使用SHA1和RSA与java.security.Signature对MessageDigest和Cipher [英] Using SHA1 and RSA with java.security.Signature vs. MessageDigest and Cipher

问题描述

我正在努力了解Java java.security.Signature 类中的内容。如果我计算一个SHA1消息摘要，然后使用RSA加密该摘要，我得到一个不同的结果，要求签名类签署相同的事情：

I'm trying to understand what the Java java.security.Signature class does. If I compute an SHA1 message digest, and then encrypt that digest using RSA, I get a different result to asking the Signature class to sign the same thing:

// Generate new key
KeyPair keyPair = KeyPairGenerator.getInstance("RSA").generateKeyPair();
PrivateKey privateKey = keyPair.getPrivate();
String plaintext = "This is the message being signed";

// Compute signature
Signature instance = Signature.getInstance("SHA1withRSA");
instance.initSign(privateKey);
instance.update((plaintext).getBytes());
byte[] signature = instance.sign();

// Compute digest
MessageDigest sha1 = MessageDigest.getInstance("SHA1");
byte[] digest = sha1.digest((plaintext).getBytes());

// Encrypt digest
Cipher cipher = Cipher.getInstance("RSA");
cipher.init(Cipher.ENCRYPT_MODE, privateKey);
byte[] cipherText = cipher.doFinal(digest);

// Display results
System.out.println("Input data: " + plaintext);
System.out.println("Digest: " + bytes2String(digest));
System.out.println("Cipher text: " + bytes2String(cipherText));
System.out.println("Signature: " + bytes2String(signature));

结果（例如）：

输入数据：这是正在签名的消息

摘要：62b0a9ef15461c82766fb5bdaae9edbe4ac2e067

加密文本：057dc0d2f7f54acc95d3cf5cba9f944619394711003bdd12 ...

签名： 7177c74bbbb871cc0af92e30d2808ebae146f25d3fd8ba1622 ...

Input data: This is the message being signed
Digest: 62b0a9ef15461c82766fb5bdaae9edbe4ac2e067
Cipher text: 057dc0d2f7f54acc95d3cf5cba9f944619394711003bdd12...
Signature: 7177c74bbbb871cc0af92e30d2808ebae146f25d3fd8ba1622...

我必须对什么签名正在做的一个根本误解 - 我跟踪通过它，它似乎是在一个 MessageDigest 对象上调用更新，其中算法按照我的预期设置为SHA1，然后获取摘要，然后进行加密。结果有什么不同？

I must have a fundamental misunderstanding of what Signature is doing - I've traced through it, and it appears to be calling update on a MessageDigest object, with the algorithm set to SHA1 as I would expect, then getting the digest, then doing the encryption. What's making the results differ?

编辑：

Leonidas让我检查签名计划应该是我认为的。 RFC 中定义的签名有两种类型：

Leonidas made me check whether the signature scheme is supposed to do what I think it does. There are two types of signature defined in the RFC:

• RSASSA -PKCS1-v1_5


• RSASSA-PSS

• RSASSA-PKCS1-v1_5
• RSASSA-PSS

第一个这些（PKCS1）是我上面描述的那个。它使用哈希函数创建摘要，然后使用私钥对结果进行加密。

The first of these (PKCS1) is the one I describe above. It uses a hash function to create a digest, and then encrypts the result with a private key.

第二个算法使用随机盐值，更安全但不确定。如果重复使用相同的密钥，则从上述代码生成的签名不会改变，所以我不认为它可以是PSS。

The second algorithm uses a random salt value, and is more secure but non-deterministic. The signature produced from the code above does not change if the same key is used repeatedly, so I don't think it can be PSS.

编辑： / strong>

这是我使用的 bytes2string 方法：

private static String bytes2String(byte[] bytes) {
    StringBuilder string = new StringBuilder();
    for (byte b : bytes) {
        String hexString = Integer.toHexString(0x00FF & b);
        string.append(hexString.length() == 1 ? "0" + hexString : hexString);
    }
    return string.toString();
}

推荐答案

出了什么事我很愚蠢Leonidas是对的，它不只是加密的哈希值，它是与摘要连接的散列算法的ID：

OK, I've worked out what's going on. I was being stupid. Leonidas is right, it's not just the hash that gets encrypted, it's the ID of the hash algorithm concatenated with the digest:

  DigestInfo ::= SEQUENCE {
      digestAlgorithm AlgorithmIdentifier,
      digest OCTET STRING
  }

这是为什么他们是不同的。

Which is why they are different.

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