I'm not exactly sure how passes are signed, but in most digital signature schemes, you only sign the hash of the message, not the actual contents. Therefore you could conceivably do this in a privacy preserving way by only passing in the hash to be signed, which would allow the server to generate a valid signature without knowing the contents.
I'm not exactly sure how passes are signed, but in most digital signature schemes, you only sign the hash of the message, not the actual contents. Therefore you could conceivably do this in a privacy preserving way by only passing in the hash to be signed, which would allow the server to generate a valid signature without knowing the contents.