Javacard key derivation (hmac) using javacardx.security.derivation

Question

i am trying to figure out how the javacard derivation functions work, and i must admit i am lost as to how to set up the functions.

I figured that the DerivationFunction can be used to derive key, and using ALG_KDF_HKDF i could specify the desired HMAC based key derivation. However, i do not understand how do i set the secret and salt. The KDFHmacSpec interface only specifies getters. It does not specify how to set the values. So far i fail to figure out how i get to set these.

I want to use it to generate XOR key (of preferably specified length), based on a secret and some random public value.

This is for javacard 3+.

Answer 1

The one thing that I can come up with is that it is indeed an interface. Interfaces you can implement in a class that you define yourself.

This has the drawback that you may have to copy arrays such as the salt (which is not used all that much for key derivation functions) into a transient memory array.

The advantage is that other key derivation parameters can simply be part of the class definition if they don't need to be changed.

I'd advice you to create a single, reusable, mutable class for your HKDF function.