feat: delta CRL

revocation/crl/errors.go

@@ -17,3 +17,6 @@ import "errors"
 
 // ErrCacheMiss is returned when a cache miss occurs.
 var ErrCacheMiss = errors.New("cache miss")
+
+// errDeltaCRLNotFound is returned when a delta CRL is not found.
+var errDeltaCRLNotFound = errors.New("delta CRL not found")

revocation/crl/fetcher.go

@@ -18,13 +18,18 @@
 import (
 	"context"
 	"crypto/x509"
+	"crypto/x509/pkix"
 	"encoding/asn1"
 	"errors"
 	"fmt"
 	"io"
 	"net/http"
 	"net/url"
+	"slices"
 	"time"
+
+	"golang.org/x/crypto/cryptobyte"
+	cryptobyte_asn1 "golang.org/x/crypto/cryptobyte/asn1"
 )
 
 // oidFreshestCRL is the object identifier for the distribution point
@@ -84,9 +89,8 @@
 	if f.Cache != nil {
 		bundle, err := f.Cache.Get(ctx, url)
 		if err == nil {
-			// check expiry
-			nextUpdate := bundle.BaseCRL.NextUpdate
-			if !nextUpdate.IsZero() && !time.Now().After(nextUpdate) {
+			// check expiry of base CRL and delta CRL
+			if isEffective(bundle.BaseCRL) && (bundle.DeltaCRL == nil || isEffective(bundle.DeltaCRL)) {
 				return bundle, nil
 			}
 		} else if !errors.Is(err, ErrCacheMiss) && !f.DiscardCacheError {
@@ -109,6 +113,11 @@
 	return bundle, nil
 }
 
+// isEffective checks if the CRL is effective by checking the NextUpdate time.
+func isEffective(crl *x509.RevocationList) bool {
+	return !crl.NextUpdate.IsZero() && !time.Now().After(crl.NextUpdate)
+}
+
 // fetch downloads the CRL from the given URL.
 func (f *HTTPFetcher) fetch(ctx context.Context, url string) (*Bundle, error) {
 	// fetch base CRL
@@ -117,19 +126,110 @@
 		return nil, err
 	}
 
-	// check delta CRL
-	// TODO: support delta CRL https://github.com/notaryproject/notation-core-go/issues/228
-	for _, ext := range base.Extensions {
-		if ext.Id.Equal(oidFreshestCRL) {
-			return nil, errors.New("delta CRL is not supported")
-		}
+	// fetch delta CRL from base CRL extension
+	deltaCRL, err := f.fetchDeltaCRL(ctx, base.Extensions)
+	if err != nil && !errors.Is(err, errDeltaCRLNotFound) {
+		return nil, err
 	}
 
 	return &Bundle{
-		BaseCRL: base,
+		BaseCRL:  base,
+		DeltaCRL: deltaCRL,
 	}, nil
 }
 
+// fetchDeltaCRL fetches the delta CRL from the given extensions of base CRL.
+//
+// It returns errDeltaCRLNotFound if the delta CRL is not found.
+func (f *HTTPFetcher) fetchDeltaCRL(ctx context.Context, extensions []pkix.Extension) (*x509.RevocationList, error) {
+	idx := slices.IndexFunc(extensions, func(ext pkix.Extension) bool {
+		return ext.Id.Equal(oidFreshestCRL)
+	})
+	if idx < 0 {
+		return nil, errDeltaCRLNotFound
+	}
+	// RFC 5280, 4.2.1.15
+	//    id-ce-freshestCRL OBJECT IDENTIFIER ::=  { id-ce 46 }
+	//
+	//    FreshestCRL ::= CRLDistributionPoints
+	urls, err := parseCRLDistributionPoint(extensions[idx].Value)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse Freshest CRL extension: %w", err)
+	}
+	if len(urls) == 0 {
+		return nil, errDeltaCRLNotFound
+	}
+
+	var (
+		lastError error
+		deltaCRL  *x509.RevocationList
+	)
+	for _, cdpURL := range urls {
+		// RFC 5280, 5.2.6
+		// Delta CRLs from the base CRL have the same scope as the base
+		// CRL, so the URLs are for redundancy and should be tried in
+		// order until one succeeds.
+		deltaCRL, lastError = fetchCRL(ctx, cdpURL, f.httpClient)
+		if lastError == nil {
+			return deltaCRL, nil
+		}
+	}
+	return nil, lastError
+}
+
+// parseCRLDistributionPoint parses the CRL extension and returns the CRL URLs
+//
+// value is the raw value of the CRL distribution point extension
+func parseCRLDistributionPoint(value []byte) ([]string, error) {
+	var urls []string
+	// borrowed from crypto/x509: https://cs.opensource.google/go/go/+/refs/tags/go1.23.4:src/crypto/x509/parser.go;l=700-743
+	//
+	// RFC 5280, 4.2.1.13
+	//
+	// CRLDistributionPoints ::= SEQUENCE SIZE (1..MAX) OF DistributionPoint
+	//
+	// DistributionPoint ::= SEQUENCE {
+	//     distributionPoint       [0]     DistributionPointName OPTIONAL,
+	//     reasons                 [1]     ReasonFlags OPTIONAL,
+	//     cRLIssuer               [2]     GeneralNames OPTIONAL }
+	//
+	// DistributionPointName ::= CHOICE {
+	//     fullName                [0]     GeneralNames,
+	//     nameRelativeToCRLIssuer [1]     RelativeDistinguishedName }
+	val := cryptobyte.String(value)
+	if !val.ReadASN1(&val, cryptobyte_asn1.SEQUENCE) {
+		return nil, errors.New("x509: invalid CRL distribution points")
+	}
+	for !val.Empty() {
+		var dpDER cryptobyte.String
+		if !val.ReadASN1(&dpDER, cryptobyte_asn1.SEQUENCE) {
+			return nil, errors.New("x509: invalid CRL distribution point")
+		}
+		var dpNameDER cryptobyte.String
+		var dpNamePresent bool
+		if !dpDER.ReadOptionalASN1(&dpNameDER, &dpNamePresent, cryptobyte_asn1.Tag(0).Constructed().ContextSpecific()) {
+			return nil, errors.New("x509: invalid CRL distribution point")
+		}
+		if !dpNamePresent {
+			continue
+		}
+		if !dpNameDER.ReadASN1(&dpNameDER, cryptobyte_asn1.Tag(0).Constructed().ContextSpecific()) {
+			return nil, errors.New("x509: invalid CRL distribution point")
+		}
+		for !dpNameDER.Empty() {
+			if !dpNameDER.PeekASN1Tag(cryptobyte_asn1.Tag(6).ContextSpecific()) {
+				break
+			}
+			var uri cryptobyte.String
+			if !dpNameDER.ReadASN1(&uri, cryptobyte_asn1.Tag(6).ContextSpecific()) {
+				return nil, errors.New("x509: invalid CRL distribution point")
+			}
+			urls = append(urls, string(uri))
+		}
+	}
+	return urls, nil
+}
+
 func fetchCRL(ctx context.Context, crlURL string, client *http.Client) (*x509.RevocationList, error) {
 	// validate URL
 	parsedURL, err := url.Parse(crlURL)