HMACSHA1 Class
Computes a Hash-based Message Authentication Code (HMAC) using the SHA1 hash function.
Assembly: mscorlib (in mscorlib.dll)
System.Security.Cryptography.HashAlgorithm
System.Security.Cryptography.KeyedHashAlgorithm
System.Security.Cryptography.HMAC
System.Security.Cryptography.HMACSHA1
| Name | Description | |
|---|---|---|
![]() | HMACSHA1() | Initializes a new instance of the HMACSHA1 class with a randomly generated key. |
![]() | HMACSHA1(Byte()) | Initializes a new instance of the HMACSHA1 class with the specified key data. |
![]() | HMACSHA1(Byte(), Boolean) | Initializes a new instance of the HMACSHA1 class with the specified key data and a value that specifies whether to use the managed version of the SHA1 algorithm. |
| Name | Description | |
|---|---|---|
![]() | BlockSizeValue | Gets or sets the block size to use in the hash value.(Inherited from HMAC.) |
![]() | CanReuseTransform | Gets a value indicating whether the current transform can be reused.(Inherited from HashAlgorithm.) |
![]() | CanTransformMultipleBlocks | When overridden in a derived class, gets a value indicating whether multiple blocks can be transformed.(Inherited from HashAlgorithm.) |
![]() | Hash | Gets the value of the computed hash code.(Inherited from HashAlgorithm.) |
![]() | HashName | Gets or sets the name of the hash algorithm to use for hashing.(Inherited from HMAC.) |
![]() | HashSize | Gets the size, in bits, of the computed hash code.(Inherited from HashAlgorithm.) |
![]() | InputBlockSize | When overridden in a derived class, gets the input block size.(Inherited from HashAlgorithm.) |
![]() | Key | Gets or sets the key to use in the hash algorithm.(Inherited from HMAC.) |
![]() | OutputBlockSize | When overridden in a derived class, gets the output block size.(Inherited from HashAlgorithm.) |
| Name | Description | |
|---|---|---|
![]() | Clear() | Releases all resources used by the HashAlgorithm class.(Inherited from HashAlgorithm.) |
![]() | ComputeHash(Byte()) | Computes the hash value for the specified byte array.(Inherited from HashAlgorithm.) |
![]() | ComputeHash(Byte(), Int32, Int32) | Computes the hash value for the specified region of the specified byte array.(Inherited from HashAlgorithm.) |
![]() | ComputeHash(Stream) | Computes the hash value for the specified Stream object.(Inherited from HashAlgorithm.) |
![]() | Dispose() | Releases all resources used by the current instance of the HashAlgorithm class.(Inherited from HashAlgorithm.) |
![]() | Dispose(Boolean) | |
![]() | Equals(Object) | Determines whether the specified object is equal to the current object.(Inherited from Object.) |
![]() | Finalize() | Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.(Inherited from Object.) |
![]() | GetHashCode() | Serves as the default hash function. (Inherited from Object.) |
![]() | GetType() | |
![]() | HashCore(Byte(), Int32, Int32) | |
![]() | HashFinal() | When overridden in a derived class, finalizes the hash computation after the last data is processed by the cryptographic stream object.(Inherited from HMAC.) |
![]() | Initialize() | |
![]() | MemberwiseClone() | |
![]() | ToString() | Returns a string that represents the current object.(Inherited from Object.) |
![]() | TransformBlock(Byte(), Int32, Int32, Byte(), Int32) | Computes the hash value for the specified region of the input byte array and copies the specified region of the input byte array to the specified region of the output byte array.(Inherited from HashAlgorithm.) |
![]() | TransformFinalBlock(Byte(), Int32, Int32) | Computes the hash value for the specified region of the specified byte array.(Inherited from HashAlgorithm.) |
| Name | Description | |
|---|---|---|
![]() | HashSizeValue | Represents the size, in bits, of the computed hash code.(Inherited from HashAlgorithm.) |
![]() | HashValue | Represents the value of the computed hash code.(Inherited from HashAlgorithm.) |
![]() | KeyValue | The key to use in the hash algorithm.(Inherited from KeyedHashAlgorithm.) |
![]() | State | Represents the state of the hash computation.(Inherited from HashAlgorithm.) |
HMACSHA1 is a type of keyed hash algorithm that is constructed from the SHA1 hash function and used as an HMAC, or hash-based message authentication code. The HMAC process mixes a secret key with the message data, hashes the result with the hash function, mixes that hash value with the secret key again, and then applies the hash function a second time. The output hash is 160 bits in length.
An HMAC can be used to determine whether a message sent over an insecure channel has been tampered with, provided that the sender and receiver share a secret key. The sender computes the hash value for the original data and sends both the original data and hash value as a single message. The receiver recalculates the hash value on the received message and checks that the computed HMAC matches the transmitted HMAC.
Any change to the data or the hash value results in a mismatch, because knowledge of the secret key is required to change the message and reproduce the correct hash value. Therefore, if the original and computed hash values match, the message is authenticated.
The SHA-1 (Secure Hash Algorithm, also called SHS, Secure Hash Standard) is a cryptographic hash algorithm published by the United States Government. It produces a 160-bit hash value from an arbitrary length string.
HMACSHA1 accepts keys of any size, and produces a hash sequence that is 160 bits in length.
The following code example shows how to sign a file by using the HMACSHA1 object and then how to verify the file.
Imports System Imports System.IO Imports System.Security.Cryptography Public Class HMACSHA1example Public Shared Sub Main(ByVal Fileargs() As String) Dim dataFile As String Dim signedFile As String 'If no file names are specified, create them. If Fileargs.Length < 2 Then dataFile = "text.txt" signedFile = "signedFile.enc" If Not File.Exists(dataFile) Then ' Create a file to write to. Using sw As StreamWriter = File.CreateText(dataFile) sw.WriteLine("Here is a message to sign") End Using End If Else dataFile = Fileargs(0) signedFile = Fileargs(1) End If Try ' Create a random key using a random number generator. This would be the ' secret key shared by sender and receiver. Dim secretkey() As Byte = New [Byte](63) {} 'RNGCryptoServiceProvider is an implementation of a random number generator. Using rng As New RNGCryptoServiceProvider() ' The array is now filled with cryptographically strong random bytes. rng.GetBytes(secretkey) ' Use the secret key to encode the message file. SignFile(secretkey, dataFile, signedFile) ' Take the encoded file and decode VerifyFile(secretkey, signedFile) End Using Catch e As IOException Console.WriteLine("Error: File not found", e) End Try End Sub '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 Shared Sub SignFile(ByVal key() As Byte, ByVal sourceFile As String, ByVal destFile As String) ' Initialize the keyed hash object. Using myhmac As New HMACSHA1(key) Using inStream As New FileStream(sourceFile, FileMode.Open) Using outStream As New FileStream(destFile, FileMode.Create) ' Compute the hash of the input file. Dim hashValue As Byte() = myhmac.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. Dim bytesRead As Integer ' read 1K at a time Dim buffer(1023) As Byte Do ' Read from the wrapping CryptoStream. bytesRead = inStream.Read(buffer, 0, 1024) outStream.Write(buffer, 0, bytesRead) Loop While bytesRead > 0 End Using End Using End Using Return End Sub 'SignFile ' 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 Shared Function VerifyFile(ByVal key() As Byte, ByVal sourceFile As String) As Boolean Dim err As Boolean = False ' Initialize the keyed hash object. Using hmac As New HMACSHA1(key) ' Create an array to hold the keyed hash value read from the file. Dim storedHash(hmac.HashSize / 8) As Byte ' Create a FileStream for the source file. Using inStream As New FileStream(sourceFile, FileMode.Open) ' Read in the storedHash. inStream.Read(storedHash, 0, storedHash.Length - 1) ' 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. Dim computedHash As Byte() = hmac.ComputeHash(inStream) ' compare the computed hash with the stored value Dim i As Integer For i = 0 To storedHash.Length - 2 If computedHash(i) <> storedHash(i) Then err = True End If Next i End Using End Using If err Then 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 If End Function 'VerifyFile End Class 'HMACSHA1example 'end VerifyFile 'end class
Available since 1.1
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Any public static ( Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.




