Use constant-time comparison of verification codes

Rate limits should make timing attacks or brute-forcing impossible regardless

Author: eager-signal

src/main/java/org/signal/registration/sender/LastDigitsOfPhoneNumberVerificationCodeSender.java

@@ -16,6 +16,7 @@
 import java.util.Locale;
 import java.util.Optional;
 import org.signal.registration.Environments;
+import org.signal.registration.util.Strings;
 
 /**
  * A trivial verification code "sender" that never actually sends codes, but instead always uses the last six digits of
@@ -60,7 +61,7 @@ public AttemptData sendVerificationCode(final MessageTransport messageTransport,
 
   @Override
   public boolean checkVerificationCode(final String verificationCode, final byte[] senderData) {
-    return verificationCode.equals(new String(senderData, StandardCharsets.UTF_8));
+    return Strings.equalsConstantTime(verificationCode, new String(senderData, StandardCharsets.UTF_8));
   }
 
   public static String getVerificationCode(final Phonenumber.PhoneNumber phoneNumber) {

src/main/java/org/signal/registration/sender/fictitious/FictitiousNumberVerificationCodeSender.java

@@ -21,6 +21,7 @@
 import org.signal.registration.sender.MessageTransport;
 import org.signal.registration.sender.VerificationCodeGenerator;
 import org.signal.registration.sender.VerificationCodeSender;
+import org.signal.registration.util.Strings;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -107,7 +108,7 @@ public boolean checkVerificationCode(final String verificationCode, final byte[]
       final String expectedVerificationCode =
           FictitiousNumberVerificationCodeSessionData.parseFrom(senderData).getVerificationCode();
 
-      return expectedVerificationCode.equals(verificationCode);
+      return Strings.equalsConstantTime(expectedVerificationCode, verificationCode);
     } catch (final InvalidProtocolBufferException e) {
       logger.error("Failed to parse stored session data", e);
       throw new UncheckedIOException(e);

src/main/java/org/signal/registration/sender/infobip/classic/InfobipSmsSender.java

@@ -24,7 +24,6 @@
 import java.util.Locale;
 import java.util.Optional;
 import javax.annotation.Nullable;
-import org.apache.commons.lang3.StringUtils;
 import org.signal.registration.sender.ApiClientInstrumenter;
 import org.signal.registration.sender.AttemptData;
 import org.signal.registration.sender.ClientType;
@@ -39,6 +38,7 @@
 import org.signal.registration.sender.infobip.InfobipClassicSessionData;
 import org.signal.registration.sender.infobip.InfobipExceptions;
 import org.signal.registration.sender.infobip.InfobipSenderConfiguration;
+import org.signal.registration.util.Strings;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -195,7 +195,7 @@ private static String checkSenderRejectedAndExtractMessageId(@Nullable final Lis
   public boolean checkVerificationCode(final String verificationCode, final byte[] senderData) {
     try {
       final String storedVerificationCode = InfobipClassicSessionData.parseFrom(senderData).getVerificationCode();
-      return StringUtils.equals(verificationCode, storedVerificationCode);
+      return Strings.equalsConstantTime(verificationCode, storedVerificationCode);
     } catch (final InvalidProtocolBufferException e) {
       logger.error("Failed to parse stored session data", e);
       throw new UncheckedIOException(e);

src/main/java/org/signal/registration/sender/messagebird/classic/MessageBirdSmsSender.java

@@ -21,7 +21,6 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Optional;
-import org.apache.commons.lang3.StringUtils;
 import org.signal.registration.sender.ApiClientInstrumenter;
 import org.signal.registration.sender.AttemptData;
 import org.signal.registration.sender.ClientType;
@@ -35,6 +34,7 @@
 import org.signal.registration.sender.messagebird.MessageBirdClassicSessionData;
 import org.signal.registration.sender.messagebird.MessageBirdExceptions;
 import org.signal.registration.sender.messagebird.MessageBirdSenderConfiguration;
+import org.signal.registration.util.Strings;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -167,7 +167,7 @@ public AttemptData sendVerificationCode(final MessageTransport messageTransport,
   public boolean checkVerificationCode(final String verificationCode, final byte[] senderData) {
     try {
       final String storedVerificationCode = MessageBirdClassicSessionData.parseFrom(senderData).getVerificationCode();
-      return StringUtils.equals(verificationCode, storedVerificationCode);
+      return Strings.equalsConstantTime(verificationCode, storedVerificationCode);
     } catch (final InvalidProtocolBufferException e) {
       logger.error("Failed to parse stored session data", e);
       throw new UncheckedIOException(e);

src/main/java/org/signal/registration/sender/messagebird/classic/MessageBirdVoiceSender.java

@@ -19,7 +19,6 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Optional;
-import org.apache.commons.lang3.StringUtils;
 import org.signal.registration.sender.ApiClientInstrumenter;
 import org.signal.registration.sender.AttemptData;
 import org.signal.registration.sender.ClientType;
@@ -32,6 +31,7 @@
 import org.signal.registration.sender.messagebird.MessageBirdClassicSessionData;
 import org.signal.registration.sender.messagebird.MessageBirdExceptions;
 import org.signal.registration.sender.messagebird.MessageBirdSenderConfiguration;
+import org.signal.registration.util.Strings;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -163,7 +163,7 @@ public AttemptData sendVerificationCode(final MessageTransport messageTransport,
   public boolean checkVerificationCode(final String verificationCode, final byte[] sessionData) {
     try {
       final String storedVerificationCode = MessageBirdClassicSessionData.parseFrom(sessionData).getVerificationCode();
-      return StringUtils.equals(verificationCode, storedVerificationCode);
+      return Strings.equalsConstantTime(verificationCode, storedVerificationCode);
     } catch (final InvalidProtocolBufferException e) {
       logger.error("Failed to parse stored session data", e);
       throw new UncheckedIOException(e);

src/main/java/org/signal/registration/sender/prescribed/PrescribedVerificationCodeSender.java

@@ -25,6 +25,7 @@
 import org.signal.registration.sender.MessageTransport;
 import org.signal.registration.sender.SenderRejectedRequestException;
 import org.signal.registration.sender.VerificationCodeSender;
+import org.signal.registration.util.Strings;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -102,7 +103,7 @@ public boolean checkVerificationCode(final String verificationCode, final byte[]
       final String expectedVerificationCode =
           PrescribedVerificationCodeSessionData.parseFrom(senderData).getVerificationCode();
 
-      return StringUtils.equals(verificationCode, expectedVerificationCode);
+      return Strings.equalsConstantTime(verificationCode, expectedVerificationCode);
     } catch (final InvalidProtocolBufferException e) {
       logger.error("Failed to parse stored session data", e);
       throw new UncheckedIOException(e);

src/main/java/org/signal/registration/sender/sinch/classic/SinchSmsSender.java

@@ -35,6 +35,7 @@
 import org.signal.registration.sender.sinch.SinchClassicSessionData;
 import org.signal.registration.sender.sinch.SinchExceptions;
 import org.signal.registration.sender.sinch.SinchSenderConfiguration;
+import org.signal.registration.util.Strings;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -148,7 +149,7 @@ public boolean checkVerificationCode(final String verificationCode, final byte[]
       throws SenderRejectedRequestException {
     try {
       final String storedVerificationCode = SinchClassicSessionData.parseFrom(senderData).getVerificationCode();
-      return storedVerificationCode.equals(verificationCode);
+      return Strings.equalsConstantTime(storedVerificationCode, verificationCode);
     } catch (final InvalidProtocolBufferException e) {
       logger.error("Failed to parse stored session data", e);
       throw new UncheckedIOException(e);

src/main/java/org/signal/registration/sender/twilio/classic/AbstractTwilioProvidedCodeSender.java

@@ -11,9 +11,9 @@
 import com.twilio.type.PhoneNumber;
 import java.io.UncheckedIOException;
 import java.util.Optional;
-import org.apache.commons.lang3.StringUtils;
 import org.signal.registration.sender.AttemptData;
 import org.signal.registration.sender.VerificationCodeSender;
+import org.signal.registration.util.Strings;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -39,7 +39,7 @@ protected static PhoneNumber twilioNumberFromPhoneNumber(final Phonenumber.Phone
   @Override
   public boolean checkVerificationCode(final String verificationCode, final byte[] senderData) {
     try {
-      return StringUtils.equals(verificationCode,
+      return Strings.equalsConstantTime(verificationCode,
           TwilioProvidedCodeSessionData.parseFrom(senderData).getVerificationCode());
     } catch (final InvalidProtocolBufferException e) {
       logger.error("Failed to parse stored session data", e);

src/main/java/org/signal/registration/util/Strings.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.registration.util;
+
+import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
+
+public class Strings {
+
+  private Strings() {
+  }
+
+  /**
+   * Compares two strings in constant time
+   *
+   * @see java.security.MessageDigest#isEqual(byte[], byte[])
+   */
+  public static boolean equalsConstantTime(final String s1, final String s2) {
+    return MessageDigest.isEqual(s1.getBytes(StandardCharsets.UTF_8), s2.getBytes(StandardCharsets.UTF_8));
+  }
+
+}

src/test/java/org/signal/registration/sender/TestVerificationCodeSender.java

@@ -1,6 +1,7 @@
 package org.signal.registration.sender;
 
 import com.google.i18n.phonenumbers.Phonenumber;
+import org.signal.registration.util.Strings;
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
 import java.util.List;
@@ -56,6 +57,6 @@ public AttemptData sendVerificationCode(
 
   @Override
   public boolean checkVerificationCode(final String verificationCode, final byte[] senderData) {
-    return verificationCode.equals(new String(senderData, StandardCharsets.UTF_8));
+    return Strings.equalsConstantTime(verificationCode, new String(senderData, StandardCharsets.UTF_8));
   }
 }