OpenSSL - asn1parse how to use offset-hlentgth-length
I have this project on MD5 collision that I'm working on and I have a problem understanding something. I have a certificate in DER encoding. I need to "export" from it the header, the prefix, the public modulus etc., using xxd. I can't understand how exactly I have to read the data an1parse (or dumpasn1) gives me, so I can put the right values for -s and -l to xxd.
For example, for the public modulus, I put in -s the byte where the BIT STRING starts? or where the actual public modulus starts? And that means that the prefix will contain everything until the point where the public modulus starts?
The professor has given us an example of how to do it, but in the part of the xxd commands he has given us only the commands without the structure of the file he used.
In the second image, why is he creating a .pfx2 with 6 bytes more, which are from the public modulus "block"?
(Someone would think he has explained all this, but no he hasn't.)
And last question, if I understand correctly what we want to do here is that the first thing we want to apply the MD5 collision on is the public modulus, but instead he uses .pfx2 on fastcoll.
I've attached the whole file with the commands for better understanding.X.rtf