RC4 Cipher

The RC4 Cipher (Rivest Cipher 4) is a symmetric stream cipher designed to encrypt data by generating a pseudorandom keystream that is combined with plaintext using bitwise XOR operations. Unlike block ciphers such as AES or DES, RC4 operates on data one byte at a time, making it fast and simple to implement. It was widely used in protocols like SSL/TLS and WEP, though it has since fallen out of favor due to discovered vulnerabilities.

RC4 relies entirely on a secret key to initialize its internal state. From this key, a keystream is generated and XORed with the plaintext to produce ciphertext. Because it is a stream cipher, the same process is used for both encoding and decoding. This tool supports output formatting in Base64 or Hexadecimal for readability and transport.

RC4 Cipher: Encoding Example

With the following configuration:

• Key: d42b710d4f2c7fe50ec39e30325c9ba4
• Format: Base64

Encoding the plaintext hello world produces:

Plaintext: hello world
Key:       d42b710d4f2c7fe50ec39e30325c9ba4

Ciphertext (Base64):
U2FsdGVkX1+LPzkDMoQQrbhzkT5tlek2Jtxi

The RC4 algorithm generates a keystream based on the provided key and combines it with the plaintext using XOR operations. The resulting binary data is then encoded into Base64 for display.

RC4 Cipher: Decoding Example

Using the same key, the ciphertext can be reversed back into the original plaintext:

Ciphertext: U2FsdGVkX1+LPzkDMoQQrbhzkT5tlek2Jtxi
Key:        d42b710d4f2c7fe50ec39e30325c9ba4

Plaintext:
hello world

Since RC4 is a stream cipher, decoding is performed by applying the same keystream generation and XOR process. The key must match exactly for successful decryption.

RC4 Cipher: Usage Notes

• Symmetric Operation: RC4 uses the same function for encoding and decoding. The distinction depends only on whether plaintext or ciphertext is provided.
• Key Sensitivity: Even a small change in the key produces a completely different keystream and output.
• No IV in Basic Form: This implementation does not explicitly expose an initialization vector; security depends entirely on the key.
• Output Format: Base64 is commonly used for transport, while Hex shows raw byte values.
• Security Consideration: RC4 is considered deprecated for modern cryptographic use due to biases in its keystream. It should only be used for educational or legacy compatibility purposes.

Despite its historical significance and simplicity, RC4 has largely been replaced by more secure algorithms like AES. However, it remains a useful example of how stream ciphers operate and how keystream-based encryption differs from block-based systems.