Loading a PKCS#1 public key using M2Crypto

[Marc Aymerich]

On Thursday, January 17, 2013 5:39:57 PM UTC+1, Piet van Oostrum wrote:

> > Converting to X.501 isn't difficult (assuming this is a 2048 bit key):
> 
> > Get rid of the 'RSA' in header and trailer
> 
> > Prepend X.501 header 'MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8A' to the data
> 
> > Reformat the lines to 64 characters.
> 
> 
> 
> This solution is a bit restricted as it only works if the key is 2048
> 
> bits and uses an exponent of 65537 (which is the default). Otherwise it
> 
> fails.
> 
> 
> 
> Here is a robust solution that works for all PKCS#1 keys. Instead of
> 
> using a fixed X.501 header it calculates the header. We could do a
> 
> complete ASN.1 encoding, but most of the parts are fixed. The only
> 
> variable parts are two length fields. So I just plug these into the
> 
> fixed stuff. This saves using one of the ASN.1 libraries. We do have to
> 
> work in binary (DER format) instead of base64, however.
> 

Thank you very much Piet, 
I'm just starting to grasp these cryptography related concepts and your code is helping me a lot to understand how to handle these keys in a low level.

I'm updating my code incorporating your new contribution!

Just to let you know, during my previous research I had found a python-Crypto related solution that also uses DER and ASN.1 [1], but it uses a different approach (I guess). I suspect that this approach is also possible with M2Crypto because it has a method for constructing RSA keys [2]. 

[1] http://stackoverflow.com/a/10574723
[2] http://www.heikkitoivonen.net/m2crypto/api/M2Crypto.RSA-module.html#new_pub_key


Thanks again!
Marc

PS: Sorry for my email format, I'm using google groups and it seems to ignore any mailing best practice.