What it's for

A hash function maps data

• from arbitrary length
• to fixed length

A cryptographic hash function

is a hash function that

• is deterministic
• is quick to compute one way
• is infeasible to compute the other way
• is therefore often called a one-way function
• changes extensively, even if the input only has small changes
• makes it infeasible to find two different messages with the same hash

Use cases:

• Verifying file & message integrity
• Verifying and creating digital signatures
• Password verification
• Proof-of-Work (you know, the blockchain thingy)
• File identification (e.g. in git, mercurial or for file sharing with magnet links)

How it works

a very simple one-way function

M % K = H

              e.g.:

              23     % 5 =   3
              42523  % 5 =   3
              42     % 5 =   2
              23542  % 5 =   2
            

How to do it the cryptographic way?

Some examples:

• MD5
• SHA-1
• SHA-3

Remember: MD5 and SHA-1 are already broken

Even better ways (for password hashing):

Using key-derivation functions

• not originally developd for password hashing
• but fulfill all characteristics for password hashing functions
• and often better protect against brute-force attacks
• examples (in order of increasing security):
  • PBKDF2
  • bcrypt
  • scrypt
  • Argon2

Usage example

PBKDF2

Getting the module

• Of course there is already a module
• named pbkdf2
• and provided by Darsey Litzenberger
• with instructions at https://www.dlitz.net/software/python-pbkdf2

Let's install:

              
                $ pip3 install pbkdf2
              
            

Let's get derived!

              
                #!/usr/bin/python3

                from pbkdf2 import PBKDF2
                import os
                import base64

                passphrase = "This is my aweseomest most annoyingly long passphrase with no extra security features whatsoever!"

                salt = os.urandom(8)
                derivedKey = PBKDF2(passphrase, salt).read(32)

                print("The derived key is:")
                print(base64.b64encode(derivedKey))
              
            

Using it for encryption

              
                #!/usr/bin/python3

                from pbkdf2 import PBKDF2
                from Crypto.Cipher import AES
                from Crypto import Random
                import base64

                passphrase = "This is my aweseomest most annoyingly long passphrase with no extra security features whatsoever!"

                salt = Random.new().read(8)
                derivedKey = PBKDF2(passphrase, salt).read(32)

                initialisationVector = Random.new().read(16)
                cipher = AES.new(derivedKey, AES.MODE_CBC, initialisationVector)

                # Now lets encipher something
                encryptedMessage = base64.b64encode(cipher.encrypt("Hey there! It's nothing important, still secret!"))

                print("The encrypted message is:")
                print(encryptedMessage)
              
            

Using it for password hashing

              
                #!/usr/bin/python3

                from pbkdf2 import crypt

                passphrase = "This is my aweseomest most annoyingly long passphrase with no extra security features whatsoever!"
                otherPass = "ladida_so_funny"

                print("The crypted passphrase:")
                print(crypt(passphrase))

                print("The crypted other passphrase (with default 400 iterations):")
                print(crypt(otherPass))

                print("The crypted other passphrase, but with 500 iterations")
                print(crypt(otherPass, iterations=500))

                print("The crypted other passphrase, but with 1024 iterations")
                print(crypt(otherPass, iterations=1024))

                print("The crypted other passphrase, but with 4096 iterations")
                print(crypt(otherPass, iterations=4096))
              
            

What you have to remember:

#!

Not everything that looks shiny ...

... shines when it comes to security

Some guidlines:

• The ECRYPT Hash Function Website: The Hash Function Zoo
• Wikipedia: Hash function security summary
• Michele Preziuso @ Medium: Password Hashing: Scrypt, Bcrypt and ARGON2