ietf-cms: add support for user-supplied signed attributes

Some use cases for CMS (notably the code signatures in Apple signed MachO
binaries) require additional attributes to be signed. This change adds a
SignWithAttrs method to cms.SignedData that accepts additional signed
attributes to include.

ietf-cms/protocol/protocol.go

@@ -626,6 +626,11 @@ func NewSignedData(eci EncapsulatedContentInfo) (*SignedData, error) {
 
 // AddSignerInfo adds a SignerInfo to the SignedData.
 func (sd *SignedData) AddSignerInfo(chain []*x509.Certificate, signer crypto.Signer) error {
+	return sd.AddSignerInfoWithAttrs(nil, chain, signer)
+}
+
+// AddSignerInfoWithAttrs adds a SignerInfo to the SignedData, allowing for the caller to supply extra Attributes to sign.
+func (sd *SignedData) AddSignerInfoWithAttrs(attrs []Attribute, chain []*x509.Certificate, signer crypto.Signer) error {
 	// figure out which certificate is associated with signer.
 	pub, err := x509.MarshalPKIXPublicKey(signer.Public())
 	if err != nil {
@@ -712,7 +717,7 @@ func (sd *SignedData) AddSignerInfo(chain []*x509.Certificate, signer crypto.Sig
 	}
 
 	// sort attributes to match required order in marshaled form
-	si.SignedAttrs, err = sortAttributes(stAttr, mdAttr, ctAttr)
+	si.SignedAttrs, err = sortAttributes(append([]Attribute{stAttr, mdAttr, ctAttr}, attrs...)...)
 	if err != nil {
 		return err
 	}

ietf-cms/protocol/protocol_test.go

@@ -88,6 +88,99 @@ func TestSignerInfo(t *testing.T) {
 	}
 }
 
+func TestSignerInfoWithExtraAttrs(t *testing.T) {
+	priv, cert, err := pkcs12.Decode(fixturePFX, "asdf")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	msg := []byte("hello, world!")
+
+	eci, err := NewEncapsulatedContentInfo(oid.ContentTypeData, msg)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	sd, err := NewSignedData(eci)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	extraAttr, err := NewAttribute(asn1.ObjectIdentifier{1, 2, 3, 4, 5, 6}, []byte("this is an attribute"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	chain := []*x509.Certificate{cert}
+	if err = sd.AddSignerInfoWithAttrs(Attributes{extraAttr}, chain, priv.(*ecdsa.PrivateKey)); err != nil {
+		t.Fatal(err)
+	}
+
+	der, err := sd.ContentInfoDER()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	ci, err := ParseContentInfo(der)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	sd2, err := ci.SignedDataContent()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	msg2, err := sd2.EncapContentInfo.DataEContent()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(msg, msg2) {
+		t.Fatal()
+	}
+
+	attrs := sd2.SignerInfos[0].SignedAttrs
+	found := false
+	for _, attr := range attrs {
+		if attr.Type.Equal(extraAttr.Type) {
+			if !bytes.Equal(attr.RawValue.FullBytes, extraAttr.RawValue.FullBytes) {
+				t.Fatalf("Extra signed attribute data round trip mismatch: want %#v, got %#v", extraAttr, attr)
+			}
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Fatalf("Extra attribute not present in signer info")
+	}
+
+	// Make detached
+	sd.EncapContentInfo.EContent = asn1.RawValue{}
+
+	der, err = sd.ContentInfoDER()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	ci, err = ParseContentInfo(der)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	sd2, err = ci.SignedDataContent()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	msg2, err = sd2.EncapContentInfo.DataEContent()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if msg2 != nil {
+		t.Fatal()
+	}
+}
+
 func TestEncapsulatedContentInfo(t *testing.T) {
 	ci, _ := ParseContentInfo(fixtureSignatureOpenSSLAttached)
 	sd, _ := ci.SignedDataContent()

ietf-cms/sign.go

@@ -3,6 +3,8 @@ package cms
 import (
 	"crypto"
 	"crypto/x509"
+
+	"github.com/github/smimesign/ietf-cms/protocol"
 )
 
 // Sign creates a CMS SignedData from the content and signs it with signer. At
@@ -47,3 +49,8 @@ func SignDetached(data []byte, chain []*x509.Certificate, signer crypto.Signer)
 func (sd *SignedData) Sign(chain []*x509.Certificate, signer crypto.Signer) error {
 	return sd.psd.AddSignerInfo(chain, signer)
 }
+
+// SignWithAttrs adds a signature with extra signed attributes to the SignedData.
+func (sd *SignedData) SignWithAttrs(attrs protocol.Attributes, chain []*x509.Certificate, signer crypto.Signer) error {
+	return sd.psd.AddSignerInfoWithAttrs(attrs, chain, signer)
+}