Format of exported private keys

[ab1234cd5678 at gmx.biz]

Hi,

I plan to produce a webmailer that does use my private key in a
Javascript implementation.

To achieve this I came up with the idea to have a textarea field on
the form where I can paste my exported secret key and use.

Unfortunately I can't find any information about the structure of the
exported key. So I know it is armored. It contains a checksum as well.
But how is the checksum calculated, how do I distinguish between PGP
keys and X.509 keys?

Could anyone give me a pointer?

-- 
Thanks in advance,
Raymund