feat: add Web Bot Auth support

https://datatracker.ietf.org/doc/html/draft-meunier-http-message-signatures-directory

https://datatracker.ietf.org/doc/html/draft-meunier-web-bot-auth-architecture

Author: fguillot

internal/botauth/botauth.go

@@ -0,0 +1,286 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package botauth // import "miniflux.app/v2/internal/botauth"
+
+// Resources:
+//
+// https://datatracker.ietf.org/doc/html/draft-meunier-http-message-signatures-directory
+// https://datatracker.ietf.org/doc/html/draft-meunier-web-bot-auth-architecture
+// https://developers.cloudflare.com/bots/reference/bot-verification/web-bot-auth/
+// https://github.com/thibmeu/http-message-signatures-directory
+
+import (
+	"crypto/ed25519"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/url"
+	"strconv"
+	"strings"
+	"time"
+
+	"miniflux.app/v2/internal/crypto"
+)
+
+const (
+	signatureValidity = 3600 // 1 hour validity
+)
+
+var GlobalInstance *botAuth
+
+type jsonWebKey struct {
+	KeyType   string `json:"kty"`
+	Curve     string `json:"crv"`
+	PublicKey string `json:"x"`
+}
+
+type jsonWebKeySet struct {
+	Keys []jsonWebKey `json:"keys"`
+}
+
+type keyPair struct {
+	privateKey []byte
+	publicKey  []byte
+	publicJWK  *jsonWebKey
+	thumbprint string
+}
+
+func NewKeyPair(privateKey, publicKey []byte) (*keyPair, error) {
+	if len(privateKey) != ed25519.PrivateKeySize {
+		return nil, fmt.Errorf("invalid private key size: got %d instead of %d", len(privateKey), ed25519.PrivateKeySize)
+	}
+	if len(publicKey) != ed25519.PublicKeySize {
+		return nil, fmt.Errorf("invalid public key size: got %d instead of %d", len(publicKey), ed25519.PublicKeySize)
+	}
+
+	publicJWK := &jsonWebKey{
+		KeyType:   "OKP",
+		Curve:     "Ed25519",
+		PublicKey: base64.RawURLEncoding.EncodeToString(publicKey),
+	}
+
+	thumbprint, err := computeJWKThumbprint(publicJWK)
+	if err != nil {
+		return nil, fmt.Errorf("failed to calculate JWK thumbprint: %w", err)
+	}
+
+	return &keyPair{
+		privateKey: privateKey,
+		publicKey:  publicKey,
+		publicJWK:  publicJWK,
+		thumbprint: thumbprint,
+	}, nil
+}
+
+type KeyPairs []*keyPair
+
+func (kps KeyPairs) jsonWebKeySet() jsonWebKeySet {
+	var keys []jsonWebKey
+	for _, kp := range kps {
+		keys = append(keys, *kp.publicJWK)
+	}
+	return jsonWebKeySet{Keys: keys}
+}
+
+type botAuth struct {
+	directoryURL string
+	keys         KeyPairs
+}
+
+func NewBothAuth(directoryURL string, keys KeyPairs) (*botAuth, error) {
+	if !strings.HasPrefix(directoryURL, "https://") {
+		return nil, fmt.Errorf("directory URL %q must start with https://", directoryURL)
+	}
+
+	if len(keys) == 0 {
+		return nil, fmt.Errorf("at least one key pair is required")
+	}
+
+	return &botAuth{
+		directoryURL: directoryURL,
+		keys:         keys,
+	}, nil
+}
+
+func (ba *botAuth) DirectoryURL() string {
+	absoluteURL, err := url.JoinPath(ba.directoryURL, "/.well-known/http-message-signatures-directory")
+	if err != nil {
+		return ba.directoryURL
+	}
+	return absoluteURL
+}
+
+func (ba *botAuth) ServeKeyDirectory(w http.ResponseWriter, r *http.Request) {
+	body, err := json.Marshal(ba.keys.jsonWebKeySet())
+	if err != nil {
+		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
+		return
+	}
+
+	created := time.Now().Unix()
+	expires := created + signatureValidity
+	signatures := make([]string, len(ba.keys))
+	signatureInputs := make([]string, len(ba.keys))
+
+	for i, key := range ba.keys {
+		signatureMetadata := []signatureMetadata{
+			{name: "alg", value: "ed25519"},
+
+			// https://datatracker.ietf.org/doc/html/draft-meunier-http-message-signatures-directory-01#section-5.2-6.6.1
+			{name: "keyid", value: key.thumbprint},
+
+			// https://datatracker.ietf.org/doc/html/draft-meunier-http-message-signatures-directory-01#section-5.2-6.8.1
+			{name: "tag", value: "http-message-signatures-directory"},
+
+			// https://datatracker.ietf.org/doc/html/draft-meunier-http-message-signatures-directory-01#section-5.2-6.2.1
+			{name: "created", value: created},
+
+			// https://datatracker.ietf.org/doc/html/draft-meunier-http-message-signatures-directory-01#section-5.2-6.4.1
+			{name: "expires", value: expires},
+		}
+
+		signatureComponents := []signatureComponent{
+			{name: "@authority", value: r.Host},
+		}
+
+		signatureParams := generateSignatureParams(signatureComponents, signatureMetadata)
+
+		signature, err := signComponents(key.privateKey, signatureComponents, signatureParams)
+		if err != nil {
+			http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
+			return
+		}
+
+		signatureLabel := `sig` + strconv.Itoa(i+1)
+		signatureInputs[i] = signatureLabel + `=` + signatureParams
+		signatures[i] = signatureLabel + `=:` + signature + `:`
+	}
+
+	// https://datatracker.ietf.org/doc/html/draft-meunier-http-message-signatures-directory-01#name-application-http-message-si
+	w.Header().Set("Content-Type", "application/http-message-signatures-directory+json")
+	w.Header().Set("Signature-Input", strings.Join(signatureInputs, ", "))
+	w.Header().Set("Signature", strings.Join(signatures, ", "))
+
+	// Verifiers can cache keys directory for 1 day.
+	w.Header().Set("Cache-Control", "max-age=86400")
+
+	w.WriteHeader(http.StatusOK)
+	w.Write(body)
+}
+
+func (ba *botAuth) SignRequest(req *http.Request) error {
+	if len(ba.keys) == 0 {
+		return fmt.Errorf("no key pairs available to sign the request")
+	}
+
+	firstKeyPair := ba.keys[0]
+	created := time.Now().Unix()
+	expires := created + signatureValidity
+
+	// @authority component
+	// https://www.rfc-editor.org/rfc/rfc9421#section-2.2.3
+	authority := req.Host
+	if authority == "" {
+		authority = req.URL.Host
+	}
+
+	signatureAgent := `"` + ba.directoryURL + `"`
+
+	signatureMetadata := []signatureMetadata{
+		{name: "alg", value: "ed25519"},
+
+		// https://datatracker.ietf.org/doc/html/draft-meunier-web-bot-auth-architecture-02#section-4.2-5.6.1
+		{name: "keyid", value: firstKeyPair.thumbprint},
+
+		// https://datatracker.ietf.org/doc/html/draft-meunier-web-bot-auth-architecture-02#section-4.2-5.8.1
+		{name: "tag", value: "web-bot-auth"},
+
+		// https://datatracker.ietf.org/doc/html/draft-meunier-web-bot-auth-architecture-02#section-4.2-5.2.1
+		{name: "created", value: created},
+
+		// https://datatracker.ietf.org/doc/html/draft-meunier-web-bot-auth-architecture-02#section-4.2-5.4.1
+		{name: "expires", value: expires},
+
+		// https://datatracker.ietf.org/doc/html/draft-meunier-web-bot-auth-architecture-02#name-anti-replay
+		{name: "nonce", value: base64.StdEncoding.EncodeToString(crypto.GenerateRandomBytes(64))},
+	}
+
+	// https://datatracker.ietf.org/doc/html/draft-meunier-web-bot-auth-architecture-02#name-signature-agent
+	signatureComponents := []signatureComponent{
+		{name: "@authority", value: authority},
+		{name: "signature-agent", value: signatureAgent},
+	}
+
+	signatureParams := generateSignatureParams(signatureComponents, signatureMetadata)
+	signatureInput := `sig1=` + signatureParams
+
+	signature, err := signComponents(firstKeyPair.privateKey, signatureComponents, signatureParams)
+	if err != nil {
+		return fmt.Errorf("failed to sign request: %w", err)
+	}
+
+	// Add headers to request
+	req.Header.Set("Signature-Agent", signatureAgent)
+	req.Header.Set("Signature-Input", signatureInput)
+	req.Header.Set("Signature", `sig1=:`+signature+`:`)
+
+	return nil
+}
+
+// https://www.rfc-editor.org/rfc/rfc8037.html#appendix-A.3
+func computeJWKThumbprint(jwk *jsonWebKey) (string, error) {
+	canonical := `{"crv":"` + jwk.Curve + `","kty":"` + jwk.KeyType + `","x":"` + jwk.PublicKey + `"}`
+	hash := sha256.Sum256([]byte(canonical))
+	return base64.RawURLEncoding.EncodeToString(hash[:]), nil
+}
+
+type signatureMetadata struct {
+	name  string
+	value any
+}
+
+// https://www.rfc-editor.org/rfc/rfc9421#name-signature-parameters
+func generateSignatureParams(components []signatureComponent, signatureMetadata []signatureMetadata) string {
+	var componentNames []string
+
+	for _, component := range components {
+		componentNames = append(componentNames, `"`+component.name+`"`)
+	}
+
+	var metadataParts []string
+	for _, meta := range signatureMetadata {
+		switch v := meta.value.(type) {
+		case string:
+			metadataParts = append(metadataParts, meta.name+`="`+v+`"`)
+		case int64:
+			metadataParts = append(metadataParts, meta.name+`=`+strconv.FormatInt(v, 10))
+		}
+	}
+
+	return `(` + strings.Join(componentNames, ` `) + `);` + strings.Join(metadataParts, ";")
+}
+
+type signatureComponent struct {
+	name  string
+	value string
+}
+
+// https://www.rfc-editor.org/rfc/rfc9421#name-signing-request-components-
+func signComponents(privateKey ed25519.PrivateKey, components []signatureComponent, signatureParams string) (string, error) {
+	var signatureBase strings.Builder
+
+	// Build signature base
+	for _, comp := range components {
+		signatureBase.WriteString(`"` + comp.name + `": ` + comp.value + "\n")
+	}
+
+	signatureBase.WriteString(`"@signature-params": ` + signatureParams)
+
+	// Sign the signature base
+	signature := ed25519.Sign(privateKey, []byte(signatureBase.String()))
+
+	return base64.StdEncoding.EncodeToString(signature), nil
+}