Parsing an X509 Certificate

Question

I currently need to parse the CommonName from a packet. I have code that works up to a point, however am having trouble skipping over the "issuer" member of a Certificate record for TLSv1.2. I have done research on the format of the SSL records and have investigated the dump via wireshark. I've found the format is generally - Length, followed by the data. However when trying to find the issue length, I cannot seem to get it, and is inconsistent with the bytes presented. Any ideas..or a better way to skip over the issuer field, and go directly to the "subject" of an TLS 1.2 record. Coded in C..Thank you for useful responses.

Answer 1

You need to understand ASN.1. Go read this book (it is a free download). Once you have read and understood it, you can write your decoder, following the ASN.1 specification for certificates . This is doable, but requires great implementation care. In fact, this is a bad idea unless you are a demi-god of C programming.

Alternatively, use some library that already knows how to decode a certificate. Typically, OpenSSL .