02-block-ciphers

About

I've implemented two block cipher encryption/decryption systems; CBC and CTR mode of operation. I used AES for decryption and encryption of each 16-byte block. The 16-byte encryption IV is chosen at random and is prepended to the ciphertext. For CBC, PKCS#5/#7 padding scheme is used. Note that for every encryption of the same message and the same key, a different ciphertext is generated. Diffent ciphertexts can decrypt to the same message.

Dependency

• pyca/cryptography

Sample Usage

$ python script.py

-------------
BLOCK CIPHER MODE OF OPERATION
-------------
Enter preferred mode of operation [cbc/ctr]: ctr
Enter hex-encoded key: 36f18357be4dbd77f050515c73fcf9f2
Decrypt or encrypt?[d/e]: d
Enter hex-encoded cipher to decrypt: 69dda8455c7dd4254bf353b773304eec0ec7702330098ce7f7520d1cbbb20fc388d1b0adb5054dbd7370849dbf0b88d393f252e764f1f5f7ad97ef79d59ce29f5f51eeca32eabedd9afa9329

Decrypted message:
CTR mode lets you build a stream cipher from a block cipher.

Press any key to exit.

Theory

• AES (Advanced Encryption Standard)
• PKCS #5/#7 padding scheme as described in RFC 5652

01
02 02
03 03 03
04 04 04 04
05 05 05 05 05
06 06 06 06 06 06
etc.

CBC

• Cipher Block Chaining

Properties

Encryption parallelizable:  No
Decryption parallelizable:  Yes
Random read access: Yes

CTR

• Counter mode with randomized IV

Properties

Encryption parallelizable:  Yes
Decryption parallelizable:  Yes
Random read access: Yes

Test Cases

CBC

• Key: 140b41b22a29beb4061bda66b6747e14

Cipher A

• The following cipher decrypts to Basic CBC mode encryption needs padding. when the key above is used

4ca00ff4c898d61e1edbf1800618fb28
28a226d160dad07883d04e008a7897ee
2e4b7465d5290d0c0e6c6822236e1daa
fb94ffe0c5da05d9476be028ad7c1d81

Cipher B

• The following cipher decrypts to Our implementation uses rand. IV when the key above is used.

5b68629feb8606f9a6667670b75b38a5
b4832d0f26e1ab7da33249de7d4afc48
e713ac646ace36e872ad5fb8a512428a
6e21364b0c374df45503473c5242a253

CTR

• Key: 36f18357be4dbd77f050515c73fcf9f2

Cipher C

• The following cipher decrypts to CTR mode lets you build a stream cipher from a block cipher. when the key above is used.

69dda8455c7dd4254bf353b773304eec
0ec7702330098ce7f7520d1cbbb20fc3
88d1b0adb5054dbd7370849dbf0b88d3
93f252e764f1f5f7ad97ef79d59ce29f
5f51eeca32eabedd9afa9329

Cipher D

• The following cipher decrypts to Always avoid the two time pad! when the key above is used.

770b80259ec33beb2561358a9f2dc617
e46218c0a53cbeca695ae45faa8952aa
0e311bde9d4e01726d3184c34451