## Overview#

A Cryptographic Hash Function (One-Way Hash Function OWHF) is a Hash Function with some additional MUST have properties:- Preimage Resistance - implies that using the Computational Hardness Assumption it is not realistic to determine the input Message
- Second Preimage Resistance - A given Message and the Hash using the Computational Hardness Assumption it is not realistic find different Message with the same Hash
- Collision Resistance - implies that using the Computational Hardness Assumption it is not realistic to find two messages which produce the same Hash (see SHAttered)

**NOT**meet these requirements, the resulting hash is considered Cryptographically Weak.

### Most Common Cryptographic Hash Function#

The most commonly used Cryptographic Hash Function function today (2015-03-15) is SHA-1, which has output of 160 bits. (see SHAttered and SHA-1 Deprecation)### Strength of Cryptographic Hash Function#

Unlike with Ciphers, the strength of a Cryptographic Hash Function doesn’t equal the Hash length. Because of the Birthday Paradox (a well-known problem in probability theory), the strength of a Cryptographic Hash Function is at most one half of the Hash length.The harder it is to compute raises the Level Of Assurance.

Note that unlike hardness in most of complexity theory (e.g., NP-hardness), "hard" in the context of one-way functions refers to the Computational Hardness Assumption rather than worst-case hardness.

The existence of "pure" one-way functions is an open conjecture. for more information see Wikipedia:One-way_function

### More Information#

There might be more information for this subject on one of the following:- Bitcoin block
- Certificate Signature
- Crypt
- Cryptographic Primitive
- Cryptography
- Digital Signature
- HMAC-MD5
- HMAC-SHA1
- HMAC-SHA2
- Hash
- JWK-Thumbprint
- Keyed-Hash Message Authentication Code
- MD5
- Master Secret
- Message Authentication Code
- Message-Digest
- One-Way Hash Function
- PBKDF2
- Record Protocol
- SHA-1
- SHA-3
- SHAttered
- Secure Hash Algorithm
- TLS 1.3
- Time-based One-time Password Algorithm
- Verifying Certificate Signatures
- Zero-knowledge proof

- [#1] - Cryptographic Hash Functions - based on information obtained 2013-04-10
- [#1] - One-way_function - based on information obtained 2013-04-10