Add and default to Argon2 hashing (#2877)

We've previously been using Scrypt since PR #2191 which, while being a
memory-hard slow function, isn't the optimal solution according to the
OWASP recommendations. While we could get away with increasing the
parallelization parameter to 3, it's better to just switch to the
most-recommended solution if we're switching things up anyway.

For the transition, we do something similar to PR #2191 where if the
previous-algorithm's hash is successful, we re-hash with Argo2id and
store that version. By doing this, we should not need any intervention
for registrars who log in at any point during the transition period.

Much of this PR, especially the parts where we re-hash the passwords in
Argon2 instead of Scrypt upon login, is based on the code that was
eventually removed in #2310.

core/src/main/java/google/registry/flows/session/LoginFlow.java

@@ -47,6 +47,7 @@ import google.registry.model.eppinput.EppInput.Options;
 import google.registry.model.eppinput.EppInput.Services;
 import google.registry.model.eppoutput.EppResponse;
 import google.registry.model.registrar.Registrar;
+import google.registry.util.PasswordUtils;
 import google.registry.util.StopwatchLogger;
 import jakarta.inject.Inject;
 import java.util.Optional;
@@ -150,8 +151,19 @@ public class LoginFlow implements MutatingFlow {
       throw new RegistrarAccountNotActiveException();
     }
 
-    if (login.getNewPassword().isPresent()) {
-      String newPassword = login.getNewPassword().get();
+    // TODO(b/458423787): Remove this circa March 2026 after enough time has passed for the logins
+    // to have transitioned to Argon2 hashing.
+    if (login.getNewPassword().isPresent()
+        || registrar.get().getCurrentHashAlgorithm(login.getPassword()).orElse(null)
+            != PasswordUtils.HashAlgorithm.ARGON_2_ID) {
+      String newPassword =
+          login
+              .getNewPassword()
+              .orElseGet(
+                  () -> {
+                    logger.atInfo().log("Rehashing existing registrar password with ARGON_2_ID");
+                    return login.getPassword();
+                  });
       // Load fresh from database (bypassing the cache) to ensure we don't save stale data.
       Optional<Registrar> freshRegistrar = Registrar.loadByRegistrarId(login.getClientId());
       stopwatch.tick("LoginFlow reload freshRegistrar");

core/src/main/java/google/registry/model/console/User.java

@@ -37,6 +37,7 @@ import google.registry.tools.IamClient;
 import google.registry.tools.ServiceConnection;
 import google.registry.tools.server.UpdateUserGroupAction;
 import google.registry.util.PasswordUtils;
+import google.registry.util.PasswordUtils.HashAlgorithm;
 import google.registry.util.RegistryEnvironment;
 import jakarta.persistence.Column;
 import jakarta.persistence.Embeddable;
@@ -229,6 +230,10 @@ public class User extends UpdateAutoTimestampEntity implements Buildable {
         || isNullOrEmpty(registryLockPasswordHash)) {
       return false;
     }
+    return getCurrentHashAlgorithm(registryLockPassword).isPresent();
+  }
+
+  public Optional<HashAlgorithm> getCurrentHashAlgorithm(String registryLockPassword) {
     return PasswordUtils.verifyPassword(
         registryLockPassword, registryLockPasswordHash, registryLockPasswordSalt);
   }

core/src/main/java/google/registry/model/registrar/Registrar.java

@@ -67,6 +67,7 @@ import google.registry.persistence.converter.CurrencyToStringMapUserType;
 import google.registry.persistence.transaction.TransactionManager;
 import google.registry.util.CidrAddressBlock;
 import google.registry.util.PasswordUtils;
+import google.registry.util.PasswordUtils.HashAlgorithm;
 import jakarta.mail.internet.AddressException;
 import jakarta.mail.internet.InternetAddress;
 import jakarta.persistence.AttributeOverride;
@@ -672,6 +673,10 @@ public class Registrar extends UpdateAutoTimestampEntity implements Buildable, J
   }
 
   public boolean verifyPassword(String password) {
+    return getCurrentHashAlgorithm(password).isPresent();
+  }
+
+  public Optional<HashAlgorithm> getCurrentHashAlgorithm(String password) {
     return PasswordUtils.verifyPassword(password, passwordHash, salt);
   }
 

core/src/main/java/google/registry/ui/server/console/ConsoleRegistryLockAction.java

@@ -23,6 +23,7 @@ import static jakarta.servlet.http.HttpServletResponse.SC_OK;
 import static jakarta.servlet.http.HttpServletResponse.SC_UNAUTHORIZED;
 
 import com.google.common.collect.ImmutableList;
+import com.google.common.flogger.FluentLogger;
 import com.google.gson.annotations.Expose;
 import google.registry.flows.EppException;
 import google.registry.flows.domain.DomainFlowUtils;
@@ -40,6 +41,7 @@ import google.registry.request.Response;
 import google.registry.request.auth.Auth;
 import google.registry.tools.DomainLockUtils;
 import google.registry.util.EmailMessage;
+import google.registry.util.PasswordUtils.HashAlgorithm;
 import jakarta.inject.Inject;
 import jakarta.mail.internet.AddressException;
 import jakarta.mail.internet.InternetAddress;
@@ -61,13 +63,16 @@ import org.joda.time.Duration;
     auth = Auth.AUTH_PUBLIC_LOGGED_IN)
 public class ConsoleRegistryLockAction extends ConsoleApiAction {
 
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+
   static final String PATH = "/console-api/registry-lock";
   static final String VERIFICATION_EMAIL_TEMPLATE =
       """
       Please click the link below to perform the lock / unlock action on domain %s. Note: this\
        code will expire in one hour.
 
-      %s""";
+      %s\
+      """;
 
   private final DomainLockUtils domainLockUtils;
   private final GmailClient gmailClient;
@@ -114,7 +119,6 @@ public class ConsoleRegistryLockAction extends ConsoleApiAction {
         optionalPostInput.orElseThrow(() -> new IllegalArgumentException("No POST input provided"));
     String domainName = postInput.domainName();
     boolean isLock = postInput.isLock();
-    Optional<String> maybePassword = Optional.ofNullable(postInput.password());
     Optional<Long> relockDurationMillis = Optional.ofNullable(postInput.relockDurationMillis());
 
     try {
@@ -126,11 +130,25 @@ public class ConsoleRegistryLockAction extends ConsoleApiAction {
     // Passwords aren't required for admin users, otherwise we need to validate it
     boolean isAdmin = user.getUserRoles().isAdmin();
     if (!isAdmin) {
-      checkArgument(maybePassword.isPresent(), "No password provided");
-      if (!user.verifyRegistryLockPassword(maybePassword.get())) {
+      checkArgument(postInput.password != null, "No password provided");
+      Optional<HashAlgorithm> hashAlgorithm = user.getCurrentHashAlgorithm(postInput.password);
+      if (hashAlgorithm.isEmpty()) {
         setFailedResponse("Incorrect registry lock password", SC_UNAUTHORIZED);
         return;
       }
+      // TODO(b/458423787): Remove this circa March 2026 after enough time has passed for the logins
+      // to have transitioned to Argon2 hashing.
+      if (hashAlgorithm.get() != HashAlgorithm.ARGON_2_ID) {
+        logger.atInfo().log("Rehashing existing registry lock password with ARGON_2_ID.");
+        tm().transact(
+                () ->
+                    tm().update(
+                            tm().loadByEntity(user)
+                                .asBuilder()
+                                .removeRegistryLockPassword()
+                                .setRegistryLockPassword(postInput.password)
+                                .build()));
+      }
     }
 
     String registryLockEmail =

core/src/main/java/google/registry/xml/xsd/fee-std-v1.xsd

@@ -10,36 +10,30 @@
   <import namespace="urn:ietf:params:xml:ns:domain-1.0" />
 
   <annotation>
-    <documentation>Extensible Provisioning Protocol
-    v1.0 extension schema for fee
-    information.</documentation>
+    <documentation>
+      Extensible Provisioning Protocol v1.0 Fee Extension
+    </documentation>
   </annotation>
 
-  <!--
-  Child elements found in EPP commands and responses
-  -->
-  <element name="check"    type="fee:checkType" />
-  <element name="chkData"  type="fee:chkDataType" />
-  <element name="create"   type="fee:transformCommandType" />
-  <element name="creData"  type="fee:transformResultType" />
-  <element name="renew"    type="fee:transformCommandType" />
-  <element name="renData"  type="fee:transformResultType" />
+  <!-- Child elements found in EPP commands and responses -->
+  <element name="check" type="fee:checkType" />
+  <element name="chkData" type="fee:chkDataType" />
+  <element name="create" type="fee:transformCommandType" />
+  <element name="creData" type="fee:transformResultType" />
+  <element name="renew" type="fee:transformCommandType" />
+  <element name="renData" type="fee:transformResultType" />
   <element name="transfer" type="fee:transformCommandType" />
-  <element name="trnData"  type="fee:transferResultType" />
-  <element name="update"   type="fee:transformCommandType" />
-  <element name="updData"  type="fee:transformResultType" />
-  <element name="delData"  type="fee:transformResultType" />
+  <element name="trnData" type="fee:transferResultType" />
+  <element name="update" type="fee:transformCommandType" />
+  <element name="updData" type="fee:transformResultType" />
+  <element name="delData" type="fee:transformResultType" />
 
-  <!--
-  client <check> command
-  -->
+  <!-- client <check> command -->
   <complexType name="checkType">
     <sequence>
-      <element name="currency"
-        type="fee:currencyType"
+      <element name="currency" type="fee:currencyType"
         minOccurs="0" />
-      <element name="command"
-  type="fee:commandCheckType"
+      <element name="command" type="fee:commandCheckType"
         maxOccurs="unbounded" />
     </sequence>
   </complexType>
@@ -61,17 +55,9 @@
     <attribute name="subphase" type="token" />
   </complexType>
 
-  <!--
-  server <check> result
-  -->
+  <!-- server <check> result -->
   <complexType name="chkDataType">
     <sequence>
-      <!--
-      Dialog is ongoing with Gavin Brown about this. His XSD has a
-      mandatory currency field. But what if the different domains being
-      checked have different currencies? I am trying to figure out what
-      he intends here. - Brian
-      -->
       <element name="currency" type="fee:currencyType" minOccurs="0"/>
       <element name="cd" type="fee:objectCDType"
         maxOccurs="unbounded" />
@@ -123,9 +109,7 @@
     <attribute name="subphase" type="token" />
   </complexType>
 
-  <!--
-    general transform (create, renew, update, transfer) command
-  -->
+  <!-- general transform (create, renew, update, transfer) command-->
   <complexType name="transformCommandType">
     <sequence>
       <element name="currency" type="fee:currencyType"
@@ -137,9 +121,7 @@
     </sequence>
   </complexType>
 
-  <!--
-    general transform (create, renew, update, delete) result
-  -->
+  <!-- general transform (create, renew, update, delete) result -->
   <complexType name="transformResultType">
     <sequence>
       <element name="currency" type="fee:currencyType" />
@@ -154,9 +136,7 @@
     </sequence>
   </complexType>
 
-  <!--
-    transfer result
-  -->
+  <!-- transfer result -->
   <complexType name="transferResultType">
     <sequence>
       <element name="currency" type="fee:currencyType" />
@@ -169,7 +149,6 @@
         maxOccurs="unbounded" />
       <element name="credit" type="fee:creditType"
         minOccurs="0" maxOccurs="unbounded" />
-
       <element name="balance" type="fee:balanceType"
         minOccurs="0" />
       <element name="creditLimit" type="fee:creditLimitType"
@@ -177,9 +156,7 @@
     </sequence>
   </complexType>
 
-  <!--
-    common types
-  -->
+  <!-- common types -->
   <simpleType name="currencyType">
     <restriction base="string">
       <pattern value="[A-Z]{3}" />

core/src/test/java/google/registry/flows/session/LoginFlowTestCase.java

@@ -14,7 +14,9 @@
 
 package google.registry.flows.session;
 
+import static com.google.common.io.BaseEncoding.base64;
 import static com.google.common.truth.Truth.assertThat;
+import static google.registry.persistence.transaction.TransactionManagerFactory.tm;
 import static google.registry.testing.DatabaseHelper.deleteResource;
 import static google.registry.testing.DatabaseHelper.loadRegistrar;
 import static google.registry.testing.DatabaseHelper.persistResource;
@@ -38,6 +40,8 @@ import google.registry.model.eppoutput.EppOutput;
 import google.registry.model.registrar.Registrar;
 import google.registry.model.registrar.Registrar.State;
 import google.registry.testing.DatabaseHelper;
+import google.registry.util.PasswordUtils;
+import google.registry.util.PasswordUtils.HashAlgorithm;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -186,6 +190,32 @@ public abstract class LoginFlowTestCase extends FlowTestCase<LoginFlow> {
     doFailingTest("login_valid.xml", RegistrarAccountNotActiveException.class);
   }
 
+  @Test
+  void testSuccess_scryptPasswordToArgon2() throws Exception {
+    String password = "foo-BAR2";
+    tm().transact(
+            () -> {
+              // The salt is not exposed by Registrar (nor should it be), so we query it directly.
+              String encodedSalt =
+                  tm().query("SELECT salt FROM Registrar WHERE registrarId = :id", String.class)
+                      .setParameter("id", registrar.getRegistrarId())
+                      .getSingleResult();
+              byte[] salt = base64().decode(encodedSalt);
+              String newHash = PasswordUtils.hashPassword(password, salt, HashAlgorithm.SCRYPT_P_1);
+              // Set password directly, as the Java method would have used Argon2.
+              tm().query("UPDATE Registrar SET passwordHash = :hash WHERE registrarId = :id")
+                  .setParameter("id", registrar.getRegistrarId())
+                  .setParameter("hash", newHash)
+                  .executeUpdate();
+            });
+    assertThat(loadRegistrar("NewRegistrar").getCurrentHashAlgorithm(password).get())
+        .isEqualTo(HashAlgorithm.SCRYPT_P_1);
+    doSuccessfulTest("login_valid.xml");
+    // Verifies that after successfully login, the password is re-hased with Scrypt.
+    assertThat(loadRegistrar("NewRegistrar").getCurrentHashAlgorithm(password).get())
+        .isEqualTo(HashAlgorithm.ARGON_2_ID);
+  }
+
   @Test
   void testFailure_incorrectPassword() {
     persistResource(getRegistrarBuilder().setPassword("diff password").build());

core/src/test/java/google/registry/ui/server/console/ConsoleRegistryLockActionTest.java

@@ -14,7 +14,9 @@
 
 package google.registry.ui.server.console;
 
+import static com.google.common.io.BaseEncoding.base64;
 import static com.google.common.truth.Truth.assertThat;
+import static google.registry.persistence.transaction.TransactionManagerFactory.tm;
 import static google.registry.testing.DatabaseHelper.createTld;
 import static google.registry.testing.DatabaseHelper.loadByEntity;
 import static google.registry.testing.DatabaseHelper.loadRegistrar;
@@ -49,6 +51,8 @@ import google.registry.testing.DeterministicStringGenerator;
 import google.registry.testing.FakeResponse;
 import google.registry.tools.DomainLockUtils;
 import google.registry.util.EmailMessage;
+import google.registry.util.PasswordUtils;
+import google.registry.util.PasswordUtils.HashAlgorithm;
 import google.registry.util.StringGenerator;
 import jakarta.mail.internet.InternetAddress;
 import java.io.IOException;
@@ -88,16 +92,17 @@ public class ConsoleRegistryLockActionTest extends ConsoleActionBaseTestCase {
     createTld("test");
     defaultDomain = persistActiveDomain("example.test");
     user =
-        new User.Builder()
-            .setEmailAddress("user@theregistrar.com")
-            .setRegistryLockEmailAddress("registrylock@theregistrar.com")
-            .setUserRoles(
-                new UserRoles.Builder()
-                    .setRegistrarRoles(
-                        ImmutableMap.of("TheRegistrar", RegistrarRole.PRIMARY_CONTACT))
-                    .build())
-            .setRegistryLockPassword("registryLockPassword")
-            .build();
+        persistResource(
+            new User.Builder()
+                .setEmailAddress("user@theregistrar.com")
+                .setRegistryLockEmailAddress("registrylock@theregistrar.com")
+                .setUserRoles(
+                    new UserRoles.Builder()
+                        .setRegistrarRoles(
+                            ImmutableMap.of("TheRegistrar", RegistrarRole.PRIMARY_CONTACT))
+                        .build())
+                .setRegistryLockPassword("registryLockPassword")
+                .build());
     action = createGetAction();
   }
 
@@ -277,6 +282,40 @@ public class ConsoleRegistryLockActionTest extends ConsoleActionBaseTestCase {
     assertThat(loadByEntity(defaultDomain).getStatusValues()).containsExactly(StatusValue.INACTIVE);
   }
 
+  @Test
+  void testPost_lock_scryptPasswordToArgon2() throws Exception {
+    tm().transact(
+            () -> {
+              // The salt is not exposed by User (nor should it be), so we query it directly.
+              String encodedSalt =
+                  tm().query(
+                          "SELECT registryLockPasswordSalt FROM User WHERE emailAddress ="
+                              + " :emailAddress",
+                          String.class)
+                      .setParameter("emailAddress", user.getEmailAddress())
+                      .getSingleResult();
+              byte[] salt = base64().decode(encodedSalt);
+              String newHash =
+                  PasswordUtils.hashPassword(
+                      "registryLockPassword", salt, HashAlgorithm.SCRYPT_P_1);
+              // Set password directly, as the Java method would have used Argon2.
+              tm().query("UPDATE User SET registryLockPasswordHash = :hash")
+                  .setParameter("hash", newHash)
+                  .executeUpdate();
+            });
+    user = loadByEntity(user);
+    assertThat(user.getCurrentHashAlgorithm("registryLockPassword").get())
+        .isEqualTo(HashAlgorithm.SCRYPT_P_1);
+    action = createDefaultPostAction(true);
+    action.run();
+    verifyEmail();
+    assertThat(response.getStatus()).isEqualTo(SC_OK);
+    assertThat(getMostRecentRegistryLockByRepoId(defaultDomain.getRepoId())).isPresent();
+    user = loadByEntity(user);
+    assertThat(user.getCurrentHashAlgorithm("registryLockPassword").get())
+        .isEqualTo(HashAlgorithm.ARGON_2_ID);
+  }
+
   @Test
   void testPost_unlock() throws Exception {
     saveRegistryLock(createDefaultLockBuilder().setLockCompletionTime(clock.nowUtc()).build());

util/src/main/java/google/registry/util/PasswordUtils.java

@@ -15,28 +15,40 @@
 package google.registry.util;
 
 import static com.google.common.io.BaseEncoding.base64;
+import static google.registry.util.PasswordUtils.HashAlgorithm.ARGON_2_ID;
+import static google.registry.util.PasswordUtils.HashAlgorithm.SCRYPT_P_1;
 import static java.nio.charset.StandardCharsets.US_ASCII;
 
 import com.google.common.base.Supplier;
+import com.google.common.flogger.FluentLogger;
 import com.google.common.primitives.Bytes;
 import java.security.SecureRandom;
 import java.util.Arrays;
+import java.util.Optional;
+import org.bouncycastle.crypto.generators.Argon2BytesGenerator;
 import org.bouncycastle.crypto.generators.SCrypt;
+import org.bouncycastle.crypto.params.Argon2Parameters;
 
 /**
- * Common utility class to handle password hashing and salting /*
+ * Common utility class to handle password hashing and salting
  *
- * <p>We use a memory-hard hashing algorithm (Scrypt) to prevent brute-force attacks on passwords.
+ * <p>We use a memory-hard hashing algorithm (Argon2id) to prevent brute-force attacks on passwords.
  *
  * <p>Note that in tests, we simply concatenate the password and salt which is much faster and
- * reduces the overall test run time by a half. Our tests are not verifying that SCRYPT is
+ * reduces the overall test run time by a half. Our tests are not verifying that Argon2 is
  * implemented correctly anyway.
  *
  * @see <a href="https://en.wikipedia.org/wiki/Scrypt">Scrypt</a>
+ * @see <a href="https://en.wikipedia.org/wiki/Argon2">Argon2</a>
  */
 public final class PasswordUtils {
 
-  private PasswordUtils() {}
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+  // Argon2 requires an "init" call on the generator each time the salt is changed. As a result,
+  // each thread needs its own generator, then we can init it with the new salt every time a new
+  // hash is required.
+  private static final ThreadLocal<Argon2BytesGenerator> argon2BytesGenerator =
+      ThreadLocal.withInitial(Argon2BytesGenerator::new);
 
   public static final Supplier<byte[]> SALT_SUPPLIER =
       () -> {
@@ -46,25 +58,101 @@ public final class PasswordUtils {
         return salt;
       };
 
-  private static byte[] hashPassword(byte[] password, byte[] salt) {
-    return RegistryEnvironment.get() == RegistryEnvironment.UNITTEST
-        ? Bytes.concat(password, salt)
-        : SCrypt.generate(password, salt, 32768, 8, 1, 256);
+  public enum HashAlgorithm {
+    /**
+     * SCrypt algorithm using a parallelization factor of 1.
+     *
+     * <p>Note that in tests, we simply concatenate the password and salt which is much faster and
+     * reduces the overall test run time by a half. Our tests are not verifying that Scrypt is
+     * implemented correctly anyway.
+     *
+     * <p>TODO(b/458423787): Remove this circa March 2026 after enough time has passed for the
+     * logins to have transitioned to Argon2 hashing.
+     *
+     * @see <a href="https://en.wikipedia.org/wiki/Scrypt">Scrypt</a>
+     */
+    @Deprecated
+    SCRYPT_P_1 {
+      @Override
+      byte[] hash(byte[] password, byte[] salt) {
+        return RegistryEnvironment.get() == RegistryEnvironment.UNITTEST
+            ? Bytes.concat(password, salt)
+            : SCrypt.generate(password, salt, 32768, 8, 1, 256);
+      }
+    },
+
+    /**
+     * Argon2id algorithm using 3 iterations and 12 MiB.
+     *
+     * <p>Note that in tests, we simply concatenate the salt and password (the inverse order from
+     * Scrypt) which is much faster and reduces the overall test run time by a half. Our tests are
+     * not verifying that Argon2 is implemented correctly anyway.
+     *
+     * @see <a href="https://en.wikipedia.org/wiki/Argon2">Argon2</a>
+     * @see <a
+     *     href="https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html#argon2id">OWASP
+     *     cheat sheet</a>
+     */
+    ARGON_2_ID {
+      @Override
+      byte[] hash(byte[] password, byte[] salt) {
+        if (RegistryEnvironment.get() == RegistryEnvironment.UNITTEST) {
+          return Bytes.concat(salt, password);
+        }
+        Argon2BytesGenerator generator = argon2BytesGenerator.get();
+        // For Argon2, the salt is part of the parameters so we must reinitialize each time
+        generator.init(
+            new Argon2Parameters.Builder(Argon2Parameters.ARGON2_id)
+                .withVersion(Argon2Parameters.ARGON2_VERSION_13)
+                .withIterations(3)
+                .withMemoryAsKB(12288)
+                .withParallelism(1)
+                .withSalt(salt)
+                .build());
+        byte[] result = new byte[256];
+        generator.generateBytes(password, result);
+        return result;
+      }
+    };
+
+    abstract byte[] hash(byte[] password, byte[] salt);
   }
 
   /** Returns the hash of the password using the provided salt. */
   public static String hashPassword(String password, byte[] salt) {
-    return base64().encode(hashPassword(password.getBytes(US_ASCII), salt));
+    return hashPassword(password, salt, ARGON_2_ID);
+  }
+
+  /** Returns the hash of the password using the provided salt and {@link HashAlgorithm}. */
+  public static String hashPassword(String password, byte[] salt, HashAlgorithm algorithm) {
+    return base64().encode(algorithm.hash(password.getBytes(US_ASCII), salt));
   }
 
   /**
    * Verifies a password by regenerating the hash with the provided salt and comparing it to the
    * provided hash.
+   *
+   * <p>This method will first try to use {@link HashAlgorithm#ARGON_2_ID} to verify the password,
+   * and falls back to {@link HashAlgorithm#SCRYPT_P_1} if the former fails.
+   *
+   * @return the {@link HashAlgorithm} used to successfully verify the password, or {@link
+   *     Optional#empty()} if neither works.
    */
-  public static boolean verifyPassword(String password, String hash, String salt) {
+  public static Optional<HashAlgorithm> verifyPassword(String password, String hash, String salt) {
     byte[] decodedHash = base64().decode(hash);
     byte[] decodedSalt = base64().decode(salt);
-    byte[] calculatedHash = hashPassword(password.getBytes(US_ASCII), decodedSalt);
-    return Arrays.equals(decodedHash, calculatedHash);
+    byte[] decodedPassword = password.getBytes(US_ASCII);
+
+    if (Arrays.equals(decodedHash, ARGON_2_ID.hash(decodedPassword, decodedSalt))) {
+      logger.atInfo().log("ARGON_2_ID hash verified.");
+      return Optional.of(ARGON_2_ID);
+    }
+    if (Arrays.equals(decodedHash, SCRYPT_P_1.hash(decodedPassword, decodedSalt))) {
+      logger.atInfo().log("SCRYPT_P_1 hash verified.");
+      return Optional.of(SCRYPT_P_1);
+    }
+    return Optional.empty();
   }
+
+  private PasswordUtils() {}
 }

util/src/test/java/google/registry/util/PasswordUtilsTest.java

@@ -16,6 +16,8 @@ package google.registry.util;
 
 import static com.google.common.io.BaseEncoding.base64;
 import static com.google.common.truth.Truth.assertThat;
+import static google.registry.util.PasswordUtils.HashAlgorithm.ARGON_2_ID;
+import static google.registry.util.PasswordUtils.HashAlgorithm.SCRYPT_P_1;
 import static google.registry.util.PasswordUtils.SALT_SUPPLIER;
 import static google.registry.util.PasswordUtils.hashPassword;
 import static google.registry.util.PasswordUtils.verifyPassword;
@@ -47,12 +49,22 @@ final class PasswordUtilsTest {
   }
 
   @Test
-  void testVerify_scrypt_default() {
+  void testVerify_argon2_default() {
     byte[] salt = SALT_SUPPLIER.get();
     String password = "mySuperSecurePassword";
     String hashedPassword = hashPassword(password, salt);
-    assertThat(hashedPassword).isEqualTo(hashPassword(password, salt));
-    assertThat(verifyPassword(password, hashedPassword, base64().encode(salt))).isTrue();
+    assertThat(hashedPassword).isEqualTo(hashPassword(password, salt, ARGON_2_ID));
+    assertThat(verifyPassword(password, hashedPassword, base64().encode(salt)))
+        .hasValue(ARGON_2_ID);
+  }
+
+  @Test
+  void testVerify_scrypt() {
+    byte[] salt = SALT_SUPPLIER.get();
+    String password = "mySuperSecurePassword";
+    String hashedPassword = hashPassword(password, salt, SCRYPT_P_1);
+    assertThat(verifyPassword(password, hashedPassword, base64().encode(salt)))
+        .hasValue(SCRYPT_P_1);
   }
 
   @Test
@@ -60,6 +72,6 @@ final class PasswordUtilsTest {
     byte[] salt = SALT_SUPPLIER.get();
     String password = "mySuperSecurePassword";
     String hashedPassword = hashPassword(password, salt);
-    assertThat(verifyPassword(password + "a", hashedPassword, base64().encode(salt))).isFalse();
+    assertThat(verifyPassword(password + "a", hashedPassword, base64().encode(salt))).isEmpty();
   }
 }