Use salt length equals hash, but verify auto salt length too in RSA P…

…SS sign methods.

Fixes dgrijalva#285.

rsa_pss.go

@@ -12,9 +12,14 @@ import (
 type SigningMethodRSAPSS struct {
 	*SigningMethodRSA
 	Options *rsa.PSSOptions
+	// VerifyOptions is optional. If set overrides Options for rsa.VerifyPPS.
+	// Used to accept tokens signed with rsa.PSSSaltLengthAuto, what doesn't follow
+	// https://tools.ietf.org/html/rfc7518#section-3.5 but was used previously.
+	// See https://github.com/dgrijalva/jwt-go/issues/285#issuecomment-437451244 for details.
+	VerifyOptions *rsa.PSSOptions
 }
 
-// Specific instances for RS/PS and company
+// Specific instances for RS/PS and company.
 var (
 	SigningMethodPS256 *SigningMethodRSAPSS
 	SigningMethodPS384 *SigningMethodRSAPSS
@@ -24,13 +29,15 @@ var (
 func init() {
 	// PS256
 	SigningMethodPS256 = &SigningMethodRSAPSS{
-		&SigningMethodRSA{
+		SigningMethodRSA: &SigningMethodRSA{
 			Name: "PS256",
 			Hash: crypto.SHA256,
 		},
-		&rsa.PSSOptions{
+		Options: &rsa.PSSOptions{
+			SaltLength: rsa.PSSSaltLengthEqualsHash,
+		},
+		VerifyOptions: &rsa.PSSOptions{
 			SaltLength: rsa.PSSSaltLengthAuto,
-			Hash:       crypto.SHA256,
 		},
 	}
 	RegisterSigningMethod(SigningMethodPS256.Alg(), func() SigningMethod {
@@ -39,13 +46,15 @@ func init() {
 
 	// PS384
 	SigningMethodPS384 = &SigningMethodRSAPSS{
-		&SigningMethodRSA{
+		SigningMethodRSA: &SigningMethodRSA{
 			Name: "PS384",
 			Hash: crypto.SHA384,
 		},
-		&rsa.PSSOptions{
+		Options: &rsa.PSSOptions{
+			SaltLength: rsa.PSSSaltLengthEqualsHash,
+		},
+		VerifyOptions: &rsa.PSSOptions{
 			SaltLength: rsa.PSSSaltLengthAuto,
-			Hash:       crypto.SHA384,
 		},
 	}
 	RegisterSigningMethod(SigningMethodPS384.Alg(), func() SigningMethod {
@@ -54,13 +63,15 @@ func init() {
 
 	// PS512
 	SigningMethodPS512 = &SigningMethodRSAPSS{
-		&SigningMethodRSA{
+		SigningMethodRSA: &SigningMethodRSA{
 			Name: "PS512",
 			Hash: crypto.SHA512,
 		},
-		&rsa.PSSOptions{
+		Options: &rsa.PSSOptions{
+			SaltLength: rsa.PSSSaltLengthEqualsHash,
+		},
+		VerifyOptions: &rsa.PSSOptions{
 			SaltLength: rsa.PSSSaltLengthAuto,
-			Hash:       crypto.SHA512,
 		},
 	}
 	RegisterSigningMethod(SigningMethodPS512.Alg(), func() SigningMethod {
@@ -94,7 +105,12 @@ func (m *SigningMethodRSAPSS) Verify(signingString, signature string, key interf
 	hasher := m.Hash.New()
 	hasher.Write([]byte(signingString))
 
-	return rsa.VerifyPSS(rsaKey, m.Hash, hasher.Sum(nil), sig, m.Options)
+	opts := m.Options
+	if m.VerifyOptions != nil {
+		opts = m.VerifyOptions
+	}
+
+	return rsa.VerifyPSS(rsaKey, m.Hash, hasher.Sum(nil), sig, opts)
 }
 
 // Implements the Sign method from SigningMethod

rsa_pss_test.go

@@ -7,8 +7,10 @@ import (
 	"io/ioutil"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/dgrijalva/jwt-go"
+	"github.com/dgrijalva/jwt-go/test"
 )
 
 var rsaPSSTestData = []struct {
@@ -94,3 +96,55 @@ func TestRSAPSSSign(t *testing.T) {
 		}
 	}
 }
+
+func TestRSAPSSSaltLengthCompatibility(t *testing.T) {
+	// Fails token verify, if salt length is auto.
+	ps256SaltLengthEqualsHash := &jwt.SigningMethodRSAPSS{
+		SigningMethodRSA: jwt.SigningMethodPS256.SigningMethodRSA,
+		Options: &rsa.PSSOptions{
+			SaltLength: rsa.PSSSaltLengthEqualsHash,
+		},
+	}
+
+	// Behaves as before https://github.com/dgrijalva/jwt-go/issues/285 fix.
+	ps256SaltLengthAuto := &jwt.SigningMethodRSAPSS{
+		SigningMethodRSA: jwt.SigningMethodPS256.SigningMethodRSA,
+		Options: &rsa.PSSOptions{
+			SaltLength: rsa.PSSSaltLengthAuto,
+		},
+	}
+	if !verify(jwt.SigningMethodPS256, makeToken(ps256SaltLengthEqualsHash)) {
+		t.Error("SigningMethodPS256 should accept salt length that is defined in RFC")
+	}
+	if !verify(ps256SaltLengthEqualsHash, makeToken(jwt.SigningMethodPS256)) {
+		t.Error("Sign by SigningMethodPS256 should have salt length that is defined in RFC")
+	}
+	if !verify(jwt.SigningMethodPS256, makeToken(ps256SaltLengthAuto)) {
+		t.Error("SigningMethodPS256 should accept auto salt length to be compatible with previous versions")
+	}
+	if !verify(ps256SaltLengthAuto, makeToken(jwt.SigningMethodPS256)) {
+		t.Error("Sign by SigningMethodPS256 should be accepted by previous versions")
+	}
+	if verify(ps256SaltLengthEqualsHash, makeToken(ps256SaltLengthAuto)) {
+		t.Error("Auto salt length should be not accepted, when RFC salt length is required")
+	}
+}
+
+func makeToken(method jwt.SigningMethod) string {
+	token := jwt.NewWithClaims(method, jwt.StandardClaims{
+		Issuer:   "example",
+		IssuedAt: time.Now().Unix(),
+	})
+	privateKey := test.LoadRSAPrivateKeyFromDisk("test/sample_key")
+	signed, err := token.SignedString(privateKey)
+	if err != nil {
+		panic(err)
+	}
+	return signed
+}
+
+func verify(signingMethod jwt.SigningMethod, token string) bool {
+	segments := strings.Split(token, ".")
+	err := signingMethod.Verify(strings.Join(segments[:2], "."), segments[2], test.LoadRSAPublicKeyFromDisk("test/sample_key.pub"))
+	return err == nil
+}