Cryptography

LECTURE 7 - CRYPTOGRAPHY

Cryptography is the practice and study of techniques for secure communication in the presence of third parties.

It is about constructing and analyzing protocols that overcome the influence of adversaries and which are related to various aspects in information security such as data confidentiality, data integrity, authentication, and non-repudiation.

Encryption is the process of encoding messages (or information) in such a way that unauthorized parties cannot access.

In an encryption scheme, the message (plaintext) is encrypted using an encryption algorithm, turning it into an unreadable (ciphertext). This is usually done with the use of an encryption key, which specifies how the message is to be encoded.

An authorized party, however, is able to decode the ciphertext using a decryption algorithm, that usually requires a secret decryption key.

An encryption scheme usually needs a key-generation algorithm to randomly produce keys.

There are two basic types of encryption schemes:

• Symmetric-key and

• Public-key encryption.

Symmetric-key

In symmetric-key schemes, the encryption and decryption keys are the same. Thus communicating parties must agree on a secret key before they wish to communicate.

Symmetric-key encryption can use either stream ciphers or block ciphers.

• Stream ciphers encrypt the digits one at a time.

• Block ciphers take a number of bits and encrypt them as a single unit, padding the plaintext so that it is a multiple of the block size. Blocks of 64 bits have been commonly used. The Advanced Encryption Standard (AES) algorithm approved by NIST in December 2001 uses 128-bit blocks.

In public-key schemes, the encryption key is published for anyone to use and encrypt messages. However, only the receiving party has access to the decryption key and is capable of reading the encrypted messages.

There are two main uses for public-key cryptography:

• Public-key encryption, in which a...