feat(a2acrypto): add AgentCard JWS signing and verification (fixes #141)

a2aclient/agentcard/resolver.go

@@ -24,6 +24,7 @@ import (
 	"time"
 
 	"github.com/a2aproject/a2a-go/v2/a2a"
+	"github.com/a2aproject/a2a-go/v2/a2acrypto"
 	"github.com/a2aproject/a2a-go/v2/log"
 )
 
@@ -62,6 +63,8 @@ type Resolver struct {
 	Client *http.Client
 	// CardParser can be used to configure AgentCard parsing.
 	CardParser Parser
+	// Verifier optionally verifies AgentCard signatures after resolution.
+	Verifier *a2acrypto.Verifier
 }
 
 // NewResolver is a [Resolver] constructor function.
@@ -131,6 +134,22 @@ func (r *Resolver) Resolve(ctx context.Context, baseURL string, opts ...ResolveO
 		return nil, fmt.Errorf("card parsing failed: %w", err)
 	}
 
+	if r.Verifier != nil && len(card.Signatures) > 0 {
+		var verified bool
+		var lastErr error
+		for _, sig := range card.Signatures {
+			if err := r.Verifier.Verify(card, &sig); err == nil {
+				verified = true
+				break
+			} else {
+				lastErr = err
+			}
+		}
+		if !verified {
+			return nil, fmt.Errorf("agent card signature verification failed: %w", lastErr)
+		}
+	}
+
 	return card, nil
 }
 

a2aclient/factory.go

@@ -22,6 +22,7 @@ import (
 	"strings"
 
 	"github.com/a2aproject/a2a-go/v2/a2a"
+	"github.com/a2aproject/a2a-go/v2/a2acrypto"
 	"github.com/a2aproject/a2a-go/v2/log"
 
 	"golang.org/x/mod/semver"
@@ -33,6 +34,7 @@ type Factory struct {
 	config       Config
 	interceptors []CallInterceptor
 	transports   map[transportKey]TransportFactory
+	cardVerifier *a2acrypto.Verifier
 }
 
 type transportKey struct {
@@ -109,6 +111,21 @@ func (f *Factory) CreateFromCard(ctx context.Context, card *a2a.AgentCard) (*Cli
 		protocolVersion: a2a.ProtocolVersion(selected.semver[1:]),
 	}
 	client.card.Store(card)
+	if f.cardVerifier != nil && len(card.Signatures) > 0 {
+		var verified bool
+		var lastErr error
+		for _, sig := range card.Signatures {
+			if err := f.cardVerifier.Verify(card, &sig); err == nil {
+				verified = true
+				break
+			} else {
+				lastErr = err
+			}
+		}
+		if !verified {
+			return nil, fmt.Errorf("agent card signature verification failed: %w", lastErr)
+		}
+	}
 	return client, nil
 }
 
@@ -272,6 +289,14 @@ func WithDefaultsDisabled() FactoryOption {
 	return defaultsDisabledOpt{}
 }
 
+// WithCardVerifier sets a verifier to validate AgentCard signatures when creating clients.
+// If set, signed cards are verified in CreateFromCard; unsigned cards pass through.
+func WithCardVerifier(v *a2acrypto.Verifier) FactoryOption {
+	return factoryOptionFn(func(f *Factory) {
+		f.cardVerifier = v
+	})
+}
+
 // NewFactory creates a new Factory applying the provided configurations.
 func NewFactory(options ...FactoryOption) *Factory {
 	f := &Factory{

a2acrypto/a2acrypto.go

@@ -0,0 +1,71 @@
+// Copyright 2026 The A2A Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package a2acrypto provides utilities for AgentCard JWS signing and verification.
+package a2acrypto
+
+import (
+	"crypto"
+)
+
+// KeyResolver resolves a key ID and JWKS URL into a public key for verification.
+type KeyResolver interface {
+	// ResolveKey looks up the public key for the given key ID (kid) and JWKS endpoint (jku).
+	ResolveKey(kid, jku string) (crypto.PublicKey, error)
+}
+
+// VerifierConfig configures signature verification.
+type VerifierConfig struct {
+	KeyResolver KeyResolver
+}
+
+// Verifier verifies AgentCard JWS signatures.
+type Verifier struct {
+	kr KeyResolver
+}
+
+// NewVerifier creates a Verifier using the provided configuration.
+func NewVerifier(config VerifierConfig) *Verifier {
+	return &Verifier{kr: config.KeyResolver}
+}
+
+// SignerConfig configures AgentCard signing.
+type SignerConfig struct {
+	PrivateKey crypto.Signer
+	KeyID      string
+	Algorithm  string
+	JWKSURL    string
+}
+
+// Signer creates JWS signatures for AgentCards.
+type Signer struct {
+	key       crypto.Signer
+	kid       string
+	algorithm string
+	jwksURL   string
+}
+
+// NewSigner creates a Signer using the provided configuration.
+func NewSigner(config SignerConfig) *Signer {
+	alg := config.Algorithm
+	if alg == "" {
+		alg = inferAlgorithm(config.PrivateKey)
+	}
+	return &Signer{
+		key:       config.PrivateKey,
+		kid:       config.KeyID,
+		algorithm: alg,
+		jwksURL:   config.JWKSURL,
+	}
+}

a2acrypto/alg.go

@@ -0,0 +1,61 @@
+// Copyright 2025 The A2A Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package a2acrypto
+
+import (
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/elliptic"
+	"crypto/rsa"
+	"fmt"
+)
+
+func inferAlgorithm(key crypto.Signer) string {
+	switch pub := key.Public().(type) {
+	case *ecdsa.PublicKey:
+		switch pub.Curve {
+		case elliptic.P256():
+			return "ES256"
+		case elliptic.P384():
+			return "ES384"
+		case elliptic.P521():
+			return "ES512"
+		default:
+			return ""
+		}
+	case ed25519.PublicKey:
+		return "EdDSA"
+	case *rsa.PublicKey:
+		return "RS256"
+	default:
+		return ""
+	}
+}
+
+func algToHash(alg string) (crypto.Hash, error) {
+	switch alg {
+	case "ES256", "RS256":
+		return crypto.SHA256, nil
+	case "ES384", "RS384":
+		return crypto.SHA384, nil
+	case "ES512", "RS512":
+		return crypto.SHA512, nil
+	case "EdDSA":
+		return crypto.Hash(0), nil
+	default:
+		return 0, fmt.Errorf("unsupported algorithm: %s", alg)
+	}
+}