Support other hash algs for pre-signed timestamp besides SHA256

There are three relevant hash algs used in this codebase:

* The certificate hash alg, which specifies how the CA certificate
  hashed the to-be-signed certificate
* The message hash alg, specified in the request, which says how the
  timestamp message was hashed
* The timestamp hash alg, which specifies how the timestamp signer
  should hash the pre-signed timestamp structure

The latter of these three was not configurable. We had a previous
approach that used the certificate hash alg, but this does not have to
match the timestamp hash alg.

Signed-off-by: Hayden Blauzvern <[email protected]>

cmd/timestamp-server/app/root.go

@@ -59,6 +59,7 @@ func init() {
 	rootCmd.PersistentFlags().BoolVar(&enablePprof, "enable-pprof", false, "enable pprof for profiling on port 6060")
 	rootCmd.PersistentFlags().BoolVar(&httpPingOnly, "http-ping-only", false, "serve only /ping in the http server")
 	rootCmd.PersistentFlags().String("timestamp-signer", "memory", "Timestamping authority signer. Valid options include: [kms, tink, memory, file]. Memory and file-based signers should only be used for testing")
+	rootCmd.PersistentFlags().String("timestamp-signer-hash", "sha256", "Hash algorithm used by the signer. Must match the hash algorithm specified for a KMS or Tink key. Valid options include: [sha256, sha384, sha512]. Ignored for Memory signer.")
 	// KMS flags
 	rootCmd.PersistentFlags().String("kms-key-resource", "", "KMS key for signing timestamp responses. Valid options include: [gcpkms://resource, azurekms://resource, hashivault://resource, awskms://resource]")
 	// Tink flags

pkg/api/api.go

@@ -34,15 +34,21 @@ import (
 )
 
 type API struct {
-	tsaSigner    crypto.Signer       // the signer to use for timestamping
-	certChain    []*x509.Certificate // timestamping cert chain
-	certChainPem string              // PEM encoded timestamping cert chain
+	tsaSigner     crypto.Signer       // the signer to use for timestamping
+	tsaSignerHash crypto.Hash         // hash algorithm used to hash pre-signed timestamps
+	certChain     []*x509.Certificate // timestamping cert chain
+	certChainPem  string              // PEM encoded timestamping cert chain
 }
 
 func NewAPI() (*API, error) {
 	ctx := context.Background()
 
-	tsaSigner, err := signer.NewCryptoSigner(ctx, viper.GetString("timestamp-signer"),
+	tsaSignerHash, err := signer.HashToAlg(viper.GetString("timestamp-signer-hash"))
+	if err != nil {
+		return nil, errors.Wrap(err, "error getting hash")
+	}
+	tsaSigner, err := signer.NewCryptoSigner(ctx, tsaSignerHash,
+		viper.GetString("timestamp-signer"),
 		viper.GetString("kms-key-resource"),
 		viper.GetString("tink-key-resource"), viper.GetString("tink-keyset-path"),
 		viper.GetString("tink-hcvault-token"),
@@ -81,9 +87,10 @@ func NewAPI() (*API, error) {
 	}
 
 	return &API{
-		tsaSigner:    tsaSigner,
-		certChain:    certChain,
-		certChainPem: string(certChainPEM),
+		tsaSigner:     tsaSigner,
+		tsaSignerHash: tsaSignerHash,
+		certChain:     certChain,
+		certChainPem:  string(certChainPEM),
 	}, nil
 }
 

pkg/api/timestamp.go

@@ -173,7 +173,7 @@ func TimestampResponseHandler(params ts.GetTimestampResponseParams) middleware.R
 		ExtraExtensions:   req.Extensions,
 	}
 
-	resp, err := tsStruct.CreateResponseWithOpts(api.certChain[0], api.tsaSigner, crypto.SHA256)
+	resp, err := tsStruct.CreateResponseWithOpts(api.certChain[0], api.tsaSigner, api.tsaSignerHash)
 	if err != nil {
 		return handleTimestampAPIError(params, http.StatusInternalServerError, err, failedToGenerateTimestampResponse)
 	}

pkg/signer/file.go

@@ -30,21 +30,21 @@ type File struct {
 	crypto.Signer
 }
 
-func NewFileSigner(keyPath, keyPass string) (*File, error) {
+func NewFileSigner(keyPath, keyPass string, hash crypto.Hash) (*File, error) {
 	opaqueKey, err := pemutil.Read(keyPath, pemutil.WithPassword([]byte(keyPass)))
 	if err != nil {
 		return nil, fmt.Errorf("file: provide a valid signer, %s is not valid: %w", keyPath, err)
 	}
 	// Cannot use signature.LoadSignerVerifier because the SignerVerifier interface does not extend crypto.Signer
 	switch pk := opaqueKey.(type) {
 	case *rsa.PrivateKey:
-		signer, err := signature.LoadRSAPKCS1v15SignerVerifier(pk, crypto.SHA256)
+		signer, err := signature.LoadRSAPKCS1v15SignerVerifier(pk, hash)
 		if err != nil {
 			return nil, err
 		}
 		return &File{signer}, nil
 	case *ecdsa.PrivateKey:
-		signer, err := signature.LoadECDSASignerVerifier(pk, crypto.SHA256)
+		signer, err := signature.LoadECDSASignerVerifier(pk, hash)
 		if err != nil {
 			return nil, err
 		}

pkg/signer/file_test.go

@@ -15,6 +15,7 @@
 package signer
 
 import (
+	"crypto"
 	"crypto/ecdsa"
 	"crypto/ed25519"
 	"crypto/elliptic"
@@ -88,7 +89,7 @@ func TestNewFileSigner(t *testing.T) {
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
 			tc := tc
-			_, err := NewFileSigner(tc.keyPath, tc.keyPass)
+			_, err := NewFileSigner(tc.keyPath, tc.keyPass, crypto.SHA256)
 			if tc.wantErr != (err != nil) {
 				t.Errorf("NewFileSigner() expected %t, got err %s", tc.wantErr, err)
 			}

pkg/signer/memory_test.go

@@ -30,7 +30,7 @@ import (
 func TestNewTimestampingCertWithChain(t *testing.T) {
 	ctx := context.Background()
 
-	signer, err := NewCryptoSigner(ctx, "memory", "", "", "", "", "", "")
+	signer, err := NewCryptoSigner(ctx, crypto.Hash(0), "memory", "", "", "", "", "", "")
 	if err != nil {
 		t.Fatalf("new signer: %v", err)
 	}

pkg/signer/signer.go

@@ -20,6 +20,7 @@ import (
 	"crypto/elliptic"
 	"crypto/rand"
 	"fmt"
+	"strings"
 
 	"github.com/sigstore/sigstore/pkg/signature"
 	"github.com/sigstore/sigstore/pkg/signature/kms"
@@ -36,15 +37,15 @@ const TinkScheme = "tink"
 const MemoryScheme = "memory"
 const FileScheme = "file"
 
-func NewCryptoSigner(ctx context.Context, signer, kmsKey, tinkKmsKey, tinkKeysetPath, hcVaultToken, fileSignerPath, fileSignerPasswd string) (crypto.Signer, error) {
+func NewCryptoSigner(ctx context.Context, hash crypto.Hash, signer, kmsKey, tinkKmsKey, tinkKeysetPath, hcVaultToken, fileSignerPath, fileSignerPasswd string) (crypto.Signer, error) {
 	switch signer {
 	case MemoryScheme:
 		sv, _, err := signature.NewECDSASignerVerifier(elliptic.P256(), rand.Reader, crypto.SHA256)
 		return sv, err
 	case FileScheme:
-		return NewFileSigner(fileSignerPath, fileSignerPasswd)
+		return NewFileSigner(fileSignerPath, fileSignerPasswd, hash)
 	case KMSScheme:
-		signer, err := kms.Get(ctx, kmsKey, crypto.SHA256)
+		signer, err := kms.Get(ctx, kmsKey, hash) // hash is ignored for all KMS providers except Hashivault
 		if err != nil {
 			return nil, err
 		}
@@ -60,3 +61,19 @@ func NewCryptoSigner(ctx context.Context, signer, kmsKey, tinkKmsKey, tinkKeyset
 		return nil, fmt.Errorf("unsupported signer type: %s", signer)
 	}
 }
+
+func HashToAlg(signerHashAlg string) (crypto.Hash, error) {
+	lowercaseAlg := strings.ToLower(signerHashAlg)
+	var hash crypto.Hash
+	switch lowercaseAlg {
+	case "sha256":
+		hash = crypto.SHA256
+	case "sha384":
+		hash = crypto.SHA384
+	case "sha512":
+		hash = crypto.SHA512
+	default:
+		return crypto.Hash(0), fmt.Errorf("unsupported hash algorithm: %s", lowercaseAlg)
+	}
+	return hash, nil
+}

pkg/tests/server.go

@@ -28,6 +28,7 @@ import (
 
 func createServer(t *testing.T) string {
 	viper.Set("timestamp-signer", "memory")
+	viper.Set("timestamp-signer-hash", "sha256")
 	// unused port
 	apiServer := server.NewRestAPIServer("localhost", 0, []string{"http"}, false, 10*time.Second, 10*time.Second)
 	server := httptest.NewServer(apiServer.GetHandler())