast_rng? If this value is zero, we do not + * need to test the owner. */ + pid_t owner; +#endif + struct cbuf_t { + /** The seed (key and IV) that we will use the next time that we refill + * cbuf_t. */ + 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_t. + */ +CTASSERT(sizeof(struct cbuf_t) == 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, you should probably look at get_thread_fast_rng(). Alternatively, + * use crypto_rand(), wrap this in a mutex. + **/ +crypto_fast_rng_t * +crypto_fast_rng_new_from_seed(const uint8_t *seed) +{ + unsigned inherit = INHERIT_RES_KEEP; + /* 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, + &inherit); + 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; +#ifdef CHECK_PID + if (inherit == INHERIT_RES_KEEP) { + /* This value will neither be dropped nor zeroed after fork, so we need to + * check our pid to make sure we are not sharing it across a fork. This + * can be expensive if the pid value isn't cached, sadly. + */ + result->owner = getpid(); + } +#elif defined(_WIN32) + /* Windows can't fork(), so there's no need to noinherit. */ +#else + /* We decided above that noinherit would always do _something_. Assert here + * that we were correct. */ + tor_assertf(inherit != INHERIT_RES_KEEP, + "We failed to create a non-inheritable memory region, even " + "though we believed such a failure to be impossible! This is " + "probably a bug in Tor support for your platform; please report " + "it."); +#endif /* defined(CHECK_PID) || ... */ + return result; +} + +#ifdef TOR_UNIT_TESTS +/** + * Unit tests only: prevent a crypto_fast_rng_t from ever mixing in more + * entropy. + */ +void +crypto_fast_rng_disable_reseed(crypto_fast_rng_t *rng) +{ + rng->n_till_reseed = -1; +} +#endif /* defined(TOR_UNIT_TESTS) */ + +/** + * 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: mix additional entropy into <b>rng</b> by using our XOF to mix the + * old value for the seed with some additional bytes from + * crypto_strongest_rand(). + **/ +static void +crypto_fast_rng_add_entopy(crypto_fast_rng_t *rng) +{ + 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); +} + +/** + * 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) +{ + rng->n_till_reseed--; + if (rng->n_till_reseed == 0) { + /* It's time to reseed the RNG. */ + crypto_fast_rng_add_entopy(rng); + rng->n_till_reseed = RESEED_AFTER; + } else if (rng->n_till_reseed < 0) { +#ifdef TOR_UNIT_TESTS + /* Reseeding is disabled for testing; never do it on this prng. */ + rng->n_till_reseed = -1; +#else + /* If testing is disabled, this shouldn't be able to become negative. */ + tor_assert_unreached(); +#endif /* defined(TOR_UNIT_TESTS) */ + } + /* 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) +{ +#ifdef CHECK_PID + if (rng->owner) { + /* Note that we only need to do this check when we have owner set: that + * is, when our attempt to block inheriting failed, and the result was + * INHERIT_RES_KEEP. + * + * If the result was INHERIT_RES_DROP, then any attempt to access the rng + * memory after forking will crash. + * + * If the result was INHERIT_RES_ZERO, then forking will set the bytes_left + * and n_till_reseed fields to zero. This function will call + * crypto_fast_rng_refill(), which will in turn reseed the PRNG. + * + * So we only need to do this test in the case when mmap_anonymous() + * returned INHERIT_KEEP. We avoid doing it needlessly, since getpid() is + * often a system call, and that can be slow. + */ + tor_assert(rng->owner == getpid()); + } +#endif /* defined(CHECK_PID) */ + + 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 /* defined(TOR_UNIT_TESTS) */ + +/** + * Thread-local instance for our fast RNG. + **/ +static tor_threadlocal_t thread_rng; + +/** + * Return a per-thread fast RNG, initializing it if necessary. + * + * You do not need to free this yourself. + * + * It is NOT safe to share this value across threads. + **/ +crypto_fast_rng_t * +get_thread_fast_rng(void) +{ + crypto_fast_rng_t *rng = tor_threadlocal_get(&thread_rng); + + if (PREDICT_UNLIKELY(rng == NULL)) { + rng = crypto_fast_rng_new(); + tor_threadlocal_set(&thread_rng, rng); + } + + return rng; +} + +/** + * Used when a thread is exiting: free the per-thread fast RNG if needed. + * Invoked from the crypto subsystem's thread-cleanup code. + **/ +void +destroy_thread_fast_rng(void) +{ + crypto_fast_rng_t *rng = tor_threadlocal_get(&thread_rng); + if (!rng) + return; + crypto_fast_rng_free(rng); + tor_threadlocal_set(&thread_rng, NULL); +} + +#ifdef TOR_UNIT_TESTS +/** + * Replace the current thread's rng with <b>rng</b>. For use by the + * unit tests only. Returns the previous thread rng. + **/ +crypto_fast_rng_t * +crypto_replace_thread_fast_rng(crypto_fast_rng_t *rng) +{ + crypto_fast_rng_t *old_rng = tor_threadlocal_get(&thread_rng); + tor_threadlocal_set(&thread_rng, rng); + return old_rng; +} +#endif /* defined(TOR_UNIT_TESTS) */ + +/** + * Initialize the global thread-local key that will be used to keep track + * of per-thread fast RNG instances. Called from the crypto subsystem's + * initialization code. + **/ +void +crypto_rand_fast_init(void) +{ + tor_threadlocal_init(&thread_rng); +} + +/** + * Initialize the global thread-local key that will be used to keep track + * of per-thread fast RNG instances. Called from the crypto subsystem's + * shutdown code. + **/ +void +crypto_rand_fast_shutdown(void) +{ + destroy_thread_fast_rng(); + tor_threadlocal_destroy(&thread_rng); +} |