1. Hashing Hash are the auxiliary values that are used in cryptography.
A hash is a special function that performs one way encryption meaning that once the algorithm is processed, there is no feasible way to take the cipher text and retrieve the plain text that was used to generate it.
The hash code is a function of all bits of the message and provides an error detection capability.
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2. A change in any bit or bits result in a change hash value.
Popular Hash Functions:
Secure Hash Function(SHA)
Message Digest of varying functions like MD2, MD4 ,MD5
A hash value h is generated by a function H of the form
h=H(M)
Where M is variable length message and H(M) is the fix length hash value.

3. Hash value should have following properties for message authentication
1. H can be applied to a block of data of any size.
2. H produces a fixed length output.
3. H(X) is relatively easy to compute for any given x making both hardware and software implementation practical.
4. For any given value of h , it is computationally infeasible to find
x such that H(X) = h This is referred to as the one way property.
5. For any given block of x, it is computationally infeasible to find
y ≠ x with H(y) = H(x) This is referred to as weak collision resistance.
6. For any given block of x, it is computationally infeasible to find (m , x ) such a that with H(m) = H(y) This is referred to as strong collision resistance.

4. Advantages
It is more efficient to compute a digital signature using a document‘s message digest.
A digest can be made public without revealing the contents of the document from which it derives.
It is used for digital authentication must have certain properties that make it secure enough for cryptographic use.
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