Question

我正在尝试用Java编写非常简单的merkle-tree实现。

我正在使用比特币区块链上block 170中的txid值作为参考，因此我可以看到正确的结果。

与该块对应的txid如下：

b1fea52486ce0c62bb442b530a3f0132b826c74e473d1f2c220bfa78111c5082

f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16

据我了解，比特币的merkle-tree实现方式如下：

将区块中的交易分成几对
字节交换txids
连接txids
对连接的对进行双散列

警告：

If there's no additional pairs of txids, concatenate the result of the first pair after double hashing with itself and repeat

我的代码在switch语句中，如下所示：

case "test merkle root": {
    // txid A
    String A = "b1fea52486ce0c62bb442b530a3f0132b826c74e473d1f2c220bfa78111c5082";

    // txid A byte-swapped
    String A_little = MainChain.swapEndianness(A);

    // txid B
    String B = "f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16";

    // txid B byte-swapped
    String B_little = MainChain.swapEndianness(B);

    // txid A + B concatenated
    String AB_little = A_little + B_little;

    // double hash of byte-swapped concatenated A+B
    String ABdoubleHash =  SHA256.generateSHA256Hash(SHA256.generateSHA256Hash(AB_little));

    // double hash concatenated with itself
    String ABAB_little = ABdoubleHash + ABdoubleHash;

    // double hash of self-concatenated double-hashed txid
    String merkleRootLittleEndian = SHA256.generateSHA256Hash(SHA256.generateSHA256Hash(ABAB_little));

    // print result byte-swapped back to big-endian
    System.out.println("Merkle root: " + MainChain.swapEndianness(merkleRootLittleEndian));
}

我编写的swapEndianness方法不是真正的“字节级”交换，而是仅更改String的顺序，如下所示：

public static String swapEndianness(String hash) {
        char[] hashAsCharArray = hash.toCharArray();
        StringBuilder sb = new StringBuilder();
        for (int i = hash.length() - 1; i > 0; i-=2) {
            sb.append(hashAsCharArray[i - 1]);
            sb.append(hashAsCharArray[i]);
        }
        return sb.toString();
    }

这两个txid的Merkle根的预期结果是：

7dac2c5666815c17a3b36427de37bb9d2e2c5ccec3f8633eb91a4205cb4c10ff

但是，我最终得到的结果是：

3b40cab1157838cc41b08e27641f65d245957ab07b3504d94bc2d355abaed06c

我没有得到我期望的结果，是因为我在做字节交换时作弊，是因为我错过了一步，还是因为我的代码中有错误（或这些错误的某种组合）？任何帮助将不胜感激！

Answer 1

您不了解Java中的byte []是什么。您的示例中的字符串是byte []的“十六进制”表示。参见How do I initialize a byte array in Java?

public class MerkleTree {
    static MessageDigest digest;
    public static void main(String[] args) throws NoSuchAlgorithmException {
        digest = MessageDigest.getInstance("SHA-256");
        new MerkleTree().run();
    }
    private void run() {
        // txid A
        byte[] A = hexStringToByteArray("b1fea52486ce0c62bb442b530a3f0132b826c74e473d1f2c220bfa78111c5082");
        System.out.println(Arrays.toString(A));
        // txid A byte-swapped
        byte[] A_little = swapEndianness(A);
        System.out.println(Arrays.toString(A_little));

        // txid B
        byte[] B = hexStringToByteArray("f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16");
        System.out.println(Arrays.toString(B));

        // txid B byte-swapped
        byte[] B_little = swapEndianness(B);
        System.out.println(Arrays.toString(B_little));

        // txid A + B concatenated
        byte[] AB_little = Arrays.copyOf(A_little, A_little.length + B_little.length);
        System.arraycopy(B_little, 0, AB_little, A_little.length, B_little.length);
        System.out.println(Arrays.toString(AB_little));

        // double hash of byte-swapped concatenated A+B
        byte[] ABdoubleHash = SHA256(SHA256(AB_little));
        System.out.println(Arrays.toString(ABdoubleHash));

        // print result byte-swapped back to big-endian
        byte[] result = swapEndianness(ABdoubleHash);
        System.out.println(Arrays.toString(result));
        System.out.println(getHex(result));         
    }
    byte[] swapEndianness(byte[] hash) {
        byte[] result = new byte[hash.length];
        for (int i = 0; i < hash.length; i++) {
            result[i] = hash[hash.length-i-1];
        }
        return result;
    }
    byte[] SHA256(byte[] obytes) {
        return digest.digest(obytes);
    }
    byte[] hexStringToByteArray(String s) {
        int len = s.length();
        byte[] data = new byte[len / 2];
        for (int i = 0; i < len; i += 2) {
            data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
                                 + Character.digit(s.charAt(i+1), 16));
        }
        return data;
    }
    private static final String    HEXES    = "0123456789abcdef";
    String getHex(byte[] raw) {
        final StringBuilder hex = new StringBuilder(2 * raw.length);
        for (final byte b : raw) {
            hex.append(HEXES.charAt((b & 0xF0) >> 4)).append(HEXES.charAt((b & 0x0F)));
        }
        return hex.toString();
    }
}

最后，请参考Java code To convert byte to Hexadecimal

编辑：这在资源上会更好一些，因为您经常想做很多这类事情。

    // txid A byte-swapped
    byte[] A = swapEndianness(
            hexStringToByteArray("b1fea52486ce0c62bb442b530a3f0132b826c74e473d1f2c220bfa78111c5082")
        );
    // txid B byte-swapped
    byte[] B = swapEndianness(
            hexStringToByteArray("f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16")
        );
    // txid A + B concatenated
    byte[] AB = Arrays.copyOf(A, A.length + B.length);
    System.arraycopy(B, 0, AB, A.length, B.length);

    // print result byte-swapped back to big-endian
    String result = getHex(swapEndianness(SHA256(SHA256(AB))));
    System.out.println(result);         
    }
    byte[] swapEndianness(byte[] hash) {
        for (int i = 0; i < hash.length/2; i++) {
            byte t = hash[hash.length-i-1];
            hash[hash.length-i-1] = hash[i]; 
            hash[i] = t; 
        }
        return hash;
    }

Java中的简单merkle-tree实现

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