Use RSA-PSS for signatures in PublicKeyAuthenticator

[thack1]

The signatures for authentication use RSA PKCS #1 v1.5 Padding which has no proof of security. Switch to RSA-PSS as standarized in PKCS #1 v2.1. RSA-PSS has a proof of security in the ROM.

[thack1]

Probably not provided by Java by default. Use Bouncycastle.