Added support for ECDH in enveloped data with ECDSA certificates.

- fixed parsing of choice(was compatible with Apple mail.app which tagged kari explicitly)
- minor fixes

cms/cms.go

@@ -65,7 +65,11 @@ func (cms *CMS) Encrypt(data []byte, recipients []*x509.Certificate) (der []byte
 	var reciInfos []protocol.RecipientInfo
 
 	for _, recipient := range recipients {
-		rInfo := protocol.NewRecipientInfo(recipient, key)
+		var rInfo protocol.RecipientInfo
+		rInfo, err = protocol.NewRecipientInfo(recipient, key)
+		if err != nil {
+			return
+		}
 		reciInfos = append(reciInfos, rInfo)
 	}
 
@@ -90,7 +94,11 @@ func (cms *CMS) AuthEncrypt(data []byte, recipients []*x509.Certificate) (der []
 	var reciInfos []protocol.RecipientInfo
 
 	for _, recipient := range recipients {
-		rInfo := protocol.NewRecipientInfo(recipient, key)
+		var rInfo protocol.RecipientInfo
+		rInfo, err = protocol.NewRecipientInfo(recipient, key)
+		if err != nil {
+			return
+		}
 		reciInfos = append(reciInfos, rInfo)
 	}
 

cms/cms_test.go

@@ -3,10 +3,13 @@ package cms
 import (
 	"bytes"
 	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
 	"crypto/tls"
 	"crypto/x509"
 	"crypto/x509/pkix"
-	"log"
+	"strings"
 	"testing"
 
 	openssl "github.com/InfiniteLoopSpace/go_S-MIME/openssl"
@@ -26,162 +29,216 @@ var (
 		CommonName: "leaf.example.com",
 	}))
 
+	ecKey, _ = ecdsa.GenerateKey(elliptic.P384(), rand.Reader)
+
+	leafECC = intermediate.Issue(pki.Subject(pkix.Name{
+		CommonName: "leaf.example.com",
+	}), pki.PrivateKey(ecKey))
+
 	keyPair = tls.Certificate{
 		Certificate: [][]byte{leaf.Certificate.Raw, intermediate.Certificate.Raw, root.Certificate.Raw},
 		PrivateKey:  leaf.PrivateKey.(crypto.PrivateKey),
+		Leaf:        leaf.Certificate,
 	}
+
+	keyPairECC = tls.Certificate{
+		Certificate: [][]byte{leafECC.Certificate.Raw, intermediate.Certificate.Raw, root.Certificate.Raw},
+		PrivateKey:  leafECC.PrivateKey.(crypto.PrivateKey),
+		Leaf:        leafECC.Certificate,
+	}
+
+	keyPairs = []tls.Certificate{}
+
+	keyPairsOpenssl = []tls.Certificate{}
 )
 
+func TestMain(m *testing.M) {
+
+	keyPairs = []tls.Certificate{keyPair, keyPairECC}
+
+	version, err := openssl.Openssl(nil, "version")
+
+	if err == nil {
+		keyPairsOpenssl = append(keyPairsOpenssl, keyPair)
+	}
+
+	if strings.HasPrefix(string(version), "OpenSSL 1.1") {
+		openssl.SMIME = "cms"
+		keyPairsOpenssl = append(keyPairsOpenssl, keyPairECC)
+	}
+
+	m.Run()
+}
+
 func TestAuthEnrypt(t *testing.T) {
 
-	cms, err := New(keyPair)
-	if err != nil {
-		t.Error(err)
-	}
+	for _, keypair := range keyPairs {
+		cms, err := New(keypair)
+		if err != nil {
+			t.Error(err)
+		}
 
-	plaintext := []byte("Hallo Welt!")
+		plaintext := []byte("Hallo Welt!")
 
-	ciphertext, err := cms.AuthEncrypt(plaintext, []*x509.Certificate{leaf.Certificate})
-	if err != nil {
-		t.Error(err)
-	}
+		ciphertext, err := cms.AuthEncrypt(plaintext, []*x509.Certificate{keypair.Leaf})
+		if err != nil {
+			t.Error(err)
+		}
 
-	plain, err := cms.AuthDecrypt(ciphertext)
-	if err != nil {
-		log.Fatal(err)
-	}
+		plain, err := cms.AuthDecrypt(ciphertext)
+		if err != nil {
+			t.Error(err)
+		}
 
-	if !bytes.Equal(plaintext, plain) {
-		t.Fatal("Encryption and decryption are not inverse")
+		if !bytes.Equal(plaintext, plain) {
+			t.Fatal("Encryption and decryption are not inverse")
+		}
 	}
 }
 
 func TestEnryptDecrypt(t *testing.T) {
 
-	cms, err := New(keyPair)
-	if err != nil {
-		t.Error(err)
-	}
+	for _, keypair := range keyPairs {
+		cms, err := New(keypair)
+		if err != nil {
+			t.Error(err)
+		}
 
-	plaintext := []byte("Hallo Welt!")
+		plaintext := []byte("Hallo Welt!")
 
-	ciphertext, err := cms.Encrypt(plaintext, []*x509.Certificate{leaf.Certificate})
-	if err != nil {
-		t.Error(err)
-	}
+		ciphertext, err := cms.Encrypt(plaintext, []*x509.Certificate{keypair.Leaf})
+		if err != nil {
+			t.Error(err)
+		}
 
-	plain, err := cms.Decrypt(ciphertext)
-	if err != nil {
-		log.Fatal(err)
-	}
+		plain, err := cms.Decrypt(ciphertext)
+		if err != nil {
+			t.Error(err)
+		}
 
-	if !bytes.Equal(plaintext, plain) {
-		t.Fatal("Encryption and decryption are not inverse")
+		if !bytes.Equal(plaintext, plain) {
+			t.Fatal("Encryption and decryption are not inverse")
+		}
 	}
 }
 
 func TestSignVerify(t *testing.T) {
-	cms, err := New(keyPair)
-	if err != nil {
-		t.Error(err)
-	}
 
-	cms.roots.AddCert(root.Certificate)
+	for _, keypair := range keyPairs {
+		cms, err := New(keypair)
+		if err != nil {
+			t.Error(err)
+		}
 
-	msg := []byte("Hallo Welt!")
+		cms.roots.AddCert(root.Certificate)
 
-	der, err := cms.Sign(msg)
-	if err != nil {
-		t.Error(err)
-	}
+		msg := []byte("Hallo Welt!")
 
-	_, err = cms.Verify(der)
-	if err != nil {
-		t.Error(err)
+		der, err := cms.Sign(msg)
+		if err != nil {
+			t.Error(err)
+		}
+
+		_, err = cms.Verify(der)
+		if err != nil {
+			t.Error(err)
+		}
 	}
 }
 
 func TestEncryptOpenSSL(t *testing.T) {
-	message := []byte("Hallo Welt!")
 
-	der, err := openssl.Encrypt(message, leaf.Certificate, "-outform", "DER")
-	if err != nil {
-		t.Error(err)
-	}
+	for _, keypair := range keyPairsOpenssl {
+		message := []byte("Hallo Welt!")
 
-	cms, err := New(keyPair)
-	plain, err := cms.Decrypt(der)
-	if err != nil {
-		t.Error(err)
-	}
+		der, err := openssl.Encrypt(message, keypair.Leaf, "-outform", "DER")
+		if err != nil {
+			t.Error(err)
+		}
 
-	if !bytes.Equal(message, plain) {
-		t.Fatal("Encryption and decryption are not inverse")
+		cms, err := New(keypair)
+		plain, err := cms.Decrypt(der)
+		if err != nil {
+			t.Error(err)
+		}
+
+		if !bytes.Equal(message, plain) {
+			t.Fatal("Encryption and decryption are not inverse")
+		}
 	}
 }
 
 func TestDecryptOpenSSL(t *testing.T) {
-	message := []byte("Hallo Welt!")
 
-	cms, _ := New()
-	ciphertext, err := cms.Encrypt(message, []*x509.Certificate{leaf.Certificate})
-	if err != nil {
-		t.Error(err)
-	}
+	for _, keypair := range keyPairsOpenssl {
+		message := []byte("Hallo Welt!")
 
-	plain, err := openssl.Decrypt(ciphertext, leaf.PrivateKey, "-inform", "DER")
-	if err != nil {
-		t.Error(err)
-	}
+		cms, _ := New()
+		ciphertext, err := cms.Encrypt(message, []*x509.Certificate{keypair.Leaf})
+		if err != nil {
+			t.Error(err)
+		}
 
-	if !bytes.Equal(message, plain) {
-		t.Fatal("Encryption and decryption are not inverse")
+		plain, err := openssl.Decrypt(ciphertext, keypair.PrivateKey, "-inform", "DER")
+		if err != nil {
+			t.Error(err)
+		}
+
+		if !bytes.Equal(message, plain) {
+			t.Fatal("Encryption and decryption are not inverse")
+		}
 	}
 }
 
 func TestSignOpenSSL(t *testing.T) {
-	message := []byte("Hallo Welt")
 
-	sig, err := openssl.SignDetached(message, leaf.Certificate, leaf.PrivateKey, []*x509.Certificate{intermediate.Certificate}, "-outform", "DER")
-	if err != nil {
-		t.Error(err)
-	}
+	for _, keypair := range keyPairsOpenssl {
+		message := []byte("Hallo Welt")
 
-	cms, err := New()
-	if err != nil {
-		t.Error(err)
-	}
-	cms.roots.AddCert(root.Certificate)
+		sig, err := openssl.SignDetached(message, keypair.Leaf, keypair.PrivateKey, []*x509.Certificate{intermediate.Certificate}, "-outform", "DER")
+		if err != nil {
+			t.Error(err)
+		}
 
-	_, err = cms.Verify(sig)
-	if err != nil {
-		t.Error(err)
+		cms, err := New()
+		if err != nil {
+			t.Error(err)
+		}
+		cms.roots.AddCert(root.Certificate)
+
+		_, err = cms.Verify(sig)
+		if err != nil {
+			t.Error(err)
+		}
 	}
 }
 
 func TestVerifyOpenSSL(t *testing.T) {
-	cms, err := New(keyPair)
-	if err != nil {
-		t.Error(err)
-	}
 
-	cms.TimeStamp = true
+	for _, keypair := range keyPairsOpenssl {
+		cms, err := New(keypair)
+		if err != nil {
+			t.Error(err)
+		}
 
-	cms.roots.AddCert(root.Certificate)
+		cms.TimeStamp = true
 
-	msg := []byte("Hallo Welt!")
+		cms.roots.AddCert(root.Certificate)
 
-	der, err := cms.Sign(msg)
-	if err != nil {
-		t.Error(err)
-	}
+		msg := []byte("Hallo Welt!")
 
-	sig, err := openssl.Verify(der, root.Certificate, "-inform", "DER")
-	if err != nil {
-		t.Error(err)
-	}
+		der, err := cms.Sign(msg)
+		if err != nil {
+			t.Error(err)
+		}
 
-	if !bytes.Equal(msg, sig) {
-		t.Fatal("Signed message and message do not agree!")
+		sig, err := openssl.Verify(der, root.Certificate, "-inform", "DER")
+		if err != nil {
+			t.Error(err)
+		}
+
+		if !bytes.Equal(msg, sig) {
+			t.Fatal("Signed message and message do not agree!")
+		}
 	}
 }

cms/protocol/ecdh.go

@@ -0,0 +1,218 @@
+package protocol
+
+import (
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/tls"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/asn1"
+	"encoding/binary"
+	"errors"
+	"math/big"
+
+	"github.com/InfiniteLoopSpace/go_S-MIME/oid"
+)
+
+var errUnsupported = errors.New("Unsupported hash function")
+
+// ECDHsharedSecret computes shared secret with ephemeral static ECDH
+func ECDHsharedSecret(curve elliptic.Curve, priv []byte, pubX, pubY *big.Int) []byte {
+
+	x, _ := curve.ScalarMult(pubX, pubY, priv)
+
+	return x.Bytes()
+}
+
+// ANSIx963KDF implents ANSI X9.63 key derivation function
+func ANSIx963KDF(sharedSecret, sharedInfo []byte, keyLen int, hash crypto.Hash) (key []byte, err error) {
+
+	ctr := make([]byte, 4)
+	ctr[3] = 0x01
+	if hash == 0 || !hash.Available() {
+		return nil, errUnsupported
+	}
+	h := hash.New()
+
+	for i := 0; i < keyLen/hash.Size()+1; i++ {
+		h.Reset()
+		h.Write(sharedSecret)
+		h.Write(ctr)
+		h.Write(sharedInfo)
+		key = append(key, h.Sum(nil)...)
+
+		// Increment counter
+		for i := len(ctr) - 1; i >= 0; i-- {
+			ctr[i]++
+			if ctr[i] != 0 {
+				break
+			}
+		}
+	}
+
+	return key[:keyLen], nil
+}
+
+func encryptKeyECDH(key []byte, recipient *x509.Certificate) (kari KeyAgreeRecipientInfo, err error) {
+
+	keyWrapAlgorithm := oid.KeyWrap{KeyWrapAlgorithm: oid.AES128Wrap}
+	keyEncryptionAlgorithm := oid.DHSinglePassstdDHsha256kdfscheme
+	hash := oid.KDFHashAlgorithm[keyEncryptionAlgorithm.String()]
+
+	kari.UKM = make([]byte, 8)
+	rand.Read(kari.UKM)
+
+	kari.Version = 3
+	kari.Originator.OriginatorKey.Algorithm = pkix.AlgorithmIdentifier{Algorithm: oid.ECPublicKey}
+
+	// check recipient key
+
+	if recipient.PublicKeyAlgorithm != x509.ECDSA {
+		err = errors.New("Recipient certficiate has wrong public key algorithm, expected ECDSA")
+		return
+	}
+
+	pubKey, ok := recipient.PublicKey.(*ecdsa.PublicKey)
+	if !ok {
+		err = errors.New("Can not parse public key of recipient")
+		return
+	}
+
+	// genrate ephemeral public key and key encryption key
+
+	priv, x, y, err := elliptic.GenerateKey(pubKey.Curve, rand.Reader)
+	if err != nil {
+		return
+	}
+
+	ephPubKey := elliptic.Marshal(pubKey.Curve, x, y)
+	kari.Originator.OriginatorKey.PublicKey = asn1.BitString{Bytes: ephPubKey, BitLength: len(ephPubKey) * 8}
+
+	sharedSecret := ECDHsharedSecret(pubKey.Curve, priv, pubKey.X, pubKey.Y)
+
+	keyLenBigEnd := make([]byte, 4)
+	binary.BigEndian.PutUint32(keyLenBigEnd, uint32(keyWrapAlgorithm.KeyLen())*8)
+	sharedInfo := ECCCMSSharedInfo{KeyInfo: keyWrapAlgorithm.AlgorithmIdentifier(),
+		SuppPubInfo: keyLenBigEnd,
+		EntityUInfo: kari.UKM}
+
+	sharedInfoDER, err := asn1.Marshal(sharedInfo)
+
+	kek, err := ANSIx963KDF(sharedSecret, sharedInfoDER, keyWrapAlgorithm.KeyLen(), hash)
+	if err != nil {
+		return
+	}
+
+	// encrypt key
+
+	keyWrapAlgorithm.KEK = kek
+	encKey, err := keyWrapAlgorithm.Wrap(key)
+	if err != nil {
+		return
+	}
+
+	keyWrapAlgorithmIdentifier, err := RawValue(keyWrapAlgorithm.AlgorithmIdentifier())
+	if err != nil {
+		return
+	}
+
+	kari.KeyEncryptionAlgorithm = pkix.AlgorithmIdentifier{Algorithm: keyEncryptionAlgorithm,
+		Parameters: keyWrapAlgorithmIdentifier}
+
+	ias, err := NewIssuerAndSerialNumber(recipient)
+	karID := KeyAgreeRecipientIdentifier{IAS: ias}
+
+	kari.RecipientEncryptedKeys = append(kari.RecipientEncryptedKeys, RecipientEncryptedKey{RID: karID, EncryptedKey: encKey})
+
+	return
+}
+
+// ECCCMSSharedInfo ECC-CMS-SharedInfo ::= SEQUENCE {
+//	keyInfo         AlgorithmIdentifier,
+//	entityUInfo [0] EXPLICIT OCTET STRING OPTIONAL,
+//	suppPubInfo [2] EXPLICIT OCTET STRING  }
+type ECCCMSSharedInfo struct {
+	KeyInfo     pkix.AlgorithmIdentifier
+	EntityUInfo []byte `asn1:"optional,explicit,tag:0"`
+	SuppPubInfo []byte `asn1:"explicit,tag:2"`
+}
+
+func (kari *KeyAgreeRecipientInfo) decryptKey(keyPair tls.Certificate) (key []byte, err error) {
+
+	// check for ec key
+
+	if kari.Version != 3 {
+		err = errors.New("Version not supported")
+		return
+	}
+
+	if !kari.Originator.OriginatorKey.Algorithm.Algorithm.Equal(oid.ECPublicKey) {
+		err = errors.New("Orginator key algorithm not supported")
+		return
+	}
+
+	pubKey, ok := keyPair.Leaf.PublicKey.(*ecdsa.PublicKey)
+	if !ok {
+		err = errors.New("Can not parse public key of recipient")
+		return
+	}
+
+	x, y := elliptic.Unmarshal(pubKey.Curve, kari.Originator.OriginatorKey.PublicKey.Bytes)
+
+	// genrate ephemeral public key and key encryption key
+
+	priv := keyPair.PrivateKey.(*ecdsa.PrivateKey)
+
+	privateKeyBytes := keyPair.PrivateKey.(*ecdsa.PrivateKey).D.Bytes()
+	paddedPrivateKey := make([]byte, (priv.Curve.Params().N.BitLen()+7)/8)
+	copy(paddedPrivateKey[len(paddedPrivateKey)-len(privateKeyBytes):], privateKeyBytes)
+
+	sharedSecret := ECDHsharedSecret(pubKey.Curve, paddedPrivateKey, x, y)
+
+	hash, ok := oid.KDFHashAlgorithm[kari.KeyEncryptionAlgorithm.Algorithm.String()]
+	if !ok {
+		err = errors.New("Unsupported key derivation hash algorithm")
+		return
+	}
+
+	var keyWrapAlgorithmIdentifier pkix.AlgorithmIdentifier
+	asn1.Unmarshal(kari.KeyEncryptionAlgorithm.Parameters.FullBytes, &keyWrapAlgorithmIdentifier)
+	keyWrapAlgorithm := oid.KeyWrap{KeyWrapAlgorithm: keyWrapAlgorithmIdentifier.Algorithm}
+
+	//
+
+	keyLenBigEnd := make([]byte, 4)
+	binary.BigEndian.PutUint32(keyLenBigEnd, uint32(keyWrapAlgorithm.KeyLen())*8)
+	sharedInfo := ECCCMSSharedInfo{KeyInfo: keyWrapAlgorithmIdentifier,
+		SuppPubInfo: keyLenBigEnd,
+		EntityUInfo: kari.UKM}
+
+	sharedInfoDER, err := asn1.Marshal(sharedInfo)
+
+	kek, err := ANSIx963KDF(sharedSecret, sharedInfoDER, keyWrapAlgorithm.KeyLen(), hash)
+	if err != nil {
+		return
+	}
+
+	keyWrapAlgorithm.KEK = kek
+
+	// encrypt key
+
+	ias, err := NewIssuerAndSerialNumber(keyPair.Leaf)
+	if err != nil {
+		return
+	}
+
+	for i := range kari.RecipientEncryptedKeys {
+		if kari.RecipientEncryptedKeys[i].RID.IAS.Equal(ias) {
+			key, err = keyWrapAlgorithm.UnWrap(kari.RecipientEncryptedKeys[i].EncryptedKey)
+			return
+		}
+	}
+
+	err = ErrNoKeyFound
+
+	return
+}

cms/protocol/envelopeddata.go

@@ -58,7 +58,7 @@ func (ed *EnvelopedData) decryptKey(keyPair tls.Certificate) (key []byte, err er
 	for i := range ed.RecipientInfos {
 
 		key, err = ed.RecipientInfos[i].decryptKey(keyPair)
-		if key != nil {
+		if key != nil || err != ErrNoKeyFound {
 			return
 		}
 	}

cms/protocol/reciepientinfo.go

@@ -33,8 +33,14 @@ type RecipientInfo struct {
 
 func (recInfo *RecipientInfo) decryptKey(keyPair tls.Certificate) (key []byte, err error) {
 
-	return recInfo.KTRI.decryptKey(keyPair)
+	key, err = recInfo.KTRI.decryptKey(keyPair)
+	if key != nil {
+		return
+	}
 
+	key, err = recInfo.KARI.decryptKey(keyPair)
+
+	return
 }
 
 //KeyTransRecipientInfo ::= SEQUENCE {
@@ -102,7 +108,7 @@ type RecipientIdentifier struct {
 }
 
 // NewRecipientInfo creates RecipientInfo for giben recipient and key.
-func NewRecipientInfo(recipient *x509.Certificate, key []byte) RecipientInfo {
+func NewRecipientInfo(recipient *x509.Certificate, key []byte) (info RecipientInfo, err error) {
 	version := 0 //issuerAndSerialNumber
 
 	rid := RecipientIdentifier{}
@@ -118,24 +124,35 @@ func NewRecipientInfo(recipient *x509.Certificate, key []byte) RecipientInfo {
 		rid.SKI = recipient.SubjectKeyId
 	}
 
-	kea := oid.PublicKeyAlgorithmToEncrytionAlgorithm[recipient.PublicKeyAlgorithm]
-	if _, ok := oid.PublicKeyAlgorithmToEncrytionAlgorithm[recipient.PublicKeyAlgorithm]; !ok {
-		log.Fatal("NewRecipientInfo: PublicKeyAlgorithm not supported")
+	switch recipient.PublicKeyAlgorithm {
+	case x509.RSA:
+		var encrypted []byte
+		encrypted, err = encryptKeyRSA(key, recipient)
+		if err != nil {
+			return
+		}
+		info = RecipientInfo{
+			KTRI: KeyTransRecipientInfo{
+				Version:                version,
+				Rid:                    rid,
+				KeyEncryptionAlgorithm: pkix.AlgorithmIdentifier{Algorithm: oid.EncryptionAlgorithmRSA},
+				EncryptedKey:           encrypted,
+			}}
+	case x509.ECDSA:
+		var kari KeyAgreeRecipientInfo
+		kari, err = encryptKeyECDH(key, recipient)
+		if err != nil {
+			return
+		}
+		info = RecipientInfo{KARI: kari}
+	default:
+		err = errors.New("Public key algorithm not supported")
 	}
 
-	encrypted, _ := encryptKey(key, recipient)
-
-	info := RecipientInfo{
-		KTRI: KeyTransRecipientInfo{
-			Version:                version,
-			Rid:                    rid,
-			KeyEncryptionAlgorithm: kea,
-			EncryptedKey:           encrypted,
-		}}
-	return info
+	return
 }
 
-func encryptKey(key []byte, recipient *x509.Certificate) ([]byte, error) {
+func encryptKeyRSA(key []byte, recipient *x509.Certificate) ([]byte, error) {
 	if pub := recipient.PublicKey.(*rsa.PublicKey); pub != nil {
 		return rsa.EncryptPKCS1v15(rand.Reader, pub, key)
 	}

cms/protocol/signeddata.go

@@ -379,8 +379,12 @@ func (sd *SignedData) Verify(Opts x509.VerifyOptions, detached []byte) (chains [
 				return
 			}
 		}
-
-		err = cert.CheckSignature(signer.X509SignatureAlgorithm(), signedMessage, signer.Signature)
+		var sigAlg x509.SignatureAlgorithm
+		sigAlg, err = signer.X509SignatureAlgorithm()
+		if err != nil {
+			return
+		}
+		err = cert.CheckSignature(sigAlg, signedMessage, signer.Signature)
 		if err != nil {
 			return
 		}

cms/protocol/signerinfo.go

@@ -6,6 +6,7 @@ import (
 	"crypto/x509"
 	"crypto/x509/pkix"
 	"encoding/asn1"
+	"fmt"
 	"time"
 
 	asn "github.com/InfiniteLoopSpace/go_S-MIME/asn1"
@@ -83,13 +84,18 @@ func (si SignerInfo) Hash() (crypto.Hash, error) {
 
 // X509SignatureAlgorithm gets the x509.SignatureAlgorithm that should be used
 // for verifying this SignerInfo's signature.
-func (si SignerInfo) X509SignatureAlgorithm() x509.SignatureAlgorithm {
+func (si SignerInfo) X509SignatureAlgorithm() (sigAlg x509.SignatureAlgorithm, err error) {
 	var (
 		sigOID    = si.SignatureAlgorithm.Algorithm.String()
 		digestOID = si.DigestAlgorithm.Algorithm.String()
 	)
+	sigAlg, ok := oid.SignatureAlgorithms[sigOID][digestOID]
 
-	return oid.SignatureAlgorithms[sigOID][digestOID]
+	if !ok {
+		err = fmt.Errorf("Signature algorithm with OID %s in combination with digest with OID %s not supported", sigOID, digestOID)
+	}
+
+	return
 }
 
 // GetContentTypeAttribute gets the signed ContentType attribute from the