Crypto U4, Stream Cipher

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Encryption:

Uses a key that is usually shorter than the plaintext.

Keystream generator takes key and uses a mathematical process to generate a pseudorandom stream of bits called the keystream.

Keystream bits are XOR'd with the plaintext bits to give a resulting ciphertext.

Keystream generators are deterministic generators.

Decryption:

The receiver XOR's the ciphertext bits with the plaintext stream bits.

Key Management

Compared to OTP, the key is easier to manage.

The keystream is pseudorandomly generated which is simpler than OTP (where the key is randomly generated).

You should never reuse the same bits of a keystream.

To avoid this, you can use different parts of a keystream to encrypt a message (reusing the same key).

tend to be adopted in high security apps in closed networking environments

Properties (positive)

no error propagation

ideal where communication has to go over channels of poor quality

fast because built on XOR

on-the-fly-encryption - data spends the least amount of time as plaintext

Properties (negative)

need for synchronization between sender and receiver

need procedures for resynchronization: time based, based on special communication,

many are proprietary

less versatile than block ciphers... only used for encryption

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