Added Python convenience for different nonce sizes. Statistical tests of output randomness.

tests/test_chacha.py

@@ -1,17 +1,50 @@
 from secrets import randbelow, token_bytes
 
+import numpy as np
 from cryptography.hazmat.primitives.ciphers import Cipher
 from cryptography.hazmat.primitives.ciphers.algorithms import ChaCha20
+from scipy.stats import chisquare
 
 from randquik import cha
 
 
+def test_cha_generate_statistical():
+    """Requests in multiple of ChaCha20 block size 64 bytes"""
+    ROUNDS = 100
+    min_p = 1 - 0.99 ** (1 / ROUNDS)  # 99 % confidence over the entire test
+    print(min_p)
+
+    for round in range(ROUNDS):
+        # First round both zero, use 8-byte nonce as round counter
+        key = bytes(32)
+        nonce = round.to_bytes(8, "little")
+        iv = bytes(8) + nonce  # Cryptography module requires IV (counter, nonce)
+        # Varying sizes to test internal processing that occurs in 64 bit blocks
+        # and with SIMD implementations also 256 or 512 bytes at a time.
+        N = 10000 + randbelow(10000)
+        ct0, ct1 = bytearray(N), bytearray(N)
+        Cipher(ChaCha20(key, iv), None, None).encryptor().update_into(bytes(N), ct0)
+        cha.generate(ct1, key, nonce)
+        assert len(ct0) == len(ct1)
+        assert ct0.hex() == ct1.hex()
+        assert ct1.count(0)
+
+        # Zeroing out 50 bytes fail the test in ~30 rounds
+        # ct1[:50] = bytes(50)
+
+        # Test that all byte values are equivalently common (despite zero inputs)
+        observed = np.bincount(ct1, minlength=256)
+        expected = np.full(256, N / 256)
+        chi2, p_value = chisquare(observed, expected)
+        assert p_value >= min_p, f"{round=} {N=}"
+
+
 def test_cipherstreams_fullblocks():
     """Requests in multiple of ChaCha20 block size 64 bytes"""
     key = token_bytes(32)
-    nonce = token_bytes(16)
-    c0 = Cipher(ChaCha20(key, nonce), None, None).encryptor()
-    c1 = cha.Cha(key, nonce)
+    iv = token_bytes(16)
+    c0 = Cipher(ChaCha20(key, iv), None, None).encryptor()
+    c1 = cha.Cha(key, iv)
 
     for i in range(2048):
         N = 64 * (1 + randbelow(2048))
@@ -24,9 +57,9 @@ def test_cipherstreams_fullblocks():
 def test_cipherstreams_partial_updates():
     """Odd-sized requests that retain leftover buffers"""
     key = token_bytes(32)
-    nonce = token_bytes(16)
-    c0 = Cipher(ChaCha20(key, nonce), None, None).encryptor()
-    c1 = cha.Cha(key, nonce)
+    iv = token_bytes(16)
+    c0 = Cipher(ChaCha20(key, iv), None, None).encryptor()
+    c1 = cha.Cha(key, iv)
 
     for i in range(2048):
         N = 1 + randbelow(2048)
@@ -39,8 +72,9 @@ def test_cipherstreams_partial_updates():
 def test_cipherstreams_32leftover():
     """Test the special case where the second response comes entirely from the leftover buffer"""
     key = token_bytes(32)
-    nonce = token_bytes(16)
-    c0 = Cipher(ChaCha20(key, nonce), None, None).encryptor()
+    nonce = token_bytes(12)  # IETF nonce size
+    iv = bytes(4) + nonce
+    c0 = Cipher(ChaCha20(key, iv), None, None).encryptor()
     c1 = cha.Cha(key, nonce)
 
     for N in [512 - 32, 32]: