certificate.go

// Copyright 2023 The Sigstore 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 verify

import (
	"crypto/x509"
	"errors"
	"time"

	"github.com/sigstore/sigstore-go/pkg/root"
)

func VerifyLeafCertificate(observerTimestamp time.Time, leafCert x509.Certificate, trustedMaterial root.TrustedMaterial) error { // nolint: revive
	for _, ca := range trustedMaterial.FulcioCertificateAuthorities() {
		if !ca.ValidityPeriodStart.IsZero() && leafCert.NotBefore.Before(ca.ValidityPeriodStart) {
			continue
		}
		if !ca.ValidityPeriodEnd.IsZero() && leafCert.NotAfter.After(ca.ValidityPeriodEnd) {
			continue
		}

		rootCertPool := x509.NewCertPool()
		rootCertPool.AddCert(ca.Root)
		intermediateCertPool := x509.NewCertPool()
		for _, cert := range ca.Intermediates {
			intermediateCertPool.AddCert(cert)
		}

		// From spec:
		// > ## Certificate
		// > For a signature with a given certificate to be considered valid, it must have a timestamp while every certificate in the chain up to the root is valid (the so-called “hybrid model” of certificate verification per Braun et al. (2013)).

		opts := x509.VerifyOptions{
			CurrentTime:   observerTimestamp,
			Roots:         rootCertPool,
			Intermediates: intermediateCertPool,
			KeyUsages: []x509.ExtKeyUsage{
				x509.ExtKeyUsageCodeSigning,
			},
		}

		_, err := leafCert.Verify(opts)
		if err == nil {
			return nil
		}
	}

	return errors.New("leaf certificate verification failed")
}