下面的示例演示如何使用 HMACSHA256 對象，然後驗證文件的方法來對文件進行簽名。
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
註解
HMACSHA256 是一種加密哈希算法，它是從 SHA-256 哈希函數構建的，用作基於哈希的消息驗證代碼（HMAC）。 HMAC 進程將密鑰與消息數據混合，使用哈希函數對結果進行哈希運算，再次混合使用密鑰的哈希值，然後再次應用哈希函數。 輸出哈希的長度爲256位。
如果發送方和接收方共享密鑰，則可以使用 HMAC 來確定通過不安全通道發送的消息是否已被篡改。 發件人計算原始數據的哈希值，並以單個消息形式發送原始數據和哈希值。 接收方重新計算收到的消息上的哈希值，並檢查計算的 HMAC 是否與傳輸的 HMAC 匹配。
對數據或哈希值所做的任何更改都將導致不匹配，因爲更改消息和重現正確的哈希值需要使用機密密鑰知識。 因此，如果原始和計算所得的哈希值匹配，則將對消息進行身份驗證。
HMACSHA256 接受任意大小的密鑰，併產生長度爲256位的哈希序列。