How to know which private key use to decipher when there is more than one in the keystore

Question

I have a system that receives requests encoded with 2 different public keys and which I have to decipher with the respective private keys that I have in the Java keystore.

How can I know which one of the private keys in my JKS I should use to decipher? Preferably better than test-and-fail till I find the correct key?

Answer 1

Most of encryption standards (like CMS, XML Encrpyption, GPG...) require the inclusion of an unambiguous way to determine the public key used to encrypt. It could be a reference to the public key (i.e. a hash) or a certificate. With this indication it is possible to select the correct private key.

Unfortunately if the system you are using does not provide that kind of indication I don't think there is no other way than try-and-fail.