217 changes: 217 additions & 0 deletions services/auth/httpsign.go
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// Copyright 2021 The Gitea Authors. All rights reserved.
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE file.

package auth

import (
"bytes"
"encoding/base64"
"errors"
"fmt"
"net/http"
"strings"

asymkey_model "code.gitea.io/gitea/models/asymkey"
user_model "code.gitea.io/gitea/models/user"
"code.gitea.io/gitea/modules/log"
"code.gitea.io/gitea/modules/setting"

"github.com/go-fed/httpsig"
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"golang.org/x/crypto/ssh"
)

// Ensure the struct implements the interface.
var (
_ Method = &HTTPSign{}
_ Named = &HTTPSign{}
)

// HTTPSign implements the Auth interface and authenticates requests (API requests
// only) by looking for http signature data in the "Signature" header.
// more information can be found on https://github.com/go-fed/httpsig
type HTTPSign struct{}

// Name represents the name of auth method
func (h *HTTPSign) Name() string {
return "httpsign"
}

// Verify extracts and validates HTTPsign from the Signature header of the request and returns
// the corresponding user object on successful validation.
// Returns nil if header is empty or validation fails.
func (h *HTTPSign) Verify(req *http.Request, w http.ResponseWriter, store DataStore, sess SessionStore) *user_model.User {
sigHead := req.Header.Get("Signature")
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if len(sigHead) == 0 {
return nil
}

var (
publicKey *asymkey_model.PublicKey
err error
)

if len(req.Header.Get("X-Ssh-Certificate")) != 0 {
// Handle Signature signed by SSH certificates
if len(setting.SSH.TrustedUserCAKeys) == 0 {
return nil
}

publicKey, err = VerifyCert(req)
if err != nil {
log.Debug("VerifyCert on request from %s: failed: %v", req.RemoteAddr, err)
log.Warn("Failed authentication attempt from %s", req.RemoteAddr)
return nil
}
} else {
// Handle Signature signed by Public Key
publicKey, err = VerifyPubKey(req)
if err != nil {
log.Debug("VerifyPubKey on request from %s: failed: %v", req.RemoteAddr, err)
log.Warn("Failed authentication attempt from %s", req.RemoteAddr)
return nil
}
}

u, err := user_model.GetUserByID(publicKey.OwnerID)
if err != nil {
log.Error("GetUserByID: %v", err)
return nil
}

store.GetData()["IsApiToken"] = true

log.Trace("HTTP Sign: Logged in user %-v", u)

return u
}

func VerifyPubKey(r *http.Request) (*asymkey_model.PublicKey, error) {
verifier, err := httpsig.NewVerifier(r)
if err != nil {
return nil, fmt.Errorf("httpsig.NewVerifier failed: %s", err)
}

keyID := verifier.KeyId()

publicKeys, err := asymkey_model.SearchPublicKey(0, keyID)
if err != nil {
return nil, err
}

if len(publicKeys) == 0 {
return nil, fmt.Errorf("no public key found for keyid %s", keyID)
}

sshPublicKey, _, _, _, err := ssh.ParseAuthorizedKey([]byte(publicKeys[0].Content))
if err != nil {
return nil, err
}

if err := doVerify(verifier, []ssh.PublicKey{sshPublicKey}); err != nil {
return nil, err
}

return publicKeys[0], nil
}

// VerifyCert verifies the validity of the ssh certificate and returns the publickey of the signer
// We verify that the certificate is signed with the correct CA
// We verify that the http request is signed with the private key (of the public key mentioned in the certificate)
func VerifyCert(r *http.Request) (*asymkey_model.PublicKey, error) {
// Get our certificate from the header
bcert, err := base64.RawStdEncoding.DecodeString(r.Header.Get("x-ssh-certificate"))
if err != nil {
return nil, err
}

pk, err := ssh.ParsePublicKey(bcert)
if err != nil {
return nil, err
}

// Check if it's really a ssh certificate
cert, ok := pk.(*ssh.Certificate)
if !ok {
return nil, fmt.Errorf("no certificate found")
}

c := &ssh.CertChecker{
IsUserAuthority: func(auth ssh.PublicKey) bool {
marshaled := auth.Marshal()

for _, k := range setting.SSH.TrustedUserCAKeysParsed {
if bytes.Equal(marshaled, k.Marshal()) {
return true
}
}

return false
},
}

// check the CA of the cert
if !c.IsUserAuthority(cert.SignatureKey) {
return nil, fmt.Errorf("CA check failed")
}

// Create a verifier
verifier, err := httpsig.NewVerifier(r)
if err != nil {
return nil, fmt.Errorf("httpsig.NewVerifier failed: %s", err)
}

// now verify that this request was signed with the private key that matches the certificate public key
if err := doVerify(verifier, []ssh.PublicKey{cert.Key}); err != nil {
return nil, err
}

// Now for each of the certificate valid principals
for _, principal := range cert.ValidPrincipals {
// Look in the db for the public key
publicKey, err := asymkey_model.SearchPublicKeyByContentExact(r.Context(), principal)
if asymkey_model.IsErrKeyNotExist(err) {
// No public key matches this principal - try the next principal
continue
} else if err != nil {
// this error will be a db error therefore we can't solve this and we should abort
log.Error("SearchPublicKeyByContentExact: %v", err)
return nil, err
}

// Validate the cert for this principal
if err := c.CheckCert(principal, cert); err != nil {
// however, because principal is a member of ValidPrincipals - if this fails then the certificate itself is invalid
return nil, err
}

// OK we have a public key for a principal matching a valid certificate whose key has signed this request.
return publicKey, nil
}

// No public key matching a principal in the certificate is registered in gitea
return nil, fmt.Errorf("no valid principal found")
}

// doVerify iterates across the provided public keys attempting the verify the current request against each key in turn
func doVerify(verifier httpsig.Verifier, sshPublicKeys []ssh.PublicKey) error {
for _, publicKey := range sshPublicKeys {
cryptoPubkey := publicKey.(ssh.CryptoPublicKey).CryptoPublicKey()

var algos []httpsig.Algorithm

switch {
case strings.HasPrefix(publicKey.Type(), "ssh-ed25519"):
algos = []httpsig.Algorithm{httpsig.ED25519}
case strings.HasPrefix(publicKey.Type(), "ssh-rsa"):
algos = []httpsig.Algorithm{httpsig.RSA_SHA1, httpsig.RSA_SHA256, httpsig.RSA_SHA512}
}
for _, algo := range algos {
if err := verifier.Verify(cryptoPubkey, algo); err == nil {
return nil
}
}
}

return errors.New("verification failed")
}