ASN.1 and X.509 certificate parser

Keys and certificates are ASN.1 structures encoded in DER, usually wrapped in Base64 with PEM headers. When a challenge hands you a .pem file, parsing it is how you extract the modulus and exponent that the RSA attack actually needs.

The encoding is deliberately self-describing: every element carries a tag saying what it is and a length saying how far it runs, so a parser can walk the whole structure without knowing the schema. That means you can pull an integer out of an unfamiliar key format without first finding a specification for it.

From PEM to parameters

• Strip the -----BEGIN----- and -----END----- lines and Base64 decode the middle to get DER.
• Walk the DER tree: a public key is a SEQUENCE containing the algorithm identifier and a BIT STRING holding another SEQUENCE of two INTEGERs - the modulus n and exponent e.
• A private key adds d, p, q, and the CRT coefficients in the same structure.
• Feed n and e to the RSA analyzer and check whether the modulus is small, or the exponent unusual.