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// Package ocsr creates certificate signing requests for onion addresses. For
// reference, see RFC 2985 and Appendix B(2) of the CA/Browser BRs.
package ocsr
import (
"crypto"
"crypto/ed25519"
"crypto/rand"
"crypto/x509/pkix"
"encoding/asn1"
"encoding/pem"
"fmt"
"sauteed-onions.org/onion-csr/pkg/oaddr"
)
const (
minEntropyBits = 64 // CA/Browser BRs, Appendix B(2)
generalNameDNSName = 2 // RFC 5280, §4.2.1.6
)
var (
oidEd25519 = asn1.ObjectIdentifier{1, 3, 101, 112} // RFC 8410, §3
oidCASigningNonce = asn1.ObjectIdentifier{2, 23, 140, 41} // CA/Browser BRs, Appendix B(2)
oidApplicantSigningNonce = asn1.ObjectIdentifier{2, 23, 140, 42} // CA/Browser BRs, Appendix B(2)
oidExtensionRequest = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 9, 14} // RFC 2985, Appendix A
oidSubjectAltName = asn1.ObjectIdentifier{2, 5, 29, 17} // RFC 5280, §4.2.1.6.
)
// New creates a signed onion certification request in PEM format
func New(priv crypto.Signer, caNonce, applicantNonce []byte) (string, error) {
if len(applicantNonce)*8 < minEntropyBits {
return "", fmt.Errorf("applicant nonce must be at least %d bits", minEntropyBits)
}
addr, err := oaddr.NewFromSigner(priv)
if err != nil {
return "", err
}
attrs, err := newAttributes(caNonce, applicantNonce, addr.String())
if err != nil {
return "", err
}
cr, err := newCertificationRequest(priv, newCertificationRequestInfo(addr, attrs))
if err != nil {
return "", err
}
der, err := asn1.Marshal(cr)
b := &pem.Block{
Type: "CERTIFICATE REQUEST",
Bytes: der,
}
return string(pem.EncodeToMemory(b)), err
}
// certificationRequestInfo is defined in RFC 2986, §4
type certificationRequestInfo struct {
Version int
Subject []pkix.RelativeDistinguishedNameSET
SubjectPKInfo subjectPublicKeyInfo
Attributes []asn1.RawValue `asn1:"tag:0"`
}
type subjectPublicKeyInfo struct {
Algorithm pkix.AlgorithmIdentifier
PublicKey asn1.BitString
}
func newCertificationRequestInfo(pub [ed25519.PublicKeySize]byte, attrs []asn1.RawValue) certificationRequestInfo {
return certificationRequestInfo{
Version: 0,
Subject: []pkix.RelativeDistinguishedNameSET{},
SubjectPKInfo: subjectPublicKeyInfo{
Algorithm: pkix.AlgorithmIdentifier{Algorithm: oidEd25519},
PublicKey: asn1.BitString{
Bytes: pub[:],
BitLength: len(pub) * 8,
},
},
Attributes: attrs,
}
}
// newAttributes creates an encoded attribute list with three attributes:
//
// - CA signing nonce (CA/B BRs, Appendix B(2))
// - Applicant signing nonce (CA/B BRs, Appendix B(2))
// - Requested extensions in the certificate to be issued (PKCS#9). The only
// requested extension is an onion address as the subject alt name (SAN).
func newAttributes(caNonce, applicantNonce []byte, onionAddr string) ([]asn1.RawValue, error) {
attrCA, err := newAttributeNonce(oidCASigningNonce, caNonce)
if err != nil {
return nil, err
}
attrApplicant, err := newAttributeNonce(oidApplicantSigningNonce, applicantNonce)
if err != nil {
return nil, err
}
attrReqExt, err := newAttributeRequestedExtensions(onionAddr)
if err != nil {
return nil, err
}
return []asn1.RawValue{attrReqExt, attrCA, attrApplicant}, nil
}
type attribute struct {
Type asn1.ObjectIdentifier
Value [][]asn1.RawValue `asn1:"set"`
}
func newAttributeNonce(oid asn1.ObjectIdentifier, nonce []byte) (asn1.RawValue, error) {
n := asn1.RawValue{Tag: asn1.TagOctetString, Bytes: nonce}
b, err := asn1.Marshal(attribute{oid, [][]asn1.RawValue{[]asn1.RawValue{n}}})
return asn1.RawValue{FullBytes: b}, err
}
func newAttributeRequestedExtensions(onionAddr string) (asn1.RawValue, error) {
san, err := newSAN(onionAddr)
if err != nil {
return asn1.RawValue{}, err
}
b, err := asn1.Marshal(attribute{oidExtensionRequest, [][]asn1.RawValue{[]asn1.RawValue{san}}})
return asn1.RawValue{FullBytes: b}, err
}
func newSAN(onionAddr string) (asn1.RawValue, error) {
b, err := asn1.Marshal([]asn1.RawValue{asn1.RawValue{
Class: asn1.ClassContextSpecific,
Tag: generalNameDNSName,
Bytes: []byte(onionAddr),
}})
if err != nil {
return asn1.RawValue{}, err
}
b, err = asn1.Marshal(pkix.Extension{
Id: oidSubjectAltName,
Value: b,
})
return asn1.RawValue{FullBytes: b}, err
}
// certificationRequest is defined in RFC 2986, §4
type certificationRequest struct {
CertificationRequestInfo certificationRequestInfo
SignatureAlgorithm pkix.AlgorithmIdentifier
SignatureValue asn1.BitString
}
func newCertificationRequest(priv crypto.Signer, cri certificationRequestInfo) (certificationRequest, error) {
msg, err := asn1.Marshal(cri)
if err != nil {
return certificationRequest{}, err
}
sig, err := priv.Sign(rand.Reader, msg, crypto.Hash(0))
if err != nil {
return certificationRequest{}, err
}
return certificationRequest{
CertificationRequestInfo: cri,
SignatureAlgorithm: pkix.AlgorithmIdentifier{Algorithm: oidEd25519},
SignatureValue: asn1.BitString{
Bytes: sig,
BitLength: len(sig) * 8,
},
}, nil
}
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