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what-is-hashing.md

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What is Hashing?

Hashing, in cryptography is the process of mapping a given arbitrary amount of data to a fixed length digest. Since it is deterministic in nature, the same input given will produce the same output value every time. It is a mathematical algorithm that maps the input to a fixed length value and is designed to be a one-way (non-invertible) function.

For e.g. consider the following hash function h(), that returns the ASCII value and next two characters in series, for the first character of the given input:

String Hash
a 97bc
cat 99de
at 97bc

But this cannot be considered as a “good” hash function, because we can see that we get similar hashes for words starting with the same letter of the alphabet. A perfect hash function maps every key to a different value.

The ideal cryptographic hash function has the following properties:

  • should be deterministic
  • should be non-invertible
  • a small change to the message should change the hash extensively so that the old and new value appear completely unrelated.
  • No two strings must have the same hash value i.e. h(x1) ≠h(x2) for x1 and x2

Since the input length is unbounded (i.e. it can be any length) and the output is bounded by a fixed length, there will always be cases where two different inputs generate the same output. This is called a collision. It is not possible for a hash function to be collision-free. However, cryptographic hash functions can be resistant to collision, i.e. it is very difficult to find two inputs that generate the same output.A non-perfect but a good example of hashing algorithm is the SHA-3. Several variants like SHA3-256 and SHA3-512 exists. Several other hashing algorithms exist like SHA-1 and MD5 which are obsolete now.

The following table shows a collision resistant set of hashes.

String SHA-256 Hash
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua 29c03d361dde0dbff070f8efad6da05fcb72e0cc825c301ab8f0596224bc23c7
lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua c1f947fd7d47254945c9131be3254609634c84f709b60bb37c6a8f3377300392
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. 973153f86ec2da1748e63f0cf85b89835b42f8ee8018c549868a1308a19f6ca3