testcrypt: allow random_read() to use a full PRNG.

This still isn't the true random generator used in the live tools:
it's deterministic, for repeatable testing. The Python side of
testcrypt can now call random_make_prng(), which will instantiate a
PRNG with the given seed. random_clear() still gets rid of it.

So I can still have some tests control the precise random numbers
received by the function under test, but for others (especially key
generation, with its uncertainty about how much randomness it will
actually use) I can just say 'here, have a seed, generate as much
stuff from that seed as you need'.

test/cryptsuite.py

@@ -103,6 +103,12 @@ def queued_specific_random_data(data):
     yield None
     random_clear()
 
+@contextlib.contextmanager
+def random_prng(seed):
+    random_make_prng('sha256', seed)
+    yield None
+    random_clear()
+
 def hash_str(alg, message):
     h = ssh_hash_new(alg)
     ssh_hash_update(h, message)

testcrypt.c

@@ -55,10 +55,15 @@ static bool old_keyfile_warning_given;
 void old_keyfile_warning(void) { old_keyfile_warning_given = true; }
 
 static bufchain random_data_queue;
+static prng *test_prng;
 void random_read(void *buf, size_t size)
 {
-    if (!bufchain_try_fetch_consume(&random_data_queue, buf, size))
-        fatal_error("No random data in queue");
+    if (test_prng) {
+        prng_read(test_prng, buf, size);
+    } else {
+        if (!bufchain_try_fetch_consume(&random_data_queue, buf, size))
+            fatal_error("No random data in queue");
+    }
 }
 
 uint64_t prng_reseed_time_ms(void)
@@ -673,9 +678,24 @@ static size_t random_queue_len(void)
 
 static void random_clear(void)
 {
+    if (test_prng) {
+        prng_free(test_prng);
+        test_prng = NULL;
+    }
+
     bufchain_clear(&random_data_queue);
 }
 
+static void random_make_prng(const ssh_hashalg *hashalg, ptrlen seed)
+{
+    random_clear();
+
+    test_prng = prng_new(hashalg);
+    prng_seed_begin(test_prng);
+    put_datapl(test_prng, seed);
+    prng_seed_finish(test_prng);
+}
+
 mp_int *monty_identity_wrapper(MontyContext *mc)
 {
     return mp_copy(monty_identity(mc));

testcrypt.h

@@ -290,4 +290,5 @@ FUNC2(boolean, crcda_detect, val_string_ptrlen, val_string_ptrlen)
  */
 FUNC1(void, random_queue, val_string_ptrlen)
 FUNC0(uint, random_queue_len)
+FUNC2(void, random_make_prng, hashalg, val_string_ptrlen)
 FUNC0(void, random_clear)