This is an exercise in secure symmetric-key encryption, implemented in pure Python (only built-in libraries used), expanded from Bo Zhu's (http://about.bozhu.me) AES-128 implementation at https://github.com/bozhu/AES-Python

import aes, os
key = os.urandom(16)
iv = os.urandom(16)
encrypted = aes.AES(key).encrypt_ctr(b'Attack at dawn', iv)
print(aes.AES(key).decrypt_ctr(encrypted, iv))
# b'Attack at dawn'

• AES-128, AES-192 and AES-256 implementations in pure python (very slow, but works). Results have been tested against the NIST standard (http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf)
• CBC mode for AES with PKCS#7 padding (now also PCBC, CFB, OFB and CTR thanks to @righthandabacus!)
• encrypt and decrypt functions for protecting arbitrary data with a password

Note: this implementation is not resistant to side channel attacks.

Although this is an exercise, the encrypt and decrypt functions should provide reasonable security to encrypted messages. It ensures the data is kept secret (using AES), blocks are encrypted together (CBC), the same message encrypted twice will have different ciphertexts (salt), the ciphertext hasn't been tampered with (HMAC) and the key has some defense against brute-force (PBKDF2).

The algorithm is as follows:

salt <- random(16)                                                        (1)
key_aes, key_hmac, iv <- PKBDF2(master_key, salt)                         (2)
HMAC(salt + E_key_aes(message, iv)) + salt + E_key_aes(message, iv)       (3+)

1. 16 random bytes of salt are extracted from the system's secure random number generator (usually /dev/urandom)>

2. The given master key is stretched and expanded by PKBDF2-HMAC(SHA256) using the salt from 1), to generate the AES key, HMAC key and IV (initialization vector for CBC).

3. The given message is encrypted with AES-128 using the AES key and IV from step 2), in CBC mode and PKCS#7 padding.

4. A HMAC-SHA256 is generated from the concatenation of the salt from 1) and the ciphertext from 3).

5. The final ciphertext is HMAC + salt + ciphertext.

Security overview:

• The random salt ensures the same message will map to different ciphertexts.

• The HMAC ensures the integrity of both the entire ciphertext and the PKBDF2 salt; encrypt-then-mac prevents attacks like Padding Oracle.

• Bytes from keys, iv and salt are not reused in different algorithms.

• PBKDF2 key stretching allows for relatively weak passwords to be used as AES keys and be moderately resistant to brute-force, but sacrificing performance.