I have a sample private key uploaded at omnipotente.com/private.key,
and the public key at omnipotente.com/public.cer

The password for the private key is Empresa1.

I need to use a private key in the same format as the one I uploaded
to sign some data.

So far I've tried OpenSSL (cygwin), PHP's OpenSSL, Ruby's RSA lib, and
even CryptoSys PKI Toolkit. None can decrypt the damn key (or I'm not
using them properly).

Help me find a library that can use my key to sign data. The prefered
solutions are listed in order:

1. Ruby
2. PHP
3. VB.NET/C#
4. Python/Perl
5. ActiveX (COM)/DLL library
6. Command line utility
7. *You* create a Java program I can use from the command line, or
provide detailed instructions on how to do it.
x. Anything else you can suggest...

Open source libraries are prefered, commercial ones will be considered
only if no open source is available, and not too expensive. If you
give me a commercial solution, comment first so I decide if it's a
valid answer.

Also, the platform I'll use it can be either Windows or Linux, so a
solution that works in both environments is prefered, although this is
not a requirement.

If you don't provide me with a straight valid answer but you do help
me get this done, I'll open another question with less value for you
to take.

Additional info
------------------
The government (Mexico) gave me the key, and they have a document specifying
how to sign data. Unfortunately, they don't provide any info on what
to do it with, other than mention the algorithms:

RSAPrivateEncrypt and RSAPublicDecrypt.

They go on to cite several web sites:

www.rsa.com/rsalabs/faq/3-6-6.html
www.ietf.org/rfc/rfc1321.txt
www.dcs.uky.edu/~calvert/old-classes/645/assign4.ps
web.mit.edu/crypto/arch/sgi_62/sbin/sshd

They also mention RSAwithMD5Encryption, but that's not really
necessary because an MD5 is easily generated in almost any language.

Thanks!