**Generic PKC employs two keys that are mathematically related although knowledge of one key does not allow someone to easily determine the other key. One key is used to encrypt the plaintext and the other key is used to decrypt the ciphertext. No matter which key is applied first, but both the keys are required for the process to work. Because a pair of keys are required, this approach is also called asymmetric cryptography.**

__Public-Key Cryptography__:-In PKC, one of the keys is designated the public key and may be advertised as widely as the owner wants. The other key is designated the private key and is never revealed to another party.

Public-key cryptography algorithms that are in use today for key exchange or digital signatures include :

**1) RSA :**One of the most popular encryption algorithm, invented in 1977 by three MIT scientists (Ronald Rivest, Adi Shamir, and Leonard Adleman)

The key-pair is derived from a very large number, n, that is the product of two prime numbers chosen according to special rules; these primes may be 100 or more digits in length each, yielding an n with roughly twice as many digits as the prime factors. The public key information includes n and a derivative of one of the factors of n; an attacker cannot determine the prime factors of n (and, therefore, the private key) from this information alone and that is what makes the RSA algorithm so secure.

**Hash functions, also called message digests and one-way encryption, are algorithms that, in some sense, use no key. Instead, a fixed-length hash value is computed based upon the plaintext that makes it impossible for either the contents or length of the plaintext to be recovered. Hash algorithms are typically used to provide a digital fingerprint of a file's contents, often used to ensure that the file has not been altered by an intruder or virus. Hash functions are also commonly employed by many operating systems to encrypt passwords. Hash functions, then, provide a measure of the integrity of a file.**

__Hash Functions__:-Hash algorithms that are in common use today include:

**1) Message Digest (MD) algorithms :**
A series of byte-oriented algorithms that produce a 128-bit hash value
from an arbitrary-length message.

**MD2 :** Designed for systems with limited memory, such
as smart cards.

**MD4 :** Developed by Rivest, similar to MD2 but designed
specifically for fast processing in software.

**MD5 :** Also developed by Rivest in 1991 after potential
weaknesses were reported in MD4; this scheme is similar to MD4 but is
slower because more manipulation is made to the original data.It accepts
variable length message from the user and converts it into a fixed 128-bit
message digest value.

**2) Secure Hash Algorithm (SHA) :**
Algorithm for NIST's Secure Hash Standard (SHS). SHA-1 produces a 160-bit
hash value and was originally published as FIPS 180-1 and RFC 3174. FIPS
180-2 describes five algorithms in the SHS: SHA-1 plus SHA-224, SHA-256,
SHA-384, and SHA-512 which can produce hash values that are 224, 256,
384, or 512 bits in length, respectively. SHA-224, -256, -384, and -52
are also described in RFC 4634.

**3) RIPEMD :**A series of message digests that initially came from the RIPE (RACE Integrity Primitives Evaluation) project. RIPEMD-160 was designed by Hans Dobbertin, Antoon Bosselaers, and Bart Preneel, and optimized for 32-bit processors to replace the then-current 128-bit hash functions. Other versions include RIPEMD-256, RIPEMD-320, and RIPEMD-128.

**4) HAVAL (HAsh of VAriable Length) :**
Designed by Y. Zheng, J. Pieprzyk and J. Seberry, a hash algorithm with
many levels of security. HAVAL can create hash values that are 128, 160,
192, 224, or 256 bits in length.

**5) Whirlpool :** A relatively
new hash function, designed by V. Rijmen and P.S.L.M. Barreto. Whirlpool
operates on messages less than 2256 bits in length, and produces a message
digest of 512 bits. The design of this hash function is very different
than that of MD5 and SHA-1, making it immune to the same attacks as on
those hashes.

**6) Tiger :** Designed by
Ross Anderson and Eli Biham, Tiger is designed to be secure, run efficiently
on 64-bit processors, and easily replace MD4, MD5, SHA and SHA-1 in other
applications. Tiger/192 produces a 192-bit output and is compatible with
64-bit architectures; Tiger/128 and Tiger/160 produce the first 128 and
160 bits, respectively, to provide compatibility with the other hash functions.