policy.json BYOPKI signature verification API

Signed-off-by: Qi Wang <[email protected]>

docs/containers-policy.json.5.md

@@ -329,14 +329,22 @@ This requirement requires an image to be signed using a sigstore signature with
         "oidcIssuer": "https://expected.OIDC.issuer/",
         "subjectEmail", "[email protected]",
     },
+    "pki": {
+		"caRootsPath": "/path/to/local/CARoots/file",
+        "caRootsData": "base64-encoded-CARoots-data",
+        "caIntermediatesPath": "/path/to/local/CAIntermediates/file",
+        "caIntermediatesData": "base64-encoded-CAIntermediates-data",
+        "subjectHostname": "expected-signing-hostname.example.com",
+		"subjectEmail": "[email protected]"
+    },
     "rekorPublicKeyPath": "/path/to/local/public/key/file",
     "rekorPublicKeyPaths": ["/path/to/local/public/key/one","/path/to/local/public/key/two"],
     "rekorPublicKeyData": "base64-encoded-public-key-data",
     "rekorPublicKeyDatas": ["base64-encoded-public-key-one-data","base64-encoded-public-key-two-data"],
     "signedIdentity": identity_requirement
 }
 ```
-Exactly one of `keyPath`, `keyPaths`, `keyData`, `keyDatas` and `fulcio` must be present.
+Exactly one of `keyPath`, `keyPaths`, `keyData`, `keyDatas`, `fulcio` and `pki` must be present.
 
 If `keyPath` or `keyData` is present, it contains a sigstore public key.
 Only signatures made by this key are accepted.
@@ -350,6 +358,11 @@ Both `oidcIssuer` and `subjectEmail` are mandatory,
 exactly specifying the expected identity provider,
 and the identity of the user obtaining the Fulcio certificate.
 
+If `pki` is present, the signature must be based on a non-Fulcio X.509 certificate.
+One of `caRootsPath` and `caRootsData` must be specified, containing the public key of the CA.
+Only one of `caIntermediatesPath` and `caIntermediatesData` can be present, containing the public key of the intermediate CA.
+One of `subjectEmail` and `subjectHostname` must be specified, exactly specifying the expected identity provider, and the identity of the user obtaining the certificate.
+
 At most one of `rekorPublicKeyPath`, `rekorPublicKeyPaths`, `rekorPublicKeyData` and `rekorPublicKeyDatas` can be present;
 it is mandatory if `fulcio` is specified.
 If a Rekor public key is specified,
@@ -407,6 +420,19 @@ selectively allow individual transports and scopes as desired.
                     "rekorPublicKeyPath": "/path/to/rekor.pub",
                 }
             ],
+            /* A Sigstore-signed repository using a certificate generated by the Bring Your Own Public Key Infrastructure (BYOPKI).*/
+            "hostname:5000/myns/sigstore-signed-byopki": [
+                {
+                    "type": "sigstoreSigned",
+                    "pki": {
+                        "caRootsPath": "/path/to/pki_roots_crt.pem",
+                        "caIntermediatesPath": "/path/to/pki_intermediates_crt.pem",
+                        "oidcIssuer": "https://github.com/login/oauth",
+                        "subjectHostname": "test-user.example.com"
+                        "subjectEmail": "[email protected]"
+                    }
+                }
+            ],
             /* A sigstore-signed repository, accepts signatures by /usr/bin/cosign */
             "hostname:5000/myns/sigstore-signed-allows-malicious-tag-substitution": [
                 {

signature/fixtures/dir-img-cosign-pki-valid/manifest.json

@@ -0,0 +1 @@
+{"schemaVersion":2,"mediaType":"application/vnd.oci.image.manifest.v1+json","config":{"mediaType":"application/vnd.oci.image.config.v1+json","digest":"sha256:84e2abbb0b1347753fa15b585fb2181509ee296e29eed9f4bd3fd7778d027a98","size":348},"layers":[],"annotations":{"org.opencontainers.image.base.digest":"","org.opencontainers.image.base.name":""}}

signature/fixtures/pki-cert

@@ -0,0 +1,34 @@
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+-----END CERTIFICATE-----

signature/fixtures/pki-chain

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+-----BEGIN CERTIFICATE-----
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signature/fixtures/pki_intermediates_crt.pem

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+-----END CERTIFICATE-----

signature/fixtures/pki_roots_crt.pem

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+aj4kcwBrVtJ1h3Q7VPuigeiDR/9TZUv3QEphm4GgaTM+BK0=
+-----END CERTIFICATE-----

signature/pki_cert.go

@@ -0,0 +1,100 @@
+package signature
+
+import (
+	"crypto"
+	"crypto/x509"
+	"errors"
+	"fmt"
+	"slices"
+
+	"github.com/containers/image/v5/signature/internal"
+	"github.com/sigstore/sigstore/pkg/cryptoutils"
+)
+
+type pkiTrustRoot struct {
+	caRootsCertificates        *x509.CertPool
+	caIntermediateCertificates []byte
+	subjectEmail               string
+	subjectHostname            string
+}
+
+func (p *pkiTrustRoot) validate() error {
+	if p.subjectEmail == "" && p.subjectHostname == "" {
+		return errors.New("Internal inconsistency: PKI use set up without subject email or subject hostname")
+	}
+	return nil
+}
+
+func verifyPKI(pkiTrustRoot *pkiTrustRoot, untrustedCertificateBytes []byte, untrustedIntermediateChainBytes []byte) (crypto.PublicKey, error) {
+
+	untrustedLeafCerts, err := cryptoutils.UnmarshalCertificatesFromPEM(untrustedCertificateBytes)
+	if err != nil {
+		return nil, internal.NewInvalidSignatureError(fmt.Sprintf("parsing leaf certificate: %v", err))
+	}
+	switch len(untrustedLeafCerts) {
+	case 0:
+		return nil, internal.NewInvalidSignatureError("no certificate found in signature certificate data")
+	case 1:
+		break // OK
+	default:
+		return nil, internal.NewInvalidSignatureError("unexpected multiple certificates present in signature certificate data")
+	}
+	untrustedCertificate := untrustedLeafCerts[0]
+
+	if pkiTrustRoot.subjectEmail != "" {
+		if !slices.Contains(untrustedCertificate.EmailAddresses, pkiTrustRoot.subjectEmail) {
+			return nil, internal.NewInvalidSignatureError(fmt.Sprintf("Required email %q not found (got %q)",
+				pkiTrustRoot.subjectEmail,
+				untrustedCertificate.EmailAddresses))
+		}
+	}
+
+	if pkiTrustRoot.subjectHostname != "" {
+		if err = untrustedCertificate.VerifyHostname(pkiTrustRoot.subjectHostname); err != nil {
+			return nil, internal.NewInvalidSignatureError(fmt.Sprintf("Unexpected subject hostname: %v", err))
+		}
+	}
+
+	untrustedIntermediatePool := x509.NewCertPool()
+	if len(untrustedIntermediateChainBytes) > 0 {
+		untrustedIntermediateChain, err := cryptoutils.UnmarshalCertificatesFromPEM(untrustedIntermediateChainBytes)
+		if err != nil {
+			return nil, internal.NewInvalidSignatureError(fmt.Sprintf("loading certificate chain: %v", err))
+		}
+		if len(untrustedIntermediateChain) > 1 {
+			for _, untrustedIntermediateCert := range untrustedIntermediateChain {
+				if isIntermediateCA(untrustedIntermediateCert) {
+					untrustedIntermediatePool.AddCert(untrustedIntermediateCert)
+				}
+			}
+		}
+	}
+
+	if len(pkiTrustRoot.caIntermediateCertificates) > 0 {
+		trustedIntermediates, err := cryptoutils.UnmarshalCertificatesFromPEM(pkiTrustRoot.caIntermediateCertificates)
+		if err != nil {
+			return nil, internal.NewInvalidSignatureError(fmt.Sprintf("loading trusted intermediate certificates: %v", err))
+		}
+		for _, trustedIntermediateCert := range trustedIntermediates {
+			untrustedIntermediatePool.AddCert(trustedIntermediateCert)
+		}
+	}
+
+	if _, err := untrustedCertificate.Verify(x509.VerifyOptions{
+		Intermediates: untrustedIntermediatePool,
+		Roots:         pkiTrustRoot.caRootsCertificates,
+		KeyUsages:     []x509.ExtKeyUsage{x509.ExtKeyUsageCodeSigning},
+	}); err != nil {
+		return nil, internal.NewInvalidSignatureError(fmt.Sprintf("veryfing leaf certificate failed: %v", err))
+	}
+
+	return untrustedCertificate.PublicKey, nil
+}
+
+// isIntermediateCA checks if the certificate is an intermediate CA
+func isIntermediateCA(cert *x509.Certificate) bool {
+	// The certificate is an intermediate CA if:
+	// 1. It is a CA (cert.IsCA is true).
+	// 2. It is not self-signed (Issuer != Subject).
+	return cert.IsCA && cert.Issuer.String() != cert.Subject.String()
+}