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Validating Data Integrity with Hashes using .Net

Validating Data Integrity with Hashes:
            Hashes can be used to generate code that cross pond to specific data or file. But you can’t reproduce data from the hash. All hash algorithms inherit from System.Security.Cryptography.HashAlgorithms. These algorithms include:

MD5
Represents the abstract class from which all implementations of the MD5 hash algorithm inherit.
HashSize =128
 RIPEMD160
Represents the abstract class from which all implementations of the MD160 hash algorithm inherit.
HashSize=128
SHA1
HashSize = 160 bits
SHA256
HashSize = 256 bits
SHA384
HashSize = 384 bits
SHA512
HashSize = 512 bits

Keyed Hash Algorithms:
                All Keyed Hash Algorithms inherit from System.Security.Cryptography.KeyedHashAlgorithm which in turn inherits from HashAlgorithm. These Algorithms include:

HMACSHA1: Both sender and receiver share the same secret key. It uses SHA1 to produce hash of length 20 bytes. It accepts key of all sizes.

MACTripleDES: Uses TripleDES to produce a hash of 8 bytes. It accepts key of size 8, 16, or 24 bytes.

Computing a Non-Keyed hash:

byte[] data = new byte[DATA_SIZE];
MD5 md5 = new MD5CryptoServiceProvider();
byte[] result = md5.ComputeHash(data);


Computing a Keyed Hash:

HMACSHA1 myhmacsha1 = new HMACSHA1( key );
FileStream inStream = new FileStream(sourceFile, FileMode.Open);

byte[] hashValue = myhmacsha1.ComputeHash(inStream);

myhmacsha1.Clear();

inStream.Close();

Digitally Signing Files:
Digital signature is a value that is appended to a file to secure its contents. For this purpose public key is used that is the part of the signature. The recipient receives and verifies the signature, before trusting on it. RSACryptoServiceProvider and DSACryptoServiceProvider are the two implementations for digitally signing documents. Properties involved in signing are:

SignData
Overloaded. Computes the hash value of the specified data and signs it.
SignHash
Computes the signature for the specified hash value by encrypting it with the private key.
ToXmlString 
Creates and returns an XML string containing the key of the current RSA object. (Inherited from RSA.)
VerifyData
Verifies the specified signature data by comparing it to the signature computed for the specified data.
VerifyHash
Verifies the specified signature data by comparing it to the signature computed for the specified hash value.
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