Horible Public Key Tricks

The piece presents a protest against the complexity of TLS certificates while still accepting the need to encrypt web traffic. Rather than use an obviously weak private key, the author finds a way to make a trusted certificate’s public key carry a readable message. RSA’s modulus is the product of two primes, and its bytes appear in the certificate’s base64 text. The method begins with a normal RSA key, alters the upper part of the modulus to encode text, then adjusts one prime so the result is again a product of two primes. The author uses the resulting private key to request a certificate from a certificate authority, which validates the domain but does not inspect the embedded message.

The example certificate contains a joke about weak certificate-authority keys, and the author explains how to place text so it survives base64’s encoding boundaries. RSA’s standard public exponent remains unchanged, while the modulus offers roughly 150 characters in a 2,048-bit key or 320 in a 4,096-bit key. The technique reduces the key’s randomness; adjusting the primes can also make them dangerously close, and a small private exponent could create another weakness. The generator does not check for these problems. Still, the author argues that the key can be adequate for a public website because TLS forward secrecy means later discovery of the certificate’s private key does not, by itself, expose previously recorded sessions.