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Hash codes create fixed-length binary strings to uniquely identify a set of binary data; hash codes can be used to determine that the data that has been hashed has not been altered. The .NET Framework supports the following hashing algorithms:
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MD5 SHA-1 SHA-256 SHA-384 SHA-512
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17. Security and Cryptography
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The base class for all hash code algorithms is System.Security.Cryptography.HashAlgorithm, and following the model for other cryptographic functions, the .NET Framework provides an abstract class and an implementation class for each algorithm. Table 17-8 lists the classes.
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Table 17-8. Hash Code Classes
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Abstract Class Implementation Class System.Security.Cryptography.MD5 System.Security.Cryptography.MD5CryptoServiceProvider System.Security.Cryptography.SHA1 System.Security.Cryptography.SHA1CryptoServiceProvider System.Security.Cryptography.SHA256 System.Security.Cryptography.SHA256Managed System.Security.Cryptography.SHA384 System.Security.Cryptography.SHA384Managed System.Security.Cryptography.SHA512 System.Security.Cryptography.SHA512Managed Hash codes are generated using the ComputeHash method derived from the HashAlgorithm class; this method will generate a hash code either from a byte array or by reading data from an instance of System.IO.Stream. There are no class members to verify a hash code directly; to validate a hash code, the ComputeHash method must be called with the candidate data and the result compared with the original code. The following example demonstrates how to generate an MD5 hash code and print it to the console:
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using System; using System.Security.Cryptography; using System.Text; class MD5Demo { MD5Demo() { byte[] x_message = Encoding.Default.GetBytes("C# for Java Developers"); // create the MD5 hash code provider MD5CryptoServiceProvider x_md5_provider = new MD5CryptoServiceProvider(); // generate the hash code for the message byte[] x_hashcode = x_md5_provider.ComputeHash(x_message); // print out the hashcode foreach (byte x_byte in x_hashcode) { Console.Write(x_byte); } } static void Main(string[] args) { new MD5Demo() ; } }
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Digital signatures provide verification that data has originated from a certain individual; digital signatures rely on asymmetrical cryptography, and the processing that is required to
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17. Security and Cryptography
generate a signature means that digital signatures are usually used to sign a hash code generated from the source data. See the preceding section in this chapter for more information on generating hash codes. The base class for signatures is System.Security.Cryptography.AsymmetricSignatureFormatter. Unlike the other classes in the cryptography library, the signature classes do not provide abstract and implementation classes for each algorithm. The .NET Framework supports the signature formats RSA PKCS #1 version 1.5 and DSA PKCS #1 version 1.5. For each signature format, the .NET Framework includes a formatter class (to produce a signature) and a deformatter class (to validate a signature). The constructors for the formatter and deformatter classes accept an instance of an asymmetrical algorithm implementation class, as detailed in Table 17-9.
Table 17-9. Digital Signature Formatter Classes
Signature Formatter/Deformatter Class Format RSA PKCS RSAPKCS1SignatureFormatter #1 RSAPKCS1SignatureDeformatter DSA PKCS DSASignatureFormatter #1 DSASignatureDeformatter
Constructor Algorithm Class RSACryptoServiceProvider DSACryptoServiceProvider
The following example demonstrates how to produce an RSA digital signature for an MD5 hash code and then verify the signature:
using System; using System.Security.Cryptography; using System.Text; class RSASignatureDemo{ RSASignatureDemo() { byte[] x_message = Encoding.Default.GetBytes("C# for Java Developers"); // create the MD5 hash code provider MD5CryptoServiceProvider x_md5_provider = new MD5CryptoServiceProvider(); // generate the hash code for the message byte[] x_hashcode = x_md5_provider.ComputeHash(x_message); RSACryptoServiceProvider x_rsa_provider = new RSACryptoServiceProvider(); RSAPKCS1SignatureFormatter x_formatter = new RSAPKCS1SignatureFormatter(x_rsa_provider); x_formatter.SetHashAlgorithm("MD5"); byte[] x_signature = x_formatter.CreateSignature(x_hashcode);
17. Security and Cryptography // print out the hashcode foreach (byte x_byte in x_signature) { Console.Write(x_byte); } RSAPKCS1SignatureDeformatter x_deformatter = new RSAPKCS1SignatureDeformatter(x_rsa_provider); x_deformatter.SetHashAlgorithm("MD5"); bool x_verified = x_deformatter.VerifySignature(x_hashcode, x_signature); Console.WriteLine("Verified: " + x_verified); } static void Main(string[] args) { new RSASignatureDemo () ; } }
Note that the SetHashAlgorithm method must be called to specify the format of the hash code that is to be signed or verified. Table 17-10 lists the mapping between the hash code algorithms and the string names that can be used with this method.
Table 17-10. String Representations of Hash Code Algorithms
Hash Algorithm MD5 SHA-1 SHA-256 SHA-348 SHA-512
String Name MD5 SHA1 SHA256 SHA-256 SHA348 SHA-348 SHA512 SHA-512
Summary
The support for security in the .NET platform is tightly integrated into the platform and allows a great deal of flexibility. The use of the Code Access Security model allows programmers to provide administrators with the information necessary to implement trust policies. The support for cryptography is well structured and easy to use, offering highperformance cipher implementations for applications that work with sensitive data.
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