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SECURITY

Programming Hashing Algorithms (part 5) - Validating Hash Codes

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5. Validating Hash Codes

Now that you have seen how to create hash codes (playing the role of Alice), we will show you how to validate them, playing the role of Bob. Remember that Bob receives a hash code and a message from Alice. He generates his own hash code and checks that it matches the one that he received. Example 1 shows a simple approach to validating hash codes:

Example 1. Validating a hash code
# C#

using System;
using System.Text;
using System.Security.Cryptography;

class HashCodeValidation {

private static byte[] ParseHashCodeString(string p_hash_code_string) {
// split the hash code on spaces
string[] x_elements = p_hash_code_string.Split(' ');
// create a byte array to hold the elements
byte[] x_hash_code_array = new byte[x_elements.Length];
// parse each string element into a byte
for (int i = 0; i < x_elements.Length; i++) {
x_hash_code_array[i] = Byte.Parse(x_elements[i],
System.Globalization.NumberStyles.HexNumber);
}
// return the byte array
return x_hash_code_array;
}

private static bool CompareHashCodes(byte[] x_hash_code1,
byte[] x_hash_code2) {

// check that the hash codes are the same length
if (x_hash_code1.Length == x_hash_code2.Length) {
// run through the hash code and check
// each value in turn
for (int i = 0; i < x_hash_code1.Length; i++) {
if (x_hash_code1[i] != x_hash_code2[i]) {
// the byte at this location is different
// in each hash code
return false;
}
}
// the hash codes are the same
return true;
} else {
// the hash codes contain different numbers of
// bytes and so cannot be the same
return false;
}
}

public static bool ValidateHashCode(string p_hash_algorithm,
string p_hash_string, byte[] p_data) {

// parse the hash code string into a byte array
byte[] x_hash_code = ParseHashCodeString(p_hash_string);

// create the hashing algorithm object using the
// name argument
HashAlgorithm x_hash_alg = HashAlgorithm.Create(p_hash_algorithm);

// compare the hash codes
return CompareHashCodes(x_hash_code, x_hash_alg.ComputeHash(p_data));
}

static void Main(string[] args) {
// define the message data that Alice sent to Bob
string x_message_data = "Programming .NET Security";
// covert the message data into a byte array
byte[] x_message_bytes = Encoding.Default.GetBytes(x_message_data);
// define the hash code that Alice sent to Bob
string x_hash_string =
"E1 62 9F C2 96 85 C3 A4 5B 94 97 57 D8 9C 65 78";
// define the name of the hashing algorithm that Alice used
string x_algorithm_name = "MD5";

// validate the hash code and write out the result
bool x_result = ValidateHashCode(x_algorithm_name,
x_hash_string, x_message_bytes);

if (x_result) {
Console.WriteLine("The hash code is valid");
} else {
Console.WriteLine("The hash code is invalid");
}
}
}
# Visual Basic .NET

Imports System.Security.Cryptography
Imports System.Text

Module HashCodeValidation

Public Function ParseHashCodeString(ByVal p_hash_code_string _
As String) As Byte( )
' split the hash code on spaces
Dim x_elements( ) As String = p_hash_code_string.Split(" "c)
' create a byte array to hold the elements
Dim x_hash_code_array( ) As Byte = New Byte(x_elements.Length - 1) {}
' parse each string element into a byte
Dim i As Integer
For i = 0 To x_elements.Length - 1 Step i + 1
x_hash_code_array(i) = Byte.Parse(x_elements(i), _
System.Globalization.NumberStyles.HexNumber)
Next
' return the bye array
Return x_hash_code_array
End Function

Private Function CompareHashCodes(ByVal x_hash_code1( ) As Byte, _
ByVal x_hash_code2( ) As Byte) As Boolean
' check that the hash codes are the same length
If x_hash_code1.Length = x_hash_code2.Length Then
' run through the hash code and check
' each value in turn
Dim i As Integer
For i = 0 To x_hash_code1.Length - 1 Step i + 1
If x_hash_code1(i) <> x_hash_code2(i) Then
' the byte at this location is different
' in each hash code
Return False
End If
Next
' the hash codes are the same
Return True
Else
' the hash codes contain different numbers of
' bytes and so cannot be the same
Return False
End If
End Function

Public Function ValidateHashCode(ByVal p_hash_algorithm As String, _
ByVal p_hash_string As String, ByVal p_data( ) As Byte) As Boolean

' parse the hash code string into a byte array
Dim x_hash_code( ) As Byte = ParseHashCodeString(p_hash_string)

' create the hashing algorithm object using the
' name argument
Dim x_hash_alg As HashAlgorithm = HashAlgorithm.Create(p_hash_algorithm)

' compare the hash codes
Return CompareHashCodes(x_hash_code, x_hash_alg.ComputeHash(p_data))
End Function

Sub Main( )
' define the message data that Alice sent to Bob
Dim x_message_data As String = "Programming .NET Security"
' covert the message data into a byte array
Dim x_message_bytes( ) As Byte = Encoding.Default.GetBytes(x_message_data)
' define the hash code that Alice sent to Bob
Dim x_hash_string As String = _
"E1 62 9F C2 96 85 C3 A4 5B 94 97 57 D8 9C 65 78"
' define the name of the hashing algorithm that Alice used
Dim x_algorithm_name As String = "MD5"

' validate the hash code and write out the result
Dim x_result As Boolean = ValidateHashCode(x_algorithm_name, _
x_hash_string, x_message_bytes)

If (x_result) Then
Console.WriteLine("The hash code is valid")
Else
Console.WriteLine("The hash code is invalid")
End If
End Sub

End Module


The first step in validating the hash code is to parse the information that Bob has received. The format we have used to express the hash codes is the most commonly used, but you may encounter other formats.

The ParseHashCodeString method accepts a string containing a formatted hash code and produces a byte array. The CompareHashCodes method takes care of comparing two arrays of bytes to ensure that they contain the same hash code.

The ValidateHashCode method uses the other two methods to validate a hash code, taking three arguments:

  1. The name of the algorithm that Alice used to create the hash code

  2. The hash code Alice created, expressed as a string of hexadecimal bytes

  3. The message from Alice, expressed as a string

Parse the hash code string into a byte array, and then create an instance of the HashAlgorithm class based on the algorithm name. Assume that Alice and Bob agree on the algorithm that was used to create the hash codes, or that Alice picks one and indicates which algorithm she has used by sending some information along with the message. Notice how you cannot create an instance of any supported algorithm using the same code statements; this is another benefit of instantiating algorithms by name.

Finally, calculate a hash code for the message data and compare it to the one that Bob received from Alice. The example includes a Main method to demonstrate how to validate the hash code that you created previously.

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