SHA Introduced

SHA (Secure Hash Algorithm) is a family of cryptographic hash functions designed to securely process data into a fixed-size output that appears random.

The Big Picture

Imagine you have a huge pile of documents and you need a quick way to create a unique "fingerprint" for each document. SHA (Secure Hash Algorithm) creates such fingerprints, called hash values, that uniquely represent your documents. Even a tiny change in the document will produce a completely different fingerprint.

Core Concepts

1. Hash Function: A process that takes an input (or 'message') and returns a fixed-size string of bytes.
2. Fixed-size Output: No matter the size of the input, the output hash is always the same size (e.g., SHA-256 always produces a 256-bit hash).
3. Deterministic: The same input will always produce the same output.
4. Avalanche Effect: A small change in the input drastically changes the output.
5. Collision Resistance: It’s infeasible to find two different inputs that produce the same output.

Detailed Walkthrough

1. Input Processing: The input message is divided into fixed-size blocks.
2. Initial State: The algorithm starts with an initial state (a set of initial values).
3. Compression Function: Each block is processed through a compression function that mixes the data with the current state.
4. Final Output: After all blocks are processed, the final state is transformed into the hash value.

Types of SHA

1. SHA-1: Produces a 160-bit hash value. It's no longer considered secure for many applications.
2. SHA-2: Includes SHA-224, SHA-256, SHA-384, and SHA-512, producing 224, 256, 384, and 512-bit hash values respectively.
3. SHA-3: A newer standard with similar hash sizes to SHA-2 but different internal algorithms.

Understanding Through an Example

Let's say we use SHA-256 to hash the message "HELLO".

1. Initial Message: "HELLO"

2. Padding: The message is padded to ensure its length is a multiple of the block size (512 bits for SHA-256).

3. Initial Hash Values: SHA-256 starts with specific constants.

4. Processing:

   • The message is split into blocks.
   • Each block is processed through a series of logical functions and mixed with the current state.

5. Final Hash: The result after processing all blocks might look like: 185f8db32271fe25f561a6fc938b2e264306ec304eda518007d1764826381969

Conclusion and Summary

SHA is a family of cryptographic hash functions used to produce fixed-size, unique fingerprints of data. These functions ensure data integrity by providing a way to detect changes to the original input, due to their deterministic nature, fixed-size output, and resistance to collisions.

Test Your Understanding

1. Why is it important that a hash function exhibits the avalanche effect?
2. What are the differences between SHA-1, SHA-2, and SHA-3?
3. How does collision resistance contribute to the security of a hash function?

Reference

• "Cryptography and Network Security" by William Stallings
• Secure Hash Algorithm: https://en.wikipedia.org/wiki/Secure_Hash_Algorithm