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// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
import (
"crypto"
"crypto/rsa"
"crypto/sha1"
"crypto/x509"
"crypto/x509/pkix"
"encoding/asn1"
"io"
"math/big"
"time"
)
// signPKCS7 does the minimal amount of work necessary to embed an RSA
// signature into a PKCS#7 certificate.
//
// We prepare the certificate using the x509 package, read it back in
// to our custom data type and then write it back out with the signature.
func signPKCS7(rand io.Reader, priv *rsa.PrivateKey, msg []byte) ([]byte, error) {
const serialNumber = 0x5462c4dd // arbitrary
name := pkix.Name{CommonName: "gomobile"}
template := &x509.Certificate{
SerialNumber: big.NewInt(serialNumber),
SignatureAlgorithm: x509.SHA1WithRSA,
Subject: name,
}
b, err := x509.CreateCertificate(rand, template, template, priv.Public(), priv)
if err != nil {
return nil, err
}
c := certificate{}
if _, err := asn1.Unmarshal(b, &c); err != nil {
return nil, err
}
h := sha1.New()
h.Write(msg)
hashed := h.Sum(nil)
signed, err := rsa.SignPKCS1v15(rand, priv, crypto.SHA1, hashed)
if err != nil {
return nil, err
}
content := pkcs7SignedData{
ContentType: oidSignedData,
Content: signedData{
Version: 1,
DigestAlgorithms: []pkix.AlgorithmIdentifier{{
Algorithm: oidSHA1,
Parameters: asn1.RawValue{Tag: 5},
}},
ContentInfo: contentInfo{Type: oidData},
Certificates: c,
SignerInfos: []signerInfo{{
Version: 1,
IssuerAndSerialNumber: issuerAndSerialNumber{
Issuer: name.ToRDNSequence(),
SerialNumber: serialNumber,
},
DigestAlgorithm: pkix.AlgorithmIdentifier{
Algorithm: oidSHA1,
Parameters: asn1.RawValue{Tag: 5},
},
DigestEncryptionAlgorithm: pkix.AlgorithmIdentifier{
Algorithm: oidRSAEncryption,
Parameters: asn1.RawValue{Tag: 5},
},
EncryptedDigest: signed,
}},
},
}
return asn1.Marshal(content)
}
type pkcs7SignedData struct {
ContentType asn1.ObjectIdentifier
Content signedData `asn1:"tag:0,explicit"`
}
// signedData is defined in rfc2315, section 9.1.
type signedData struct {
Version int
DigestAlgorithms []pkix.AlgorithmIdentifier `asn1:"set"`
ContentInfo contentInfo
Certificates certificate `asn1:"tag0,explicit"`
SignerInfos []signerInfo `asn1:"set"`
}
type contentInfo struct {
Type asn1.ObjectIdentifier
// Content is optional in PKCS#7 and not provided here.
}
// certificate is defined in rfc2459, section 4.1.
type certificate struct {
TBSCertificate tbsCertificate
SignatureAlgorithm pkix.AlgorithmIdentifier
SignatureValue asn1.BitString
}
// tbsCertificate is defined in rfc2459, section 4.1.
type tbsCertificate struct {
Version int `asn1:"tag:0,default:2,explicit"`
SerialNumber int
Signature pkix.AlgorithmIdentifier
Issuer pkix.RDNSequence // pkix.Name
Validity validity
Subject pkix.RDNSequence // pkix.Name
SubjectPKI subjectPublicKeyInfo
}
// validity is defined in rfc2459, section 4.1.
type validity struct {
NotBefore time.Time
NotAfter time.Time
}
// subjectPublicKeyInfo is defined in rfc2459, section 4.1.
type subjectPublicKeyInfo struct {
Algorithm pkix.AlgorithmIdentifier
SubjectPublicKey asn1.BitString
}
type signerInfo struct {
Version int
IssuerAndSerialNumber issuerAndSerialNumber
DigestAlgorithm pkix.AlgorithmIdentifier
DigestEncryptionAlgorithm pkix.AlgorithmIdentifier
EncryptedDigest []byte
}
type issuerAndSerialNumber struct {
Issuer pkix.RDNSequence // pkix.Name
SerialNumber int
}
// Various ASN.1 Object Identifies, mostly from rfc3852.
var (
oidPKCS7 = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 7}
oidData = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 7, 1}
oidSignedData = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 7, 2}
oidSHA1 = asn1.ObjectIdentifier{1, 3, 14, 3, 2, 26}
oidRSAEncryption = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 1}
)
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