[Arnavion]

I have a similar story to OP's. I had made a service that provided access to cryptographic keys but did not reveal the key material directly. Instead it had an RPC API for requesting a key "handle" for the key you wanted to use, and API for performing operations like encrypt or sign that took that key handle, performed the operation inside the service and returned the result. The key handle was to be treated as opaque and implemented as a base64-encoded blob containing the key ID and a signature (for tamper-proofing).

One day a coworker working on another project that would use my service contacted me to complain that the keys from my service were malformed. Turned out they had noticed that the return value was base64-encoded so they assumed it was a base64-encoded key, so they wrote code to base64-decode it and load the result into their language's crypto library to perform those operations directly. They figured that the service's API for doing those operations was just there to be convenient for callers that didn't have access to a crypto library.

[taeric]

We could probably make a drinking club for teams that have been bitten by stuff like this. :)

[groestl]

I'll join :) For past war stories, because these days, I sign parameters that should not be tapered with ;)