Implement http signatures support for the API (#17565)

Fixes #12338

This allows use to talk to the API with our ssh certificate (and/or ssh-agent) without needing to fetch an API key or tokens.
It will just automatically work when users have added their ssh principal in gitea.

This needs client code in tea
Update: also support normal pubkeys

ref: https://tools.ietf.org/html/draft-cavage-http-signatures

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>
Signed-off-by: Andrew Thornton <art27@cantab.net>

1 files changed, 217 insertions, 0 deletions

diff --git a/services/auth/httpsign.go b/services/auth/httpsign.go
new file mode 100644
index 0000000000..67053d2b77
--- /dev/null
+++ b/services/auth/httpsign.go
@@ -0,0 +1,217 @@
+// 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"
+	"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")
+	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")
+}