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// Copyright 2024 OpenPubkey
//
// 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.
//
// SPDX-License-Identifier: Apache-2.0
package sshcert
import (
"context"
"crypto/rand"
"encoding/json"
"fmt"
"time"
"github.com/lestrrat-go/jwx/v2/jwk"
"github.com/openpubkey/openpubkey/pktoken"
"github.com/openpubkey/openpubkey/verifier"
"golang.org/x/crypto/ssh"
)
type SshCertSmuggler struct {
SshCert *ssh.Certificate
}
func New(pkt *pktoken.PKToken, accessToken []byte, principals []string) (*SshCertSmuggler, error) {
// TODO: assumes email exists in ID Token,
// this will break for OPs like Azure that do not have email as a claim
var claims struct {
Email string `json:"email"`
}
if err := json.Unmarshal(pkt.Payload, &claims); err != nil {
return nil, err
}
pubkeySsh, err := sshPubkeyFromPKT(pkt)
if err != nil {
return nil, err
}
pktCom, err := pkt.Compact()
if err != nil {
return nil, err
}
extensions := map[string]string{
"permit-X11-forwarding": "",
"permit-agent-forwarding": "",
"permit-port-forwarding": "",
"permit-pty": "",
"permit-user-rc": "",
"openpubkey-pkt": string(pktCom),
}
if accessToken != nil {
extensions["openpubkey-act"] = string(accessToken)
}
sshSmuggler := SshCertSmuggler{
SshCert: &ssh.Certificate{
Key: pubkeySsh,
CertType: ssh.UserCert,
KeyId: claims.Email,
ValidPrincipals: principals,
ValidBefore: ssh.CertTimeInfinity,
Permissions: ssh.Permissions{
Extensions: extensions,
},
},
}
return &sshSmuggler, nil
}
func NewFromAuthorizedKey(certType string, certB64 string) (*SshCertSmuggler, error) {
if certPubkey, _, _, _, err := ssh.ParseAuthorizedKey([]byte(certType + " " + certB64)); err != nil {
return nil, err
} else {
sshCert, ok := certPubkey.(*ssh.Certificate)
if !ok {
return nil, fmt.Errorf("parsed SSH authorized_key is not an SSH certificate")
}
opkcert := &SshCertSmuggler{
SshCert: sshCert,
}
return opkcert, nil
}
}
func (s *SshCertSmuggler) SignCert(signerMas ssh.MultiAlgorithmSigner) (*ssh.Certificate, error) {
if err := s.SshCert.SignCert(rand.Reader, signerMas); err != nil {
return nil, err
}
return s.SshCert, nil
}
func (s *SshCertSmuggler) VerifyCaSig(caPubkey ssh.PublicKey) error {
certCopy := *(s.SshCert)
certCopy.Signature = nil
certBytes := certCopy.Marshal()
certBytes = certBytes[:len(certBytes)-4] // Drops signature length bytes (see crypto.ssh.certs.go)
return caPubkey.Verify(certBytes, s.SshCert.Signature)
}
func (s *SshCertSmuggler) GetPKToken() (*pktoken.PKToken, error) {
pktCom, ok := s.SshCert.Extensions["openpubkey-pkt"]
if !ok {
return nil, fmt.Errorf("cert is missing required openpubkey-pkt extension")
}
pkt, err := pktoken.NewFromCompact([]byte(pktCom))
if err != nil {
return nil, fmt.Errorf("openpubkey-pkt extension in cert failed deserialization: %w", err)
}
return pkt, nil
}
func (s *SshCertSmuggler) GetAccessToken() string {
// Generally we don't expect this to be set, but if it is, we return it
if accessToken, ok := s.SshCert.Extensions["openpubkey-act"]; ok {
return accessToken
}
return ""
}
func (s *SshCertSmuggler) VerifySshPktCert(ctx context.Context, pktVerifier verifier.Verifier) (*pktoken.PKToken, error) {
pkt, err := s.GetPKToken()
if err != nil {
return nil, fmt.Errorf("openpubkey-pkt extension in cert failed deserialization: %w", err)
}
ctxWithTimeout, cancel := context.WithTimeout(ctx, 30*time.Second)
defer cancel()
err = pktVerifier.VerifyPKToken(ctxWithTimeout, pkt)
if err != nil {
return nil, err
}
cic, err := pkt.GetCicValues()
if err != nil {
return nil, err
}
upk := cic.PublicKey()
cryptoCertKey := (s.SshCert.Key.(ssh.CryptoPublicKey)).CryptoPublicKey()
jwkCertKey, err := jwk.FromRaw(cryptoCertKey)
if err != nil {
return nil, err
}
if jwk.Equal(jwkCertKey, upk) {
return pkt, nil
} else {
return nil, fmt.Errorf("public key 'upk' in PK Token does not match public key in certificate")
}
}
func sshPubkeyFromPKT(pkt *pktoken.PKToken) (ssh.PublicKey, error) {
cic, err := pkt.GetCicValues()
if err != nil {
return nil, err
}
upk := cic.PublicKey()
var rawkey any
if err := upk.Raw(&rawkey); err != nil {
return nil, err
}
return ssh.NewPublicKey(rawkey)
}
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