The **GET_KEYLENGTH** case simply returns the length of the key stored
on the card. This version of the applet only accepts 1024 bit keys but could
be extended to accept other lengths.

The **GET_PUBKEY** case returns the public key stored on the card.
This version of the applet does not use the public key for anything and
just stores it in an ordinary file, named "sh".

The **DECRYPT** case is used by ssh to decrypt the challenge using the
private key stored on the card. The key itself is never revealed.