CAST5 (CAST-128) Encryption/Decryption

CAST5, also known as CAST-128, is a symmetric block cipher designed by Carlisle Adams and Stafford Tavares in 1996. It was published as RFC 2144 and became widely used in the PGP (Pretty Good Privacy) encryption software.

CAST5 uses a 64-bit block size and supports variable key lengths from 40 to 128 bits. The algorithm consists of 12 or 16 rounds depending on the key size. CAST5 was designed to be resistant to differential and linear cryptanalysis.

Key Features

• Variable key length: 40-128 bits (5-16 bytes)
• 64-bit block size for efficient processing
• 12 rounds for keys ≤80 bits, 16 rounds for larger keys
• Royalty-free algorithm with no patent restrictions

Encryption Modes

• CBC: Cipher Block Chaining - Each block is XORed with the previous ciphertext block before encryption. Requires IV. Recommended for most use cases.
• ECB: Electronic Codebook - Each block is encrypted independently. No IV required. Not recommended for encrypting large amounts of data due to pattern leakage.
• CTR: Counter - Converts block cipher to stream cipher, allows parallel processing
• CFB: Cipher Feedback - Stream cipher mode, no padding required
• OFB: Output Feedback - Stream cipher mode, error propagation resistant
• RAW: Single Block - Direct encryption without chaining, for single 8-byte block only

Security Considerations

• Use 128-bit keys for maximum security
• Always use CBC mode with unique IVs for each encryption
• Generate cryptographically secure random keys and IVs
• For highly sensitive data, consider modern alternatives like AES-256

Common Use Cases

• PGP/OpenPGP email encryption compatibility
• Legacy system interoperability
• File encryption for moderate security requirements
• VPN and secure communication protocols

References

• RFC 2144 - The CAST-128 Encryption Algorithm
• Wikipedia - CAST-128