# Post-Quantum Cryptography in Go

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This repository is parameter selection and lightweight wrapper around a number of Go cryptographic libraries. Its purpose isn't to implement primitives, rather to unify the API surface of existing libraries; limited to the tiny subset needed by the Dark Bio project.

The library is opinionated. Parameters and primitives were selected to provide matching levels of security in a post-quantum world. APIs were designed to make the library easy to use and hard to misuse. Flexibility will always be rejected in favor of safety.

![](./docs/overview.png)

- Digital signatures
  - **xDSA ([RFC-DRAFT](https://datatracker.ietf.org/doc/html/draft-ietf-lamps-pq-composite-sigs))**: `MLDSA`, `EdDSA`, `SHA512`
    - **EdDSA ([RFC-8032](https://datatracker.ietf.org/doc/html/rfc8032))**: `Ed25519`
    - **MLDSA ([RFC-9881](https://datatracker.ietf.org/doc/html/rfc9881))**: Security level 3 (`ML-DSA-65`)
  - **RSA ([RFC-8017](https://datatracker.ietf.org/doc/html/rfc8017))**: 2048-bit, `SHA256`
- Encryption
  - **xHPKE ([RFC-9180](https://datatracker.ietf.org/doc/html/rfc9180))**: `X-WING`, `HKDF`, `SHA256`, `ChaCha20`, `Poly1305`, `dark-bio-v1:` domain prefix
    - **X-WING ([RFC-DRAFT](https://datatracker.ietf.org/doc/html/draft-connolly-cfrg-xwing-kem))**: `MLKEM`, `ECC`
      - **ECC ([RFC-7748](https://datatracker.ietf.org/doc/html/rfc7748))**: `X25519`
      - **MLKEM ([FIPS-203](https://csrc.nist.gov/pubs/fips/203/final))**: Security level 3 (`ML-KEM-768`)
  - **STREAM (*RFC N/A*, [Age](https://github.com/FiloSottile/age))**: `ChaCha20`, `Poly1305`, `16B` tag, `64KB` chunk
- Key derivation
  - **Argon2 ([RFC-9106](https://datatracker.ietf.org/doc/html/rfc9106))**: `id` variant
  - **HKDF ([RFC-5869](https://datatracker.ietf.org/doc/html/rfc5869))**: `SHA256`
- Serialization
  - **CBOR ([RFC-8949](https://datatracker.ietf.org/doc/html/rfc8949))**: restricted to `bool`,`null`, `integer`, `text`, `bytes`, `array`, `map[int]`, `option`
  - **COSE ([RFC-9052](https://datatracker.ietf.org/doc/html/rfc9052))**: `COSE_Sign1`, `COSE_Encrypt0`, `dark-bio-v1:` domain prefix
- Credential / Attestation
  - **CWT ([RFC-8392](https://datatracker.ietf.org/doc/html/rfc8392))**: `xDSA`, `xHPKE`
    - **EAT ([RFC-9711](https://datatracker.ietf.org/doc/html/rfc9711))**

## Quick start

Signatures come from `xdsa`, encryption from `xhpke`, and `cose` wraps both into COSE envelopes using the Dark Bio wire profile documented in the `cose` package.

```sh
go get github.com/dark-bio/crypto-go
```

COSE signing and verification and xHPKE encryption and decryption use an application domain that both sides must agree on. It is prefixed with `dark-bio-v1:` internally and binds the operation to one purpose. Choose distinct domains for distinct purposes. Raw `xdsa` signatures carry no such application domain, which is why the `cose` envelopes are the recommended entry point.

```go
func example() (string, error) {
	// Long term identities, one for signing and one for receiving
	signer := xdsa.GenerateKey()
	recipient := xhpke.GenerateKey()
	domain, drift := []byte("example"), uint64(60)

	// A detached signature over a message that travels separately
	signature, err := cose.SignDetached("payload", signer, domain)
	if err != nil {
		return "", err
	}
	if err := cose.VerifyDetached(signature, "payload", signer.PublicKey(), domain, &drift); err != nil {
		return "", err
	}

	// Sign and encrypt a payload to the recipient, then open and verify it back.
	// The second argument is authenticated but must be supplied separately.
	padding := cose.BucketPadding{Floor: 8192, Step: 20}
	sealed, err := cose.Seal("payload", "metadata", signer, recipient.PublicKey(), domain, padding)
	if err != nil {
		return "", err
	}
	return cose.Open[string](sealed, "metadata", recipient, signer.PublicKey(), domain, &drift)
}
```

Sealing pads the signed envelope with zeros inside the encryption, as many as
the sender's `cose.Padding` policy picks. `cose.NoPadding{}` adds none, and
`cose.BucketPadding` pads to the smallest of a series of sizes that starts at
`Floor` and grows by `1/Step` of each. A nil policy or a parameter below 1
panics. Receivers strip any number of zeros without knowing the policy, and
refuse a nonzero byte.

## CBOR struct tags

The `cbor` package uses Go struct tags to generate encoders and decoders for structs. By default, structs are represented as maps, with the possibility of requesting array encoding.

In map encoding mode, all keys are integers. This is a deliberate restriction to support maps but still force non-wasteful encoding. Each field requires `cbor:"N,key"`. To encode a struct as an array, use `cbor:"_,array"`.

## Siblings

This is a sibling package with the Rust [`github.com/dark-bio/crypto-rs`](https://github.com/dark-bio/crypto-rs); as in, both repositories implement the same feature sets and API surfaces at the same version points. This naturally means PRs merged into one project necessarily have to have a counter-PR in the other project.

## Bindings

The Rust sibling currently has a Flutter binding [`github.com/dark-bio/crypto-fl`](https://github.com/dark-bio/crypto-fl) that exposes the same API surface and versioning; implemented by wrapping the Rust code via FFI rather than reimplementing it.

The Rust sibling also has a TypeScript binding [`github.com/dark-bio/crypto-ts`](https://github.com/dark-bio/crypto-ts) that also exposes the same API surface and versioning; implemented by wrapping the Rust code via WASM rather than reimplementing it.

## Acknowledgements

Shoutout to Filippo Valsorda ([@filosottile](https://github.com/filosottile)) for lots of tips and nudges on what kind of cryptographic primitives to use and how to combine them properly; and also for his work in general on cryptography standards.

Naturally, many thanks to the authors of all the libraries this project depends on.

## License

This library is licensed under the [BSD 3-Clause License](https://github.com/dark-bio/crypto-go/blob/main/LICENSE).