RC4

RC4, or Rivest Cipher 4, is a symmetric stream cipher designed by Ron Rivest for RSA Data Security in 1987. It generates a stream of pseudorandom bytes that are XORed with plaintext to produce ciphertext. Its simplicity and speed in software contributed to widespread use in many protocols.

RC4 uses a variable-length key from 1 to 256 bytes to initialize a 256-byte state array S, a permutation of 0–255. The key scheduling algorithm (KSA) shuffles S using the key; the pseudo-random generation algorithm (PRGA) then iterates with two indices, producing a keystream byte for each input byte.

During PRGA, i and j are updated, S is swapped, and the output byte K is taken as S[(S[i] + S[j]) mod 256]. The plaintext byte P is encrypted as C = P XOR K.

RC4 has known weaknesses: early keystream biases and key-dependent patterns. Reusing the same key across sessions or improper protocol usage can enable attacks. These weaknesses have led to practical compromises of confidentiality in some deployments, prompting caution in modern use.

RC4 was widely deployed in SSL/TLS and wireless security in earlier decades, but analyses uncovered vulnerabilities and standards gradually deprecated it. By the mid-2010s, major browsers and security guidelines advised against RC4 in favor of stronger ciphers.

RC4 is now considered obsolete for new designs. Its biases and the availability of secure alternatives such as AES-GCM and ChaCha20-Poly1305 have led to broad recommendations to avoid RC4.