feat: add kakao OIDC (#1381)

Adds support for OIDC login with Kakao via `signInWithIdToken()`.

Fixes:
- #1358

internal/api/provider/kakao.go

@@ -12,6 +12,7 @@ import (
 const (
 	defaultKakaoAuthBase = "kauth.kakao.com"
 	defaultKakaoAPIBase  = "kapi.kakao.com"
+	IssuerKakao          = "https://kauth.kakao.com"
 )
 
 type kakaoProvider struct {

internal/api/provider/oidc.go

@@ -59,6 +59,8 @@ func ParseIDToken(ctx context.Context, provider *oidc.Provider, config *oidc.Con
 		token, data, err = parseAppleIDToken(token)
 	case IssuerLinkedin:
 		token, data, err = parseLinkedinIDToken(token)
+	case IssuerKakao:
+		token, data, err = parseKakaoIDToken(token)
 	default:
 		if IsAzureIssuer(token.Issuer) {
 			token, data, err = parseAzureIDToken(token)
@@ -312,6 +314,43 @@ func parseAzureIDToken(token *oidc.IDToken) (*oidc.IDToken, *UserProvidedData, e
 	return token, &data, nil
 }
 
+type KakaoIDTokenClaims struct {
+	jwt.StandardClaims
+
+	Email    string `json:"email"`
+	Nickname string `json:"nickname"`
+	Picture  string `json:"picture"`
+}
+
+func parseKakaoIDToken(token *oidc.IDToken) (*oidc.IDToken, *UserProvidedData, error) {
+	var claims KakaoIDTokenClaims
+
+	if err := token.Claims(&claims); err != nil {
+		return nil, nil, err
+	}
+
+	var data UserProvidedData
+
+	if claims.Email != "" {
+		data.Emails = append(data.Emails, Email{
+			Email:    claims.Email,
+			Verified: true,
+			Primary:  true,
+		})
+	}
+
+	data.Metadata = &Claims{
+		Issuer:            token.Issuer,
+		Subject:           token.Subject,
+		Name:              claims.Nickname,
+		PreferredUsername: claims.Nickname,
+		ProviderId:        token.Subject,
+		Picture:           claims.Picture,
+	}
+
+	return token, &data, nil
+}
+
 func parseGenericIDToken(token *oidc.IDToken) (*oidc.IDToken, *UserProvidedData, error) {
 	var data UserProvidedData
 

internal/api/token_oidc.go

@@ -74,6 +74,12 @@ func (p *IdTokenGrantParams) getProvider(ctx context.Context, config *conf.Globa
 		issuer = config.External.Keycloak.URL
 		acceptableClientIDs = append(acceptableClientIDs, config.External.Keycloak.ClientID...)
 
+	case p.Provider == "kakao" || p.Issuer == provider.IssuerKakao:
+		cfg = &config.External.Kakao
+		providerType = "kakao"
+		issuer = provider.IssuerKakao
+		acceptableClientIDs = append(acceptableClientIDs, config.External.Kakao.ClientID...)
+
 	default:
 		log.WithField("issuer", p.Issuer).WithField("client_id", p.ClientID).Warn("Use of POST /token with arbitrary issuer and client_id is deprecated for security reasons. Please switch to using the API with provider only!")