Implementing-cryptography-using-python

60 Chapter 2 ■ Cryptographic Protocols and Perfect Secrecy
Message Authentication Codes
Hash-based Message Authentication Code (HMAC) is a key-based message
digest algorithm that can be used for verifying the integrity of the message, to
verify the authenticity of the sender of the message, or both. HMAC has been
widely adopted for use in various systems and domains, such as server-toserver
communications, Web Service APIs, etc. A well-known use of HMAC
is in Amazon’s AWS API calls where the signature is generated using HMAC.
HMAC can use a variety of hashing algorithms, like MD5, SHA1, SHA256,
etc. The HMAC function is not process intensive, so it has been widely accepted,
and it is easy to implement in mobile and embedded devices while maintaining
decent security. The following code example shows how to generate an HMAC-
MD5 digest with Python:
import hmac
from hashlib import md5
key = b'DECLARATION'
h = hmac.new(key,b'',md5)
# add content
h.update('We hold these truths to be self-evident, that all men are
created equal')
# print the HMAC digest
print (h.hexdigest())
Perfect Forward Secrecy
In our exploration of cryptography, perfect forward secrecy (PFS), also known
as forward secrecy (FS), is a set of key agreement protocols that gives the participants
in the message exchange assurances that their session keys will not be
compromised even if the private key of the server is compromised. PFS protects
past cryptographic sessions against future compromises of passwords or secret
keys. The compromise of a single session key will not affect any data other than
that exchanged in the particular session by generating a unique session key for
each individual session; PFS further protects data on the transport layer of a
network that uses common SSL/TLS protocols such as OpenSSL. In the past,
OpenSSL was affected by the Heartbleed exploit. If PFS is used, encrypted
communications and sessions recorded that may have become compromised
cannot be used to decrypt future communications.