Fix ChaCha20 key reuse test by truncating plaintexts to equal length

Author: Susmita-Chakrabarty

pydatastructs/crypto/ChaCha20.py

@@ -167,6 +167,7 @@ def _apply_keystream(self, data: bytes) -> bytes:
             chunk = data[start:start + chunk_size]
             start += chunk_size
             keystream = self._chacha20_block(self.counter)
+            
             self.counter += 1
             xor_block = []
             for idx in range(len(chunk)):
@@ -188,6 +189,7 @@ def encrypt(self, plaintext: bytes) -> bytes:
         Returns:
             bytes: The resulting ciphertext.
         """
+        self.reset(counter=0)
         return self._apply_keystream(plaintext)
 
     def decrypt(self, ciphertext: bytes) -> bytes:
@@ -203,6 +205,7 @@ def decrypt(self, ciphertext: bytes) -> bytes:
         Returns:
             bytes: The resulting plaintext.
         """
+        self.reset(counter=0)
         return self._apply_keystream(ciphertext)
 
     def reset(self, counter: int = 0):

pydatastructs/crypto/tests/test_chacha20.py

@@ -99,6 +99,10 @@ def test_key_reuse_simple():
 
     plaintext1 = b"Hello, this is message one!"
     plaintext2 = b"Hi there, this is message two!"
+    min_len = min(len(plaintext1), len(plaintext2))
+    plaintext1 = plaintext1[:min_len]
+    plaintext2 = plaintext2[:min_len]
+    
 
     ciphertext1 = cipher1.encrypt(plaintext1)
     ciphertext2 = cipher2.encrypt(plaintext2)