Key generator

For a key generator to be cryptographically secure, its output must have several properties:

• Uncorrelated sequences, no sequence of any given length should be correlated to any other sequence of the algorithm's output
• Long period, the sequence should not repeat for a very long time
• Uniform distribution, the output bits should be uniformly distributed
• Unpredictability, it should be computationally infeasible to predict future output given past output

Key generators typically rely on sources of entropy to seed their algorithms, which may be hardware-based (such as electronic noise or timing variations) or software-based.

Symmetric key generators

Symmetric key generators produce a single shared key used for both encryption and decryption. These generators often use pseudorandom number generators (PRNGs) seeded with entropy from various sources. Modern standards such as NIST SP 800-90 specify approved random bit generators for this purpose.

Keystream generators

In stream ciphers, a keystream generator produces a continuous stream of pseudorandom bits that are combined with the plaintext using the XOR operation. The keystream generator takes a relatively short key (typically 80-256 bits) and an initialization vector (IV) and expands them into a much longer keystream.

Common key generator implementations include:

• Linear-feedback shift registers (LFSRs), widely used in hardware implementations due to their simplicity, though typically combined with non-linear functions to improve security
• A5/1, the stream cipher used for GSM mobile phone encryption, based on three irregularly clocked LFSRs
• Trivium, an eSTREAM finalist stream cipher using three interconnected shift registers
• Grain, a lightweight stream cipher using both linear and non-linear feedback shift registers
• Solitaire (or Pontifex) cipher, a manual keystream generator using a deck of playing cards