Hmac-sha256签名和验证

小灰灰 2023-02-22 07:58 12阅读 0赞

下面的示例演示如何使用 HMACSHA256 对象，然后验证文件的方法来对文件进行签名。

C\#

using System;  
using System.IO;  
using System.Security.Cryptography;

public class HMACSHA256example  
\{

public static void Main(string\[\] Fileargs)  
\{  
string dataFile;  
string signedFile;  
//If no file names are specified, create them.  
if (Fileargs.Length < 2)  
\{  
dataFile = @"text.txt";  
signedFile = "signedFile.enc";

if (!File.Exists(dataFile))  
\{  
// Create a file to write to.  
using (StreamWriter sw = File.CreateText(dataFile))  
\{  
sw.WriteLine("Here is a message to sign");  
\}  
\}  
\}  
else  
\{  
dataFile = Fileargs\[0\];  
signedFile = Fileargs\[1\];  
\}  
try  
\{  
// Create a random key using a random number generator. This would be the  
// secret key shared by sender and receiver.  
byte\[\] secretkey = new Byte\[64\];  
//RNGCryptoServiceProvider is an implementation of a random number generator.  
using (RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider())  
\{  
// The array is now filled with cryptographically strong random bytes.  
rng.GetBytes(secretkey);

// Use the secret key to sign the message file.  
SignFile(secretkey, dataFile, signedFile);

// Verify the signed file  
VerifyFile(secretkey, signedFile);  
\}  
\}  
catch (IOException e)  
\{  
Console.WriteLine("Error: File not found", e);  
\}  
\} //end main  
// Computes a keyed hash for a source file and creates a target file with the keyed hash  
// prepended to the contents of the source file.  
public static void SignFile(byte\[\] key, String sourceFile, String destFile)  
\{  
// Initialize the keyed hash object.  
using (HMACSHA256 hmac = new HMACSHA256(key))  
\{  
using (FileStream inStream = new FileStream(sourceFile, FileMode.Open))  
\{  
using (FileStream outStream = new FileStream(destFile, FileMode.Create))  
\{  
// Compute the hash of the input file.  
byte\[\] hashValue = hmac.ComputeHash(inStream);  
// Reset inStream to the beginning of the file.  
inStream.Position = 0;  
// Write the computed hash value to the output file.  
outStream.Write(hashValue, 0, hashValue.Length);  
// Copy the contents of the sourceFile to the destFile.  
int bytesRead;  
// read 1K at a time  
byte\[\] buffer = new byte\[1024\];  
do  
\{  
// Read from the wrapping CryptoStream.  
bytesRead = inStream.Read(buffer, 0, 1024);  
outStream.Write(buffer, 0, bytesRead);  
\} while (bytesRead > 0);  
\}  
\}  
\}  
return;  
\} // end SignFile

// Compares the key in the source file with a new key created for the data portion of the file. If the keys  
// compare the data has not been tampered with.  
public static bool VerifyFile(byte\[\] key, String sourceFile)  
\{  
bool err = false;  
// Initialize the keyed hash object.  
using (HMACSHA256 hmac = new HMACSHA256(key))  
\{  
// Create an array to hold the keyed hash value read from the file.  
byte\[\] storedHash = new byte\[hmac.HashSize / 8\];  
// Create a FileStream for the source file.  
using (FileStream inStream = new FileStream(sourceFile, FileMode.Open))  
\{  
// Read in the storedHash.  
inStream.Read(storedHash, 0, storedHash.Length);  
// Compute the hash of the remaining contents of the file.  
// The stream is properly positioned at the beginning of the content,  
// immediately after the stored hash value.  
byte\[\] computedHash = hmac.ComputeHash(inStream);  
// compare the computed hash with the stored value

for (int i = 0; i < storedHash.Length; i++)  
\{  
if (computedHash\[i\] != storedHash\[i\])  
\{  
err = true;  
\}  
\}  
\}  
\}  
if (err)  
\{  
Console.WriteLine("Hash values differ! Signed file has been tampered with!");  
return false;  
\}  
else  
\{  
Console.WriteLine("Hash values agree -- no tampering occurred.");  
return true;  
\}  
\} //end VerifyFile  
\} //end class

注解  
HMACSHA256 是一种加密哈希算法，它是从 SHA-256 哈希函数构建的，用作基于哈希的消息验证代码（HMAC）。 HMAC 进程将密钥与消息数据混合，使用哈希函数对结果进行哈希运算，再次混合使用密钥的哈希值，然后再次应用哈希函数。 输出哈希的长度为256位。

**如果发送方和接收方共享密钥，则可以使用 HMAC 来确定通过不安全通道发送的消息是否已被篡改。 发件人计算原始数据的哈希值，并以单个消息形式发送原始数据和哈希值。 接收方重新计算收到的消息上的哈希值，并检查计算的 HMAC 是否与传输的 HMAC 匹配。**

对数据或哈希值所做的任何更改都将导致不匹配，因为更改消息和重现正确的哈希值需要使用机密密钥知识。 因此，如果原始和计算所得的哈希值匹配，则将对消息进行身份验证。

HMACSHA256 接受任意大小的密钥，并产生长度为256位的哈希序列。