Doing it the hard way requires roughly 1.5 times key length number of modular multiplies (assuming about half the bits are ones and half zeroes) so if the shortcutted public key operation takes 17 units of time the non-shortcutted private key operation takes about 1500 (assuming a 1000 bit key).
Does this also apply to the old style keys or only in case of CRT type keys? Because, then, in any case I will have that problem when using the public key.
Also consider: what happens in the future when you want to move to a 2048 or 4096 bit key? Do you have to wait for a more capable Java card to be marketed?
The JavaCard supports RSA 2048 bits, and as of version 2.2 there is support of ECC up to 192 bits, but only for signature.
I have a 2.1 card, but it is my intention not to predefine specific key types or lengths, these are chosen when the key is generated, such that newer card will support the new algorithms.
The main problem as I see it is that for things to work, the input buffer must grow as longer keys are used or I must support sessions.
Watch out for elliptic curve because a "message" usually takes sending TWO group elements, instead of just one as for RSA. This makes the message length twice as long as you otherwise might imagine. If you're just coding a session key for the real data this increase is minimal, but for the kind of embedded computation you're thinking of this can be a real gotcha.
I have only been introduced to ECC, twice as long encrypted output is ok, the problem seems to be with decryption, then I might run out of space. Thanks for the info!
Cheers, Erik
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