c# - 如何从 SHA-1 字节数组生成 Guid？

Question

我有这个代码来生成一个 SHA-1 哈希：

SHA1 sha1 = SHA1CryptoServiceProvider.Create();
Byte[] myStringBytes = ASCIIEncoding.Default.GetBytes(myString);
Byte[] hash = sha1.ComputeHash(myStringBytes);

有没有办法hash变成一个 Guid（我猜是类型 5，与 SHA-1 一致）？

Answer 1

您可以使用此基于rfc4122的C# 代码。

为了防止链接腐烂，这里有一些代码：

public static Guid Create(Guid namespaceId, string name)
{
    if (name == null)
        throw new ArgumentNullException("name");

    // convert the name to a sequence of octets (as defined by the standard or conventions of its namespace) (step 3)
    // ASSUME: UTF-8 encoding is always appropriate
    byte[] nameBytes = Encoding.UTF8.GetBytes(name);

    // convert the namespace UUID to network order (step 3)
    byte[] namespaceBytes = namespaceId.ToByteArray();
    SwapByteOrder(namespaceBytes);

    // comput the hash of the name space ID concatenated with the name (step 4)
    byte[] hash;
    using (HashAlgorithm algorithm =  SHA1.Create())
    {
        algorithm.TransformBlock(namespaceBytes, 0, namespaceBytes.Length, null, 0);
        algorithm.TransformFinalBlock(nameBytes, 0, nameBytes.Length);
        hash = algorithm.Hash;
    }

    // most bytes from the hash are copied straight to the bytes of the new GUID (steps 5-7, 9, 11-12)
    byte[] newGuid = new byte[16];
    Array.Copy(hash, 0, newGuid, 0, 16);

    // set the four most significant bits (bits 12 through 15) of the time_hi_and_version field to the appropriate 4-bit version number from Section 4.1.3 (step 8)
    newGuid[6] = (byte)((newGuid[6] & 0x0F) | (5 << 4));

    // set the two most significant bits (bits 6 and 7) of the clock_seq_hi_and_reserved to zero and one, respectively (step 10)
    newGuid[8] = (byte)((newGuid[8] & 0x3F) | 0x80);

    // convert the resulting UUID to local byte order (step 13)
    SwapByteOrder(newGuid);
    return new Guid(newGuid);
}

/// <summary>
/// The namespace for fully-qualified domain names (from RFC 4122, Appendix C).
/// </summary>
public static readonly Guid DnsNamespace = new Guid("6ba7b810-9dad-11d1-80b4-00c04fd430c8");

/// <summary>
/// The namespace for URLs (from RFC 4122, Appendix C).
/// </summary>
public static readonly Guid UrlNamespace = new Guid("6ba7b811-9dad-11d1-80b4-00c04fd430c8");

/// <summary>
/// The namespace for ISO OIDs (from RFC 4122, Appendix C).
/// </summary>
public static readonly Guid IsoOidNamespace = new Guid("6ba7b812-9dad-11d1-80b4-00c04fd430c8");

// Converts a GUID (expressed as a byte array) to/from network order (MSB-first).
internal static void SwapByteOrder(byte[] guid)
{
    SwapBytes(guid, 0, 3);
    SwapBytes(guid, 1, 2);
    SwapBytes(guid, 4, 5);
    SwapBytes(guid, 6, 7);
}

private static void SwapBytes(byte[] guid, int left, int right)
{
    byte temp = guid[left];
    guid[left] = guid[right];
    guid[right] = temp;
}

Answer 2

正如 Justin 所指出的，Guid 每次都应该是唯一的，而哈希每次都会为相同的值提供一致的结果。

现在我想补充一点，Guid 和哈希（如果不是所有算法的话，大多数算法）都会发生冲突，尽管我的直觉是哈希比 Guid 更容易发生冲突……尽管这可能取决于散列的大小（即 128 位、256 位、512 位等）。

您将遇到的另一个问题是byte[]来自 SHA1 哈希的长度为 20 字节，而 Guid 长度为 16 字节，因此，从 SHA1 哈希创建 Guid 将不准确。

例子：

string myString = "Hello World";
SHA1 sha1 = SHA1CryptoServiceProvider.Create();
Byte[] myStringBytes = ASCIIEncoding.Default.GetBytes(myString);
Byte[] hash = sha1.ComputeHash(myStringBytes);
Console.WriteLine(new Guid(hash.Take(16).ToArray()));

上面的示例将根据您的哈希创建一个 Guid，尽管它使用 LINQ 从哈希数组中获取 16 个字节（因此不准确......最后 4 个字节被简单地省略了）

MD5 是一个 16 字节的散列，所以这似乎比 SHA1 更适合转换为 Guid。

例子：

string myString = "Hello World";
MD5 md5 = MD5.Create();
Byte[] myStringBytes = ASCIIEncoding.Default.GetBytes(myString);
Byte[] hash = md5.ComputeHash(myStringBytes);
Console.WriteLine(new Guid(hash));

这会从 MD5 散列生成准确的 Guid，尽管我会说，这一切都是 MD5 散列的 Guid 表示......数据中应该没有实际的变化byte[]。