SSI

The SSI library provides a simple and modular API to sign and verify claims exchanged between applications using Decentralized Identifiers (DIDs). SSI is embedded in the cross-platform didkit library as a core dependency.

This library supports the two main families of verifiable claims:

• JSON Web Tokens (JWT) where claims are encoded into JSON and secured using JSON Web Signatures; and
• W3C's Verifiable Credentials (VCs), a Linked-Data-based model where claims (VCs) can be interpreted as RDF datasets. VC supports multiple signature formats provided by SSI:
  • VC over JWT (JWT-VC), a restricted form of JWT following the VC data model; or
  • Data Integrity, encoding the claims and their proof in the same JSON-LD document using a wide variety of cryptographic suites.

Basic Usage

SSI provides various functions to parse, verify, create and sign various kind of claims. This section shows how to use these functions in combination with JSON Web Signatures (or Tokens) and Verifiable Credentials.

Verification

The simplest type of claim to load and verify is probably JSON Web Signatures (JWSs), often use to encode JSON Web Tokens (JWTs). To represent such claims SSI provides the JwsBuf type representing a JWS in compact textual form. One can load a JWS using new and verify it using verify.

use ssi::prelude::*;

// Load a JWT from the file system.
let jwt = JwsBuf::new(
  std::fs::read_to_string("examples/files/claims.jwt")
  .expect("unable to load JWT")
).expect("invalid JWS");

// Setup a verification method resolver, in charge of retrieving the
// public key used to sign the JWT.
// Here we use the example `ExampleDIDResolver` resolver, enabled with the
// `example` feature.
let vm_resolver = ExampleDIDResolver::default().into_vm_resolver::<AnyJwkMethod>();

// Setup the verification parameters.
let params = VerificationParameters::from_resolver(vm_resolver);

// Verify the JWT.
assert!(jwt.verify(&params).await.expect("verification failed").is_ok())

Verifiable Credentials

Verifiable Credential are much more complex as they require interpreting the input claims and proofs, such as Data-Integrity proofs as Linked-Data using JSON-LD. This operation is highly configurable. SSI provide functions exposing various levels of implementation details that you can tweak as needed. The simplest of them is any_credential_from_json_str that will simply load a VC from a string, assuming it is signed using any Data-Integrity proof supported by SSI.

use ssi::prelude::*;

let vc = ssi::claims::vc::v1::data_integrity::any_credential_from_json_str(
  &std::fs::read_to_string("examples/files/vc.jsonld")
  .expect("unable to load VC")
).expect("invalid VC");

// Setup a verification method resolver, in charge of retrieving the
// public key used to sign the JWT.
let vm_resolver = ExampleDIDResolver::default().into_vm_resolver();

// Setup the verification parameters.
let params = VerificationParameters::from_resolver(vm_resolver);

assert!(vc.verify(&params).await.expect("verification failed").is_ok());

Signature & Custom Claims

In the previous section we have seen how to load and verify arbitrary claims. This section shows how to create and sign custom claims. With SSI, any Rust type can serve as claims as long as it complies to certain conditions such as implementing serialization/deserialization functions using serde. Don't forget to enable the derive feature for serde.

In the following example, we create a custom type MyClaims and sign it as a JWT.

use serde::{Serialize, Deserialize};
use ssi::prelude::*;

// Defines the shape of our custom claims.
#[derive(Serialize, Deserialize)]
pub struct MyClaims {
  name: String,
  email: String
}

// Create JWT claims from our custom ("private") claims.
let claims = JWTClaims::from_private_claims(MyClaims {
  name: "John Smith".to_owned(),
  email: "[email protected]".to_owned()
});

// Create a random signing key, and turn its public part into a DID URL.
let mut key = JWK::generate_p256(); // requires the `p256` feature.
let did = DIDJWK::generate_url(&key.to_public());
key.key_id = Some(did.into());

// Sign the claims.
let jwt = claims.sign(&key).await.expect("signature failed");

// Create a verification method resolver, which will be in charge of
// decoding the DID back into a public key.
let vm_resolver = DIDJWK.into_vm_resolver::<AnyJwkMethod>();

// Setup the verification parameters.
let params = VerificationParameters::from_resolver(vm_resolver);

// Verify the JWT.
assert!(jwt.verify(&params).await.expect("verification failed").is_ok());

// Print the JWT.
println!("{jwt}")

Verifiable Credential

We can use a similar technique to sign a VC with custom claims. The SpecializedJsonCredential type provides a customizable implementation of the VC data-model 1.1 where you can set the credential type yourself.

use static_iref::uri;
use serde::{Serialize, Deserialize};
use ssi::prelude::*;

// Defines the shape of our custom claims.
#[derive(Serialize, Deserialize)]
pub struct MyCredentialSubject {
  #[serde(rename = "https://example.org/#name")]
  name: String,

  #[serde(rename = "https://example.org/#email")]
  email: String
}

let credential = ssi::claims::vc::v1::JsonCredential::<MyCredentialSubject>::new(
  Some(uri!("https://example.org/#CredentialId").to_owned()), // id
  uri!("https://example.org/#Issuer").to_owned().into(), // issuer
  DateTime::now(), // issuance date
  vec![MyCredentialSubject {
    name: "John Smith".to_owned(),
    email: "[email protected]".to_owned()
  }]
);

// Create a random signing key, and turn its public part into a DID URL.
let key = JWK::generate_p256(); // requires the `p256` feature.
let did = DIDJWK::generate_url(&key.to_public());

// Create a verification method resolver, which will be in charge of
// decoding the DID back into a public key.
let vm_resolver = DIDJWK.into_vm_resolver();

// Create a signer from the secret key.
// Here we use the simple `SingleSecretSigner` signer type which always uses
// the same provided secret key to sign messages.
let signer = SingleSecretSigner::new(key.clone()).into_local();

// Turn the DID URL into a verification method reference.
let verification_method = did.into_iri().into();

// Automatically pick a suitable Data-Integrity signature suite for our key.
let cryptosuite = AnySuite::pick(&key, Some(&verification_method))
  .expect("could not find appropriate cryptosuite");

let vc = cryptosuite.sign(
  credential,
  &vm_resolver,
  &signer,
  ProofOptions::from_method(verification_method)
).await.expect("signature failed");

It is critical that custom claims can be interpreted as Linked-Data. In the above example this is done by specifying a serialization URL for each field of MyCredentialSubject. This can also be done by creating a custom JSON-LD context and embed it to credential using either SpecializedJsonCredential's context field or leveraging its context type parameter.

Data-Models

The examples above are using the VC data-model 1.1, but you ssi also has support for:

• VC data-model 2.0
• A wrapper type to accept both

Features

Security Audits

ssi has undergone the following security reviews:

• March 14th, 2022 - Trail of Bits | Summary of Findings

Testing

Testing SSI requires the RDF canonicalization test suite, which is embedded as a git submodule.

$ git submodule update --init
$ cargo test --workspace