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Stream Cipher

Stream cipher is a symmetric encryption process that encrypts each bit in the plaintext message one bit at a time, but uses a pseudorandom string (keystream) of cipher digits to encrypt the input plaintext with.

  • can make a block cipher act as a stream cipher by setting the block size to one bit
  • better for:
    • encrypting data of unknown size
    • or data moving in a continuous stream
  • faster than block ciphers
  • typically use simple operations to encrypt each bit or byte
    • e.g., exclusive-or (XOR ⊕)
      • is true if either input is true, but not both
  • inspired by the one-time pad cipher
  • a stream cipher converts an encryption key into a keystream
    • a keystream is a continuous bit stream
  • using a stream cipher to protect any potentially unlimited-length plaintext requires that the keystream be truly random across its entire use with that plaintext
    • makes it somewhat impractical to exchange and keep the keystreams secret
    • instead, special hardware or software keystream generators take a fixed-length key, typically 128-bits, and use it as an input, along with other cryptovariables, to continuously generate the keystream
      • is a function that uses a seed value (encryption key itself) as input, producing encryption values to be combined with each bit or byte of the input plaintext
      • most keystream generators produce pseudorandom keystreams
      • thus, is possible to become insecure with use
    • key distribution and management becomes the most important element for preserving security
  • can work on any length of input plaintext
  • found widespread use in mobile communications systems in which the plaintext input is often of unbounded length and is bursty in nature
    • produced in sets of data of unpredictable size separated by unpredictable short or long periods of no signal on the link
    • these characteristics meant block ciphers were ill-suited
  • practical stream ciphers use a fixed-length key to encrypt messages of variable length
    • some ciphers can be broken if attacker possesses enough ciphertexts encrypted with the same key
    • changing the key periodically can increase security

Types of Stream Ciphers

Synchronous

Synchronous ciphers require the sender and receiver to remain in perfect synchronization in order to decrypt the stream.

  • if bits are added or dropped from the stream, the decryption will fail from that point on
  • receiver needs to be able to detect the loss of synchronization and either:
    • try various offsets to resync
    • or wait for distinctive marker inserted by the sender to enable the receiver to resync
  • a single-character error in the ciphertext will result in only a single-character error in the decrypted plaintext

Self-Synchronizing

Self-synchronizing ciphers have the property that after at most characters (being a property of the cipher), the receiver will automatically recover from dropped or added characters in the ciphertext stream.

  • advantage in situations which data can be dropped or added to the ciphertext stream
  • suffer from the problem that should a character be corrupted, the error will propagate, affecting up to the next characters

Usage

  • strengths and weaknesses derive from how you use it
    • impractical to have a keystream that never repeats
    • may define a key repeat period in bits
      • depending on your operational security needs
  • key to operation of stream ciphers is their use of an initialization vector (IV)
    • is used along with the encryption key to start up the keystream generator function in some algorithms
  • RC4 does not use an IV

How It Works

  • stream cipher takes a key of bits in length and stretches it into a long keystream
    • keystream is a string of bits used like a one-time pad
  • this keystream is then XORed with the plaintext to product ciphertext
  • same keystream is used to recover from
  • use of keystream is identical to that of the pad in a one-time pad
  • operation of a stream cipher:
    • is the key
    • is the resulting keystream
  • Given keystream , and plaintext
    • we generate the ciphertext by XORing the corresponding bits:
    • to decrypt , the keystream is used again:

Instances of Stream Ciphers

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