Module signable

This module defines traits used for signing and verifying structs from the MLS protocol spec.

Type-Enforced Verification

This module contains four traits, each describing the property they enable upon implementation: Signable, SignedStruct, Verifiable and VerifiedStruct.

Each trait represents the state of a struct in a sender-receiver flow with the following transitions.

• the signer creates an instance of a struct that implements Signable
• the signer signs it, consuming the Signable struct and producing a SignedStruct
• the signer serializes the struct and sends it to the verifier
• the verifier deserializes the byte-string into a struct implementing Verifiable
• the verifier verifies the struct, consuming the Verifiable struct and producing a VerifiedStruct

Using this process, we can ensure that only structs implementing SignedStruct are sent over the wire and only structs implementing VerifiedStruct are used on the verifier side as input for further processing functions.

For the type-safety to work, it is important that Signable and SignedStruct are implemented by distinct structs. The same goes for Verifiable and VerifiedStruct. In addition, only the SignedStruct should implement the [tls_codec::Serialize] trait. Similarly, only the Verifiable struct should implement the [tls_codec::Deserialize] trait.

Enums

• SignatureError
  Signature generation and verification errors. The only information relayed with this error is whether the signature verification or generation failed.

Traits

• Signable
  The Signable trait is implemented by all struct that are being signed. The implementation has to provide the unsigned_payload function.
• SignedStruct
  This trait must be implemented by all structs that contain a self-signature.
• Verifiable
  The verifiable trait must be implemented by any struct that is signed with a credential. The actual verify method is provided. The unsigned_payload and signature functions have to be implemented for each struct, returning the serialized payload and the signature respectively.
• VerifiedStruct
  This marker trait must be implemented by all structs that contain a verified self-signature.