计算SHA-256哈希时前导零

Question

我试图比较同一文件的SHA-256哈希值与Python和Java。但是，在某些情况下，Python哈希值具有前导零，而Java版本不具有前导零。例如，在两个程序中散列somefile.txt会产生：

Python：000c3720cf1066fcde30876f498f060b0b3ad4e21abd473588f1f31f10fdd890

Java：c3720cf1066fcde30876f498f060b0b3ad4e21abd473588f1f31f10fdd890

简单地删除前导0并进行比较是否安全？或者是否存在不产生前导零的实现？

Python代码

def sha256sum(filename):
    h  = hashlib.sha256()
    b  = bytearray(128*1024)
    mv = memoryview(b)
    with open(filename, 'rb', buffering=0) as f:
        for n in iter(lambda : f.readinto(mv), 0):
            h.update(mv[:n])
    return h.hexdigest()

print(sha256sum('/somepath/somefile.txt'))

# 000c3720cf1066fcde30876f498f060b0b3ad4e21abd473588f1f31f10fdd890

Java代码

public static String calculateSHA256(File updateFile) {
    MessageDigest digest;
    try {
        digest = MessageDigest.getInstance("SHA-256");
    } catch (NoSuchAlgorithmException e) {
        Log.e(TAG, "Exception while getting digest", e);
        return null;
    }

    InputStream is;
    try {
        is = new FileInputStream(updateFile);
    } catch (FileNotFoundException e) {
        Log.e(TAG, "Exception while getting FileInputStream", e);
        return null;
    }

    byte[] buffer = new byte[8192];
    int read;
    try {
        while ((read = is.read(buffer)) > 0) {
            digest.update(buffer, 0, read);
        }
        byte[] shaSum = digest.digest();
        BigInteger bigInt = new BigInteger(1, shaSum);
        String output = bigInt.toString(16);
        return output;
    } catch (IOException e) {
        throw new RuntimeException("Unable to process file for SHA256", e);
    } finally {
        try {
            is.close();
        } catch (IOException e) {
            Log.e(TAG, "Exception on closing SHA256 input stream", e);
        }
    }
}

Log.i("Output", calculateSHA256(somefile))

// I/Output: c3720cf1066fcde30876f498f060b0b3ad4e21abd473588f1f31f10fdd890

Answer 1

BigInteger转换忽略了SHA-256哈希中的前导零。相反，你应该直接编码byte[]。正如建议in this answer你可以使用String.format()：

StringBuilder sb = new StringBuilder();
for (byte b : shaSum) {
    sb.append(String.format("%02X", b));
}
return sb.toString();

当编码为十六进制字符串时，SHA-256值有64个字符，根据wiki example：

SHA256（ “”）

0x e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855