DigestAlgorithm & SigningCertificate attribute

README.md

@@ -71,8 +71,9 @@ err = sign.Sign(input_file, output_file, rdr, size, sign.SignData{
         CertType:   sign.CertificationSignature,
         DocMDPPerm: sign.AllowFillingExistingFormFieldsAndSignaturesPerms,
     },
-    Signer:            privateKey, // crypto.Signer
-    Certificate:       certificate, // x509.Certificate
+    Signer:            privateKey,         // crypto.Signer
+    DigestAlgorithm:   crypto.SHA256,      // hash algorithm for the digest creation
+    Certificate:       certificate,        // x509.Certificate
     CertificateChains: certificate_chains, // x509.Certificate.Verify()
     TSA: sign.TSA{
         URL: "https://freetsa.org/tsr",

sign.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"crypto"
 	"flag"
 	"fmt"
 	"log"
@@ -142,6 +143,7 @@ func main() {
 				DocMDPPerm: sign.AllowFillingExistingFormFieldsAndSignaturesPerms,
 			},
 			Signer:            pkey,
+			DigestAlgorithm:   crypto.SHA256,
 			Certificate:       cert,
 			CertificateChains: certificate_chains,
 			TSA: sign.TSA{

sign/helpers.go

@@ -1,6 +1,8 @@
 package sign
 
 import (
+	"crypto"
+	"encoding/asn1"
 	"errors"
 	"fmt"
 	"io"
@@ -140,3 +142,28 @@ func writePartFromSourceFileToTargetFile(input_file io.ReadSeeker, output_file i
 
 	return nil
 }
+
+var hashOIDs = map[crypto.Hash]asn1.ObjectIdentifier{
+	crypto.SHA1:   asn1.ObjectIdentifier([]int{1, 3, 14, 3, 2, 26}),
+	crypto.SHA256: asn1.ObjectIdentifier([]int{2, 16, 840, 1, 101, 3, 4, 2, 1}),
+	crypto.SHA384: asn1.ObjectIdentifier([]int{2, 16, 840, 1, 101, 3, 4, 2, 2}),
+	crypto.SHA512: asn1.ObjectIdentifier([]int{2, 16, 840, 1, 101, 3, 4, 2, 3}),
+}
+
+func getHashAlgorithmFromOID(target asn1.ObjectIdentifier) crypto.Hash {
+	for hash, oid := range hashOIDs {
+		if oid.Equal(target) {
+			return hash
+		}
+	}
+	return crypto.Hash(0)
+}
+
+func getOIDFromHashAlgorithm(target crypto.Hash) asn1.ObjectIdentifier {
+	for hash, oid := range hashOIDs {
+		if hash == target {
+			return oid
+		}
+	}
+	return nil
+}

sign/pdfsignature.go

@@ -2,6 +2,7 @@ package sign
 
 import (
 	"bytes"
+	"crypto"
 	"crypto/x509"
 	"encoding/asn1"
 	"encoding/hex"
@@ -13,6 +14,8 @@ import (
 
 	"github.com/digitorus/pkcs7"
 	"github.com/digitorus/timestamp"
+	"golang.org/x/crypto/cryptobyte"
+	cryptobyte_asn1 "golang.org/x/crypto/cryptobyte/asn1"
 )
 
 const signatureByteRangePlaceholder = "/ByteRange[0 ********** ********** **********]"
@@ -121,6 +124,39 @@ func (context *SignContext) fetchRevocationData() error {
 	return nil
 }
 
+func (context *SignContext) createSigningCertificateAttribute() (*pkcs7.Attribute, error) {
+	hash := context.SignData.DigestAlgorithm.New()
+	hash.Write(context.SignData.Certificate.Raw)
+
+	var b cryptobyte.Builder
+	b.AddASN1(cryptobyte_asn1.SEQUENCE, func(b *cryptobyte.Builder) { // SigningCertificate
+		b.AddASN1(cryptobyte_asn1.SEQUENCE, func(b *cryptobyte.Builder) { // []ESSCertID, []ESSCertIDv2
+			b.AddASN1(cryptobyte_asn1.SEQUENCE, func(b *cryptobyte.Builder) { // ESSCertID, ESSCertIDv2
+				if context.SignData.DigestAlgorithm.HashFunc() != crypto.SHA1 &&
+					context.SignData.DigestAlgorithm.HashFunc() != crypto.SHA256 { // default SHA-256
+					b.AddASN1(cryptobyte_asn1.SEQUENCE, func(b *cryptobyte.Builder) { // AlgorithmIdentifier
+						b.AddASN1ObjectIdentifier(getOIDFromHashAlgorithm(context.SignData.DigestAlgorithm))
+					})
+				}
+				b.AddASN1OctetString(hash.Sum(nil)) // certHash
+			})
+		})
+	})
+
+	sse, err := b.Bytes()
+	if err != nil {
+		return nil, err
+	}
+	signingCertificate := pkcs7.Attribute{
+		Type:  asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 9, 16, 2, 47}, // SigningCertificateV2
+		Value: asn1.RawValue{FullBytes: sse},
+	}
+	if context.SignData.DigestAlgorithm.HashFunc() == crypto.SHA1 {
+		signingCertificate.Type = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 9, 16, 2, 12} // SigningCertificate
+	}
+	return &signingCertificate, nil
+}
+
 func (context *SignContext) createSignature() ([]byte, error) {
 	if _, err := context.OutputBuffer.Seek(0, 0); err != nil {
 		return nil, err
@@ -140,12 +176,19 @@ func (context *SignContext) createSignature() ([]byte, error) {
 		return nil, fmt.Errorf("new signed data: %w", err)
 	}
 
+	signed_data.SetDigestAlgorithm(getOIDFromHashAlgorithm(context.SignData.DigestAlgorithm))
+	signingCertificate, err := context.createSigningCertificateAttribute()
+	if err != nil {
+		return nil, fmt.Errorf("new signed data: %w", err)
+	}
+
 	signer_config := pkcs7.SignerInfoConfig{
 		ExtraSignedAttributes: []pkcs7.Attribute{
 			{
 				Type:  asn1.ObjectIdentifier{1, 2, 840, 113583, 1, 1, 8},
 				Value: context.SignData.RevocationData,
 			},
+			*signingCertificate,
 		},
 	}
 

sign/sign.go

@@ -34,6 +34,7 @@ type SignData struct {
 	ObjectId           uint32
 	Signature          SignDataSignature
 	Signer             crypto.Signer
+	DigestAlgorithm    crypto.Hash
 	Certificate        *x509.Certificate
 	CertificateChains  [][]*x509.Certificate
 	TSA                TSA