RC Cipher Family Encryption/Decryption

The RC cipher family was designed by Ron Rivest at MIT and RSA Security. 'RC' originally stood for 'Rivest Cipher' or 'Ron's Code'. The family includes RC2, RC4, RC5, and RC6, each with distinct characteristics.

RC2 is a 64-bit block cipher from 1987. RC4 is a stream cipher widely used in SSL/TLS and WEP. RC5 introduced a novel parameterized structure. RC6 was designed as an AES candidate with enhanced diffusion.

RC2 (Rivest Cipher 2)

RC2 is a 64-bit block cipher designed in 1987. It uses a variable-length key (1-128 bytes) and has a unique 'effective key bits' parameter that can limit the key search space.

Originally kept secret, RC2 was designed for export compatibility with 40-bit effective key length. Due to known vulnerabilities, it should only be used for legacy system compatibility.

RC4 (Rivest Cipher 4)

RC4 is a stream cipher designed in 1987. It generates a pseudo-random stream of bytes (keystream) that is XORed with the plaintext. Known for its simplicity and speed, it was widely used in SSL/TLS, WEP, and WPA protocols.

Due to discovered vulnerabilities (especially in the first bytes of the keystream), RC4 has been deprecated from modern security protocols. The 'RC4-Drop' variant discards the initial keystream bytes to mitigate these issues.

RC5 (Rivest Cipher 5)

RC5, published in 1994, introduced a parameterized design with three variables: word size (w), number of rounds (r), and key length (b). This flexibility allows RC5 to be adapted to different security requirements.

RC5 uses data-dependent rotations as its primary non-linear operation, making it simple yet effective. Common configurations include RC5-32/12/16 (32-bit words, 12 rounds, 16-byte key).

RC6 (Rivest Cipher 6)

RC6 was submitted as an AES candidate in 1998 and was one of the five finalists. It extends RC5 with integer multiplication for better diffusion and uses four working registers instead of two.

With a fixed 128-bit block size and support for 128/192/256-bit keys, RC6 provides excellent security. Although Rijndael was selected as AES, RC6 remains a respected and secure algorithm.

Key Features

• RC2: Legacy block cipher with variable key length, for compatibility purposes only
• RC4: Extremely fast stream cipher, ideal for real-time applications
• RC5: Parameterized design allows flexible security/performance tradeoffs
• RC6: Modern block cipher with strong security guarantees
• All algorithms are patent-free and freely available for any use

Encryption Modes (RC5/RC6)

• CBC: Cipher Block Chaining - Each block is XORed with the previous ciphertext block. Recommended for most use cases.
• ECB: Electronic Codebook - Each block encrypted independently. Not recommended for sensitive data.
• CFB: Cipher Feedback - Converts block cipher to stream cipher. Self-synchronizing.
• OFB: Output Feedback - Generates keystream independently. No error propagation.

Security Considerations

• RC4 should NOT be used for new applications due to known vulnerabilities
• RC4-Drop (discarding first 768-3072 bytes) mitigates some attacks but is not recommended
• RC5 with sufficient rounds (12+) and key length (128+ bits) is considered secure
• RC6 provides excellent security comparable to AES

Use Cases

• Legacy system compatibility and migration
• Educational and research purposes
• Embedded systems with RC5 (simple implementation)
• High-security applications with RC6

References

• Wikipedia - RC2
• Wikipedia - RC4
• Wikipedia - RC5
• Wikipedia - RC6
• RFC 2268 - RC2 Block Cipher
• RFC 2040 - RC5 Specification