## About SHA1 hash

SHA-1 (Secure Hash Algorithm 1) is a cryptographic hash function designed by the National Security Agency (NSA) and published by the National Institute of Standards and Technology (NIST) in 1993. It is part of the Secure Hash Algorithm family, which also includes SHA-0, SHA-2, and SHA-3. SHA-1 was widely used in various security protocols and applications, but it is now considered insecure due to vulnerabilities that allow for collision attacks.

### Characteristics of SHA-1

**Fixed-Length Output**:- SHA-1 produces a fixed-length output of 160 bits (20 bytes), regardless of the input data's length.

**Input Padding**:- The input message is padded so that its length is congruent to 448 modulo 512. Padding involves appending a single '1' bit, followed by '0' bits until the message length is 64 bits short of a multiple of 512. The length of the original message (before padding) is appended as a 64-bit integer.

**Processing in Blocks**:- SHA-1 processes the input message in 512-bit (64-byte) blocks.

**Initialization Vector (IV)**:- SHA-1 starts with a predefined initial state composed of five 32-bit words:
- H0 = 0x67452301
- H1 = 0xEFCDAB89
- H2 = 0x98BADCFE
- H3 = 0x10325476
- H4 = 0xC3D2E1F0

- SHA-1 starts with a predefined initial state composed of five 32-bit words:
**Compression Function**:- SHA-1's compression function consists of four rounds, each with 20 iterations, making a total of 80 iterations per 512-bit block. Each round uses a different logical function and constant:
- Round 1 (0-19): Uses the function f = (B & C) | (~B & D) with a constant K = 0x5A827999
- Round 2 (20-39): Uses the function f = B ^ C ^ D with a constant K = 0x6ED9EBA1
- Round 3 (40-59): Uses the function f = (B & C) | (B & D) | (C & D) with a constant K = 0x8F1BBCDC
- Round 4 (60-79): Uses the function f = B ^ C ^ D with a constant K = 0xCA62C1D6

- These rounds involve modular addition, bitwise operations (shifts and rotations), and message schedule arrays.

- SHA-1's compression function consists of four rounds, each with 20 iterations, making a total of 80 iterations per 512-bit block. Each round uses a different logical function and constant:

### Algorithm Steps

**Initialization**:- Initialize the state variables (H0, H1, H2, H3, H4) to the predefined values.

**Padding**:- Pad the input message according to the specified padding rules.

**Processing**:- Divide the padded message into 512-bit blocks.
- For each block, perform the 80 iterations of the compression function, updating the state variables.

**Output**:- After processing all blocks, concatenate the state variables to produce the final 160-bit hash value.

### Security and Usage

**Security**:**Collision Vulnerability**: SHA-1 is vulnerable to collision attacks, where two different inputs can produce the same hash value. These vulnerabilities significantly undermine its security and reliability for ensuring data integrity and authenticity.**Preimage and Second-Preimage Attacks**: While less practical than collision attacks, SHA-1 is also susceptible to preimage and second-preimage attacks.- Due to these vulnerabilities, SHA-1 is considered deprecated and unsuitable for cryptographic purposes such as SSL/TLS certificates, digital signatures, and other applications requiring high security.

**Usage**:- Despite its known weaknesses, SHA-1 was widely used in the past and may still be encountered in some legacy systems and applications. However, it is strongly recommended to transition to more secure hash functions like SHA-256 (part of the SHA-2 family) or SHA-3 for any new or existing security-critical applications.

### Summary

SHA-1 is a cryptographic hash function that produces a 160-bit output. Although it was widely adopted due to its efficiency and simplicity, significant security vulnerabilities, particularly susceptibility to collision attacks, have led to its deprecation. Modern applications should avoid using SHA-1 and instead utilize more secure hash functions such as those in the SHA-2 or SHA-3 families.