Block ciphers process input in fixed-size blocks, producing an output block for each input. In contrast, stream ciphers handle data sequentially, processing input elements continuously and generating output one element at a time. While block ciphers are widely used, there are specific applications where stream ciphers are more suitable, which are discussed further in the text. Here, we focus on the popular symmetric stream cipher, RC4, beginning with an examination of stream cipher structure.
Stream Cipher Structure
A typical stream cipher encrypts data one byte at a time, though it can operate on bits or larger units. The structure involves inputting a key into a pseudorandom bit generator that creates a seemingly random stream of 8-bit numbers. This pseudorandom stream, unpredictable without the key, is combined with the plaintext stream using bitwise XOR operations. For example, XORing a generator byte `01101100` with a plaintext byte `11001100` results in the ciphertext byte `10100000`. Decryption uses the same pseudorandom sequence:
- Plaintext 11001100
- Keystream 01101100
- Ciphertext 10100000
Key design considerations for stream ciphers include having a long encryption sequence period to complicate cryptanalysis, ensuring the keystream closely resembles a true random number stream, and using a sufficiently long key to prevent brute-force attacks.
Advantages and Applications of Stream Ciphers
Properly designed, a stream cipher can be as secure as a block cipher of the same key length. Stream ciphers are typically faster and require less code than block ciphers. However, using the same key for different plaintexts with a stream cipher can lead to simple cryptanalysis. XORing two ciphertext streams encrypted under the same key reveals the XOR of their plaintexts, which can be analyzed if the plaintext characteristics are known.
Stream ciphers are ideal for encrypting continuous data streams, such as those over a communication channel. In contrast, block ciphers are better for applications involving data blocks, like file transfers or email. Nonetheless, either cipher can be adaptable across various applications.
The RC4 Algorithm
Developed by Ron Rivest in 1987 for RSA Security, RC4 is a widely-used stream cipher based on random permutation and supports variable key sizes with byte-oriented operations. Its period is generally longer than 10^100, ensuring rapid software execution through minimal machine operations per byte. RC4's applications include SSL/TLS standards for secure web communication and wireless protocols like WEP and WPA.
RC4 initializes with a state vector S, a permutation of numbers 0-255. A byte `k` is generated from S for encryption/decryption, continually permuting S entries. Initialization involves setting S values from 0 to 255 and creating a temporary vector T, filled by repeating the key K as needed. The initial permutation involves swapping S entries guided by T values.
Stream Generation and Security
Stream generation, after initializing S, involves cyclic manipulation of S entries to produce a keystream `k` to XOR with plaintext or ciphertext. Despite multiple analyses, RC4 remains secure with sufficiently long keys. The WEP protocol, however, demonstrated vulnerability to attacks due to inadequate key generation, not RC4's intrinsic design. This highlights the importance of designing secure systems integrating cryptographic functions and their protocols.
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