The Pursuit of HappYness

[background]
 To encipher small units of data, such as characters or bits, sevearl stream ciphers have been
designed from scratch. These stream ciphers are more efficient for real-time processing.

[RC4]
- used in many data communication and networking protocol.
1. variable key size, byte-oriented stream cipher
2. step
 - Initialization
 - Key Stream Generation
 - Encryption or Decryption
3. Security Issues
 - Like many other ciphers, it is recommended the different keys be used for different sessions.

[A5/1]
- used in GSM, a network for mobile telephone communication
1. Key generator
 - uses three LFSR(Linear Feedback Shift Register)s with 19,22, and 23bits

2. Initialization 
 => 64-bit의 secret key 와 22-bit의 frame number를 이용한다.
 <순서>
 1. set all bits in three LFSRs to 0
 2. leftmost bit (XOR) K[0...63] - 64번 반복
 3. clocking 한다
  (clocking means that each LFSR goes through one shifting process)
 4. leftmost bit (XOR) FrameNumber[0...21] - 22번 반복
 5. clocking 한다
 6. 100 cycles 동안, 어떤 LFSR 이 clocking 되어야 하는지 체크한다.

3. Majority Function
 - Initialization 한 값에서 LFSR의 값을 살펴본다.
 - if Majority(1,0,1) = 1 -> LFSR1, LFSR3 are shifted, but LFSR2 is not

[Summary]
* RC/4 is a byte-oriented stream cipher in which a byte(8bits) of a plaintext is exclusive-ored
with a byte of a key to produce a byte of a ciphertext. The secret key, from which the one-byte
keys in the key stream are generated, can contain anywhere from 1 to 256 bytes. The key stream
generator is based on the permutaton of a state of 256 bytes.

* A5/1 is a strea cipher used in mobilie telephone communication. A5/1 creates a bit stream out
of a 64-bit key using three LFSRs.