diff --git a/changes/ticket29732 b/changes/ticket29732
new file mode 100644
index 0000000000..bb72361c48
--- /dev/null
+++ b/changes/ticket29732
@@ -0,0 +1,5 @@
+  o Minor features (testing):
+    - Tor's unit test code now contains a standard set of functions to
+      replace the PRNG with a deterministic or reproducible version for
+      testing. Previously, various tests implemented this in various ways.
+      Implements ticket 29732.
diff --git a/src/lib/crypt_ops/crypto_rand.h b/src/lib/crypt_ops/crypto_rand.h
index c51d6a4480..528f238fa5 100644
--- a/src/lib/crypt_ops/crypto_rand.h
+++ b/src/lib/crypt_ops/crypto_rand.h
@@ -92,6 +92,10 @@ void crypto_rand_fast_shutdown(void);
 #if defined(TOR_UNIT_TESTS)
 /* Used for white-box testing */
 size_t crypto_fast_rng_get_bytes_used_per_stream(void);
+/* For deterministic prng implementations */
+void crypto_fast_rng_disable_reseed(crypto_fast_rng_t *rng);
+/* To override the prng for testing. */
+crypto_fast_rng_t *crypto_replace_thread_fast_rng(crypto_fast_rng_t *rng);
 #endif
 
 #ifdef CRYPTO_RAND_PRIVATE
diff --git a/src/lib/crypt_ops/crypto_rand_fast.c b/src/lib/crypt_ops/crypto_rand_fast.c
index 01817c618f..b71ade81bd 100644
--- a/src/lib/crypt_ops/crypto_rand_fast.c
+++ b/src/lib/crypt_ops/crypto_rand_fast.c
@@ -95,8 +95,13 @@ 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;
+   * 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.bytes? */
   uint16_t bytes_left;
 #ifdef CHECK_PID
@@ -181,6 +186,18 @@ crypto_fast_rng_new_from_seed(const uint8_t *seed)
   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
+
 /**
  * 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,
@@ -192,6 +209,26 @@ 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.
@@ -202,22 +239,19 @@ cipher_from_seed(const uint8_t *seed)
 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--;
+  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
   }
   /* Now fill rng->buf with output from our stream cipher, initialized from
    * that seed value. */
@@ -363,6 +397,20 @@ destroy_thread_fast_rng(void)
   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
+
 /**
  * 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
diff --git a/src/test/include.am b/src/test/include.am
index 497aa320a4..022cdbe035 100644
--- a/src/test/include.am
+++ b/src/test/include.am
@@ -89,6 +89,7 @@ src_test_test_SOURCES += \
 	src/test/log_test_helpers.c \
 	src/test/hs_test_helpers.c \
 	src/test/rend_test_helpers.c \
+	src/test/rng_test_helpers.c \
 	src/test/test.c \
 	src/test/test_accounting.c \
 	src/test/test_addr.c \
@@ -211,6 +212,7 @@ endif
 src_test_test_slow_SOURCES =
 if UNITTESTS_ENABLED
 src_test_test_slow_SOURCES += \
+	src/test/rng_test_helpers.c \
 	src/test/test_slow.c \
 	src/test/test_crypto_slow.c \
 	src/test/test_process_slow.c \
@@ -319,6 +321,7 @@ noinst_HEADERS+= \
 	src/test/hs_test_helpers.h \
 	src/test/log_test_helpers.h \
 	src/test/rend_test_helpers.h \
+	src/test/rng_test_helpers.h \
 	src/test/test.h \
 	src/test/ptr_helpers.h \
 	src/test/test_helpers.h \
diff --git a/src/test/rng_test_helpers.c b/src/test/rng_test_helpers.c
new file mode 100644
index 0000000000..262d380bda
--- /dev/null
+++ b/src/test/rng_test_helpers.c
@@ -0,0 +1,226 @@
+/* Copyright (c) 2018-2019, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file rng_test_helpers.c
+ * \brief Helpers for overriding PRNGs during unit tests.
+ *
+ * We define two PRNG overrides: a "reproducible PRNG" where the seed is
+ * chosen randomly but the stream can be replayed later on in case a bug is
+ * found, and a "deterministic PRNG" where the seed is fixed in the unit
+ * tests.
+ *
+ * Obviously, this code is testing-only.
+ */
+
+#include "orconfig.h"
+#include "core/or/or.h"
+
+#include "lib/crypt_ops/crypto_rand.h"
+
+#include "test/rng_test_helpers.h"
+
+#ifndef TOR_UNIT_TESTS
+#error "No. Never link this code into Tor proper."
+#endif
+
+/**
+ * True iff the RNG is currently replaced.  Prevents double-replacement.
+ **/
+static bool rng_is_replaced = false;
+
+/**
+ * Mutex to protect deterministic prng.
+ *
+ * Note that if you actually _use_ the prng from two threads at the same time,
+ * the results will probably be nondeterministic anyway.
+ */
+static tor_mutex_t *rng_mutex = NULL;
+
+/**
+ * Cached old value for the thread prng.
+ **/
+static crypto_fast_rng_t *stored_fast_rng = NULL;
+
+/** replacement for crypto_strongest_rand that delegates to crypto_rand. */
+static void
+mock_crypto_strongest_rand(uint8_t *out, size_t len)
+{
+  crypto_rand((char *)out, len);
+}
+
+/* This is the seed of the deterministic randomness. */
+static uint8_t rng_seed[16];
+static crypto_xof_t *rng_xof = NULL;
+
+/**
+ * Print the seed for our PRNG to stdout.  We use this when we're
+ **/
+void
+testing_dump_reproducible_rng_seed(void)
+{
+  printf("\n"
+         "Seed: %s\n",
+         hex_str((const char*)rng_seed, sizeof(rng_seed)));
+}
+
+/** Produce deterministic randomness for the stochastic tests using the global
+ * rng_xof output.
+ *
+ * This function produces deterministic data over multiple calls iff it's
+ * called in the same call order with the same 'n' parameter.
+ * If not, outputs will deviate. */
+static void
+crypto_rand_deterministic(char *out, size_t n)
+{
+  tor_assert(rng_xof);
+  tor_mutex_acquire(rng_mutex);
+  crypto_xof_squeeze_bytes(rng_xof, (uint8_t*)out, n);
+  tor_mutex_release(rng_mutex);
+}
+
+/**
+ * Implementation helper: override our crypto_rand() PRNG with a given seed of
+ * length <b>seed_len</b>.  Overlong seeds are truncated; short ones are
+ * padded.
+ **/
+static void
+enable_deterministic_rng_impl(const uint8_t *seed, size_t seed_len)
+{
+  tor_assert(!rng_is_replaced);
+  tor_assert(crypto_rand == crypto_rand__real);
+
+  memset(rng_seed, 0, sizeof(rng_seed));
+  memcpy(rng_seed, seed, MIN(seed_len, sizeof(rng_seed)));
+
+  rng_mutex = tor_mutex_new();
+
+  crypto_xof_free(rng_xof);
+  rng_xof = crypto_xof_new();
+  crypto_xof_add_bytes(rng_xof, rng_seed, sizeof(rng_seed));
+  MOCK(crypto_rand, crypto_rand_deterministic);
+  MOCK(crypto_strongest_rand_, mock_crypto_strongest_rand);
+
+  uint8_t fast_rng_seed[CRYPTO_FAST_RNG_SEED_LEN];
+  memset(fast_rng_seed, 0xff, sizeof(fast_rng_seed));
+  memcpy(fast_rng_seed, rng_seed, MIN(sizeof(rng_seed),
+                                      sizeof(fast_rng_seed)));
+  crypto_fast_rng_t *fast_rng = crypto_fast_rng_new_from_seed(fast_rng_seed);
+  crypto_fast_rng_disable_reseed(fast_rng);
+  stored_fast_rng = crypto_replace_thread_fast_rng(fast_rng);
+
+  rng_is_replaced = true;
+}
+
+/**
+ * Replace our get_thread_fast_rng(), crypto_rand() and
+ * crypto_strongest_rand() prngs with a variant that generates all of its
+ * output deterministically from a randomly chosen seed.  In the event of an
+ * error, you can log the seed later on with
+ * testing_dump_reproducible_rng_seed.
+ **/
+void
+testing_enable_reproducible_rng(void)
+{
+  uint8_t seed[16];
+  crypto_rand((char*)seed, sizeof(seed));
+  enable_deterministic_rng_impl(seed, sizeof(seed));
+}
+
+/**
+ * Replace our get_thread_fast_rng(), crypto_rand() and
+ * crypto_strongest_rand() prngs with a variant that generates all of its
+ * output deterministically from a fixed seed.  This variant is mainly useful
+ * for cases when we don't want coverage to change between runs.
+ *
+ * USAGE NOTE: Test correctness SHOULD NOT depend on the specific output of
+ * this "rng".  If you need a specific output, use
+ * testing_enable_prefilled_rng() instead.
+ **/
+void
+testing_enable_deterministic_rng(void)
+{
+  static const uint8_t quotation[] =
+    "What will it be? A tree? A weed? "
+    "Each one is started from a seed."; // -- Mary Ann Hoberman
+  enable_deterministic_rng_impl(quotation, sizeof(quotation));
+}
+
+static uint8_t *prefilled_rng_buffer = NULL;
+static size_t prefilled_rng_buflen;
+static size_t prefilled_rng_idx;
+
+/**
+ * crypto_rand() replacement that returns canned data.
+ **/
+static void
+crypto_rand_prefilled(char *out, size_t n)
+{
+  tor_mutex_acquire(rng_mutex);
+  while (n) {
+    size_t n_to_copy = MIN(prefilled_rng_buflen - prefilled_rng_idx, n);
+    memcpy(out, prefilled_rng_buffer + prefilled_rng_idx, n_to_copy);
+    out += n_to_copy;
+    n -= n_to_copy;
+    prefilled_rng_idx += n_to_copy;
+
+    if (prefilled_rng_idx == prefilled_rng_buflen) {
+      prefilled_rng_idx = 0;
+    }
+  }
+  tor_mutex_release(rng_mutex);
+}
+
+/**
+ * Replace our crypto_rand() and crypto_strongest_rand() prngs with a variant
+ * that yields output from a buffer.  If it reaches the end of the buffer, it
+ * starts over.
+ *
+ * Note: the get_thread_fast_rng() prng is not replaced by this; we'll need
+ * more code to support that.
+ **/
+void
+testing_enable_prefilled_rng(const void *buffer, size_t buflen)
+{
+  tor_assert(buflen > 0);
+  rng_mutex = tor_mutex_new();
+
+  prefilled_rng_buffer = tor_memdup(buffer, buflen);
+  prefilled_rng_buflen = buflen;
+  prefilled_rng_idx = 0;
+
+  MOCK(crypto_rand, crypto_rand_prefilled);
+  MOCK(crypto_strongest_rand_, mock_crypto_strongest_rand);
+}
+
+/**
+ * Reset the position in the prefilled RNG buffer to the start.
+ */
+void
+testing_prefilled_rng_reset(void)
+{
+  tor_mutex_acquire(rng_mutex);
+  prefilled_rng_idx = 0;
+  tor_mutex_release(rng_mutex);
+}
+
+/**
+ * Undo the overrides for our PRNG.  To be used at the end of testing.
+ *
+ * Note that this function should be safe to call even if the rng has not
+ * yet been replaced.
+ **/
+void
+testing_disable_rng_override(void)
+{
+  crypto_xof_free(rng_xof);
+  tor_free(prefilled_rng_buffer);
+  UNMOCK(crypto_rand);
+  UNMOCK(crypto_strongest_rand_);
+  tor_mutex_free(rng_mutex);
+
+  crypto_fast_rng_t *rng = crypto_replace_thread_fast_rng(stored_fast_rng);
+  crypto_fast_rng_free(rng);
+
+  rng_is_replaced = false;
+}
diff --git a/src/test/rng_test_helpers.h b/src/test/rng_test_helpers.h
new file mode 100644
index 0000000000..907099450d
--- /dev/null
+++ b/src/test/rng_test_helpers.h
@@ -0,0 +1,26 @@
+/* Copyright (c) 2017-2019, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#ifndef TOR_RNG_TEST_HELPERS_H
+#define TOR_RNG_TEST_HELPERS_H
+
+#include "core/or/or.h"
+
+void testing_enable_deterministic_rng(void);
+void testing_enable_reproducible_rng(void);
+void testing_enable_prefilled_rng(const void *buffer, size_t buflen);
+
+void testing_prefilled_rng_reset(void);
+
+void testing_disable_rng_override(void);
+
+#define testing_disable_reproducible_rng() \
+  testing_disable_rng_override()
+#define testing_disable_deterministic_rng() \
+  testing_disable_rng_override()
+#define testing_disable_prefilled_rng() \
+  testing_disable_rng_override()
+
+void testing_dump_reproducible_rng_seed(void);
+
+#endif /* !defined(TOR_RNG_TEST_HELPERS_H) */
diff --git a/src/test/test.c b/src/test/test.c
index fbc30fb64e..be5cb12b1e 100644
--- a/src/test/test.c
+++ b/src/test/test.c
@@ -12,6 +12,7 @@
 #include "lib/crypt_ops/crypto_dh.h"
 #include "lib/crypt_ops/crypto_rand.h"
 #include "app/config/or_state_st.h"
+#include "test/rng_test_helpers.h"
 
 #include <stdio.h>
 #ifdef HAVE_FCNTL_H
@@ -354,18 +355,6 @@ test_onion_queues(void *arg)
   tor_free(onionskin);
 }
 
-static crypto_cipher_t *crypto_rand_aes_cipher = NULL;
-
-// Mock replacement for crypto_rand: Generates bytes from a provided AES_CTR
-// cipher in <b>crypto_rand_aes_cipher</b>.
-static void
-crypto_rand_deterministic_aes(char *out, size_t n)
-{
-  tor_assert(crypto_rand_aes_cipher);
-  memset(out, 0, n);
-  crypto_cipher_crypt_inplace(crypto_rand_aes_cipher, out, n);
-}
-
 static void
 test_circuit_timeout(void *arg)
 {
@@ -397,8 +386,7 @@ test_circuit_timeout(void *arg)
 
   // Use a deterministic RNG here, or else we'll get nondeterministic
   // coverage in some of the circuitstats functions.
-  MOCK(crypto_rand, crypto_rand_deterministic_aes);
-  crypto_rand_aes_cipher = crypto_cipher_new("xyzzyplughplover");
+  testing_enable_deterministic_rng();
 
   circuitbuild_running_unit_tests();
 #define timeout0 (build_time_t)(30*1000.0)
@@ -534,8 +522,8 @@ test_circuit_timeout(void *arg)
   circuit_build_times_free_timeouts(&final);
   or_state_free(state);
   teardown_periodic_events();
-  UNMOCK(crypto_rand);
-  crypto_cipher_free(crypto_rand_aes_cipher);
+
+  testing_disable_deterministic_rng();
 }
 
 /** Test encoding and parsing of rendezvous service descriptors. */
diff --git a/src/test/test_addr.c b/src/test/test_addr.c
index fb8df5f0fb..3a1a7b6997 100644
--- a/src/test/test_addr.c
+++ b/src/test/test_addr.c
@@ -11,6 +11,7 @@
 #include "feature/client/addressmap.h"
 #include "test/log_test_helpers.h"
 #include "lib/net/resolve.h"
+#include "test/rng_test_helpers.h"
 
 #ifdef HAVE_SYS_UN_H
 #include <sys/un.h>
@@ -945,27 +946,6 @@ test_virtaddrmap(void *data)
   ;
 }
 
-static const char *canned_data = NULL;
-static size_t canned_data_len = 0;
-
-/* Mock replacement for crypto_rand() that returns canned data from
- * canned_data above. */
-static void
-crypto_canned(char *ptr, size_t n)
-{
-  if (canned_data_len) {
-    size_t to_copy = MIN(n, canned_data_len);
-    memcpy(ptr, canned_data, to_copy);
-    canned_data += to_copy;
-    canned_data_len -= to_copy;
-    n -= to_copy;
-    ptr += to_copy;
-  }
-  if (n) {
-    crypto_rand_unmocked(ptr, n);
-  }
-}
-
 static void
 test_virtaddrmap_persist(void *data)
 {
@@ -973,6 +953,8 @@ test_virtaddrmap_persist(void *data)
   const char *a, *b, *c;
   tor_addr_t addr;
   char *ones = NULL;
+  const char *canned_data;
+  size_t canned_data_len;
 
   addressmap_init();
 
@@ -991,7 +973,7 @@ test_virtaddrmap_persist(void *data)
                 "1234567890" // the second call returns this.
                 "abcdefghij"; // the third call returns this.
   canned_data_len = 30;
-  MOCK(crypto_rand, crypto_canned);
+  testing_enable_prefilled_rng(canned_data, canned_data_len);
 
   a = addressmap_register_virtual_address(RESOLVED_TYPE_HOSTNAME,
                                           tor_strdup("quuxit.baz"));
@@ -1001,9 +983,9 @@ test_virtaddrmap_persist(void *data)
   tt_assert(b);
   tt_str_op(a, OP_EQ, "gezdgnbvgy3tqojq.virtual");
   tt_str_op(b, OP_EQ, "mfrggzdfmztwq2lk.virtual");
+  testing_disable_prefilled_rng();
 
   // Now try something to get us an ipv4 address
-  UNMOCK(crypto_rand);
   tt_int_op(0,OP_EQ, parse_virtual_addr_network("192.168.0.0/16",
                                                 AF_INET, 0, NULL));
   a = addressmap_register_virtual_address(RESOLVED_TYPE_IPV4,
@@ -1020,22 +1002,23 @@ test_virtaddrmap_persist(void *data)
 
   // Try some canned entropy and verify all the we discard duplicates,
   // addresses that end with 0, and addresses that end with 255.
-  MOCK(crypto_rand, crypto_canned);
   canned_data = "\x01\x02\x03\x04" // okay
                 "\x01\x02\x03\x04" // duplicate
                 "\x03\x04\x00\x00" // bad ending 1
                 "\x05\x05\x00\xff" // bad ending 2
                 "\x05\x06\x07\xf0"; // okay
   canned_data_len = 20;
+  testing_enable_prefilled_rng(canned_data, canned_data_len);
+
   a = addressmap_register_virtual_address(RESOLVED_TYPE_IPV4,
                                           tor_strdup("wumble.onion"));
   b = addressmap_register_virtual_address(RESOLVED_TYPE_IPV4,
                                           tor_strdup("wumpus.onion"));
   tt_str_op(a, OP_EQ, "192.168.3.4");
   tt_str_op(b, OP_EQ, "192.168.7.240");
+  testing_disable_prefilled_rng();
 
   // Now try IPv6!
-  UNMOCK(crypto_rand);
   tt_int_op(0,OP_EQ, parse_virtual_addr_network("1010:F000::/20",
                                                 AF_INET6, 0, NULL));
   a = addressmap_register_virtual_address(RESOLVED_TYPE_IPV6,
@@ -1051,7 +1034,7 @@ test_virtaddrmap_persist(void *data)
   tt_assert(!strcmpstart(b, "[1010:f"));
 
   // Try IPv6 with canned entropy, to make sure we detect duplicates.
-  MOCK(crypto_rand, crypto_canned);
+
   canned_data = "acanthopterygian" // okay
                 "cinematographist" // okay
                 "acanthopterygian" // duplicate
@@ -1060,6 +1043,8 @@ test_virtaddrmap_persist(void *data)
                 "cinematographist" // duplicate
                 "coadministration"; // okay
   canned_data_len = 16 * 7;
+  testing_enable_prefilled_rng(canned_data, canned_data_len);
+
   a = addressmap_register_virtual_address(RESOLVED_TYPE_IPV6,
                                           tor_strdup("wuffle.baz"));
   b = addressmap_register_virtual_address(RESOLVED_TYPE_IPV6,
@@ -1072,9 +1057,11 @@ test_virtaddrmap_persist(void *data)
 
   // Try address exhaustion: make sure we can actually fail if we
   // get too many already-existing addresses.
+  testing_disable_prefilled_rng();
   canned_data_len = 128*1024;
   canned_data = ones = tor_malloc(canned_data_len);
   memset(ones, 1, canned_data_len);
+  testing_enable_prefilled_rng(canned_data, canned_data_len);
   // There is some chance this one will fail if a previous random
   // allocation gave out the address already.
   a = addressmap_register_virtual_address(RESOLVED_TYPE_IPV4,
@@ -1091,7 +1078,7 @@ test_virtaddrmap_persist(void *data)
   expect_single_log_msg_containing("Ran out of virtual addresses!");
 
  done:
-  UNMOCK(crypto_rand);
+  testing_disable_prefilled_rng();
   tor_free(ones);
   addressmap_free_all();
   teardown_capture_of_logs();
diff --git a/src/test/test_extorport.c b/src/test/test_extorport.c
index f5d16af921..cfdd11d161 100644
--- a/src/test/test_extorport.c
+++ b/src/test/test_extorport.c
@@ -18,6 +18,7 @@
 
 #include "test/test.h"
 #include "test/test_helpers.h"
+#include "test/rng_test_helpers.h"
 
 #ifdef HAVE_SYS_STAT_H
 #include <sys/stat.h>
@@ -302,16 +303,6 @@ test_ext_or_cookie_auth(void *arg)
   tor_free(client_hash2);
 }
 
-static void
-crypto_rand_return_tse_str(char *to, size_t n)
-{
-  if (n != 32) {
-    TT_FAIL(("Asked for %d bytes, not 32", (int)n));
-    return;
-  }
-  memcpy(to, "te road There is always another ", 32);
-}
-
 static void
 test_ext_or_cookie_auth_testvec(void *arg)
 {
@@ -326,7 +317,7 @@ test_ext_or_cookie_auth_testvec(void *arg)
   memcpy(ext_or_auth_cookie, "Gliding wrapt in a brown mantle," , 32);
   ext_or_auth_cookie_is_set = 1;
 
-  MOCK(crypto_rand, crypto_rand_return_tse_str);
+  testing_enable_prefilled_rng("te road There is always another ", 32);
 
   tt_int_op(0, OP_EQ,
             handle_client_auth_nonce(client_nonce, 32, &client_hash, &reply,
@@ -351,7 +342,7 @@ test_ext_or_cookie_auth_testvec(void *arg)
                  "33b3cd77ff79bd80c2074bbf438119a2");
 
  done:
-  UNMOCK(crypto_rand);
+  testing_disable_prefilled_rng();
   tor_free(reply);
   tor_free(client_hash);
   tor_free(mem_op_hex_tmp);
@@ -414,9 +405,9 @@ do_ext_or_handshake(or_connection_t *conn)
   CONTAINS("\x01\x00", 2);
   WRITE("\x01", 1);
   WRITE("But when I look ahead up the whi", 32);
-  MOCK(crypto_rand, crypto_rand_return_tse_str);
+  testing_enable_prefilled_rng("te road There is always another ", 32);
   tt_int_op(0, OP_EQ, connection_ext_or_process_inbuf(conn));
-  UNMOCK(crypto_rand);
+  testing_disable_prefilled_rng();
   tt_int_op(TO_CONN(conn)->state, OP_EQ,
             EXT_OR_CONN_STATE_AUTH_WAIT_CLIENT_HASH);
   CONTAINS("\xec\x80\xed\x6e\x54\x6d\x3b\x36\xfd\xfc\x22\xfe\x13\x15\x41\x6b"
@@ -481,9 +472,9 @@ test_ext_or_handshake(void *arg)
   tt_int_op(0, OP_EQ, connection_ext_or_process_inbuf(conn));
   /* send the rest of the nonce. */
   WRITE("ahead up the whi", 16);
-  MOCK(crypto_rand, crypto_rand_return_tse_str);
+  testing_enable_prefilled_rng("te road There is always another ", 32);
   tt_int_op(0, OP_EQ, connection_ext_or_process_inbuf(conn));
-  UNMOCK(crypto_rand);
+  testing_disable_prefilled_rng();
   /* We should get the right reply from the server. */
   CONTAINS("\xec\x80\xed\x6e\x54\x6d\x3b\x36\xfd\xfc\x22\xfe\x13\x15\x41\x6b"
            "\x02\x9f\x1a\xde\x76\x10\xd9\x10\x87\x8b\x62\xee\xb7\x40\x38\x21"
@@ -582,7 +573,7 @@ test_ext_or_handshake(void *arg)
 
  done:
   UNMOCK(connection_write_to_buf_impl_);
-  UNMOCK(crypto_rand);
+  testing_disable_prefilled_rng();
   if (conn)
     connection_free_minimal(TO_CONN(conn));
 #undef CONTAINS
diff --git a/src/test/test_hs_descriptor.c b/src/test/test_hs_descriptor.c
index 86965d7d66..5a3fd46dbe 100644
--- a/src/test/test_hs_descriptor.c
+++ b/src/test/test_hs_descriptor.c
@@ -21,6 +21,7 @@
 #include "test/hs_test_helpers.h"
 #include "test/test_helpers.h"
 #include "test/log_test_helpers.h"
+#include "test/rng_test_helpers.h"
 
 #ifdef HAVE_CFLAG_WOVERLENGTH_STRINGS
 DISABLE_GCC_WARNING(overlength-strings)
@@ -30,13 +31,6 @@ DISABLE_GCC_WARNING(overlength-strings)
 #include "test_hs_descriptor.inc"
 ENABLE_GCC_WARNING(overlength-strings)
 
-/* Mock function to fill all bytes with 1 */
-static void
-mock_crypto_strongest_rand(uint8_t *out, size_t out_len)
-{
-  memset(out, 1, out_len);
-}
-
 /* Test certificate encoding put in a descriptor. */
 static void
 test_cert_encoding(void *arg)
@@ -799,7 +793,7 @@ test_build_authorized_client(void *arg)
                 client_pubkey_b16,
                 strlen(client_pubkey_b16));
 
-  MOCK(crypto_strongest_rand_, mock_crypto_strongest_rand);
+  testing_enable_prefilled_rng("\x01", 1);
 
   hs_desc_build_authorized_client(subcredential,
                                   &client_auth_pk, &auth_ephemeral_sk,
@@ -815,7 +809,7 @@ test_build_authorized_client(void *arg)
  done:
   tor_free(desc_client);
   tor_free(mem_op_hex_tmp);
-  UNMOCK(crypto_strongest_rand_);
+  testing_disable_prefilled_rng();
 }
 
 struct testcase_t hs_descriptor[] = {
diff --git a/src/test/test_prob_distr.c b/src/test/test_prob_distr.c
index 37cfdae7d9..747c3d98e6 100644
--- a/src/test/test_prob_distr.c
+++ b/src/test/test_prob_distr.c
@@ -33,6 +33,7 @@
 #include "lib/math/prob_distr.h"
 #include "lib/math/fp.h"
 #include "lib/crypt_ops/crypto_rand.h"
+#include "test/rng_test_helpers.h"
 
 #include <float.h>
 #include <math.h>
@@ -1117,49 +1118,14 @@ test_psi_dist_sample(const struct dist *dist)
   }
 }
 
-/* This is the seed of the deterministic randomness */
-static uint8_t rng_seed[16];
-static crypto_xof_t *rng_xof = NULL;
-
-/** Initialize the seed of the deterministic randomness. */
-static void
-init_deterministic_rand(void)
-{
-  crypto_rand((char*)rng_seed, sizeof(rng_seed));
-  crypto_xof_free(rng_xof);
-  rng_xof = crypto_xof_new();
-  crypto_xof_add_bytes(rng_xof, rng_seed, sizeof(rng_seed));
-}
-
-static void
-teardown_deterministic_rand(void)
-{
-  crypto_xof_free(rng_xof);
-}
-
 static void
 dump_seed(void)
 {
   printf("\n"
          "NOTE: This is a stochastic test, and we expect it to fail from\n"
          "time to time, with some low probability. If you see it fail more\n"
-         "than one trial in 100, though, please tell us.\n\n"
-         "Seed: %s\n",
-         hex_str((const char*)rng_seed, sizeof(rng_seed)));
-}
-
-/** Produce deterministic randomness for the stochastic tests using the global
- *  deterministic_rand_counter seed
- *
- *  This function produces deterministic data over multiple calls iff it's
- *  called in the same call order with the same 'n' parameter (which is the
- *  case for the psi test). If not, outputs will deviate. */
-static void
-crypto_rand_deterministic(char *out, size_t n)
-{
-  /* Use a XOF to squeeze bytes out of that silly counter */
-  tor_assert(rng_xof);
-  crypto_xof_squeeze_bytes(rng_xof, (uint8_t*)out, n);
+         "than one trial in 100, though, please tell us.\n\n");
+  testing_dump_reproducible_rng_seed();
 }
 
 static void
@@ -1199,8 +1165,7 @@ test_stochastic_uniform(void *arg)
   };
   bool ok = true, tests_failed = true;
 
-  init_deterministic_rand();
-  MOCK(crypto_rand, crypto_rand_deterministic);
+  testing_enable_reproducible_rng();
 
   ok &= test_psi_dist_sample(&uniform01.base);
   ok &= test_psi_dist_sample(&uniform_pos.base);
@@ -1217,8 +1182,7 @@ test_stochastic_uniform(void *arg)
   if (tests_failed) {
     dump_seed();
   }
-  teardown_deterministic_rand();
-  UNMOCK(crypto_rand);
+  testing_disable_reproducible_rng();
 }
 
 static bool
@@ -1288,8 +1252,7 @@ test_stochastic_genpareto(void *arg)
   bool tests_failed = true;
   (void) arg;
 
-  init_deterministic_rand();
-  MOCK(crypto_rand, crypto_rand_deterministic);
+  testing_enable_reproducible_rng();
 
   ok = test_stochastic_genpareto_impl(0, 1, -0.25);
   tt_assert(ok);
@@ -1312,8 +1275,7 @@ test_stochastic_genpareto(void *arg)
   if (tests_failed) {
     dump_seed();
   }
-  teardown_deterministic_rand();
-  UNMOCK(crypto_rand);
+  testing_disable_reproducible_rng();
 }
 
 static void
@@ -1324,8 +1286,7 @@ test_stochastic_geometric(void *arg)
 
   (void) arg;
 
-  init_deterministic_rand();
-  MOCK(crypto_rand, crypto_rand_deterministic);
+  testing_enable_reproducible_rng();
 
   ok = test_stochastic_geometric_impl(0.1);
   tt_assert(ok);
@@ -1342,8 +1303,7 @@ test_stochastic_geometric(void *arg)
   if (tests_failed) {
     dump_seed();
   }
-  teardown_deterministic_rand();
-  UNMOCK(crypto_rand);
+  testing_disable_reproducible_rng();
 }
 
 static void
@@ -1353,8 +1313,7 @@ test_stochastic_logistic(void *arg)
   bool tests_failed = true;
   (void) arg;
 
-  init_deterministic_rand();
-  MOCK(crypto_rand, crypto_rand_deterministic);
+  testing_enable_reproducible_rng();
 
   ok = test_stochastic_logistic_impl(0, 1);
   tt_assert(ok);
@@ -1371,8 +1330,7 @@ test_stochastic_logistic(void *arg)
   if (tests_failed) {
     dump_seed();
   }
-  teardown_deterministic_rand();
-  UNMOCK(crypto_rand);
+  testing_disable_reproducible_rng();
 }
 
 static void
@@ -1382,8 +1340,7 @@ test_stochastic_log_logistic(void *arg)
   bool tests_failed = true;
   (void) arg;
 
-  init_deterministic_rand();
-  MOCK(crypto_rand, crypto_rand_deterministic);
+  testing_enable_reproducible_rng();
 
   ok = test_stochastic_log_logistic_impl(1, 1);
   tt_assert(ok);
@@ -1400,8 +1357,7 @@ test_stochastic_log_logistic(void *arg)
   if (tests_failed) {
     dump_seed();
   }
-  teardown_deterministic_rand();
-  UNMOCK(crypto_rand);
+  testing_disable_reproducible_rng();
 }
 
 static void
@@ -1411,8 +1367,7 @@ test_stochastic_weibull(void *arg)
   bool tests_failed = true;
   (void) arg;
 
-  init_deterministic_rand();
-  MOCK(crypto_rand, crypto_rand_deterministic);
+  testing_enable_reproducible_rng();
 
   ok = test_stochastic_weibull_impl(1, 0.5);
   tt_assert(ok);
@@ -1431,7 +1386,7 @@ test_stochastic_weibull(void *arg)
   if (tests_failed) {
     dump_seed();
   }
-  teardown_deterministic_rand();
+  testing_disable_reproducible_rng();
   UNMOCK(crypto_rand);
 }