RC4

RC4 is a stream cipher designed by Ron Rivest in 1987, notable for its simplicity, speed, and historical ubiquity. It was a proprietary trade secret of RSA Security until 1994, when its source code was anonymously posted to the Cypherpunks mailing list. Despite its widespread deployment — in WEP, TLS, and countless applications — RC4 was never formally published or peer-reviewed, and its security was assumed rather than demonstrated. That assumption proved false.

RC4 operates by maintaining a 256-byte state array and two pointers, generating keystream bytes through a permutation update that is simple enough to fit in a few lines of code. This minimalism made it attractive for constrained environments and fast software implementations. But the same simplicity that enabled performance also enabled analysis. Researchers discovered persistent statistical biases in RC4's output: the first few bytes are non-random in predictable ways, and the second byte has a bias toward zero with probability roughly 1/128 instead of the expected 1/256.

These biases are not theoretical. In 2013, the RC4 NOMORE attack demonstrated that the same statistical weaknesses could be exploited in practice to decrypt TLS sessions in hours. The attack did not require breaking the key; it required observing enough ciphertexts encrypted under the same key to detect the biases and recover plaintext. This is not a failure of brute-force resistance but a failure of the cipher's randomness: RC4 does not generate a sufficiently unpredictable keystream for modern cryptographic requirements.

The lesson of RC4 is not merely that a cipher can be broken. It is that a cipher can survive for decades in global infrastructure while being fundamentally unsuitable for its purpose. The algorithm was simple, fast, and easy to implement — and these are the qualities that made it dangerous. Complexity in cryptography is often criticized, but RC4 demonstrates that excessive simplicity can be equally hazardous when it leaves the design with no room for error margin. The Bar Mitzvah attack on RC4 exploited its statistical biases in a way that made the weakness practically exploitable at scale, not merely theoretically detectable.