What Is RC4 Algorithm & How Its Work ?

The Rivest Cipher (RC) algorithms are a set of symmetric-key cryptography algorithms created by Ron Rivest. There are total 6 types of Rivest Cipher (RC) algorithms but Rivest Cipher RC4 algorithm is the most popular and widely used encryption among all 6 RC encryption.
RC4: is a stream cipher, symmetric key encryption algorithm. It encrypts messages one byte at a time via an algorithm. RC4 is the most popular among all RC algorithms. It’s simple to apply, and it works quickly, even on very large pieces of data. RC4 algorithm is used by standards such as IEEE 802.11 within WEP and application like TLS/SSL also used RC4 encryption.

Working of RC4 Algorithm

RC4 Algorithm Working

RC4 algorithm is a variable key-size stream cipher using either 64-bit or 128-bit sizes. The cipher uses a permutation and two 8-bit index-pointers to generate the keystream. The permutation itself is done with the Key Scheduling Algorithm (KSA) that then is entered into a Pseudo-Random Generation Algorithm (PRG), which generates a bitstream. Bit stream that the RC4 generates is as long as the plaintext stream. Then through the Exclusive OR (X-OR) operation, the bitstream and the plaintext generate the ciphertext.

When we put a plaintext and an encryption key in RC4 algorithm engine, then it generates keystream bytes with the help of the both Key Scheduling Algorithm (KSA) and the Pseudo-Random Generation Algorithm. The X-OR operation is executed byte-by-byte, and the byte output is the encrypted text.

Variants of RC4 Algorithm

  • Spritz – Spritz is used to create cryptographic hash functions and deterministic random bit generator.
  • RC4A – This is a variant that was proposed to be faster and stronger than the average RC4 cipher. But there were issues with randomization of numbers in the cipher.
  • VMPC – Variably Modified Permutation Composition (VMPC) is a version of RC4. But also struggling with number randomization.
  • RC4A+ — a more detailed, longer and advanced version of RC4, and RC4A with a three-phase key schedule, and has proven to be more secure.

Other Types of Rivest Cipher

Rivest Cipher 1 (RC1)

The main idea of RC1 algorithm was to design a Symmetric Key cryptography algorithm that could be used by the users to protect their data as it passes through the network. But it was never published. Because different variants were designed and continuous research has been carried out by the researchers.

Rivest Cipher 2 (RC2)

RC2 was a 64-bit block cipher introduced in 1987. It was considered as a proposal for the DES replacement. The input and output block size of this algorithm was 64-bit each with variable key size from 1 byte to 128 bytes.

Rivest Cipher 3 (RC3)

RC3 was broken before ever being used. RC3 was broken before ever being used. When the RC3 algorithm was being developed at RSA security, it was broken at the same time. Hence, it was not used.

Rivest Cipher 5 (RC5)

RC5 is a block cipher algorithm based on the symmetric key, designed by Ronald Rivest for RSA Data in 1994. In RC5 the size of plain text block can be of 32 bits, 64 bits or 138 bits and length of the key can be of 0 to 2040 bits. The output generated by RC5 is the cipher text which has the size the same as plain text size. The main features of RC5 are, it is quite fast as it uses only primitive computer operations and it requires less memory for execution.

Rivest Cipher 6 (RC6)

It is a block cipher which uses 128 bit block size and supports key sizes of 128, 192 and 256 bits. RC6 was developed for an advanced encryption standard (AES) competition but was not selected by NIST. RC6 is an improvement of the RC5 Algorithm. It provides even better security against attacks which may be possible in the RC5 Algorithm. It uses 4 register which make it faster than its previous version.

Comparison between RC Algorithms

Block size64 bits2064 bits32, 64,128 bits128, 256 bits
Key size8-128 bits1-256 bits0-2048 bits128,192,256 bits
Rounds16 rounds256 rounds0-255 rounds20 recommended
Possible AttacksDifferential, LinearBeastDifferentialCorrelation

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