Implement PKCE (Proof Key for Code Exchange) support

PKCE was advertised in the discovery document but not actually implemented.
This commit adds full PKCE support:

- Store code_challenge and code_challenge_method in authorization code JWT
- Accept code_verifier parameter in token endpoint
- Validate code_verifier against stored code_challenge
- Support both S256 (SHA256) and plain code challenge methods
- PKCE validation is required when code_challenge is present

This prevents authorization code interception attacks by requiring
the client to prove possession of the code_verifier that was used
to generate the code_challenge.

internal/service/oidc_service.go

@@ -3,6 +3,7 @@ package service
 import (
 	"crypto/rand"
 	"crypto/rsa"
+	"crypto/sha256"
 	"crypto/x509"
 	"encoding/base64"
 	"encoding/json"
@@ -154,7 +155,7 @@ func (oidc *OIDCService) ValidateScope(client *model.OIDCClient, requestedScopes
 	return validScopes, nil
 }
 
-func (oidc *OIDCService) GenerateAuthorizationCode(userContext *config.UserContext, clientID string, redirectURI string, scopes []string, nonce string) (string, error) {
+func (oidc *OIDCService) GenerateAuthorizationCode(userContext *config.UserContext, clientID string, redirectURI string, scopes []string, nonce string, codeChallenge string, codeChallengeMethod string) (string, error) {
 	code := uuid.New().String()
 
 	// Store authorization code in a temporary structure
@@ -171,22 +172,33 @@ func (oidc *OIDCService) GenerateAuthorizationCode(userContext *config.UserConte
 
 	// For now, we'll encode it as a JWT for stateless operation
 	claims := jwt.MapClaims{
-		"code":        code,
-		"username":    userContext.Username,
-		"email":       userContext.Email,
-		"name":        userContext.Name,
-		"provider":    userContext.Provider,
-		"client_id":   clientID,
+		"code":         code,
+		"username":     userContext.Username,
+		"email":        userContext.Email,
+		"name":         userContext.Name,
+		"provider":     userContext.Provider,
+		"client_id":    clientID,
 		"redirect_uri": redirectURI,
-		"scopes":      scopes,
-		"exp":         time.Now().Add(10 * time.Minute).Unix(),
-		"iat":         time.Now().Unix(),
+		"scopes":       scopes,
+		"exp":          time.Now().Add(10 * time.Minute).Unix(),
+		"iat":          time.Now().Unix(),
 	}
-	
+
 	if nonce != "" {
 		claims["nonce"] = nonce
 	}
 
+	// Store PKCE challenge if provided
+	if codeChallenge != "" {
+		claims["code_challenge"] = codeChallenge
+		if codeChallengeMethod != "" {
+			claims["code_challenge_method"] = codeChallengeMethod
+		} else {
+			// Default to plain if method not specified
+			claims["code_challenge_method"] = "plain"
+		}
+	}
+
 	token := jwt.NewWithClaims(jwt.SigningMethodRS256, claims)
 	codeToken, err := token.SignedString(oidc.privateKey)
 	if err != nil {
@@ -197,7 +209,7 @@ func (oidc *OIDCService) GenerateAuthorizationCode(userContext *config.UserConte
 	return codeToken, nil
 }
 
-func (oidc *OIDCService) ValidateAuthorizationCode(codeToken string, clientID string, redirectURI string) (*config.UserContext, []string, string, error) {
+func (oidc *OIDCService) ValidateAuthorizationCode(codeToken string, clientID string, redirectURI string) (*config.UserContext, []string, string, string, string, error) {
 	token, err := jwt.Parse(codeToken, func(token *jwt.Token) (interface{}, error) {
 		if _, ok := token.Method.(*jwt.SigningMethodRSA); !ok {
 			return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
@@ -206,31 +218,31 @@ func (oidc *OIDCService) ValidateAuthorizationCode(codeToken string, clientID st
 	})
 
 	if err != nil {
-		return nil, nil, "", fmt.Errorf("failed to parse authorization code: %w", err)
+		return nil, nil, "", "", "", fmt.Errorf("failed to parse authorization code: %w", err)
 	}
 
 	if !token.Valid {
-		return nil, nil, "", errors.New("invalid authorization code")
+		return nil, nil, "", "", "", errors.New("invalid authorization code")
 	}
 
 	claims, ok := token.Claims.(jwt.MapClaims)
 	if !ok {
-		return nil, nil, "", errors.New("invalid token claims")
+		return nil, nil, "", "", "", errors.New("invalid token claims")
 	}
 
 	// Verify client_id and redirect_uri match
 	if claims["client_id"] != clientID {
-		return nil, nil, "", errors.New("client_id mismatch")
+		return nil, nil, "", "", "", errors.New("client_id mismatch")
 	}
 
 	if claims["redirect_uri"] != redirectURI {
-		return nil, nil, "", errors.New("redirect_uri mismatch")
+		return nil, nil, "", "", "", errors.New("redirect_uri mismatch")
 	}
 
 	// Check expiration
 	exp, ok := claims["exp"].(float64)
 	if !ok || time.Now().Unix() > int64(exp) {
-		return nil, nil, "", errors.New("authorization code expired")
+		return nil, nil, "", "", "", errors.New("authorization code expired")
 	}
 
 	userContext := &config.UserContext{
@@ -251,8 +263,41 @@ func (oidc *OIDCService) ValidateAuthorizationCode(codeToken string, clientID st
 	}
 
 	nonce := getStringClaim(claims, "nonce")
+	codeChallenge := getStringClaim(claims, "code_challenge")
+	codeChallengeMethod := getStringClaim(claims, "code_challenge_method")
 
-	return userContext, scopes, nonce, nil
+	return userContext, scopes, nonce, codeChallenge, codeChallengeMethod, nil
+}
+
+func (oidc *OIDCService) ValidatePKCE(codeChallenge string, codeChallengeMethod string, codeVerifier string) error {
+	if codeChallenge == "" {
+		// PKCE not used, validation passes
+		return nil
+	}
+
+	if codeVerifier == "" {
+		return errors.New("code_verifier required when code_challenge is present")
+	}
+
+	switch codeChallengeMethod {
+	case "S256":
+		// Compute SHA256 hash of code_verifier
+		hash := sha256.Sum256([]byte(codeVerifier))
+		// Base64URL encode (without padding)
+		computedChallenge := base64.RawURLEncoding.EncodeToString(hash[:])
+		if computedChallenge != codeChallenge {
+			return errors.New("code_verifier does not match code_challenge")
+		}
+	case "plain":
+		// Direct comparison
+		if codeVerifier != codeChallenge {
+			return errors.New("code_verifier does not match code_challenge")
+		}
+	default:
+		return fmt.Errorf("unsupported code_challenge_method: %s", codeChallengeMethod)
+	}
+
+	return nil
 }
 
 func (oidc *OIDCService) GenerateAccessToken(userContext *config.UserContext, clientID string, scopes []string) (string, error) {
@@ -485,14 +530,14 @@ func (oidc *OIDCService) SyncClientsFromConfig(clients map[string]config.OIDCCli
 		err = oidc.config.Database.Where("client_id = ?", clientID).First(&existingClient).Error
 
 		client := model.OIDCClient{
-			ClientID:     clientID,
-			ClientSecret: clientSecret,
-			ClientName:   clientName,
-			RedirectURIs: string(redirectURIsJSON),
-			GrantTypes:   string(grantTypesJSON),
+			ClientID:      clientID,
+			ClientSecret:  clientSecret,
+			ClientName:    clientName,
+			RedirectURIs:  string(redirectURIsJSON),
+			GrantTypes:    string(grantTypesJSON),
 			ResponseTypes: string(responseTypesJSON),
-			Scopes:       string(scopesJSON),
-			UpdatedAt:    now,
+			Scopes:        string(scopesJSON),
+			UpdatedAt:     now,
 		}
 
 		if errors.Is(err, gorm.ErrRecordNotFound) {
@@ -556,4 +601,3 @@ func getStringClaim(claims jwt.MapClaims, key string) string {
 	}
 	return ""
 }
-