Implementing-cryptography-using-python

206 Chapter 7 ■ Message Integrity
choosing the length of the truncated message digest and information about its
security implications for the cryptographic application that uses it, see SP 800-107.
import hashlib
hasher = hashlib.md5()
# from http://csrc.nist.gov/groups/STM/cavp/documents/mac/
hmactestvectors.zip
with open('hmactestvectors.zip', 'rb') as afile:
buf = afile.read()
hasher.update(buf)
print(hasher.hexdigest())
The preceding Python should produce the following HMAC:
054d8addf01353605068508266eb2f19
To examine a NIST-supported SHA256 hash, review the following. It has
many similarities, but this example incorporates a BLOCKSIZE:
import hashlib
BLOCKSIZE = 65536
hasher = hashlib.sha256()
with open('hmactestvectors.zip', 'rb') as afile:
buf = afile.read(65536)
while len(buf) > 0:
hasher.update(buf)
buf = afile.read(BLOCKSIZE)
print(hasher.hexdigest())
The second example will produce the following HMAC:
418c3837d38f249d6668146bd0090db24dd3c02d2e6797e3de33860a387ae4bd
CBC-MAC
The cipher block chaining message authentication code (CBC-MAC) is used in
cryptography to construct a MAC from a block cipher. The initial message gets
encrypted with a block cipher algorithm in CBC mode. The CBC mode creates
a chain of blocks such that each block depends on the proper encryption of the
previous block. CBC sets up an interdependence that ensures that a change to
any portion of the plaintext bit causes the last block to be encrypted in a way
that cannot be predicted or counteracted without knowing the key to the block
cipher. The CBC-MAC encryption process is shown in Figure 7.2.