CAST6, also known as CAST-256, is a symmetric key block cipher algorithm designed for encryption and decryption of data. And considered a strong encryption algorithm, offering robust security against various cryptographic attacks. Its strength primarily depends on the key size chosen during encryption, with larger key sizes providing higher levels of security.

History:

• was developed by Carlisle Adams and Stafford Tavares at Entrust, Inc., and was first published in 1998.

• It was designed as an evolution of the CAST-128 cipher, addressing some of its security concerns and supporting larger key and block sizes.

• is based on Feistel network structure and employs both substitution-permutation network (SPN) and Feistel network techniques.

Features:

• Key and Block Sizes: supports key sizes ranging from 128 to 256 bits, with a fixed block size of 128 bits.

• Rounds: The number of rounds in CAST6 varies depending on the key size, with 12 rounds for 128-bit keys, 16 rounds for 192-bit keys, and 20 rounds for 256-bit keys.

• Security: CAST6 is considered to offer strong security and resistance against various cryptographic attacks, including differential and linear cryptanalysis.

• Performance: It is known for its relatively high performance in terms of both encryption and decryption speed.

Operation:

• Encryption: In CAST6 encryption, the input block is divided into two halves, and each half undergoes a series of rounds of mixing, permutation, and substitution operations using the key schedule derived from the encryption key. The result of each round is then XORed with the other half of the block, and the process is repeated for the specified number of rounds.

• Decryption: Decryption in CAST6 is essentially the same as encryption but in reverse order. The input block undergoes the same series of rounds, but the round keys are used in reverse order to recover the original plaintext.

Replacement:

• is a robust and widely used encryption algorithm. However, with the advancement of cryptographic techniques and the emergence of new standards, it has been largely superseded by more modern symmetric encryption algorithms such as AES (Advanced Encryption Standard). AES is a widely adopted and standardized encryption algorithm chosen by the U.S. National Institute of Standards and Technology (NIST) as the successor to DES (Data Encryption Standard).

Key Size:

• supports key sizes ranging from 128 to 256 bits. Generally, larger key sizes result in stronger encryption, as they increase the number of possible keys an attacker would need to try in a brute-force attack to successfully decrypt the data.

Resistance to Cryptanalysis:

• has been designed to resist known cryptographic attacks, such as differential and linear cryptanalysis. These attacks attempt to exploit patterns and statistical properties of the encryption algorithm to recover the plaintext or the secret key. CAST6's design aims to thwart such attacks, making it highly resilient.

Key Handling:

• employs a key schedule derived from the encryption key to generate round keys used during encryption and decryption. The key schedule ensures that each round of encryption uses a unique subkey, enhancing security. Additionally, CAST6 follows a Feistel network structure, which further strengthens its resistance to cryptanalysis.

Brute-Force Attacks:

• involve systematically trying all possible keys until the correct one is found. The feasibility of a brute-force attack on CAST6 depends on the key size. For example, with a 128-bit key, there are 2^128 possible keys, making exhaustive search attacks computationally infeasible with current technology. As the key size increases, the difficulty of brute-force attacks grows exponentially.

CAST6 and AES-256 are both symmetric encryption algorithms designed to secure data by transforming it into an unreadable format that can only be decrypted with the correct key. While they share this common goal, there are several differences between them:

1. Algorithm Structure:

   • AES (Advanced Encryption Standard) is a block cipher with a fixed block size of 128 bits.

   • CAST6 (or CAST-256) is also a block cipher but supports variable block sizes of 64 to 256 bits.

2. Key Sizes:

   • AES supports key sizes of 128, 192, and 256 bits. AES-256 specifically uses a 256-bit key.

   • CAST6 supports key sizes ranging from 128 to 256 bits.

3. Round Structure:

   • AES operates through multiple rounds of encryption, with the number of rounds depending on the key size (10 rounds for AES-128, 12 rounds for AES-192, and 14 rounds for AES-256).

   • CAST6 also operates through multiple rounds, typically 48 rounds regardless of the key size.

4. Security Analysis:

   • AES has undergone extensive cryptanalysis and is widely regarded as secure. It has withstood many years of scrutiny and remains a trusted encryption standard.

   • CAST6 has also been analyzed for security and has shown resilience against known attacks. However, it has received less attention and scrutiny compared to AES.

5. Adoption and Standardization:

   • AES is the standard encryption algorithm adopted by governments and industries worldwide. It's widely supported by hardware and software implementations.

   • CAST6 has seen limited adoption and is not as widely standardized or implemented as AES.

Code overview:

// Payload {msf, cobalt,,,, you name it}
byte[] buf = new byte[6] {0xfc,0x48,0x83,0xe4,0xf0,0xe8};

// Your Encryption Key

string keyString = "Ir0nHu1k0xFF!";

// Ensure key size

int keySize = Math.Max(10, Math.Min(32, keyString.Length));

// Convert key string to byte array and ensure the key size is 32 bytes

byte[] key = new byte[32];

Array.Copy(Encoding.UTF8.GetBytes(keyString), key, Math.Min(keySize, keyString.Length));

// Initialize CAST6 encryption algorithm

Cast6Engine engine = new Cast6Engine();

KeyParameter keyParam = new KeyParameter(key);

ParametersWithIV keyParamWithIV = new ParametersWithIV(keyParam, new byte[engine.GetBlockSize()]);

BufferedBlockCipher cipher = new PaddedBufferedBlockCipher(new CbcBlockCipher(engine));

cipher.Init(true, keyParamWithIV);

// Encrypt the data

byte[] encrypted = EncryptData(cipher, buf);