Implementing-cryptography-using-python

Chapter 3 ■ Classical Cryptography 67
Hashing Passwords
Python, like many other languages, provides you a way to call hashing functions
that accept a message of any length and return a fixed-length result that is
referred to as a message digest or hash code. Hashing functions use specific hashing
algorithms but do not use secret keys. If the exact message is entered into
a hashing function, the same hash code will be produced.
There are several reasons to use hash codes in communications. They can
assist in ensuring the confidentiality and the integrity of the message. You can
use a number of hashing algorithms, including MD5, SHA-3, SHA-512, HAVAL,
and RIPEMD-160, just to name a few. The most important properties of hash
algorithms are that the output is not predictable, different messages do not
produce the same hash code, messages are not reversible, and given the same
value, a hash algorithm should always produce the same result.
The following example shows the SHA-512 hash values for the words password
and Password. Notice that even though only the first letter was capitalized, the
entire message digest is different.
Here is the hash value for password:
b109f3bbbc244eb82441917ed06d618b9008dd09b3befd1b5e07394c706a8bb980b1d778
5e5976ec049b46df5f1326af5a2ea6d103fd07c95385ffab0cacbc86
Here is the hash value for Password:
e6c83b282aeb2e022844595721cc00bbda47cb24537c1779f9bb84f04039e1676e6ba857
3e588da1052510e3aa0a32a9e55879ae22b0c2d62136fc0a3e85f8bb
To examine the hashed password using SHA-512, type the following into
Python:
import hashlib
plaintext_password = b'Password'
hashed_sha512 = hashlib.sha512(plaintext_password).hexdigest()
print(hashed_sha512)
There are two primary ways to attack hash functions: through cryptanalysis
and through brute force. Storing passwords in the database in their hash code
form still offers malicious users a way to figure out passwords. A rainbow table,
which is a precomputed table for reversing cryptographic hash functions, can
be used to crack password hashes. Rainbow tables are used in recovering a
plaintext password up to a certain length consisting of a limited set of characters.
This is where salting and stretching come in.
Salting Passwords
You have learned that you need to use cryptographic hashing to minimize
readability when storing passwords, but just hashing alone isn’t enough. Salting
is the process of adding or concatenating a random chunk of bits to the end of