tessera-sign

README

Tessera Sign

Signs a Tessera bill of materials with a hybrid post-quantum signature, so a document can be shown to have come from you and to have not changed since.

tessera-sign keygen --out ./keys
tessera-sign sign   --key ./keys/signing.key model.cdx.json
tessera-sign verify --pub ./keys/signing.pub model.cdx.json model.cdx.json.sig

Why this is a separate repository

Tessera itself has zero third-party dependencies, and that is a promise made to every program that embeds it — pinned by a test. Post-quantum signatures need a vetted cryptographic library, and nobody should be hand-rolling one. Keeping the signing in its own module means a service that only wants to read model files never acquires a crypto dependency it did not ask for, and this module can take one without touching that guarantee.

The algorithm choice

Signatures are hybrid: every document is signed twice, independently, and both signatures must verify. If either algorithm is later broken, the other still holds. That is the conservative composition, and it is also the only one that satisfies every major authority at once rather than optimising for one:

Component	Algorithm	Why
Post-quantum	ML-DSA-87 (FIPS 204, Category 5)	NSA CNSA 2.0's designated general-purpose signature, at its highest parameter set
Classical	ECDSA P-384 (FIPS 186-5)	ANSSI requires a classical algorithm alongside any post-quantum one; BSI TR-02102-1 concurs
Digest	SHA-384 (FIPS 180-4)	The weakest digest CNSA 2.0 permits

A pure ML-DSA signature would satisfy the NSA and fail ANSSI's hybrid requirement. A pure ECDSA signature satisfies neither for the long term. Signing twice costs a few kilobytes and removes the argument.

SLH-DSA-SHA2-256s (FIPS 205) is also available with --conservative. It is hash-based, so it rests on strictly weaker assumptions than ML-DSA's lattices, which is why BSI favours it; the signatures are much larger and slower to produce. Use it when the assumption matters more than the size.

What a signature covers

The signature is over the document's bytes. The document contains a SHA-256 and SHA-384 digest of every file the model is made of, so signing it transitively covers the artifact — provided the document is then checked against the artifact, which is what tessera verify does. Signing a document nobody verifies proves only that the document is unaltered, not that it is true.

The intended sequence is therefore:

tessera bom ./model --format cyclonedx > model.cdx.json   # describe
tessera-sign sign --key signing.key model.cdx.json        # attest
tessera-sign verify --pub signing.pub model.cdx.json model.cdx.json.sig
tessera verify model.cdx.json ./model                     # confirm it still holds

Licence

Apache-2.0.