Native library reference. Extended CLI examples use jacs-compat. This source reference can include unreleased changes; see the package guide for installation and MCP profiles for available tools.

Writing a Custom Evidence Adapter

Evidence adapters normalize external bytes into claims and evidence references. They belong to the retained native Rust jacs crate with the attestation feature enabled.

The built-in A2A and Email adapters are digest-only: they do not verify A2A/JACS signatures, DKIM, S/MIME, PGP, sender identity or signer trust. Their claims use self-asserted assurance and names ending in evidence-digest-recorded. Use the corresponding protocol verifier before asserting protocol or identity verification.

Trait contract

External implementations import the public types from jacs:

#![allow(unused)]
fn main() {
use jacs::attestation::adapters::EvidenceAdapter;
use jacs::attestation::types::{Claim, EvidenceRef, EvidenceVerificationResult};
use jacs::error::JacsError;
use serde_json::Value;
}

The required method signatures are:

fn kind(&self) -> &str;
fn normalize(&self, raw: &[u8], metadata: &Value)
    -> Result<(Vec<Claim>, EvidenceRef), JacsError>;
fn verify_evidence(&self, evidence: &EvidenceRef)
    -> Result<EvidenceVerificationResult, JacsError>;

An implementation must also implement Send + Sync + Debug. See the trait and built-in implementations for complete types and executable examples.

Normalize and register explicitly

Call your adapter's normalize() on the raw bytes and metadata, then supply the returned claims and evidence to attestation creation. The raw creation API takes already-normalized values; it does not automatically call every adapter.

Register a custom adapter on a mutable native Agent with agent.register_adapter(Box::new(adapter)). This API exists now. Registration makes matching evidence kinds available during full attestation verification; it does not require editing JACS's default adapter list.

Preserve the evidence's meaning

Compute digests over an explicitly defined byte representation and preserve that representation for later verification. Reformatting JSON, decoding strings or redacting fields can change the bytes. Keep parsing separate from authentication: decoding a JWT payload does not verify its signature, issuer, audience or expiry.

normalize() receives untrusted data and metadata. Do not promote a caller's verified=true or a successful digest computation into verified assurance. A confidence value is an assertion, not proof. State exactly which protocol checks your implementation performs and which trust policy supplies its keys.

verify_evidence() returns digest_valid, freshness_valid and a human-readable detail in Rust. Implement actual checks before reporting success. If required bytes, trust anchors or time context are unavailable, return a failure or an explicit error rather than a fabricated passing result. The built-in full verifier does not fetch URI-only evidence.

Privacy and testing

Sensitivity labels do not encrypt evidence or grant access control. JWT subject, issuer and audience fields may identify people or organizations; minimize them and avoid embedding bearer tokens or other credentials. Select URI resolution, retention and access policy in the application.

Test malformed input, digest mutations, unavailable evidence, expired or future-dated evidence, untrusted keys, incorrect issuer/audience, and round trips that preserve exact bytes. Include assertions that an unauthenticated source never produces a claim of verified identity.

See attestation creation and verification and testing.