5.2 EVER EVIL EVE

EXERCISE 5.2: EVER EVIL EVE

Go ahead and “intercept” some of the messages encrypted by the code you wrote in this section. Modify the intercepted messages and verify that your decryption mechanism correctly reports an error.

Alice’s Session

Alice first creates her plaintext. Then she uses her newly generated key and nonce to encrypt her plaintext.

import ex5_1
import os 

plaintext = b"Hi Bob, this is Alice!"
key = os.urandom(32)
nonce = os.urandom(16) 

encryptionManager = ex5_1.Encryptor(key=key, nonce=nonce)
ciphertextWithMac = encryptionManager.update_encryptor(plaintext=plaintext)
ciphertextWithMac += encryptionManager.finalize_encryptor()

print(f"Key: {key.hex(sep=' ')}")
print(f"Nonce: {nonce.hex(sep=' ')}")
print(f"Plaintext: {plaintext.hex(sep=' ')}")
print(f"Ciphertext With MAC: {ciphertextWithMac.hex(sep=' ')}")
Key: 0c 72 69 82 a3 54 69 16 a8 46 b1 6d 02 17 05 80 7d 1d af 4d c5 9e 6d a1 05 ff b0 fd b8 13 3e 76
Nonce: 60 8e 6d 84 0f 11 16 76 fc a3 ed ab 12 8a 4f c0
Plaintext: 48 69 20 42 6f 62 2c 20 74 68 69 73 20 69 73 20 41 6c 69 63 65 21
Ciphertext With MAC: b4 9e ac 3b c5 71 c0 e4 35 45 8b f6 21 0d b9 9c fc 48 0c 68 49 b4 8c c4 bd 0b 35 f1 fc d1 41 b6 4c 42 a9 e0 1f 83 e3 f6 86 b3 0b 5c cb 29 2e 06 31 24 91 a7 02 74

Eve then intercepts…

Suppose Eve intercepts the ciphertextWithMac and she changes its first byte:

ciphertextWithMac = bytes.fromhex('aa') + ciphertextWithMac[1:]
print(f"Ciphertext With MAC: {ciphertextWithMac.hex(sep=' ')}")
Ciphertext With MAC: aa 9e ac 3b c5 71 c0 e4 35 45 8b f6 21 0d b9 9c fc 48 0c 68 49 b4 8c c4 bd 0b 35 f1 fc d1 41 b6 4c 42 a9 e0 1f 83 e3 f6 86 b3 0b 5c cb 29 2e 06 31 24 91 a7 02 74

Bob, then tries to verify and decrypt the ciphertext…

We expect an Exception to be raised:

decryptionManager = ex5_1.Decryptor(
    key=key,
    nonce=nonce,
    digest=ex5_1.Decryptor.get_mac(ciphertextWithMac)
)
_plaintext = decryptionManager.update_decryptor(ciphertext=ciphertextWithMac[:-32])
_plaintext += decryptionManager.finalize_decryptor()
MessageWasTamperedWithError: 
Expected: b'\xeb\x19\xb7p`p\xe7\xba\xe5\xe3z\x81}[i\xec\xa2*\xfa\xe9]\xca\xeb\xcb\x13\x0f4\xba\x1b,\x8a\xf1'.
But got: b'\x8c\xc4\xbd\x0b5\xf1\xfc\xd1A\xb6LB\xa9\xe0\x1f\x83\xe3\xf6\x86\xb3\x0b\\\xcb).\x061$\x91\xa7\x02t'