RC4 is a simple yet clever algorithm that once ruled the cryptographic world—especially in the realm of internet traffic and wireless networks. Here’s how it works and why it matters:

🧠 What Is RC4?

RC4 (Rivest Cipher 4) is a stream cipher designed by Ronald Rivest in 1987. Unlike block ciphers (like AES or DES), RC4 encrypts data one byte at a time, making it lightweight and fast.

⚙️ Key Features

• Stream Cipher: Encrypts data as a stream of bytes rather than in fixed-size blocks.
• Variable Key Length: Typically between 40 and 2048 bits.
• Simplicity: Easy to implement and very fast in both hardware and software.
• Used In: SSL/TLS (older versions), WEP, WPA (early versions), and some VPN systems.

🔄 How It Works (Simplified)

RC4’s process happens in two phases:

1. Key Scheduling Algorithm (KSA)

• Initializes a state array of 256 bytes (usually called S).
• Mixes the array using the key to create a pseudorandom permutation of bytes.

2. Pseudo-Random Generation Algorithm (PRGA)

• Generates a stream of pseudorandom bytes by shuffling elements in the state array.
• Each byte of plaintext is XORed with a byte from this stream to produce ciphertext.

⚠️ Security Concerns

RC4 used to be popular, but cryptographers found weaknesses:

• Biases in Output: The initial bytes of the stream are statistically non-random.
• Predictable Patterns: These biases made it easier for attackers to guess encrypted data.
• Deprecated: Most modern systems no longer use RC4; it’s officially prohibited in TLS protocols due to vulnerabilities.

🔐 Legacy & Impact

Though it’s no longer considered secure, RC4 laid the groundwork for understanding stream ciphers. It showed how simplicity could offer speed, but also how small flaws can lead to major vulnerabilities over time.