Questions tagged [cryptographic-hash-function]

A cryptographic hash function is a function that takes a string of bytes of variable length for input and returns a fixed number of bytes (the exact number depending on the function) such that:

• It is extremely difficult to find an input given an output.
• It is extremely difficult to find another input that gives the same output.
• Preferably, a small change in input results in a large change in output. (the avalanche effect)

Common hash functions include:

• MD5 (128-bit output, tag: md5; vulnerabilities found)
• SHA-1 (160-bit output, tag: sha-1; vulnerabilities found)
• SHA-2, including SHA-512 (512-bit output, tag: sha-512)
• RIPEMD-160 (160-bit output, tag: ripemd)

There are also some other notable hash functions which are less mature:

• SHA-3/Keccak, winner of the NIST hash function competition (tag: sha-3)

Applications

While there are many uses for cryptographic hash functions, here are a selected few:

• In combination with a salt, it can be used to store passwords. (tag: saltedhash)
• It can be used as part of an HMAC. (tag: hmac)

Related Questions

• What are the important points about cryptographic hash functions?
• Which cryptographic hash function should I choose?

Usage in Various Languages

In Python, one can use the hashlib module:

import hashlib
hashlib.md5(b'Stack Overflow').hexdigest()     # => fcb3cd0c1efd8afe09d0505e46cefa4c
hashlib.sha1(b'Stack Overflow').hexdigest()    # => 47dd9a3d44ce99e3bbd366c9c91a60fc96281e66
hashlib.sha256(b'Stack Overflow').hexdigest()  # => 37a04a3fbe1f4424d5ed539c213b8f56ff07dcf9d94a360fa8b4efb69d22c1ede93e1dc4928760bf84ffe5b176babb60344b1a10bda218aaa35eb6388a9db3ec

In Ruby, one can use digest:

require 'digest'
Digest::MD5.new.update('Stack Overflow').hexdigest        # => fcb3cd0c1efd8afe09d0505e46cefa4c
Digest::SHA1.new.update('Stack Overflow').hexdigest       # => 47dd9a3d44ce99e3bbd366c9c91a60fc96281e66
Digest::SHA2.new(512).update('Stack Overflow').hexdigest  # => 37a04a3fbe1f4424d5ed539c213b8f56ff07dcf9d94a360fa8b4efb69d22c1ede93e1dc4928760bf84ffe5b176babb60344b1a10bda218aaa35eb6388a9db3ec
Digest::RMD160.new.update('Stack Overflow').hexdigest     # => 4a275d3d6a77a69a7e0bd06241d35378348b7438

In C#, one can use System.Security.Cryptography.HashAlgorithm:

using System.Security.Cryptography;

byte[] input = /* ... */;
HashAlgorithm.Create("MD5").ComputeHash(input);        // => array of bytes
HashAlgorithm.Create("SHA1").ComputeHash(input);       // => array of bytes
HashAlgorithm.Create("SHA512").ComputeHash(input);     // => array of bytes
HashAlgorithm.Create("RIPEMD160").ComputeHash(input);  // => array of bytes

In Java, one can use java.security.MessageDigest:

import java.security.MessageDigest;

byte[] input = /* ... */;
// getInstance can throw NoSuchAlgorithmException if the algorithm is unsupported
MessageDigest md = MessageDigest.getInstance("MD5" /* or any other algorithm */);
md.digest(input);  // => array of bytes

Many other languages have mechanisms for hashing; see Rosetta Code for more:

• MD5 on Rosetta Code
• SHA-1 on Rosetta Code
• RIPEMD-160 on Rosetta Code