Hide utility class constructors

Update all utility classes so that they have a private constructor. This
prevents users from accidentally creating an instance, when they should
just use the static methods directly.

Issue gh-8945

crypto/src/main/java/org/springframework/security/crypto/argon2/Argon2EncodingUtils.java

@@ -29,12 +29,15 @@ import org.bouncycastle.util.Arrays;
  * @author Simeon Macke
  * @since 5.3
  */
-class Argon2EncodingUtils {
+final class Argon2EncodingUtils {
 
 	private static final Base64.Encoder b64encoder = Base64.getEncoder().withoutPadding();
 
 	private static final Base64.Decoder b64decoder = Base64.getDecoder();
 
+	private Argon2EncodingUtils() {
+	}
+
 	/**
 	 * Encodes a raw Argon2-hash and its parameters into the standard Argon2-hash-string
 	 * as specified in the reference implementation

crypto/src/main/java/org/springframework/security/crypto/codec/Base64.java

@@ -237,6 +237,9 @@ public final class Base64 {
 			-9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9 // Decimal 244 - 255
 	};
 
+	private Base64() {
+	}
+
 	public static byte[] decode(byte[] bytes) {
 		return decode(bytes, 0, bytes.length, NO_OPTIONS);
 	}

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

@@ -30,6 +30,9 @@ public final class Hex {
 	private static final char[] HEX = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e',
 			'f' };
 
+	private Hex() {
+	}
+
 	public static char[] encode(byte[] bytes) {
 		final int nBytes = bytes.length;
 		char[] result = new char[2 * nBytes];

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

@@ -33,6 +33,9 @@ public final class Utf8 {
 
 	private static final Charset CHARSET = StandardCharsets.UTF_8;
 
+	private Utf8() {
+	}
+
 	/**
 	 * Get the bytes of the String in UTF-8 encoded form.
 	 */

crypto/src/test/java/org/springframework/security/crypto/codec/Base64Tests.java

@@ -28,8 +28,6 @@ public class Base64Tests {
 
 	@Test
 	public void isBase64ReturnsTrueForValidBase64() {
-		new Base64(); // unused
-
 		assertThat(Base64.isBase64(new byte[] { (byte) 'A', (byte) 'B', (byte) 'C', (byte) 'D' })).isTrue();
 	}
 

crypto/src/test/java/org/springframework/security/crypto/encrypt/CryptoAssumptions.java

@@ -26,7 +26,10 @@ import org.junit.AssumptionViolatedException;
 
 import org.springframework.security.crypto.encrypt.AesBytesEncryptor.CipherAlgorithm;
 
-public class CryptoAssumptions {
+public final class CryptoAssumptions {
+
+	private CryptoAssumptions() {
+	}
 
 	public static void assumeGCMJCE() {
 		assumeAes256(CipherAlgorithm.GCM);