Pseudorandom number generator

To date there is no known method to produce true randomness, because due to the very nature of randomness, any factor determining the outcome would mean that it is not entirely random. The random number generation functions provided in all software packages are therefore pseudorandom.

One-Way hash functions

A cryptographic hash function is a transformation that takes an input (plain-text) and returns a fixed-size string (cipher-text). A one-way function is a function that is easy to compute but "hard to invert" (get plain-text from cipher-text).

Output of one way hash function is called fingerprint of the plain-text because it is shrunk value (hash) of the input.

Hash functions are SHA, RIPEMD, MD ...

One-way hash functions play fundamental role in digital signatures.

Symmetric algorithms

Symmetric-key algorithms are a class of algorithms for cryptography that use trivially related, often identical, cryptographic keys for both decryption and encryption.

The encryption key is trivially related to the decryption key, in that they may be identical or there is a simple transform to go between the two keys. The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link.

One disadvantage of symmetric-key algorithms is the requirement of a shared secret key, with one copy at each end. In order to ensure secure communications between everyone in a population of n people a total of n(n − 1)/2 keys are needed, which is the total number of possible communication channels.

Advantage: implemented efficiency in computing environments
Disadvantage: complex key management

DES (Data Encryption Standard), 3DES, AES (Advance Encryption Standard)

Substitutions

plain-text	a b c d e f g h i j k l m
cipher-text	d e f g a i j k l m n o p

Plain-text "decimal" will be encoded to "gaflpdo"

Poly-alphabetic substitutions can be used. In this case different substitution sequences are defined. For example one substitution sequence would be used for each plain-text character on an even position and other substitution sequence would be used for each character on odd position.

Transposition

The order of the characters is changed. Mathematically a bijective function is used on the characters' positions to encrypt and an inverse function to decrypt.
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Product ciphers

Combination of substitutions and transpositions.

Encode 110 010
P₀(053241)
P1(241053)
P2(543210)
S0 = S1 = S2 = S3

S:P in the middle (42137506)

Asymmetric algorithms

Key used to encrypt a message differs from the key used to decrypt it. In public key cryptography, a user has a pair of cryptographic keys—a public key and a private key. The private key is kept secret, while the public key may be widely distributed. Incoming messages would have been encrypted with the recipient's public key and can only be decrypted with his corresponding private key. The keys are related mathematically, but the private key cannot be practically derived from the public key.

Advantage: easy key management
Disadvantage: computational complexity, needs binding between public key and corresponding entity (CA - certification authorities, PKI - public key infrastructure)

RSA, DG, DSS, ECC

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Steganography and watermarking

Steganography is the art and science of writing hidden messages in such a way that no one apart from the sender and intended recipient even realizes there is a hidden message. By contrast, cryptography obscures the meaning of a message, but it does not conceal the fact that there is a message.

One of the simplest forms of steganography uses color values of pixels in graphic files. Problems with such techniques emerge if files are processed.

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