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/*
 * Copyright  2019 Google Inc.
 *
 * Licensed 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 com.google.firebase.auth;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

import com.google.api.client.auth.openidconnect.IdToken;
import com.google.api.client.auth.openidconnect.IdToken.Payload;
import com.google.api.client.auth.openidconnect.IdTokenVerifier;
import com.google.api.client.googleapis.auth.oauth2.GooglePublicKeysManager;
import com.google.api.client.json.JsonFactory;
import com.google.api.client.json.webtoken.JsonWebSignature.Header;
import com.google.api.client.util.ArrayMap;
import com.google.common.base.Joiner;
import com.google.common.base.Strings;
import com.google.firebase.ErrorCode;
import com.google.firebase.auth.internal.Utils;
import com.google.firebase.internal.Nullable;
import java.io.IOException;
import java.math.BigDecimal;
import java.security.GeneralSecurityException;
import java.security.PublicKey;
import java.util.List;

/**
 * The default implementation of the {@link FirebaseTokenVerifier} interface. Uses the Google API
 * client's {@code IdToken} API to decode and verify token strings. Can be customized to verify
 * both Firebase ID tokens and session cookies.
 */
final class FirebaseTokenVerifierImpl implements FirebaseTokenVerifier {

  private static final String RS256 = "RS256";
  private static final String FIREBASE_AUDIENCE =
      "https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit";

  private final JsonFactory jsonFactory;
  private final GooglePublicKeysManager publicKeysManager;
  private final IdTokenVerifier idTokenVerifier;
  private final String method;
  private final String shortName;
  private final String articledShortName;
  private final String docUrl;
  private final AuthErrorCode invalidTokenErrorCode;
  private final AuthErrorCode expiredTokenErrorCode;
  private final String tenantId;

  private FirebaseTokenVerifierImpl(Builder builder) {
    this.jsonFactory = checkNotNull(builder.jsonFactory);
    this.publicKeysManager = checkNotNull(builder.publicKeysManager);
    this.idTokenVerifier = checkNotNull(builder.idTokenVerifier);
    checkArgument(!Strings.isNullOrEmpty(builder.method), "method name must be specified");
    checkArgument(!Strings.isNullOrEmpty(builder.shortName), "shortName must be specified");
    checkArgument(!Strings.isNullOrEmpty(builder.docUrl), "docUrl must be specified");
    this.method = builder.method;
    this.shortName = builder.shortName;
    this.articledShortName = prefixWithIndefiniteArticle(this.shortName);
    this.docUrl = builder.docUrl;
    this.invalidTokenErrorCode = checkNotNull(builder.invalidTokenErrorCode);
    this.expiredTokenErrorCode = checkNotNull(builder.expiredTokenErrorCode);
    this.tenantId = builder.tenantId;
  }

  /**
   * Verifies that the given token string is a valid Firebase JWT. This implementation considers
   * a token string to be valid if all the following conditions are met:
   * 
    *
  1. The token string is a valid RS256 JWT.
  2. *
  3. The JWT contains a valid key ID (kid) claim.
  4. *
  5. The JWT is not expired, and it has been issued some time in the past.
  6. *
  7. The JWT contains valid issuer (iss) and audience (aud) claims as determined by the * {@code IdTokenVerifier}.
  8. *
  9. The JWT contains a valid subject (sub) claim.
  10. *
  11. The JWT is signed by a Firebase Auth backend server.
  12. *
* * @param token The token string to be verified. * @return A decoded representation of the input token string. * @throws FirebaseAuthException If the input token string does not meet any of the conditions * listed above. */ @Override public FirebaseToken verifyToken(String token) throws FirebaseAuthException { boolean isEmulatorMode = Utils.isEmulatorMode(); IdToken idToken = parse(token); checkContents(idToken, isEmulatorMode); if (!isEmulatorMode) { checkSignature(idToken); } FirebaseToken firebaseToken = new FirebaseToken(idToken.getPayload()); checkTenantId(firebaseToken); return firebaseToken; } GooglePublicKeysManager getPublicKeysManager() { return publicKeysManager; } IdTokenVerifier getIdTokenVerifier() { return idTokenVerifier; } String getMethod() { return method; } String getShortName() { return shortName; } String getArticledShortName() { return articledShortName; } String getDocUrl() { return docUrl; } private String prefixWithIndefiniteArticle(String word) { if ("aeiouAEIOU".indexOf(word.charAt(0)) < 0) { return "a " + word; } else { return "an " + word; } } private IdToken parse(String token) throws FirebaseAuthException { try { return IdToken.parse(jsonFactory, token); } catch (IllegalArgumentException | IOException e) { // Old versions of guava throw an IOException for invalid strings, while new versions // might throw an IllegalArgumentException String detailedError = String.format( "Failed to parse Firebase %s. Make sure you passed a string that represents a complete " + "and valid JWT. See %s for details on how to retrieve %s.", shortName, docUrl, articledShortName); throw newException(detailedError, invalidTokenErrorCode, e); } } private void checkSignature(IdToken token) throws FirebaseAuthException { if (!isSignatureValid(token)) { String message = String.format( "Failed to verify the signature of Firebase %s. %s", shortName, getVerifyTokenMessage()); throw newException(message, invalidTokenErrorCode); } } private void checkContents(final IdToken idToken, boolean isEmulatorMode) throws FirebaseAuthException { final Header header = idToken.getHeader(); final Payload payload = idToken.getPayload(); final long currentTimeMillis = idTokenVerifier.getClock().currentTimeMillis(); String errorMessage = null; AuthErrorCode errorCode = invalidTokenErrorCode; if (!isEmulatorMode && header.getKeyId() == null) { errorMessage = getErrorForTokenWithoutKid(header, payload); } else if (!isEmulatorMode && !RS256.equals(header.getAlgorithm())) { errorMessage = String.format( "Firebase %s has incorrect algorithm. Expected \"%s\" but got \"%s\".", shortName, RS256, header.getAlgorithm()); } else if (!idToken.verifyAudience(idTokenVerifier.getAudience())) { errorMessage = String.format( "Firebase %s has incorrect \"aud\" (audience) claim. Expected \"%s\" but got \"%s\". %s", shortName, joinWithComma(idTokenVerifier.getAudience()), joinWithComma(payload.getAudienceAsList()), getProjectIdMatchMessage()); } else if (!idToken.verifyIssuer(idTokenVerifier.getIssuers())) { errorMessage = String.format( "Firebase %s has incorrect \"iss\" (issuer) claim. Expected \"%s\" but got \"%s\". %s", shortName, joinWithComma(idTokenVerifier.getIssuers()), payload.getIssuer(), getProjectIdMatchMessage()); } else if (payload.getSubject() == null) { errorMessage = String.format( "Firebase %s has no \"sub\" (subject) claim.", shortName); } else if (payload.getSubject().isEmpty()) { errorMessage = String.format( "Firebase %s has an empty string \"sub\" (subject) claim.", shortName); } else if (payload.getSubject().length() > 128) { errorMessage = String.format( "Firebase %s has \"sub\" (subject) claim longer than 128 characters.", shortName); } else if (!idToken.verifyExpirationTime( currentTimeMillis, idTokenVerifier.getAcceptableTimeSkewSeconds())) { errorMessage = String.format( "Firebase %s has expired. Get a fresh %s and try again.", shortName, shortName); // Also set the expired error code. errorCode = expiredTokenErrorCode; } else if (!idToken.verifyIssuedAtTime( currentTimeMillis, idTokenVerifier.getAcceptableTimeSkewSeconds())) { errorMessage = String.format( "Firebase %s is not yet valid.", shortName); } if (errorMessage != null) { String detailedError = String.format("%s %s", errorMessage, getVerifyTokenMessage()); throw newException(detailedError, errorCode); } } private FirebaseAuthException newException(String message, AuthErrorCode errorCode) { return newException(message, errorCode, null); } private FirebaseAuthException newException( String message, AuthErrorCode errorCode, Throwable cause) { return new FirebaseAuthException( ErrorCode.INVALID_ARGUMENT, message, cause, null, errorCode); } private String getVerifyTokenMessage() { return String.format( "See %s for details on how to retrieve %s.", docUrl, articledShortName); } /** * Verifies the cryptographic signature on the FirebaseToken. Can block on a web request to fetch * the keys if they have expired. */ private boolean isSignatureValid(IdToken token) throws FirebaseAuthException { for (PublicKey key : fetchPublicKeys()) { if (isSignatureValid(token, key)) { return true; } } return false; } private boolean isSignatureValid(IdToken token, PublicKey key) throws FirebaseAuthException { try { return token.verifySignature(key); } catch (GeneralSecurityException e) { // This doesn't happen under usual circumstances. Seems to only happen if the crypto // setup of the runtime is incorrect in some way. throw new FirebaseAuthException( ErrorCode.UNKNOWN, String.format("Unexpected error while verifying %s: %s", shortName, e.getMessage()), e, null, invalidTokenErrorCode); } } private List fetchPublicKeys() throws FirebaseAuthException { try { return publicKeysManager.getPublicKeys(); } catch (GeneralSecurityException | IOException e) { throw new FirebaseAuthException( ErrorCode.UNKNOWN, "Error while fetching public key certificates: " + e.getMessage(), e, null, AuthErrorCode.CERTIFICATE_FETCH_FAILED); } } private String getErrorForTokenWithoutKid(IdToken.Header header, IdToken.Payload payload) { if (isCustomToken(payload)) { return String.format("%s expects %s, but was given a custom token.", method, articledShortName); } else if (isLegacyCustomToken(header, payload)) { return String.format("%s expects %s, but was given a legacy custom token.", method, articledShortName); } return String.format("Firebase %s has no \"kid\" claim.", shortName); } private String joinWithComma(Iterable strings) { return Joiner.on(',').join(strings); } private String getProjectIdMatchMessage() { return String.format( "Make sure the %s comes from the same Firebase project as the service account used to " + "authenticate this SDK.", shortName); } private boolean isCustomToken(IdToken.Payload payload) { return FIREBASE_AUDIENCE.equals(payload.getAudience()); } private boolean isLegacyCustomToken(IdToken.Header header, IdToken.Payload payload) { return "HS256".equals(header.getAlgorithm()) && new BigDecimal(0).equals(payload.get("v")) && containsLegacyUidField(payload); } private boolean containsLegacyUidField(IdToken.Payload payload) { Object dataField = payload.get("d"); if (dataField instanceof ArrayMap) { return ((ArrayMap) dataField).get("uid") != null; } return false; } private void checkTenantId(final FirebaseToken firebaseToken) throws FirebaseAuthException { String tokenTenantId = firebaseToken.getTenantId(); if (this.tenantId != null && !this.tenantId.equals(tokenTenantId)) { String message = String.format( "The tenant ID ('%s') of the token did not match the expected value ('%s')", Strings.nullToEmpty(tokenTenantId), tenantId); throw newException(message, AuthErrorCode.TENANT_ID_MISMATCH); } } static Builder builder() { return new Builder(); } static final class Builder { private JsonFactory jsonFactory; private GooglePublicKeysManager publicKeysManager; private String method; private String shortName; private IdTokenVerifier idTokenVerifier; private String docUrl; private AuthErrorCode invalidTokenErrorCode; private AuthErrorCode expiredTokenErrorCode; private String tenantId; private Builder() { } Builder setJsonFactory(JsonFactory jsonFactory) { this.jsonFactory = jsonFactory; return this; } Builder setPublicKeysManager(GooglePublicKeysManager publicKeysManager) { this.publicKeysManager = publicKeysManager; return this; } Builder setMethod(String method) { this.method = method; return this; } Builder setShortName(String shortName) { this.shortName = shortName; return this; } Builder setIdTokenVerifier(IdTokenVerifier idTokenVerifier) { this.idTokenVerifier = idTokenVerifier; return this; } Builder setDocUrl(String docUrl) { this.docUrl = docUrl; return this; } Builder setInvalidTokenErrorCode(AuthErrorCode invalidTokenErrorCode) { this.invalidTokenErrorCode = invalidTokenErrorCode; return this; } Builder setExpiredTokenErrorCode(AuthErrorCode expiredTokenErrorCode) { this.expiredTokenErrorCode = expiredTokenErrorCode; return this; } Builder setTenantId(@Nullable String tenantId) { this.tenantId = tenantId; return this; } FirebaseTokenVerifierImpl build() { return new FirebaseTokenVerifierImpl(this); } } }




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