[openssl-users] OpenSSL - asn1parse how to use offset-hlentgth-length

findor paliopipis at gmail.com
Mon Nov 30 20:02:49 UTC 2015

Hi there,
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.



Next question:
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

Can anyone help?
Thanks in advance 

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