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//
// Copyright 2021 The Sigstore Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package ssh
import (
"crypto"
"crypto/rand"
"crypto/sha256"
"crypto/sha512"
"fmt"
"hash"
"io"
"github.com/sigstore/sigstore/pkg/signature"
"golang.org/x/crypto/ssh"
)
// https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.sshsig#L81
type messageWrapper struct {
Namespace string
Reserved string
HashAlgorithm string
Hash string
}
// https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.sshsig#L34
type wrappedSig struct {
MagicHeader [6]byte
Version uint32
PublicKey string
Namespace string
Reserved string
HashAlgorithm string
Signature string
}
const (
magicHeader = "SSHSIG"
defaultHashAlgorithm = "sha512"
)
var supportedHashAlgorithms = map[string]func() hash.Hash{
"sha256": sha256.New,
"sha512": sha512.New,
}
func sign(s ssh.AlgorithmSigner, m io.Reader) (*ssh.Signature, error) {
hf := sha512.New()
if _, err := io.Copy(hf, m); err != nil {
return nil, err
}
mh := hf.Sum(nil)
sp := messageWrapper{
Namespace: "file",
HashAlgorithm: defaultHashAlgorithm,
Hash: string(mh),
}
dataMessageWrapper := ssh.Marshal(sp)
dataMessageWrapper = append([]byte(magicHeader), dataMessageWrapper...)
// ssh-rsa is not supported for RSA keys:
// https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.sshsig#L71
// We can use the default value of "" for other key types though.
algo := ""
if s.PublicKey().Type() == ssh.KeyAlgoRSA {
algo = ssh.KeyAlgoRSASHA512
}
sig, err := s.SignWithAlgorithm(rand.Reader, dataMessageWrapper, algo)
if err != nil {
return nil, err
}
return sig, nil
}
// Signer implements signature.Signer for SSH keys.
type Signer struct {
signer ssh.AlgorithmSigner
}
// PublicKey returns the public key for a Signer.
func (s *Signer) PublicKey(_ ...signature.PublicKeyOption) (crypto.PublicKey, error) {
return s.signer.PublicKey(), nil
}
// SignMessage signs the supplied message.
func (s *Signer) SignMessage(message io.Reader, _ ...signature.SignOption) ([]byte, error) {
b, err := io.ReadAll(message)
if err != nil {
return nil, err
}
sig, err := s.signer.Sign(rand.Reader, b)
if err != nil {
return nil, err
}
return Armor(sig, s.signer.PublicKey()), nil
}
var _ signature.Signer = (*Signer)(nil)
// Sign signs the supplied message with the private key.
func Sign(sshPrivateKey string, data io.Reader) ([]byte, error) {
s, err := ssh.ParsePrivateKey([]byte(sshPrivateKey))
if err != nil {
return nil, err
}
as, ok := s.(ssh.AlgorithmSigner)
if !ok {
return nil, fmt.Errorf("private key %T is not a ssh.AlgorithmSigner", s)
}
sig, err := sign(as, data)
if err != nil {
return nil, err
}
armored := Armor(sig, s.PublicKey())
return armored, nil
}
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