Cryptography Using .NET: Digital Signatures
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1. Digital Signatures. The .NET Framework supports two algorithms for digital signature
schemes: the RSA signature and the Digital Signature Algorithm (DSA.)
The required classes are DSACryptoServiceProvider and
RSACryptoServiceProvider for the DSA and RSA implementation,
respectively.
To create an instance of the DSA algorithm class, dsa, you should use
DSACryptoServiceProvider dsa = new DSACryptoServiceProvider(); and the object instantiation of the RSACryptoServiceProvider class is
similar.
The default characteristics of a digital signature provided by the
.NET framework will be different each time a message is signed, even
though the message is not changed. This randomness, in the DSA case, is caused by random numbers
generated by the .NET Framework. In the RSA case, the public-private key
pair and other performance-enhancement parameters is randomly generated
each time a message is signed, thus obtaining different signatures.
Unfortunately, how to manually set the key and some random parameters is
not documented in the MSDN library. However, this would not be a big
problem since having keys generated randomly by a computer instead of
assigning these values manually is prefered. For example, in
the RSA case, the public-private key pair used in .NET Framework is a
set of large numbers with 1024-bit keys.
2. Signing and Verifying Data. The .NET Framework provides methods for signing and verifying data,
which can be seen below.
| Algorithm
|
Action
|
Method
|
Return type
|
| DSA |
Sign |
SignData(byte[ ]
data); |
byte [ ] |
| |
Verify |
VerifyData(byte[ ] data, byte[ ] sig); |
bool |
| RSA |
Sign |
SignData(byte[ ] data, object h_alg); |
byte [ ] |
| |
Verify |
VerifyData(byte[ ] data, object h_alg, byte[ ] sig); |
bool |
The value sig denotes a byte array of the signature obtained after
signing the message data, which is found in the verifyData methods. In RSA scheme, an instance of the hashing
algorithm class need to be created fist, and then
specified in both the SignData and VerifyData methods, which is denoted
by the h_alg. The return type, bool, is a Boolean value, which
equals one if the signature is valid, and equals zero otherwise.
3. Transmitting Cryptographic Parameters Between Applications. The .NET Framework provides methods for exporting and importing public
or private parameters stored in the DSAParametersand
RSAParametersstructures. You more infirmation on this topic you
can look in the MSDN documentation for more
details, but you do not need to know about all the members of these
structures. As mentioned before, a digital signature created by .NET is
random. The DSA signature relies on random numbers. Unfortunately, the
MSDN library does not explain a definition of each parameter, and the
following notation is not conventional. The public parameters
(byte array type) are P, Q, J, G, and Seed, which are of a byte array
type. The private parameters
are X and Y. All parameters are members of the DSAParameters structure.
For the RSA signature, the public-private key pair is randomly
generated, and stored in the RSAParameters structure. The public key
members are Modulus and Exponent and the private key is D. How do
you
import or export these parameters? For example, Alice signs a message,
and then transmits her public parameters to Bob so that he can use these
parameters to verify the message if it comes from Alice.
There are two ways to achieve this. The first is to use the ExportParameters and ImportParameters methods After the
signature has been created, Alice exports her algorithm parameters as
DSAParameters para = dsa.ExportParameters(bool);
,where bool denotes a string of Boolean value (i.e., true or false). The
argument is false if you want to export only the public parameters, and
is true to export both the public and private parameters. These two
Boolean cases are used for both the DSA and RSA signatures. After
exporting parameters, you can access some parameters, e.g., the byte
array P as
para.P.
The other application (e.g., Bob’s) can simply obtain these parameters
by using
dsa_Bob.ImportParameters(para);
For RSA, the Import and export procedures are the same. You will use the ToXmlString and FromXmlString methods. The .NET framework also
provides an alternative, which is more convenient, to transmit algorithm
parameters between applications with an XML (Extensible Markup Language)
string. Analogous to the ExportParameters method, we can use the
ToXmlString(bool) method to create and return an XML string
representation of the algorithm parameters, where bool denotes a string
of Boolean value as described above. For example, the RSA key pair
parameters can be exported as
string key_pair = rsa.ToXmlString(bool);
,where rsa is an instance of the RSACryptoServiceProvider class.
Similar to the ImportParameters above, these parameters can be obtained
by using
rsa_Bob.FromXmlString(key_pair);
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