Cryptography

Algorithms and Protocols:

Encryption Algorithms

encode the information so that the original message can not be retrieved by a third party;

• secret-key algorithms;
• public-key algorithms;

Key-Exchange Algorithms

exchange secret keys over an insecure channel;

One-Way Hashing Algorithms

create a digest of a message so that the original cannot be modified and to get the same digest.

Secret-Key Encryption

The partners share a secret (the key) used to encrypt and decrypt the messages.

Simple Example -- the XOR algorithm

• Encryption Method: the message is XORed with the key;
• Decryption: the encrypted message is XORed with the same key;

Analysis;

• the algorithm is very easy to break;
• it is the algorithm used in cellular phones (160 bits key);

Real examples:

• DES, RC2, RC4, IDEA, ... .

Public-Key Encryption Algorithms

Each partner has two keys: one public and one private. A message encrypted with one key can be decrypted only with the other.

Example:

• Alice knows the public key of Bob;
• Alice encrypts the message with Bob's public key and sends the message to Bob;
• Bob decrypts the message with his private key;

Problems with the Public Key Algorithms:

• slower than Secret Key Algorithms;
• distribution of the public keys;

Example: RSA Algorithm (Ron Rivest, Adi Shamir, Leonard Adleman)

Key-Exchange Protocols

The idea is that two partners, communicating through an unsecure channel, can create an information that is known by both and cannot be determined by a third party.

Example: Diffie-Hellman Key Exchange Protocol

One-Way Hash Functions

The purpose is to generate a digest of a message that has the following properties:

is relatively short (between 128 and 256 bits);

given a message it is easy to generate the digest;

given the message digest it is difficult to reconstruct the message;

given a hash value it is difficult to find a message which has the same hash value.

Examples:

• the algorithms are intricate;
• the most often used algorithms are: MD5 (RSA) and SHA (NSA, NIST) or both (used in SSL);

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