Security programming in J2EE 6.20

Hi

I'm looking for Java code snippets or developer documentation on how to do the following things inside the SAP J2EE 6.20

1) Get a certificate out of the J2EE keystore

2) Decrypt a specific String with the public key of the certificate. The String was encryptet with the corresponding private key before.

3) Calculate a hash value (MD5 od SHA-1) for a specific String

Thanks

Raphael