Add Utf8#isEqual

This commit adds a constant-time equals method,
useful for comparing password hashes or other
sensitive material

Signed-off-by: Josh Cummings <3627351+jzheaux@users.noreply.github.com>

crypto/src/main/java/org/springframework/security/crypto/codec/Utf8.java

@@ -21,6 +21,9 @@ import java.nio.CharBuffer;
 import java.nio.charset.CharacterCodingException;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
+
+import org.jspecify.annotations.Nullable;
 
 /**
  * UTF-8 Charset encoder/decoder.
@@ -66,4 +69,21 @@ public final class Utf8 {
 		}
 	}
 
+	/**
+	 * Constant time comparison to prevent against timing attacks.
+	 * @param expected the expected {@link CharSequence}
+	 * @param actual the actual {@link CharSequence}
+	 * @return true if {@code expected} and {@code actual} are equal, false otherwise
+	 * @since 5.7.26
+	 */
+	public static boolean isEqual(@Nullable CharSequence expected, @Nullable CharSequence actual) {
+		byte[] expectedBytes = bytesUtf8(expected);
+		byte[] actualBytes = bytesUtf8(actual);
+		return MessageDigest.isEqual(expectedBytes, actualBytes);
+	}
+
+	private static byte @Nullable [] bytesUtf8(@Nullable CharSequence s) {
+		return (s != null) ? Utf8.encode(s) : null;
+	}
+
 }

crypto/src/main/java/org/springframework/security/crypto/password/PasswordEncoderUtils.java

@@ -16,8 +16,6 @@
 
 package org.springframework.security.crypto.password;
 
-import java.security.MessageDigest;
-
 import org.jspecify.annotations.Nullable;
 
 import org.springframework.security.crypto.codec.Utf8;
@@ -39,15 +37,7 @@ final class PasswordEncoderUtils {
 	 * @return
 	 */
 	static boolean equals(String expected, @Nullable String actual) {
-		byte[] expectedBytes = bytesUtf8(expected);
-		byte[] actualBytes = bytesUtf8(actual);
-		return MessageDigest.isEqual(expectedBytes, actualBytes);
-	}
-
-	private static byte @Nullable [] bytesUtf8(@Nullable String s) {
-		// need to check if Utf8.encode() runs in constant time (probably not).
-		// This may leak length of string.
-		return (s != null) ? Utf8.encode(s) : null;
+		return Utf8.isEqual(expected, actual);
 	}
 
 }

crypto/src/test/java/org/springframework/security/crypto/codec/Utf8Tests.java

@@ -37,4 +37,48 @@ public class Utf8Tests {
 		assertThat(decoded).isEqualTo("6048b75ed560785c");
 	}
 
+	@Test
+	public void isEqualWhenDifferentLengthThenFalse() {
+		assertThat(Utf8.isEqual("abc", "a")).isFalse();
+		assertThat(Utf8.isEqual("a", "abc")).isFalse();
+	}
+
+	@Test
+	public void isEqualWhenNullAndNotEmptyThenFalse() {
+		assertThat(Utf8.isEqual(null, "a")).isFalse();
+		assertThat(Utf8.isEqual("a", null)).isFalse();
+	}
+
+	@Test
+	public void isEqualWhenNullAndNullThenTrue() {
+		assertThat(Utf8.isEqual(null, null)).isTrue();
+	}
+
+	@Test
+	public void isEqualWhenNullAndEmptyThenFalse() {
+		assertThat(Utf8.isEqual(null, "")).isFalse();
+		assertThat(Utf8.isEqual("", null)).isFalse();
+	}
+
+	@Test
+	public void isEqualWhenNotEmptyAndEmptyThenFalse() {
+		assertThat(Utf8.isEqual("abc", "")).isFalse();
+		assertThat(Utf8.isEqual("", "abc")).isFalse();
+	}
+
+	@Test
+	public void isEqualWhenEmptyAndEmptyThenTrue() {
+		assertThat(Utf8.isEqual("", "")).isTrue();
+	}
+
+	@Test
+	public void isEqualWhenDifferentCaseThenFalse() {
+		assertThat(Utf8.isEqual("aBc", "abc")).isFalse();
+	}
+
+	@Test
+	public void isEqualWhenSameThenTrue() {
+		assertThat(Utf8.isEqual("abcdef", "abcdef")).isTrue();
+	}
+
 }