Added readme, added exception throw and copied attack program from tp02

TPs/TP03/py/chacha20_int_attck.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python3
+
+import sys
+
+def attack(fctxt, pos, plainAtPos, newPlainAtPos):
+    f = open(fctxt,"rb")
+    ciphertext = f.read()
+    f.close()
+
+    plainAtPos = plainAtPos.encode()
+    newPlainAtPos = newPlainAtPos.encode()
+    txt_len = len(plainAtPos)
+    diff = bytes([a ^ b for (a,b) in zip(plainAtPos,newPlainAtPos)])
+    cipher_diff = bytes([a ^ b for (a,b) in zip(diff,ciphertext[pos:pos+txt_len])])
+
+    new_ciphertext = ciphertext[:pos] + cipher_diff + ciphertext[pos+txt_len:]
+
+    with open(fctxt+".attck","wb") as f:
+        f.write(new_ciphertext)
+
+def main():
+    argv = sys.argv[1:]
+    argc = len(argv)
+    if argc < 3 or argc > 5:
+        sys.exit("Needs 4 arguments <fctxt> <pos> <ptxtAtPos> <newPtxtAtPos>")
+
+    attack(argv[0],int(argv[1]),argv[2],argv[3])
+
+if __name__ == "__main__":
+    main()

TPs/TP03/py/pbenc_chacha20_poly1305.py

@@ -40,7 +40,13 @@ def decrypt(input_file, key_file):
 
 
     cipher = ChaCha20Poly1305(key)
-    plaintext = cipher.decrypt(nonce, ciphertext, aad)
+    try:
+        plaintext = cipher.decrypt(nonce, ciphertext, aad)
+    except Exception as e:
+        print(f"Could not validate the authentication: {e}")
+        return
+    
+    
 
     with open(f"{input_file}.dec", "wb") as f:
         f.write(plaintext)