diff --git a/extmod/modtrezorcrypto/rand.c b/extmod/modtrezorcrypto/rand.c
index 385244b4..665f311e 100644
--- a/extmod/modtrezorcrypto/rand.c
+++ b/extmod/modtrezorcrypto/rand.c
@@ -56,6 +56,9 @@ void random_buffer(uint8_t *buf, size_t len)
 
 void random_permute(void *buf, size_t size, size_t count)
 {
+	if (count < 1 || size < 1) {
+		return;
+	}
 	uint8_t *d = (uint8_t *)buf;
 	uint8_t t[size];
 	for (size_t i = count - 1; i >= 1; i--) {
diff --git a/src/tests/test_crypto_random.py b/src/tests/test_crypto_random.py
new file mode 100644
index 00000000..c8f89899
--- /dev/null
+++ b/src/tests/test_crypto_random.py
@@ -0,0 +1,48 @@
+import sys
+sys.path.append('..')
+sys.path.append('../lib')
+import unittest
+from ubinascii import hexlify
+
+from trezor.crypto import random
+
+class TestCryptoRandom(unittest.TestCase):
+
+    def test_uniform(self):
+        c = {}
+        for i in range(15):
+            c[i] = 0
+        for _ in range(15000):
+            r = random.uniform(15)
+            c[r] += 1
+        for i in range(15):
+            self.assertTrue(c[r] > 900)
+            self.assertTrue(c[r] < 1100)
+
+    def test_bytes_length(self):
+        for l in range(1024 + 1):
+            lst = random.bytes(l)
+            self.assertEqual(len(lst), l)
+
+    def test_bytes_uniform(self):
+        for _ in range(100):
+            b = random.bytes(8000)
+            c = {}
+            for h in '0123456789abcdef':
+                c[h] = 0
+            for h in hexlify(b):
+                c[chr(h)] += 1
+            for h in '0123456789abcdef':
+                self.assertTrue(c[h] > 900)
+                self.assertTrue(c[h] < 1100)
+
+
+    def test_shuffle(self):
+        for l in range(256 + 1):
+            lst = list(range(l))
+            random.shuffle(lst)
+            self.assertEqual(len(lst), l)
+            self.assertEqual(sorted(lst), list(range(l)))
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/src/tests/test_crypto_sha512.py b/src/tests/test_crypto_sha512.py
index 58348e4d..94ca2dfe 100644
--- a/src/tests/test_crypto_sha512.py
+++ b/src/tests/test_crypto_sha512.py
@@ -2,7 +2,6 @@ import sys
 sys.path.append('..')
 sys.path.append('../lib')
 import unittest
-import trezor.utils
 from ubinascii import unhexlify
 
 from trezor.crypto import hashlib