RC6 is a powerful and flexible encryption algorithm that builds on its predecessor RC5, with some clever upgrades. Let’s break it down:

🔐 What Is RC6?

RC6 (Rivest Cipher 6) is a symmetric-key block cipher developed in 1998 by Ron Rivest and colleagues. It was designed as a candidate for the Advanced Encryption Standard (AES) competition and is known for its speed, simplicity, and strong security features.

⚙️ Key Features

• Block Size: 128 bits
• Key Sizes: 128, 192, or 256 bits (can be extended up to 2040 bits)
• Rounds: Typically 20 rounds of encryption
• Structure: Feistel-like network using:
  • Modular addition
  • Bitwise XOR
  • Data-dependent rotations
  • Integer multiplication (adds complexity and diffusion)

🔄 How RC6 Works (Simplified)

1. Key Expansion: The user’s key is expanded into a set of subkeys.
2. Initialization: Two of the four 32-bit registers are modified using the first subkeys.
3. Rounds:
   • Each round mixes the four registers using multiplication, addition, XOR, and rotations.
   • The registers are rotated cyclically after each round.
4. Finalization: Two registers are modified again using the final subkeys.

RC6 can be visualized as a turbocharged version of RC5, with four working registers instead of two and added multiplication for better diffusion.

🛡️ Strengths

• Highly customizable: You can tweak block size, key length, and number of rounds.
• Fast and efficient: Performs well on both hardware and software.
• Strong security: Resistant to known attacks when configured properly.

⚠️ Considerations

• Not selected as AES: Despite being a finalist, RC6 wasn’t chosen—largely due to its use of multiplication, which complicates hardware implementation.
• Patent concerns: RC6 was patented, which may have limited its adoption.