zkcmp

Zero-knowledge file possession and comparison utility.

zkcmp is a minimal Unix utility exploring whether knowledge of a file-derived secret can be demonstrated without revealing the file itself.

Status

Experimental / exploratory software.

The protocol and implementation are still evolving. Do not rely on zkcmp for security-critical applications, authentication, or protection of valuable secrets.

How it works

zkcmp derives a secret (hash) from a file:

file
  |
  v
hash
  |
  v
secret
  |
  v
commitment
  |
  v
zero-knowledge proof

The proof is constructed so that the secret itself does not need to be transmitted.

The current implementation uses a Schnorr-style proof of knowledge over a 3072-bit MODP group.

The default hash is SHA-256. SHA3-256 or BLAKE2s-256 can also be selected.

Read more about Schnorr proof.

Commands

zkcmp provides four operations:

zkcmp commit file
zkcmp prove file
zkcmp verify commitment proof
zkcmp check file proof

commit

Creates a reusable public commitment from a file.

zkcmp commit release.tar > commitment

A commitment can be published once and used as the public statement for multiple independent proofs.

prove

Creates a proof from a local file.

zkcmp prove release.tar > proof

verify

Verifies a proof against a previously created commitment.

zkcmp verify commitment proof

This is useful when a commitment has been published and multiple parties need to demonstrate knowledge of the corresponding secret.

For example:

server$ zkcmp commit release.tar > commitment

alice$ zkcmp prove release.tar > alice.proof
bob$   zkcmp prove release.tar > bob.proof

server$ zkcmp verify commit alice.proof
server$ zkcmp verify commit bob.proof

check

Checks the supplied proof against a local file.

The intended Unix/SSH workflow is:

zkcmp prove release.tar | ssh host 'zkcmp check release.tar -'

Linkability

The current commitment is deterministic.

The same file produces the same commitment when the same hash algorithm is used.

A published commitment can therefore act as a stable identifier.

An observer can recognize repeated use of the same commitment and, if they possess candidate files, can test those candidates against it.

Therefore:

A zkcmp commitment should not be considered an unlinkable or strongly hiding commitment.

A proof can therefore provide a zero-knowledge property while the public statement to which it refers remains linkable.

If a reusable public statement is not needed, the check workflow avoids publishing the commitment as a separate object.

Building

On FreeBSD:

make
make install

License

See LICENSE for licensing information.

Author

Nami Arjmandi