Horible public key tricks

The author, frustrated by the complexities and perceived fragility of SSL/TLS and its certificate authority (CA) system, embarked on a project to create a "protest certificate." This unique certificate would openly display a manipulated public key, yet remain fully trusted and functional, challenging the assumptions behind CA validation. The goal was to demonstrate how one could embed a custom message within a standard RSA public key.

• The core idea involves embedding a message into the RSA modulus n, which is the product of two large prime numbers, p and q.
• Since the public exponent e is usually fixed (e.g., 65537), n provides the "user-controlled" bits for manipulation.
• The technique starts by generating n from random p and q, then directly modifying bytes within n's base64 representation to insert the desired message.
• To maintain cryptographic validity, the q value is recalculated as n/p, and then rounded up to the next prime, ensuring the modified n can still be factored.
• This adjusted p and q pair allows for the creation of a functional private key.
• The resulting key pair is then used to generate a Certificate Signing Request (CSR), which a standard CA (like Let's Encrypt) is shown to sign successfully via ACME challenges.
• While acknowledging a reduction in key entropy and potential risks like p and q being too close, the author suggests the key should remain secure enough for public websites due to modern TLS's forward secrecy.
• The method offers space for approximately 150-320 characters in RSA-2048/4096 keys for embedding messages.

This project ingeniously highlights both the flexibility and potential blind spots within the current SSL/TLS certificate issuance process. It serves as a fascinating technical demonstration of cryptographic manipulation, subtly critiquing the practical checks CAs perform, while still providing a functional, trusted certificate.