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SECURITY

Programming .NET Security : Programming Digital Signatures (part 3) - Using the Signature Formatter Classes

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3. Using the Signature Formatter Classes

The .NET Framework specifies separate formatter and deformatter classes, as shown by the hierarchy in Figure 2. These classes format a hash code using PKCS #1, and sign the result in conjunction with the abstract and implementation classes. The functionality that these classes provide is equivalent to using the methods that we have described in the preceding sections; there is no specific advantage to using the formatter and deformatter classes.

Figure 2. The .NET Framework class hierarchy for signature formatters

The abstract AsymmetricSignatureFormatter class defines the methods described in Table 2. The abstract class does not specify how to format a hash code, and so implementation classes can define any formatting that is appropriate to an algorithm; both of the default .NET implementation classes use PKCS #1 formatting.

Table 2. AsymmetricSignatureFormatter members
Method Description
SetHashAlgorithm Specifies the name of the hashing algorithm that was used to create the hash code that will be signed; in the case of PKCS #1 formatting, the ID of the algorithm will be included in the signed data
SetKey Specifies an instance of the asymmetric signing algorithm that will be used to create the digital signature
CreateSignature Creates a digital signature from a hash code

The following statements demonstrate how to use the RSAPKCS1SignatureFormatter class to create a digital signature:

# C#

// create the plaintext
byte[] x_plaintext = Encoding.Default.GetBytes("Programming .NET Security");

// create a hash code for the plaintext, using the SHA-1 algorithm
byte[] x_hashcode = HashAlgorithm.Create("SHA1").ComputeHash(x_plaintext);

// create an instance of the DSA implementation class
DSACryptoServiceProvider x_dsa = new DSACryptoServiceProvider( );

// create the signature formatter
DSASignatureFormatter x_formatter = new DSASignatureFormatter( );
// set the instance of the DSA algorithm that will sign the data
x_formatter.SetKey(x_dsa);
// set the name of the hashing algorithm we used to create the hash code
x_formatter.SetHashAlgorithm("SHA1");
// create the formatted DSA signature
byte[] x_signature = x_formatter.CreateSignature(x_hashcode);

# Visual Basic .NET

' create the plaintext
Dim x_plaintext As Byte( ) = Encoding.Default.GetBytes("Programming .NET Security")

' create a hash code for the plaintext, using the SHA-1 algorithm
Dim x_hashcode As Byte( ) = HashAlgorithm.Create("SHA1").ComputeHash(x_plaintext)

' create an instance of the DSA implementation class
Dim x_dsa As DSACryptoServiceProvider = New DSACryptoServiceProvider( )

' create the signature formatter
Dim x_formatter As DSASignatureFormatter = New DSASignatureFormatter( )
' set the instance of the DSA algorithm that will sign the data
x_formatter.SetKey(x_dsa)
' set the name of the hashing algorithm we used to create the hash code
x_formatter.SetHashAlgorithm("SHA1")
' create the formatted DSA signature
Dim x_signature As Byte( ) = x_formatter.CreateSignature(x_hashcode)


The abstract AsymmetricSignatureDeformatter is the basis for classes that verify digital signatures created by subclasses of the AsymmetricSignatureFormatter class. Table 3 summarizes the methods of the AsymmetricSignatureDeformatter class.

Table 3. AsymmetricSignatureDeformatter members
Method Description
SetHashAlgorithm Specifies the name of the hashing algorithm that will be used to verify the signature
SetKey Specifies an instance of the asymmetric signing algorithm that will be used to verify the digital signature
VerifySignature Verifies a digital signature

The following statements demonstrate how to verify a digital signature using the DSASignatureDeformatter class:

# C#

// create the plaintext
byte[] x_plaintext = Encoding.Default.GetBytes("Programming .NET Security");

// create a hash code for the plaintext, using the SHA-1 algorithm
byte[] x_hashcode = HashAlgorithm.Create("SHA1").ComputeHash(x_plaintext);

// define the signature to verify
byte[] x_signature = new Byte[] {0x7D, 0x2B, 0xD7, 0x3D, 0x88, 0xCB, 0x1B, 0x6B,
0x04, 0x62, 0x95, 0xBE, 0x28, 0x59, 0x3E, 0xC5,
0x40, 0xDA, 0x79, 0xFE, 0x3B, 0x25, 0x08, 0x4B,
0x27, 0xF1, 0x31, 0x2A, 0x6F, 0x7C, 0x6E, 0x35,
0x45, 0x9A, 0x49, 0x4C, 0xA4, 0x5E, 0xE6, 0xA0};

// create an instance of the DSA implementation class
DSACryptoServiceProvider x_dsa = new DSACryptoServiceProvider( );

// create the signature deformatter
DSASignatureDeformatter x_deformatter = new DSASignatureDeformatter( );
// set the instance of the DSA algorithm that will verify the signature
x_deformatter.SetKey(x_dsa);
// set the name of the hashing algorithm we used to create the hash code
x_deformatter.SetHashAlgorithm("SHA1");
// verify the DSA signature
bool x_signature_valid = x_deformatter.VerifySignature(x_hashcode, x_signature);

# Visual Basic .NET

' create the plaintext
Dim x_plaintext As Byte( ) = Encoding.Default.GetBytes("Programming .NET Security")

' create a hash code for the plaintext, using the SHA-1 algorithm
Dim x_hashcode As Byte( ) = HashAlgorithm.Create("SHA1").ComputeHash(x_plaintext)

' define the signature to verify
Dim x_signature As Byte( ) = New Byte( ) {&H7D, &H2B, &HD7, &H3D, &H88, &HCB, _
&H1B, &H6B, &H4, &H62, &H95, &HBE, &H28, _
&H59, &H3E, &HC5, &H40, &HDA, &H79, &HFE, _
&H3B, &H25, &H8, &H4B, &H27, &HF1, &H31, _
&H2A, &H6F, &H7C, &H6E, &H35, &H45, &H9A, _
&H49, &H4C, &HA4, &H5E, &HE6, &HA0}

' create an instance of the DSA implementation class
Dim x_dsa As DSACryptoServiceProvider = New DSACryptoServiceProvider( )

' create the signature deformatter
Dim x_deformatter As DSASignatureDeformatter = New DSASignatureDeformatter( )
' set the instance of the DSA algorithm that will verify the signature
x_deformatter.SetKey(x_dsa)
' set the name of the hashing algorithm we used to create the hash code
x_deformatter.SetHashAlgorithm("SHA1")

' verify the DSA signature
Dim x_signature_valid As Boolean = _
x_deformatter.VerifySignature(x_hashcode, x_signature)

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