Introduction to Cryptographic Hash Functions

Cryptographic Hash Functions are a class of hash functions that are cryptographically secure. From password authentication and integrity verification to blockchain—these functions are used in a multitude of applications.

Table of Contents

1. Blockchain Basics Revisited
2. What is a Cryptographic Hash Function?
3. Properties of Cryptographic Hash Functions
4. Is 256-Bit Security Really Secure?
5. How to Compute SHA256 Sum in Bash
6. How to Obtain SHA256 Hash in Python
   

Blockchain Basics Revisited

Let's start our discussion by answering the question: "What's a Blockchain?"

A blockchain is an immutable, distributed ledger system. It's essentially a decentralized peer-to-peer network in which transactions can happen between peers without the involvement of a central authority.

• data/details of the transaction
• its hash
• hash of the previous block

The first block is called the genesis block, and it's the only block that doesn't contain the previous block's hash.

In a blockchain, the transactions are hashed using secure hashing algorithms. And here's where cryptographic hash functions enter the discussion.

Cryptographic hash functions are used to generate the hashes that uniquely identify the blocks.

What is a Cryptographic Hash Function?

A cryptographic hash function takes in an input message and maps it to a fixed length output, called the hash or the digest.

Whatever be the length of the input—it could be a single character, a string, or even a large file—the output is always of fixed length, say N.

In the following illustration, we pass two different inputs 'blockchain' and 'hello' of different lengths to the

SHA1

The

SHA1

Note: SHA stands for Secure Hashing Algorithm. Because of known security breaches in the past,

SHA1

SHA2

SHA256

SHA512

Properties of Cryptographic Hash Functions

1. Deterministic

A cryptographic hash function is deterministic. This means no matter how many times you feed in a particular input, you'll get the same output hash.

2. Computationally Efficient

3. Pre-Image Resistant or Non-Invertible

This property is based on the concept of one-way functions.

Let's take an example. You have a function

f(x) = cube(x)

f

In this case, if the output is 27, you can conclude right away that the input is 3, which is the pre-image corresponding to the output 27. Therefore, such a function

f

However, cryptographic hash functions should be pre-image resistant.

This means that you can input a message to the hashing algorithm, and obtain the hash. But it should be infeasible to obtain the input message by looking at the hash.

4. Collision Resistant

But what is collision in the context of hash functions?

Well, let's parse what collision resistance actually means.

A collision is said to occur when two input messages

M1

M2

M1

M2 

5. Exhibits Avalanche Effect

To sum up, a cryptographic hash function generates a fixed length hash that is deterministic yet random, and is cryptographically secure.

So far, you've learned what cryptographic hash functions are, and their properties. And we've mentioned that the

SHA256

Is 256-Bit Security Really Secure?

Suppose you have the desired output hash. Recall that a cryptographic hash function is deterministic. And it outputs the same hash for a specific input.

But it's also non-invertible. So the only way you can get back the input is by trying to generate the output hash at your end—through a series of random guesses.

The

SHA256

So there are 2^256 total combinations in all! And this is an insanely large number. And breaking this hash by random guessing is exponentially hard.

The Bash command

sha256sum

$ printf "I'm coding"| sha256sum
860f5cae6febaa6b9064a16d78553819de43cb1e4c5a87ab267bb1c35fb41a04 

To get the 160-bit long SHA1 hash, you can use the Bash command

sha1sum

sha256sum

sha1sum

$ printf "I'm coding" | sha1sum
cafc711fba6c8ccdcbb807e5a676e9810e5cce4c 

In the next section, we'll see how to obtain the hash under the

SHA256

How to Obtain SHA256 Hash in Python

Python ships with a built-in

hashlib

import hashlib.

• Use the

  sha256()

  constructor to instantiate a hash object
• Encode the message string, optionally include an encoding format
• Call the

  hexdigest()

  method to obtain the hex equivalent of the 256-bit hash

import hashlib
 
message = "I'm coding"
 
hash_obj = hashlib.sha256(message.encode())
 
hash_val = hash_obj.hexdigest()
 
print(hash_val)

# output: 860f5cae6febaa6b9064a16d78553819de43cb1e4c5a87ab267bb1c35fb41a04

print(len(hash_val)) 

# output: 64
# correct! 64 hexadecimal digits; total length = 64 * 4 = 256 bits

Notice how the

SHA256

import hashlib

strings = ["hello","sha256","sensitive info"]

for string in strings:
    hash_obj = hashlib.sha256(string.encode())
    hash_val = hash_obj.hexdigest()
    print(f"Hash #{strings.index(string)+1}: {hash_val}")

# Output
Hash #1: 2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824
Hash #2: 5d5b09f6dcb2d53a5fffc60c4ac0d55fabdf556069d6631545f42aa6e3500f2e
Hash #3: 034fcc03d9332ee032b5815ef69b0f21926dd2da73f0fcfd65ff90ded1700892

Summing Up the Discussion on Cryptographic Hash Functions

You've learned what cryptographic functions are what their properties are. And you've also learned how to use the Bash commands like

sha1sum

sha256sum

In addition, you've seen how to use Python's

hashlib