TYPES OF CRYPTOGRAPHIC ALGORITHMS

TYPES OF CRYPTOGRAPHIC ALGORITHMS

      There are several ways of classifying cryptographic algorithms. For purposes of this paper,they will be categorized based on the number of keys that are employed for encryption and decryption,    and further defined by their application and use. The three types of algorithms that will be discussed are:

• Secret Key Cryptography (SKC): Uses a single key for both encryption and decryption.
• Public Key Cryptography (PKC): Uses one key for encryption and another for decryption.
• Hash Functions: Uses a mathematical transformation to irreversibly "encrypt" information.

Secret Key Cryptography

With secret key cryptography, a single key is used for both encryption and decryption. , the sender uses the key (or some set of rules) to encrypt the plaintext and sends the ciphertext to the receiver. The receiver applies the same key (or ruleset) to decrypt the message and recover the plaintext. Because a single key is used for both functions, secret key cryptography is also called symmetric encryption.
With this form of cryptography, it is obvious that the key must be known to both the sender and the receiver; that, in fact, is the secret. The biggest difficulty with this approach, of course, is the distribution of the key.
Secret key cryptography schemes are generally categorized as being either stream ciphers or block ciphers. Stream ciphers operate on a single bit (byte or computer word) at a time and implement some form of feedback mechanism so that the key is constantly changing.
Block ciphers can operate in one of several modes; the following four are the most important:

1. Electronic Codebook (ECB) mode. 
2. Cipher Block Chaining (CBC) mode.
3. Cipher Feedback (CFB) mode.
4. Output Feedback (OFB) mode.

Public-Key Cryptography

Public-key cryptography has been said to be the most significant new development in cryptography in the last 300-400 years. Modern PKC was first described publicly by Stanford University professor Martin Hellman and graduate student Whitfield Diffie in 1976. Their paper described a two-key crypto system in which two parties could engage in a secure communication over a non-secure communications channel without having to share a secret key.
PKC depends upon the existence of so-called one-way functions, or mathematical functions that are easy to compute whereas their inverse function is relatively difficult to compute.

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

• RSA
• Diffie-Hellman.
• Digital Signature Algorithm (DSA).
• ElGamal.
• Public-Key Cryptography Standards (PKCS).

Hash Functions

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 algorithms that are in common use today include:

• Message Digest (MD) algorithms. 
• Secure Hash Algorithm (SHA).
• Tiger.

watch this video to know how asymmetric encryption works:

 Some more  videos :
https://www.youtube.com/watch?v=ERp8420ucGs
https://www.youtube.com/watch?v=E5FEqGYLL0o

References :

http://www.garykessler.net/library/crypto.html#fig01

https://cryptography.io/en/latest/

http://www.encryptionanddecryption.com/algorithms/encryption_algorithms.html