key_wrap.py

'''
Key wrapping and unwrapping as defined in RFC 3394.  
Also a padding mechanism that was used in openssl at one time.  
The purpose of this algorithm is to encrypt a key multiple times to add an extra layer of security.

Personally, I wouldn't recommend using this for most applications.  
Just use AES/mode CTR to encrypt your keys, the same as you would any other data.  
The time to use this code is when you need compatibility with another system that implements the RFC.  
(For example, these functions are compatible with the openssl functions of the same name.)

Performance should be reasonable, since the heavy lifting is all done in PyCrypto's AES.
'''
import struct
from math import ceil
from Crypto.Cipher import AES

QUAD = struct.Struct('>Q')

def aes_unwrap_key_and_iv(kek, wrapped):
    n = len(wrapped)/8 - 1
    #NOTE: R[0] is never accessed, left in for consistency with RFC indices
    R = [None]+[wrapped[i*8:i*8+8] for i in range(1, n+1)]
    A = QUAD.unpack(wrapped[:8])[0]
    decrypt = AES.new(kek).decrypt
    for j in range(5,-1,-1): #counting down
        for i in range(n, 0, -1): #(n, n-1, ..., 1)
            ciphertext = QUAD.pack(A^(n*j+i)) + R[i]
            B = decrypt(ciphertext)
            A = QUAD.unpack(B[:8])[0]
            R[i] = B[8:]
    return "".join(R[1:]), A

#key wrapping as defined in RFC 3394
#http://www.ietf.org/rfc/rfc3394.txt
def aes_unwrap_key(kek, wrapped, iv=0xa6a6a6a6a6a6a6a6):
    key, key_iv = aes_unwrap_key_and_iv(kek, wrapped)
    if key_iv != iv:
        raise ValueError("Integrity Check Failed: "+hex(key_iv)+" (expected "+hex(iv)+")")
    return key

#alternate initial value for aes key wrapping, as defined in RFC 5649 section 3
#http://www.ietf.org/rfc/rfc5649.txt
def aes_unwrap_key_withpad(kek, wrapped):
    if len(wrapped) == 16:
        plaintext = AES.new(kek).decrypt(wrapped)
        key, key_iv = plaintext[:8], plaintext[8:]
    else:
        key, key_iv = aes_unwrap_key_and_iv(kek, wrapped)
    key_iv = "{0:016X}".format(key_iv)
    if key_iv[:8] != "A65959A6":
        raise ValueError("Integrity Check Failed: "+key_iv[:8]+" (expected A65959A6)")
    key_len = int(key_iv[8:], 16)
    return key[:key_len]

def aes_wrap_key(kek, plaintext, iv=0xa6a6a6a6a6a6a6a6):
    n = len(plaintext)/8
    R = [None]+[plaintext[i*8:i*8+8] for i in range(0, n)]
    A = iv
    encrypt = AES.new(kek).encrypt
    for j in range(6):
        for i in range(1, n+1):
            B = encrypt(QUAD.pack(A) + R[i])
            A = QUAD.unpack(B[:8])[0] ^ (n*j + i)
            R[i] = B[8:]
    return QUAD.pack(A) + "".join(R[1:])

def aes_wrap_key_withpad(kek, plaintext):
    iv = 0xA65959A600000000 + len(plaintext)
    plaintext = plaintext + "\0" * int(8*ceil(float(len(plaintext))/8) - len(plaintext)) 
    if len(plaintext) == 8:
        return AES.new(kek).encrypt(QUAD.pack[iv] + plaintext)
    return aes_wrap_key(kek, plaintext, iv)

def test():
    #test vector from RFC 3394
    import binascii
    KEK = binascii.unhexlify("000102030405060708090A0B0C0D0E0F")
    CIPHER = binascii.unhexlify("1FA68B0A8112B447AEF34BD8FB5A7B829D3E862371D2CFE5")
    PLAIN = binascii.unhexlify("00112233445566778899AABBCCDDEEFF")
    assert aes_unwrap_key(KEK, CIPHER) == PLAIN
    assert aes_wrap_key(KEK, PLAIN) == CIPHER