PGPSecretKey.copyWithNewPassword defaults to S2K USAGE_CHECKSUM

[vanitasvitae]

Hey!

When setting a passphrase on a key which previously was unencrypted, BC chooses S2K usage USAGE_CHECKSUM which is not an ideal choice.

OpenPGP.js for instance rejects this S2K usage as insecure.

Could you instead change the implementation of PGPSecretKey.copyWithNewPassword() to default to USAGE_SHA1 as default if the previous usage was NONE?

[dghgit]

This is now done. I needed to add an extra method to allow a PGPDigestCalculator to do it first. In line with the current RFC only SHA-1 is accepted for it. Should show up on github soon.