用\0填充HMAC SHA256密钥是否应该返回相同的哈希值？

Question

hmacSHA256digest(  "secret".getBytes("UTF-8"), msg) = "244d9c89069406d40803722ec6a793e5e04c55234d9ca03039a7b505cb3f8f00"
hmacSHA256digest("secret\1".getBytes("UTF-8"), msg) = "4f94305c91ca9d8dec13ffcff7e455d6f0c49373e1bbc4035da2b500b11063fb" 

• “秘密”
• “机密\0”
• “机密\0\0”

因此，JavaX HMAC SHA256返回的是相同的散列，尽管从getBytes（“UTF-8”）返回的byte[]数组在结尾处只有几个额外的零（所以这不是UTF-8问题）：

hmacSHA256digest(   "secret".getBytes("UTF-8"), msg) 
= "244d9c89069406d40803722ec6a793e5e04c55234d9ca03039a7b505cb3f8f00"

hmacSHA256digest(   "secret\0".getBytes("UTF-8"), msg) 
= "244d9c89069406d40803722ec6a793e5e04c55234d9ca03039a7b505cb3f8f00"

hmacSHA256digest(   "secret\0\0".getBytes("UTF-8"), msg) 
= "244d9c89069406d40803722ec6a793e5e04c55234d9ca03039a7b505cb3f8f00"

对于MD5和普通SHA256的其他JavaX方法的调用，当额外的\0 s附加到秘密时，不会返回相同的哈希，因此它们通过了我们的安全测试用例，以验证不同秘密的哈希唯一性。MAC/SHA256的这个零填充秘密案例的失败是一个可能的攻击载体吗？

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.security.InvalidKeyException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;

static void testRightZeroPaddedSecretsHaveDifferentHashes() {
    try {
        byte[] msg = "msg".getBytes("UTF-8");

        // HMAC SHA256
        byte[] b3 = hmacSHA256digest(msg, "secret".getBytes("UTF-8"));
        byte[] b4 = hmacSHA256digest(msg, "secret\0".getBytes("UTF-8"));

        // Plain SHA256
        byte[] b5 = SHA256digest(msg, "secret".getBytes("UTF-8"));
        byte[] b6 = SHA256digest(msg, "secret\0".getBytes("UTF-8"));

        boolean same34 = Arrays.equals(b3, b4);
        boolean same56 = Arrays.equals(b5, b6);
        System.out.println(
                "\n" + Arrays.toString(b3) +
                "\n" + Arrays.toString(b4) +
                "\nHMAC SHA256 - identical hash results? = " + same34 +
                "\n" +
                "\n" + Arrays.toString(b5) +
                "\n" + Arrays.toString(b6) +
                "\nPlain SHA256 - identical hash results? = " + same56
        );
    } catch (Throwable e) {
        e.printStackTrace();
    }
}

static byte[] hmacSHA256digest(byte[] msg, byte[] secret) {
    try {
        SecretKeySpec keySpec = new SecretKeySpec(secret, "HmacSHA256");
        Mac mac = Mac.getInstance("HmacSHA256");
        mac.init(keySpec);
        byte[] hmac = mac.doFinal(msg);
        return hmac;
    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    } catch (InvalidKeyException e) {
        e.printStackTrace();
    }
    return null;
}

static byte[] SHA256digest(byte[] msg, byte[] secret) {
    try {
        MessageDigest digest = MessageDigest.getInstance("SHA-256");
        digest.update(msg);
        byte[] hash = digest.digest(secret);
        return hash;
    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    }
    return null;
}

和示例输出：

[-2, 79, -100, 65, -113, 104, 63, 3, 79, 106, -7, 13, 29, -43, -72, 106, -64, 53, 93, -39, 99, 50, -59, -100, -57, 69, -104, -48, 115, 97, 7, -10] 
[-2, 79, -100, 65, -113, 104, 63, 3, 79, 106, -7, 13, 29, -43, -72, 106, -64, 53, 93, -39, 99, 50, -59, -100, -57, 69, -104, -48, 115, 97, 7, -10] 
HMAC SHA256 - identical hash results? = true

[-88, 92, 89, -29, -65, -48, -127, 51, 125, -120, 78, -38, 25, 57, -91, 91, -50, 111, -33, 40, -3, 0, -95, 89, -50, -88, 39, 118, 101, -56, 91, 126] 
[-40, 39, 49, -64, 58, 40, 124, 64, 110, -100, 50, 115, -32, 114, -107, 24, -73, -17, -37, 11, 67, -26, -48, -65, 109, -24, 119, 45, 74, -31, -81, 119]
Plain SHA256 - identical hash results? = false

由于JavaX HMAC SHA256未能通过上述简单的SHA256/MD5算法的零填充秘密测试用例，有人能解释行为上的差异吗？

Answer 1

根据设计，这是HMAC构造的正确行为。

理想情况下，密钥的大小应与基础哈希算法的块大小相同。对于SHA-256，块大小是512位，所以密钥应该是64字节。

从RFC2104来看，如果一个键长于块大小，那么它将通过哈希函数传递并使用哈希作为键来缩短。如果键短于块大小，则将通过追加零来扩展键。