# Multos Forums

### Multos Trust Anchor and maximum key size for ECC

Dear all,

I would like to use different curves (domain parameters) from https://safecurves.cr.yp.to/ on Multos Trust Anchor. I succeed to support Curve25519 (256 bit) but I failed for M-383, Curve383187 and M-511.

However the chip is equiped with infineon crypt@2304T I was assumed to be able to reach 521 bit.

Is this a known limitation? Is it possible using API to know the max key size supported? Is there another way to know that?

Best regards,

Dear all,

I also failed to use curves under whose parameter are smaller than 192 bits like secp160r1, secp160k1 (it might be an issue for those two of N length) or secp128r1 or secp112r1…

Is this a known limitation?

Hi Damien,

The prime length range supported is 21 - 66 bytes inclusive. That rules out the smaller curves you tried.

Domain parameters must also be consistent with Short Weierstrass equation representation (y^2 = x^3 + A.x +B). Perhaps that is why some of the others didn’t work?

Chris.

Dear Chris,

You are right for the smaller ones.

You are also right for M511, Curve383187 and M383 since they were Montgomery curves.

I have tried and I have succeeded with secp521r1 ...

It is a pity since the safe curves are not on Short Weierstrass curve https://safecurves.cr.yp.to/

Best regards,

Damien