Hashing

Hashing serves the purpose of ensuring integrity, i.e. making it so that if something is changed you can know that it’s changed. Technically, hashing takes arbitrary input and produce a fixed-length string that has the following attributes:

1. The same input will always produce the same output.
2. Multiple disparate inputs should not produce the same output.
3. It should not be possible to go from the output to the input.
4. Any modification of a given input should result in drastic change to the hash.

Hashing is used in conjunction with authentication to produce strong evidence that a given message has not been modified. This is accomplished by taking a given input, hashing it, and then signing the hash with the sender’s private key.
When the recipient opens the message, they can then validate the signature of the hash with the sender’s public key and then hash the message themselves and compare it to the hash that was signed by the sender. If they match it is an unmodified message, sent by the correct person.

Examples: SHA-3 ↗, MD5 ↗, etc.

Hash Functions#

Hash functions are algorithms that produce a code that can’t be decrypted.

Hash functions have been around since the early days of computing. They were originally created as a way to quickly search for data. Since the beginning, these algorithms have been designed to represent data of any size as small, fixed-size values, or digests. Using a hash table, which is a data structure that’s used to store and reference hash values, these small values became a more secure and efficient way for computers to reference data.

MD5#

One of the earliest hash functions is Message Digest 5, more commonly known as MD5. Professor Ronald Rivest of the Massachusetts Institute of Technology (MIT) developed MD5 in the early 1990s as a way to verify that a file sent over a network matched its source file.
Message Digest 5 (MD5) is a cryptographic hash function that takes any input and produces a 128-bit hexadecimal number. The output of an MD5 hash function is called a digest. MD5 digests are often used to verify the integrity of files or data; however, MD5 is no longer considered secure and should not be used for sensitive applications.
Whether it’s used to convert a single email or the source code of an application, MD5 works by converting data into a 128-bit value. You might recall that a bit is the smallest unit of data measurement on a computer. Bits can either be a 0 or 1. In a computer, bits represent user input in a way that computers can interpret. In a hash table, this appears as a string of 32 characters. Altering anything in the source file generates an entirely new hash value.
Generally, the longer the hash value, the more secure it is. It wasn’t long after MD5’s creation that security practitioners discovered 128-bit digests resulted in a major vulnerability.

Hash collisions#

One of the flaws in MD5 happens to be a characteristic of all hash functions. Hash algorithms map any input, regardless of its length, into a fixed-size value of letters and numbers. What’s the problem with that? Although there are an infinite amount of possible inputs, there’s only a finite set of available outputs!
MD5 values are limited to 32 characters in length. Due to the limited output size, the algorithm is considered to be vulnerable to hash collision, an instance when different inputs produce the same hash value. Because hashes are used for authentication, a hash collision is similar to copying someone’s identity. Attackers can carry out collision attacks to fraudulently impersonate authentic data.

Next-generation hashing#

To avoid the risk of hash collisions, functions that generated longer values were needed. MD5’s shortcomings gave way to a new group of functions known as the Secure Hashing Algorithms, or SHAs.
The National Institute of Standards and Technology (NIST) approves each of these algorithms. Numbers besides each SHA function indicate the size of its hash value in bits. Except for SHA-1, which produces a 160-bit digest, these algorithms are considered to be collision-resistant. However, that doesn’t make them invulnerable to other exploits.
Five functions make up the SHA family of algorithms:

• SHA-1
• SHA-224
• SHA-256
• SHA-384
• SHA-512

SHA-256#

Secure Hash Algorithm 256 bits (SHA-256) is a cryptographic hash function that takes any input and produces a 256-bit hexadecimal number. SHA-256 is often used to verify the integrity of files or data and to create digital signatures. SHA-256 is considered very secure and is widely used in applications such as Bitcoin and blockchain technology.