Correct typos and error in all comments of the PR

* issuer,
  * authorization_endpoint,
  * token_endpoint,
  * jwks_uri
- Replace openid-flow-type with openid-response-type
- Use an Enum for openid-response-type to limit to the values 'id_token',
  'token' or 'cod'
- Treat the response type as an implicit flow type allowing use of AWS
  Cognito

extensions/guacamole-auth-jdbc/modules/guacamole-auth-jdbc-mysql/src/main/resources/org/apache/guacamole/auth/jdbc/usergroup/UserGroupMapper.xml

@@ -291,7 +291,6 @@
     <delete id="delete">
         DELETE FROM guacamole_entity
         WHERE
-            type = 'USER_GROUP'
             <choose>
                 <when test="caseSensitivity.caseSensitiveGroupNames()">
                     name = #{identifier,jdbcType=VARCHAR}
@@ -300,6 +299,7 @@
                     LOWER(name) = LOWER(#{identifier,jdbcType=VARCHAR})
                 </otherwise>
             </choose>
+            AND type = 'USER_GROUP'
     </delete>
 
     <!-- Insert single group -->

extensions/guacamole-auth-jdbc/modules/guacamole-auth-jdbc-mysql/src/main/resources/org/apache/guacamole/auth/jdbc/usergroup/UserGroupMemberUserGroupMapper.xml

@@ -96,7 +96,7 @@
             guacamole_entity.type = 'USER_GROUP'
             AND
             <choose>
-                <when test="caseSensitivity.caesSensitiveGroupNames()">
+                <when test="caseSensitivity.caseSensitiveGroupNames()">
                     guacamole_entity.name
                 </when>
                 <otherwise>

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/AuthenticationProviderService.java

@@ -24,12 +24,15 @@ import com.google.inject.Provider;
 import com.google.inject.Singleton;
 import java.net.URI;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.Map;
 import java.util.Set;
 import javax.ws.rs.core.UriBuilder;
 import org.apache.guacamole.auth.openid.conf.ConfigurationService;
+import org.apache.guacamole.auth.openid.OpenIDAuthenticationSessionManager;
 import org.apache.guacamole.auth.openid.token.TokenValidationService;
+import org.apache.guacamole.auth.openid.util.PKCEUtil;
 import org.apache.guacamole.GuacamoleException;
 import org.apache.guacamole.auth.sso.NonceService;
 import org.apache.guacamole.auth.sso.SSOAuthenticationProviderService;
@@ -40,7 +43,11 @@ import org.apache.guacamole.language.TranslatableMessage;
 import org.apache.guacamole.net.auth.Credentials;
 import org.apache.guacamole.net.auth.credentials.CredentialsInfo;
 import org.apache.guacamole.net.auth.credentials.GuacamoleInvalidCredentialsException;
+import org.apache.guacamole.net.auth.IdentifierGenerator;
 import org.jose4j.jwt.JwtClaims;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 
 /**
  * Service that authenticates Guacamole users by processing OpenID tokens.
@@ -49,10 +56,29 @@ import org.jose4j.jwt.JwtClaims;
 public class AuthenticationProviderService implements SSOAuthenticationProviderService {
 
     /**
-     * The standard HTTP parameter which will be included within the URL by all
+     * Logger for this class.
-     * OpenID services upon successful authentication and redirect.
      */
-    public static final String TOKEN_PARAMETER_NAME = "id_token";
+    private final Logger logger = LoggerFactory.getLogger(AuthenticationProviderService.class);
+
+    /**
+     * The standard HTTP parameter which will be included within the URL by all
+     * OpenID services upon successful implicit flow authentication.
+     *
+     */
+    public static final String IMPLICIT_TOKEN_PARAMETER_NAME = "id_token";
+
+    /**
+     * The standard HTTP parameter which will be included within the URL by all
+     * OpenID services upon successful code flow authentication. Used to recover
+     * the stored user state.
+     */
+    public static final String CODE_TOKEN_PARAMETER_NAME = "code";
+
+    /**
+     * The name of the query parameter that identifies an active authentication
+     * session (in-progress OpenID authentication attempt).
+     */
+    public static final String AUTH_SESSION_QUERY_PARAM = "state";
 
     /**
      * Service for retrieving OpenID configuration information.
@@ -60,6 +86,12 @@ public class AuthenticationProviderService implements SSOAuthenticationProviderS
     @Inject
     private ConfigurationService confService;
 
+    /**
+     * Manager of active OpenID authentication attempts.
+     */
+    @Inject
+    private OpenIDAuthenticationSessionManager sessionManager;
+
     /**
      * Service for validating and generating unique nonce values.
      */
@@ -78,6 +110,25 @@ public class AuthenticationProviderService implements SSOAuthenticationProviderS
     @Inject
     private Provider<SSOAuthenticatedUser> authenticatedUserProvider;
 
+    /**
+     * Return the value of the session identifier associated with the given
+     * credentials, or null if no session identifier is found in the
+     * credentials.
+     *
+     * @param credentials
+     *     The credentials from which to extract the session identifier.
+     *
+     * @return
+     *     The session identifier associated with the given credentials, or
+     *     null if no identifier is found.
+     */
+    public static String getSessionIdentifier(Credentials credentials) {
+
+        // Return the session identifier from the request params, if set, or
+        // null otherwise
+        return credentials != null ? credentials.getParameter(AUTH_SESSION_QUERY_PARAM) : null;
+    }
+
     @Override
     public SSOAuthenticatedUser authenticateUser(Credentials credentials)
             throws GuacamoleException {
@@ -86,33 +137,57 @@ public class AuthenticationProviderService implements SSOAuthenticationProviderS
         Set<String> groups = null;
         Map<String,String> tokens = Collections.emptyMap();
 
-        // Validate OpenID token in request, if present, and derive username
+        logger.debug("OpenID authentication with '{}' reponse type (ID: {}, Secret: {}, PKCE: {})",
-        String token = credentials.getParameter(TOKEN_PARAMETER_NAME);
+                  confService.getResponseType(),
+                  confService.getClientID(),
+                  confService.getClientSecret(),
+                  confService.isPKCERequired());
+
+        if (confService.isImplicitFlow()) {
+            String token = credentials.getParameter(IMPLICIT_TOKEN_PARAMETER_NAME);
             if (token != null) {
-            JwtClaims claims = tokenService.validateToken(token);
+                JwtClaims claims = tokenService.validateTokenOrCode(token, "");
                 if (claims != null) {
                     username = tokenService.processUsername(claims);
                     groups = tokenService.processGroups(claims);
                     tokens = tokenService.processAttributes(claims);
                 }
             }
+        }
+        else {
+            String verifier = null;
+            if (confService.isPKCERequired()) {
+                // Recover session
+                String identifier = getSessionIdentifier(credentials);
+                if (identifier != null) {
+                    verifier = sessionManager.getVerifier(identifier);
+                }
+            }
+            String code = credentials.getParameter("code");
+            if (code != null && (confService.isPKCERequired() == false || verifier != null)) {
+                JwtClaims claims = tokenService.validateTokenOrCode(code, verifier);
+                if (claims != null) {
+                    username = tokenService.processUsername(claims);
+                    groups = tokenService.processGroups(claims);
+                    tokens = tokenService.processAttributes(claims);
+                }
+            }
+        }
 
         // If the username was successfully retrieved from the token, produce
         // authenticated user
         if (username != null) {
-
             // Create corresponding authenticated user
             SSOAuthenticatedUser authenticatedUser = authenticatedUserProvider.get();
             authenticatedUser.init(username, credentials, groups, tokens);
             return authenticatedUser;
-
         }
 
         // Request OpenID token (will automatically redirect the user to the
         // OpenID authorization page via JavaScript)
         throw new GuacamoleInvalidCredentialsException("Invalid login.",
             new CredentialsInfo(Arrays.asList(new Field[] {
-                new RedirectField(TOKEN_PARAMETER_NAME, getLoginURI(),
+                new RedirectField(AUTH_SESSION_QUERY_PARAM, getLoginURI(),
                         new TranslatableMessage("LOGIN.INFO_IDP_REDIRECT_PENDING"))
             }))
         );
@@ -121,18 +196,45 @@ public class AuthenticationProviderService implements SSOAuthenticationProviderS
 
     @Override
     public URI getLoginURI() throws GuacamoleException {
-        return UriBuilder.fromUri(confService.getAuthorizationEndpoint())
+        UriBuilder builder = UriBuilder.fromUri(confService.getAuthorizationEndpoint())
                 .queryParam("scope", confService.getScope())
-                .queryParam("response_type", "id_token")
+                .queryParam("response_type", confService.getResponseType().toString())
                 .queryParam("client_id", confService.getClientID())
-                .queryParam("redirect_uri", confService.getRedirectURI())
+                .queryParam("redirect_uri", confService.getRedirectURI());
-                .queryParam("nonce", nonceService.generate(confService.getMaxNonceValidity() * 60000L))
+
-                .build();
+        if (confService.isImplicitFlow()) {
+            builder.queryParam("nonce", nonceService.generate(confService.getMaxNonceValidity() * 60000L));
+        }
+        else  {
+            if (confService.isPKCERequired()) {
+                String codeVerifier = PKCEUtil.generateCodeVerifier();
+                String codeChallenge;
+
+                try {
+                    codeChallenge = PKCEUtil.generateCodeChallenge(codeVerifier);
+                }
+                catch (Exception e) {
+                    throw new GuacamoleException("Unable to compute PKCE challenge", e);
+                }
+
+                // Store verifier for authenticateUser
+                OpenIDAuthenticationSession session = new OpenIDAuthenticationSession(codeVerifier,
+                            confService.getMaxPKCEVerifierValidity() * 60000L);
+                String identifier = IdentifierGenerator.generateIdentifier();
+                sessionManager.defer(session, identifier);
+
+                builder.queryParam("code_challenge", codeChallenge)
+                       .queryParam("code_challenge_method", "S256")
+                       .queryParam(AUTH_SESSION_QUERY_PARAM, identifier);
+            }
+        }
+
+        return builder.build();
     }
 
     @Override
     public void shutdown() {
-        // Nothing to clean up
+        sessionManager.shutdown();
     }
     
 }

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/OpenIDAuthenticationProviderModule.java

@@ -21,8 +21,11 @@ package org.apache.guacamole.auth.openid;
 
 import com.google.inject.AbstractModule;
 import com.google.inject.Scopes;
+import com.google.inject.Singleton;
 import org.apache.guacamole.auth.openid.conf.ConfigurationService;
 import org.apache.guacamole.auth.openid.conf.OpenIDEnvironment;
+import org.apache.guacamole.auth.openid.conf.OpenIDWellKnown;
+import org.apache.guacamole.auth.openid.OpenIDAuthenticationSessionManager;
 import org.apache.guacamole.auth.sso.NonceService;
 import org.apache.guacamole.auth.openid.token.TokenValidationService;
 import org.apache.guacamole.environment.Environment;
@@ -42,6 +45,7 @@ public class OpenIDAuthenticationProviderModule extends AbstractModule {
         bind(ConfigurationService.class);
         bind(NonceService.class).in(Scopes.SINGLETON);
         bind(TokenValidationService.class);
+        bind(OpenIDAuthenticationSessionManager.class);
 
         bind(Environment.class).toInstance(environment);
     }

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/OpenIDAuthenticationSession.java

@@ -0,0 +1,57 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.guacamole.auth.openid;
+
+import org.apache.guacamole.net.auth.AuthenticationSession;
+
+/**
+ * Representation of an in-progress OpenID authentication attempt.
+ */
+
+public class OpenIDAuthenticationSession extends AuthenticationSession {
+    /**
+     * The PKCE challenge verifier.
+     */
+    private final String verifier;
+
+    /**
+     * Creates a new AuthenticationSession representing an in-progress OpenID
+     * authentication attempt.
+     *
+     * @param expires
+     *     The number of milliseconds that may elapse before this session must
+     *     be considered invalid.
+     */
+    public OpenIDAuthenticationSession(String verifier, long expires) {
+        super(expires);
+        this.verifier = verifier;
+    }
+
+    /**
+     * Returns the stored PKCE verifier
+     *
+     * @return
+     *     The PKCE verifier
+     */
+    public String getVerifier() {
+        return verifier;
+    }
+}
+

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/OpenIDAuthenticationSessionManager.java

@@ -0,0 +1,50 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.guacamole.auth.openid;
+
+import com.google.inject.Singleton;
+import org.apache.guacamole.net.auth.AuthenticationSessionManager;
+
+/**
+ * Manager service that temporarily stores OpenID authentication attempts while
+ * the authentication flow is underway.
+ */
+@Singleton
+public class OpenIDAuthenticationSessionManager
+        extends AuthenticationSessionManager<OpenIDAuthenticationSession> {
+
+   /**
+     * Returns the stored PKCE verifier used with the identity provider
+     *
+     * @param identifier
+     *     The unique string returned by the call to defer(). For convenience,
+     *     this value may safely be null.
+     *
+     * @return
+     *     The PKCE verifier used with the identity provider
+     */
+    public String getVerifier(String identifier) {
+        OpenIDAuthenticationSession session = resume(identifier);
+        if (session != null)
+            return session.getVerifier();
+        return null;
+    }
+}
+

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/conf/ConfigurationService.java

@@ -24,8 +24,12 @@ import java.net.URI;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
+import org.apache.guacamole.auth.openid.conf.OpenIDResponseType;
+import org.apache.guacamole.auth.openid.conf.OpenIDWellKnown;
 import org.apache.guacamole.GuacamoleException;
 import org.apache.guacamole.environment.Environment;
+import org.apache.guacamole.properties.BooleanGuacamoleProperty;
+import org.apache.guacamole.properties.EnumGuacamoleProperty;
 import org.apache.guacamole.properties.IntegerGuacamoleProperty;
 import org.apache.guacamole.properties.StringGuacamoleProperty;
 import org.apache.guacamole.properties.URIGuacamoleProperty;
@@ -36,6 +40,11 @@ import org.apache.guacamole.properties.URIGuacamoleProperty;
  */
 public class ConfigurationService {
 
+    /**
+     * The default OICD reponse type
+     */
+    private static final OpenIDResponseType DEFAULT_RESPONSE_TYPE = OpenIDResponseType.ID_TOKEN;
+
     /**
      * The default claim type to use to retrieve an authenticated user's
      * username.
@@ -76,6 +85,12 @@ public class ConfigurationService {
      */
     private static final int DEFAULT_MAX_NONCE_VALIDITY = 10;
 
+    /**
+     * The default maximum amount of time that a pkce verifier generated by the
+     * Guacamole server should remain valid, in minutes.
+     */
+    private static final int DEFAULT_MAX_PKCE_VERIFIER_VALIDITY = 10;
+
     /**
      * The authorization endpoint (URI) of the OpenID service.
      */
@@ -110,6 +125,64 @@ public class ConfigurationService {
 
     };
 
+    /**
+     * The token endpoint (URI) of the OIDC service.
+     */
+    private static final URIGuacamoleProperty OPENID_TOKEN_ENDPOINT =
+            new URIGuacamoleProperty() {
+
+        @Override
+        public String getName() { return "openid-token-endpoint"; }
+    };
+
+    /**
+     * The reponse type of the OpenID service.
+     */
+    private static final EnumGuacamoleProperty<OpenIDResponseType> OPENID_RESPONSE_TYPE =
+            new EnumGuacamoleProperty<OpenIDResponseType>(OpenIDResponseType.class) {
+
+        @Override
+        public String getName() { return "openid-response-type"; }
+
+    };
+
+    /**
+     * OIDC client secret which should be submitted to the OIDC service when
+     * validating tokens with code flow
+     */
+    private static final StringGuacamoleProperty OPENID_CLIENT_SECRET =
+            new StringGuacamoleProperty() {
+
+        @Override
+        public String getName() { return "openid-client-secret"; }
+    };
+
+    /**
+     * True if "Proof Key for Code Exchange" (PKCE) must be used.
+     */
+    private static final BooleanGuacamoleProperty OPENID_PKCE_REQUIRED =
+            new BooleanGuacamoleProperty() {
+
+        @Override
+        public String getName() { return "openid-pkce-required"; }
+    };
+
+    /**
+     * The maximum amount of time that a PKCE verifier generated by the
+     * Guacamole server should remain valid, in minutes. As each OpenID request
+     * requiring PKCE has a unique verifier, this imposes an upper limit on the
+     * amount of time any particular OpenID request can result in successful
+     * authentication within Guacamole. By default, each generated PKCE verifier
+     * expires after 10 minutes.
+     */
+    private static final IntegerGuacamoleProperty OPENID_MAX_PKCE_VERIFIER_VALIDITY =
+            new IntegerGuacamoleProperty() {
+
+        @Override
+        public String getName() { return "openid-max-pkce-verifier-validity"; }
+
+    };
+
     /**
      * The claim type which contains the authenticated user's username within
      * any valid JWT.
@@ -226,6 +299,12 @@ public class ConfigurationService {
     @Inject
     private Environment environment;
     
+    /**
+     * Service for retrieving OpenID well-known data.
+     */
+    @Inject
+    private OpenIDWellKnown confWellKnown;
+
     /**
      * Returns the authorization endpoint (URI) of the OpenID service as
      * configured with guacamole.properties.
@@ -235,11 +314,16 @@ public class ConfigurationService {
      *     guacamole.properties.
      *
      * @throws GuacamoleException
-     *     If guacamole.properties cannot be parsed, or if the authorization
+     *     If guacamole.properties cannot be parsed, or it authorization and
-     *     endpoint property is missing.
+     *     well-known endpoints are property is missing.
      */
     public URI getAuthorizationEndpoint() throws GuacamoleException {
-        return environment.getRequiredProperty(OPENID_AUTHORIZATION_ENDPOINT);
+        URI authorization_endpoint = environment.getProperty(OPENID_AUTHORIZATION_ENDPOINT);
+        authorization_endpoint = authorization_endpoint == null ? confWellKnown.getAuthorizationEndpoint() : authorization_endpoint;
+        if (authorization_endpoint == null) {
+            throw new GuacamoleException("Property openid-authorization-endpoint or openid-well-known-endpoint is required");
+        }
+        return authorization_endpoint;
     }
 
     /**
@@ -288,10 +372,15 @@ public class ConfigurationService {
      *
      * @throws GuacamoleException
      *     If guacamole.properties cannot be parsed, or if the issuer property
-     *     is missing.
+     *     and well-known endpoints are property is missing.
      */
     public String getIssuer() throws GuacamoleException {
-        return environment.getRequiredProperty(OPENID_ISSUER);
+        String issuer = environment.getProperty(OPENID_ISSUER);
+        issuer = issuer == null ? confWellKnown.getIssuer() : issuer;
+        if (issuer == null) {
+            throw new GuacamoleException("Property openid-issuer or openid-well-known-endpoint is required");
+        }
+        return issuer;
     }
 
     /**
@@ -305,11 +394,117 @@ public class ConfigurationService {
      *     guacamole.properties.
      *
      * @throws GuacamoleException
-     *     If guacamole.properties cannot be parsed, or if the JWKS endpoint
+     *     If guacamole.properties cannot be parsed, or if the JWKS and
-     *     property is missing.
+     *     well-known endpoints are property is missing.
      */
     public URI getJWKSEndpoint() throws GuacamoleException {
-        return environment.getRequiredProperty(OPENID_JWKS_ENDPOINT);
+        URI jwks_uri =  environment.getProperty(OPENID_JWKS_ENDPOINT);
+        jwks_uri = jwks_uri == null ? confWellKnown.getJWKSEndpoint() : jwks_uri;
+        if (jwks_uri == null) {
+            throw new GuacamoleException("Property openid-jwks-endpoint or openid-well-known-endpoint is required");
+        }
+        return jwks_uri;
+    }
+
+    /**
+     * Returns the token endpoint (URI) of the OIDC service as
+     * configured with guacamole.properties.
+     *
+     * @return
+     *     The token endpoint of the OIDC service, as configured with
+     *     guacamole.properties.
+     *
+     * @throws GuacamoleException
+     *     If guacamole.properties cannot be parsed, or if the token and
+     *     well-known endpoints are property is missing.
+     */
+    public URI getTokenEndpoint() throws GuacamoleException {
+        URI token_endpoint = environment.getProperty(OPENID_TOKEN_ENDPOINT);
+        token_endpoint = token_endpoint == null ? confWellKnown.getTokenEndpoint() : token_endpoint;
+        if (token_endpoint == null) {
+            throw new GuacamoleException("Property openid-token-endpoint or openid-well-known-endpoint is required");
+        }
+        return token_endpoint;
+    }
+
+    /**
+     * Returns the reponse type of the OpenID service as configured with guacamole.properties.
+     *
+     * @return
+     *     The reponse type of the OpenID service, as configured with guacamole.properties. Can
+     *     be either 'id_token', 'token' or 'code'.
+     *
+     * @throws GuacamoleException
+     *     If guacamole.properties cannot be parsed.
+     */
+    public OpenIDResponseType getResponseType() throws GuacamoleException {
+        return environment.getProperty(OPENID_RESPONSE_TYPE, DEFAULT_RESPONSE_TYPE);
+    }
+
+    /**
+     * Returns true if the response type defines an implict flow
+     *
+     * @return
+     *     The whether implicit flow is used or not, as configured with guacamole.properties.
+     *
+     * @throws GuacamoleException
+     *     If guacamole.properties cannot be parsed.
+     */
+    public boolean isImplicitFlow() throws GuacamoleException {
+        OpenIDResponseType response_type = environment.getProperty(OPENID_RESPONSE_TYPE, DEFAULT_RESPONSE_TYPE);
+        return response_type != OpenIDResponseType.CODE;
+    }
+
+    /**
+     * Returns the OIDC client secret used for toen validation, as configured
+     * with guacamole.properties. This value is typically provided by the OIDC
+     * service when OIDC credentials are generated for your application, and
+     * may be null.
+     *
+     * @return
+     *     The client secret to use when communicating with the OIDC service,
+     *     as configured with guacamole.properties.
+     *
+     * @throws GuacamoleException
+     *     If guacamole.properties cannot be parsed.
+     */
+    public String getClientSecret() throws GuacamoleException {
+       return environment.getProperty(OPENID_CLIENT_SECRET);
+    }
+
+    /**
+     * Returns  a boolean value of whether "Proof Key for Code Exchange (PKCE)"
+     * will be used, as configured with guacamole.properties. The choice of
+     * whether to use PKCE is up to you, but the OIDC service must support
+     * it. By default will be false.
+     *
+     * @return
+     *     The whether to use PKCE or not, as configured with guacamole.properties.
+     *
+     * @throws GuacamoleException
+     *     If guacamole.properties cannot be parsed.
+     */
+    public boolean isPKCERequired() throws GuacamoleException {
+        Boolean enabled = environment.getProperty(OPENID_PKCE_REQUIRED);
+        return enabled != null && enabled;
+    }
+
+    /**
+     * Returns the maximum amount of time that a PKCE verifier generated by the
+     * Guacamole server should remain valid, in minutes. As each OpenID request
+     * requiring PKCE has a unique verifier, this imposes an upper limit on the
+     * amount of time any particular OpenID request can result in successful
+     * authentication within Guacamole. By default, this will be 10.
+     *
+     * @return
+     *     The maximum amount of time that a PKCE verifier generated by the
+     *     Guacamole server should remain valid, in minutes.
+     *
+     * @throws GuacamoleException
+     *     If guacamole.properties cannot be parsed.
+     */
+    public int getMaxPKCEVerifierValidity() throws GuacamoleException {
+        return environment.getProperty(OPENID_MAX_PKCE_VERIFIER_VALIDITY, DEFAULT_MAX_PKCE_VERIFIER_VALIDITY);
     }
 
     /**

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/conf/OpenIDResponseType.java

@@ -0,0 +1,67 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.guacamole.auth.openid.conf;
+
+import org.apache.guacamole.properties.EnumGuacamoleProperty.PropertyValue;
+
+/**
+ * This enum represents the valid OIDC reponse types that can be used
+ */
+ public enum OpenIDResponseType {
+     /*
+      * Response type "id_token" used for implciit flow as specified in the OpenID standard
+      */
+     @PropertyValue("id_token")
+     ID_TOKEN("id_token"),
+     
+     /* Response type "token" used for implicit flow by certain Identity Providers, notably
+      * AWS Cognito. This corresponds to the official OIDC response_type "id_token token" 
+      * that returns both the "id_token" and "access_token" parameters
+      */
+     @PropertyValue("token")
+     TOKEN("token"),
+     
+     /**
+      * Response type "code" used for code flow authentication
+      */
+     @PropertyValue("code")
+     CODE("code");
+     
+     /*
+      * The string value of the response type used
+      */
+     public final String STRING_VALUE;
+     
+    /**
+     * Initializes the response type such that it is associated with the
+     * given string value.
+     *
+     * @param value
+     *     The string value that will be associated with the enum value.
+     */
+    private OpenIDResponseType(String value) {
+        this.STRING_VALUE = value;
+    }
+    
+    @Override
+    public String toString() {
+        return STRING_VALUE;
+    }
+}

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/conf/OpenIDWellKnown.java

@@ -0,0 +1,227 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.guacamole.auth.openid.conf;
+
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+import java.io.IOException;
+import java.net.URI;
+import java.net.URL;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
+import java.util.Map;
+import javax.ws.rs.core.UriBuilder;
+import org.apache.guacamole.auth.openid.util.JsonUrlReader;
+import org.apache.guacamole.GuacamoleException;
+import org.apache.guacamole.environment.Environment;
+import org.apache.guacamole.properties.URIGuacamoleProperty;
+import org.jose4j.json.JsonUtil;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Class for retrieving well-known endpoint data.
+ */
+@Singleton
+public class OpenIDWellKnown {
+
+    /**
+     * Logger for this class.
+     */
+    private static final Logger logger = LoggerFactory.getLogger(OpenIDWellKnown.class);
+
+
+    /**
+     * The number of attempts to the well-known endpoint to get the values before giving up
+     */
+    private static final int MAX_ATTEMPTS = 24;
+
+    /**
+     * The delay between each attempt to well-known endpoint in seconds
+     */
+    private static final long DELAY_SECONDS = 5;
+
+    /**
+     * The detected issuer
+     */
+    private static String issuer = null;
+
+    /**
+     * The detected authorization endpoint
+     */
+    private static URI authorization_endpoint = null;
+
+    /**
+     * The detected token endpoint
+     */
+    private static URI token_endpoint = null;
+
+    /**
+     * The detected jwks_uri
+     */
+    private static URI jwks_uri = null;
+    
+    /**
+     * Empty constructor of the class to populate data recovered from a OIDC
+     * well-known URL. The class will be populated on injection by Guice
+     */
+    public OpenIDWellKnown() { }
+
+    /**
+     * The well-known endpoint (URI) of the OIDC service.
+     */
+    private static final URIGuacamoleProperty OPENID_WELL_KNOWN_ENDPOINT =
+            new URIGuacamoleProperty() {
+
+        @Override
+        public String getName() { return "openid-well-known-endpoint"; }
+    };
+
+    /**
+     * Returns the well-known endpoint (URI) of the OIDC service as
+     * configured with guacamole.properties.
+     *
+     * @return
+     *     The well-known endpoint of the OIDC service, as configured with
+     *     guacamole.properties.
+     *
+     * @throws GuacamoleException
+     *     If guacamole.properties cannot be parsed, or if the well-known
+     *     endpoint property is missing.
+     */
+    private URI getWellKnownEndpoint() throws GuacamoleException {
+        return environment.getProperty(OPENID_WELL_KNOWN_ENDPOINT);
+    }
+
+    /**
+     * Returns the issuer to expect for all received ID tokens, as configured
+     * from the well_known endpoint.
+     *
+     * @return
+     *     The issuer to expect for all received ID tokens, as returned by the
+     *     well-known endpoint.
+     */
+    public String getIssuer() {
+        return issuer;
+    }
+
+    /**
+     * Returns the authorization endpoint (URI) of the OpenID service as
+     * configured from the well_known endpoint.
+     *
+     * @return
+     *     The authorization endpoint of the OpenID service, as returned by the
+     *     well-known endpoint.
+     */
+    public URI getAuthorizationEndpoint() {
+        return authorization_endpoint;
+    }
+
+    /**
+     * Returns the token endpoint (URI) of the OpenID service as
+     * configured from the well_known endpoint.
+     *
+     * @return
+     *     The token endpoint of the OpenID service, as returned by the
+     *     well-known endpoint.
+     */
+    public URI getTokenEndpoint() {
+        return token_endpoint;
+    }
+
+    /**
+     * Returns the endpoint (URI) of the JWKS service which defines how
+     * received ID tokens (JWTs) shall be validated, as configured from
+     * the well-known endpoint.
+     *
+     * @return
+     *     The endpoint (URI) of the JWKS service which defines how received ID
+     *     tokens (JWTs) shall be validated, as configured from the
+     *     well-known endpoint.
+     */
+    public URI getJWKSEndpoint() {
+        return jwks_uri;
+    }
+
+    /**
+     * The Guacamole server environment.
+     */
+    @Inject
+    private Environment environment;
+
+    /*
+     * On injection, when the environment is non null, populates the OpenIDWellKnown 
+     * class by reading the json from an OIDC well-known endpoint and saves these values
+     * for later use. Use Guice to ensure environment exists before initializing.
+     */
+    @Inject
+    public void init() {
+        // Fast return if there is no well-known endpoint or its unreadable
+        try {
+            if (getWellKnownEndpoint() == null) {
+                return;
+            }
+        }
+        catch (Exception e) {
+            return;
+        }
+    
+        // Call to well-known endpoint might fail, so allow several tries before giving up
+        ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();
+        
+        Runnable task = new Runnable() {
+            int attempts = 0;
+
+            @Override
+            public void run() {
+                attempts++;
+
+                try {
+                    Map<String,Object> json = JsonUrlReader.fetch("GET", getWellKnownEndpoint(), "");
+                    issuer = (String) json.get("issuer");
+                    authorization_endpoint = UriBuilder.fromUri((String) json.get("authorization_endpoint")).build();
+                    token_endpoint = UriBuilder.fromUri((String) json.get("token_endpoint")).build();
+                    jwks_uri = UriBuilder.fromUri((String) json.get("jwks_uri")).build();
+
+                    logger.info("OIDC well-known\n" +
+                                "  issuer                 : {}\n" +
+                                "  authorization_endpoint : {}\n" +
+                                "  token_endpoint         : {}\n" +
+                                "  jwks_uri               : {}\n",
+                                issuer, authorization_endpoint, token_endpoint, jwks_uri);
+
+                    scheduler.shutdown();
+                    return;
+                }
+                catch (Exception e) {
+                    logger.debug("Rejecting well-known endpoint : {}", e.getMessage());
+                }
+
+                if (attempts >= MAX_ATTEMPTS) {
+                    logger.info("Timeout on well-known on endpoint");
+                    scheduler.shutdown();
+                }
+            }
+        };
+
+        scheduler.scheduleAtFixedRate(task, 0, DELAY_SECONDS, TimeUnit.SECONDS);
+    }
+}

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/token/TokenValidationService.java

@@ -20,6 +20,8 @@
 package org.apache.guacamole.auth.openid.token;
 
 import com.google.inject.Inject;
+import java.net.URLEncoder;
+import java.nio.charset.StandardCharsets;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
@@ -29,6 +31,7 @@ import java.util.Map;
 import java.util.Set;
 import org.apache.guacamole.GuacamoleException;
 import org.apache.guacamole.auth.openid.conf.ConfigurationService;
+import org.apache.guacamole.auth.openid.util.JsonUrlReader;
 import org.apache.guacamole.auth.sso.NonceService;
 import org.apache.guacamole.token.TokenName;
 import org.jose4j.jwk.HttpsJwks;
@@ -70,20 +73,26 @@ public class TokenValidationService {
     private NonceService nonceService;
 
     /**
-     * Validates the given ID token, returning the JwtClaims contained therein.
+     * Validates the given ID token, using implicit flow. Also validates codes,
-     * If the ID token is invalid, null is returned.
+     * exchanging them for id_tokens before validation. If the id_token or
+     * code is invalid, null is returned, otherwise the JwtClaims in the
+     * id_token are returned.
      *
      * @param token
-     *     The ID token to validate.
+     *     The ID token to validate if implicit flow or the code to exchange for
+     *     an id_token and then validate.
+     *
+     * @param verifier
+     *     A PKCE verifier or null if not used. Only used with code flow
      *
      * @return
-     *     The JWT claims contained within the given ID token if it passes tests,
+     *     The JWT claims contained within the id_token if it passes tests,
-     *     or null if the token is not valid.
+     *     or null if the id_token is not valid.
      *
      * @throws GuacamoleException
      *     If guacamole.properties could not be parsed.
      */
-    public JwtClaims validateToken(String token) throws GuacamoleException {
+    public JwtClaims validateTokenOrCode(String token, String verifier) throws GuacamoleException {
         // Validating the token requires a JWKS key resolver
         HttpsJwks jwks = new HttpsJwks(confService.getJWKSEndpoint().toString());
         HttpsJwksVerificationKeyResolver resolver = new HttpsJwksVerificationKeyResolver(jwks);
@@ -99,10 +108,16 @@ public class TokenValidationService {
                 .setVerificationKeyResolver(resolver)
                 .build();
                 
+        /* Exchange code → token */
+        if (! confService.isImplicitFlow()) {
+            token = exchangeCode(token, verifier);                
+        }
+        
         try {
             // Validate JWT
             JwtClaims claims = jwtConsumer.processToClaims(token);
 
+            if (confService.isImplicitFlow()) {
                 // Verify a nonce is present
                 String nonce = claims.getStringClaimValue("nonce");
                 if (nonce != null) {
@@ -120,6 +135,10 @@ public class TokenValidationService {
                     logger.info("Rejected OpenID token without nonce.");
                 }
             }
+            else {
+                return claims;
+            }
+        }
         // Log any failures to validate/parse the JWT
         catch (MalformedClaimException e) {
             logger.info("Rejected OpenID token with malformed claim: {}", e.getMessage());
@@ -133,6 +152,73 @@ public class TokenValidationService {
         return null;
     }
 
+    /**
+     * URLEncodes a key/value pair
+     *
+     * @param key
+     *     The key to encode
+     *
+     * @param value
+     *     The value to encode
+     *
+     * @return
+     *     The urlencoded kay/value pair
+     */
+     private String urlencode(String key, String value) {
+         StringBuilder builder = new StringBuilder();
+         return builder.append(URLEncoder.encode(key, StandardCharsets.UTF_8))
+                       .append("=")
+                       .append(URLEncoder.encode(value, StandardCharsets.UTF_8))
+                       .toString();
+     }
+
+    /**
+     * Exchanges the authorization code for tokens.
+     *
+     * @param code
+     *     The authorization code received from the IdP.
+     *
+     * @param codeVerifier
+     *     The PKCE verifier (or null if PKCE is disabled).
+     *
+     * @return
+     *     The token string returned.
+     *
+     * @throws GuacamoleException
+     *     If a valid token is not returned.
+     */
+    private String exchangeCode(String code, String verifier) throws GuacamoleException {
+
+        try {
+            StringBuilder bodyBuilder = new StringBuilder();
+            bodyBuilder.append(urlencode("grant_type", "authorization_code")).append("&");
+            bodyBuilder.append(urlencode("code", code)).append("&");
+            bodyBuilder.append(urlencode("redirect_uri", confService.getRedirectURI().toString())).append("&");
+            bodyBuilder.append(urlencode("scope", confService.getScope())).append("&");
+            bodyBuilder.append(urlencode("client_id", confService.getClientID()));
+
+            String clientSecret = confService.getClientSecret();
+            if (clientSecret != null && !clientSecret.trim().isEmpty()) {
+                bodyBuilder.append("&").append(urlencode("client_secret", clientSecret));
+            }
+
+            if (confService.isPKCERequired()) {
+                bodyBuilder.append("&").append(urlencode("code_verifier", verifier));
+            }
+
+            Map<String,Object> json = 
+                    JsonUrlReader.fetch("POST", confService.getTokenEndpoint(),
+                                        bodyBuilder.toString());
+
+            return (String) json.get("id_token");
+
+        }
+        catch (Exception e) {
+            logger.info("Rejected invalid OpenID code exchange: {}", e.getMessage(), e);
+        }
+        return null;
+    }
+
     /**
      * Parses the given JwtClaims, returning the username contained
      * therein, as defined by the username claim type given in

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/util/JsonUrlReader.java

@@ -0,0 +1,106 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.guacamole.auth.openid.util;
+
+import java.io.IOException;
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.nio.charset.StandardCharsets;
+import java.util.Map;
+import java.util.concurrent.CompletableFuture;
+import org.jose4j.json.JsonUtil;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+
+/*
+ * Utility class to open a http connection to a URL, send a body
+ * and receive a response in the form of a parsed JSON
+ */
+
+public final class JsonUrlReader {
+    /**
+     * Logger for this class.
+     */
+    private static final Logger logger = LoggerFactory.getLogger(JsonUrlReader.class);
+
+    /**
+     * Class to GET and POST to a URL and read the returned JSON. This class should
+     * not be instantiated.
+     */
+    private JsonUrlReader() {}
+
+    /**
+     * Method to POST or GET to a URL and recover the JSON in the form of a Map
+     *
+     * @param String method
+     *      The http method to use. Should be "GET", "POST" or "PATCH".
+     *
+     * @param URI uri
+     *      A URI value giving the address where to recover the JSON
+     *
+     * @param String body
+     *      A pre-encoded body string to be sent to the address if the method is
+     *      "POST" or "PATCH". Ignored if the method is "GET".
+     *
+     * @return
+     *      A Map<String,Object> containing the decoded json values.
+     */
+    public static Map<String, Object> fetch(String method, URI uri, String body) throws IOException {
+        if (uri == null || uri.toString().isEmpty()) {
+            throw new IOException("JsonUrlReader: Missing URL");
+        }
+
+        try {
+            HttpClient client = HttpClient.newHttpClient();
+            HttpRequest.Builder requestBuilder = HttpRequest.newBuilder().uri(uri);
+            if (method != "GET") {
+                // FIXME: If this function is ever used to post json bodies this header
+                // will need to be configurable
+                requestBuilder.header("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8")
+                .method(method, HttpRequest.BodyPublishers.ofString(body == null ? "" : body,
+                                                                    StandardCharsets.UTF_8));
+            }
+            else {
+                requestBuilder.GET();
+            }
+
+            // Asynchronous, non-blocking send, so that tomcat servlets are not blocked by outbound connection
+            CompletableFuture<HttpResponse<String>> future = client.sendAsync(requestBuilder.build(),
+                    HttpResponse.BodyHandlers.ofString(StandardCharsets.UTF_8));
+
+            HttpResponse<String> response = future.join();
+            int status = response.statusCode();
+            logger.debug("Response body: {}", response.body());
+            Map<String, Object> json = JsonUtil.parseJson(response.body());
+
+            if (status < 200 || status >= 300) {
+                throw new IOException("(status: " + status + "): " + json.toString());
+            }
+
+            return json;
+        }
+        catch (Exception e) {
+            throw new IOException("JsonUrlReader error: " + e.getMessage());
+        }
+    }
+}

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/util/PKCEUtil.java

@@ -0,0 +1,85 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.guacamole.auth.openid.util;
+
+import java.security.MessageDigest;
+import java.security.SecureRandom;
+
+/**
+ * Utility class for generating PKCE parameters.
+ *
+ * Supports:
+ *   - code_verifier (random Base64URL)
+ *   - code_challenge (S256)
+ */
+public final class PKCEUtil {
+    /**
+     * Get the verifier data from a cryptographically secure random source
+     */
+    private static final SecureRandom RANDOM = new SecureRandom();
+
+    /*
+     * Class to create PKCE challenges and verifiers. This class should not be instantiated
+     */
+    private PKCEUtil() {}
+
+    /**
+     * Generates a high-entropy PKCE code_verifier.
+     *
+     * @return
+     *      A 256bit or 64 byte random Base64 URL encode string
+     */
+    public static String generateCodeVerifier() {
+        byte[] bytes = new byte[64];
+        RANDOM.nextBytes(bytes);
+        return base64Url(bytes);
+    }
+
+    /**
+     * Computes the PKCE code_challenge = BASE64URL(SHA256(code_verifier)).
+     *
+     * @param String verifier
+     *      A string containing the S256 verifier calculated bu generateCodeVerifier
+     *
+     * @return
+     *      The generated S256 code challenge used for the PKCE request encoded 
+     *      in Base64 URL format.
+     */
+    public static String generateCodeChallenge(String verifier) throws Exception {
+        MessageDigest sha = MessageDigest.getInstance("SHA-256");
+        byte[] hash = sha.digest(verifier.getBytes("US-ASCII"));
+        return base64Url(hash);
+    }
+
+    /**
+     * Base64URL encoding without padding.
+     *
+     * @param bytes
+     *      The bytes to be Base64 URL encoded
+     *
+     * @return
+     *      The Base64 URL encoded string value corresponding to the bytes 
+     */
+    public static String base64Url(byte[] bytes) {
+        return java.util.Base64.getUrlEncoder()
+                .withoutPadding()
+                .encodeToString(bytes);
+    }
+}