Re: Converting SSH2-RSA key to RSA numbers

From: Unruh (unruh-spam_at_physics.ubc.ca)
Date: 04/02/05 

Date: 2 Apr 2005 17:16:40 GMT

"Dro Kulix" <peter.s.may@gmail.com> writes:

>I am working on a lightweight program that will use a simplistic
>version of RSA for short encrypted transmissions. Of course it's a
>dangerous thing to do when I'm not a crypto expert, but for the moment
>that's beside the point. What I have in hand is a basic RSA
>implementation that's already been written, which encrypts using an
>encryption exponent E and a modulus M, and decrypts using a decryption
>exponent D and the same modulus M. I assume, then, than E and M make
>up the private key, D and M make up the public key.

Uh, no, unless you want to be the only one who can encrypt and anyone in
the world can decrypt your messages. Usually people want anyone to be able
to encrypt and only you decrypt.

>Anyway, while I do have the encryption/decryption code, I don't have a
>key generator to go with it. I decided that this wasn't going to be a
>problem because there are plenty of good programs that will generate an
>(unencrypted) OpenSSH SSH2 RSA key, and that I'd only have to find out
>how to decode that format.

>I've downloaded PuTTY's source to see if I could figure it out from
>that. Turns out it's not trivial, so before I actually attempt to do
>all the code chasing necessary to figure it out, I wanted to know if
>anyone already knew enough about the key file format to save me some
>trouble. Specifically, I'm looking to write a routine that converts
>the key in an OpenSSH SSH2 RSA private key file into the aforementioned
>E, D, and M.

Look at teh source code for ssh-keygen. It will have all of the details.
That is the key generation program from openssh.

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