RSA-Blind-Signature

A RSA Blind Signature implementation using Crypto++, based on Chaum's paper.

At its core, this library is based on the example on the Crypto++ wiki.
Here is the documentation for the Crypto++ library.
Here are instructions for installing and building the underlying Crypto++ library on Linux.
The parts of the code that read the pem-formatted keys are taken from cryptopp-pem, but for better or worse this library does not have a dependency on that library.

Warning:

This library has not yet been reviewed by any particular community. Until further notice, don't use it for anything.
Additionally, one should never use the keys used for RSA Blind Signatures for any other purpose.

Scope:

This library is intended as a minimal, portable usable implementation of a commonly-cited, trusted blind-signature algorithm. At present it only supports un-encrypted keys in PEM format, and extension to other cryptographic paradigms besides RSA would likely constitute a separate project.
"Nice to have" feature requests will probably be treated as notes for an eventual overhaul.
We want to keep this simple to use, maintain, and review and verify. For this reason even pull-requests may be declined if they don't address security concerns or important usability concerns.

TODO:

• ✓ Link to instructions for installing the underlying library and getting this code running.
• ✓ Upgrade the hash to a FDH or SHA512.
• ✓ Upgrade the keys to 2048 bits or more.
• ✓ Separate into functions that can be called individually from surrounding code.
• ✓ Provide instructions for building (installing?) this code.
• Test function should clean up after itself.
• Figure out what the advertised restrictions on the character set of the messages should be.
• Make sure the on-failure behavior is consistent, and document it.

Installation:

• Download the files to your chosen directory.
• You'll need the Crypto++ library accessible at ./cryptopp810/.
• Running make will build the six executables in ./bin/, including test, which is intended to confirm that the library has build and will run.
• test will use the additional directory ./scratch/.

Useage:

There are six separate executables (to simplify system calls from other languages).
"Key" arguments are the path/filename of the respective key. "Message" is the bare messages as a string. Other arguments are the strings returned by earlier steps in the process. The executables, on success, will print their responses as text to standard-out, and will return the system's "success" value. A newline is included in the responses, but should not be included in arguments.

• blsig_get_client_secret: Generates a single-use secret for blinding a message.
  blsig_get_client_secret public_key.pem
• blsig_get_blinded_hash: Hashes the message and then blinds the hash so it can be sent to the signer.
  blsig_get_blinded_hash message client_secret public_key.pem
• blsig_get_blind_signature: Generates a "pre-signature" (or hashed signature or whatever you want to call it) without any knowledge of the message, the message-hash, or the client secret.
  blsig_get_blind_signature blinded_hash private_key.pem
• blsig_get_unblinded_signature: Un-blinds the pre-signature using the same client_secret used to generate the blinded-hash. Also verifies the signature. The client secret should not be stored once it has served its purpose once.
  blsig_get_unblinded_signature blind_signature blinded_hash client_secret public_key.pem
• blsig_verify_unblinded_signature: Confirms that a provided signature is a valid signature, by the corresponding private-key, of the provided message. Prints true for success.
  /bin/bash: s: command not found
• test: The most rudimentary of tests for the most rudimentary of libraries! It's probably not useful in a production setting, but it's a good place to start into the code.
  test