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>

go.mod

@@ -29,6 +29,7 @@ require (
 	github.com/go-chi/chi/v5 v5.0.7
 	github.com/go-chi/cors v1.2.1
 	github.com/go-enry/go-enry/v2 v2.8.2
+	github.com/go-fed/httpsig v1.1.0
 	github.com/go-git/go-billy/v5 v5.3.1
 	github.com/go-git/go-git/v5 v5.4.3-0.20210630082519-b4368b2a2ca4
 	github.com/go-ldap/ldap/v3 v3.4.3

go.sum

@@ -468,6 +468,8 @@ github.com/go-enry/go-enry/v2 v2.8.2 h1:uiGmC+3K8sVd/6DOe2AOJEOihJdqda83nPyJNtMR
 github.com/go-enry/go-enry/v2 v2.8.2/go.mod h1:GVzIiAytiS5uT/QiuakK7TF1u4xDab87Y8V5EJRpsIQ=
 github.com/go-enry/go-oniguruma v1.2.1 h1:k8aAMuJfMrqm/56SG2lV9Cfti6tC4x8673aHCcBk+eo=
 github.com/go-enry/go-oniguruma v1.2.1/go.mod h1:bWDhYP+S6xZQgiRL7wlTScFYBe023B6ilRZbCAD5Hf4=
+github.com/go-fed/httpsig v1.1.0 h1:9M+hb0jkEICD8/cAiNqEB66R87tTINszBRTjwjQzWcI=
+github.com/go-fed/httpsig v1.1.0/go.mod h1:RCMrTZvN1bJYtofsG4rd5NaO5obxQ5xBkdiS7xsT7bM=
 github.com/go-git/gcfg v1.5.0 h1:Q5ViNfGF8zFgyJWPqYwA7qGFoMTEiBmdlkcfRmpIMa4=
 github.com/go-git/gcfg v1.5.0/go.mod h1:5m20vg6GwYabIxaOonVkTdrILxQMpEShl1xiMF4ua+E=
 github.com/go-git/go-billy/v5 v5.2.0/go.mod h1:pmpqyWchKfYfrkb/UVH4otLvyi/5gJlGI4Hb3ZqZ3W0=

integrations/api_httpsig_test.go

@@ -0,0 +1,137 @@
+// Copyright 2022 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 integrations
+
+import (
+	"encoding/base64"
+	"fmt"
+	"net/http"
+	"net/url"
+	"testing"
+
+	api "code.gitea.io/gitea/modules/structs"
+
+	"github.com/go-fed/httpsig"
+	"golang.org/x/crypto/ssh"
+)
+
+const (
+	httpsigPrivateKey = `-----BEGIN OPENSSH PRIVATE KEY-----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+-----END OPENSSH PRIVATE KEY-----`
+	httpsigCertificate = `ssh-rsa-cert-v01@openssh.com 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`
+)
+
+func TestHTTPSigPubKey(t *testing.T) {
+	// Add our public key to user1
+	defer prepareTestEnv(t)()
+	session := loginUser(t, "user1")
+	token := url.QueryEscape(getTokenForLoggedInUser(t, session))
+	keysURL := fmt.Sprintf("/api/v1/user/keys?token=%s", token)
+	keyType := "ssh-rsa"
+	keyContent := "AAAAB3NzaC1yc2EAAAADAQABAAABAQCqOZB5vkRvXFXups1/0StDRdG8plbNSwsWEnNnP4Bvurxa0+z3W9B8GLKnDiLw5MbpbMNyBlpXw13GfuIeciy10DWTz0xUbiy3J3KabCaT36asIw2y7k6Z0jL0UBnrVENwq5/lUbZYqSZ4rRU744wkhh8TULpzM14npQCZwg6aEbG+MwjzddQ72fR+3BPBrKn5dTmmu8rH99O+U+Nuto81Tg7PA+NUupcHOmhdiEGq49plgVFXK98Vks5tiybL4GuzFyWgyX73Dg/QBMn2eMHt1EMv5Gs3i6GFhKKGo4rjDi9qI6PX5oDR4LTNe6cR8td8YhVD8WFZwLLl/vaYyIqd"
+	rawKeyBody := api.CreateKeyOption{
+		Title: "test-key",
+		Key:   keyType + " " + keyContent,
+	}
+	req := NewRequestWithJSON(t, "POST", keysURL, rawKeyBody)
+	session.MakeRequest(t, req, http.StatusCreated)
+
+	// parse our private key and create the httpsig request
+	sshSigner, _ := ssh.ParsePrivateKey([]byte(httpsigPrivateKey))
+	keyID := ssh.FingerprintSHA256(sshSigner.PublicKey())
+
+	// create the request
+	req = NewRequest(t, "GET", "/api/v1/admin/users")
+
+	signer, _, err := httpsig.NewSSHSigner(sshSigner, httpsig.DigestSha512, []string{httpsig.RequestTarget, "(created)", "(expires)"}, httpsig.Signature, 10)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// sign the request
+	err = signer.SignRequest(keyID, req, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// make the request
+	MakeRequest(t, req, http.StatusOK)
+}
+
+func TestHTTPSigCert(t *testing.T) {
+	// Add our public key to user1
+	defer prepareTestEnv(t)()
+	session := loginUser(t, "user1")
+
+	csrf := GetCSRF(t, session, "/user/settings/keys")
+	req := NewRequestWithValues(t, "POST", "/user/settings/keys", map[string]string{
+		"_csrf":   csrf,
+		"content": "user1",
+		"title":   "principal",
+		"type":    "principal",
+	})
+
+	session.MakeRequest(t, req, http.StatusSeeOther)
+	pkcert, _, _, _, err := ssh.ParseAuthorizedKey([]byte(httpsigCertificate))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// parse our private key and create the httpsig request
+	sshSigner, _ := ssh.ParsePrivateKey([]byte(httpsigPrivateKey))
+	keyID := "gitea"
+
+	// create our certificate signer using the ssh signer and our certificate
+	certSigner, err := ssh.NewCertSigner(pkcert.(*ssh.Certificate), sshSigner)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// create the request
+	req = NewRequest(t, "GET", "/api/v1/admin/users")
+
+	// add our cert to the request
+	certString := base64.RawStdEncoding.EncodeToString(pkcert.(*ssh.Certificate).Marshal())
+	req.Header.Add("x-ssh-certificate", certString)
+
+	signer, _, err := httpsig.NewSSHSigner(certSigner, httpsig.DigestSha512, []string{httpsig.RequestTarget, "(created)", "(expires)", "x-ssh-certificate"}, httpsig.Signature, 10)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// sign the request
+	err = signer.SignRequest(keyID, req, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// make the request
+	MakeRequest(t, req, http.StatusOK)
+}

integrations/mssql.ini.tmpl

@@ -49,6 +49,7 @@ OFFLINE_MODE     = false
 LFS_JWT_SECRET   = Tv_MjmZuHqpIY6GFl12ebgkRAMt4RlWt0v4EHKSXO0w
 APP_DATA_PATH    = integrations/gitea-integration-mssql/data
 BUILTIN_SSH_SERVER_USER = git
+SSH_TRUSTED_USER_CA_KEYS = ssh-rsa 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
 
 [attachment]
 PATH = integrations/gitea-integration-mssql/data/attachments

integrations/mysql.ini.tmpl

@@ -51,6 +51,7 @@ OFFLINE_MODE     = false
 
 LFS_START_SERVER = true
 LFS_JWT_SECRET   = Tv_MjmZuHqpIY6GFl12ebgkRAMt4RlWt0v4EHKSXO0w
+SSH_TRUSTED_USER_CA_KEYS = ssh-rsa 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
 
 [lfs]
 MINIO_BASE_PATH = lfs/

integrations/mysql8.ini.tmpl

@@ -49,6 +49,7 @@ OFFLINE_MODE     = false
 LFS_JWT_SECRET   = Tv_MjmZuHqpIY6GFl12ebgkRAMt4RlWt0v4EHKSXO0w
 APP_DATA_PATH    = integrations/gitea-integration-mysql8/data
 BUILTIN_SSH_SERVER_USER = git
+SSH_TRUSTED_USER_CA_KEYS = ssh-rsa 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
 
 [attachment]
 PATH = integrations/gitea-integration-mysql8/data/attachments

integrations/pgsql.ini.tmpl

@@ -50,6 +50,7 @@ OFFLINE_MODE     = false
 LFS_JWT_SECRET   = Tv_MjmZuHqpIY6GFl12ebgkRAMt4RlWt0v4EHKSXO0w
 APP_DATA_PATH    = integrations/gitea-integration-pgsql/data
 BUILTIN_SSH_SERVER_USER = git
+SSH_TRUSTED_USER_CA_KEYS = ssh-rsa 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
 
 [attachment]
 PATH = integrations/gitea-integration-pgsql/data/attachments

integrations/sqlite.ini.tmpl

@@ -46,6 +46,7 @@ LFS_JWT_SECRET   = Tv_MjmZuHqpIY6GFl12ebgkRAMt4RlWt0v4EHKSXO0w
 APP_DATA_PATH    = integrations/gitea-integration-sqlite/data
 ENABLE_GZIP      = true
 BUILTIN_SSH_SERVER_USER = git
+SSH_TRUSTED_USER_CA_KEYS = ssh-rsa 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
 
 [attachment]
 PATH = integrations/gitea-integration-sqlite/data/attachments

modules/ssh/ssh.go

@@ -188,8 +188,9 @@ func publicKeyHandler(ctx ssh.Context, key ssh.PublicKey) bool {
 
 			c := &gossh.CertChecker{
 				IsUserAuthority: func(auth gossh.PublicKey) bool {
+					marshaled := auth.Marshal()
 					for _, k := range setting.SSH.TrustedUserCAKeysParsed {
-						if bytes.Equal(auth.Marshal(), k.Marshal()) {
+						if bytes.Equal(marshaled, k.Marshal()) {
 							return true
 						}
 					}

routers/api/v1/api.go

@@ -592,6 +592,7 @@ func bind(obj interface{}) http.HandlerFunc {
 func buildAuthGroup() *auth.Group {
 	group := auth.NewGroup(
 		&auth.OAuth2{},
+		&auth.HTTPSign{},
 		&auth.Basic{}, // FIXME: this should be removed once we don't allow basic auth in API
 	)
 	if setting.Service.EnableReverseProxyAuth {

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")
+}