Hi Mark,

Thanks for posting at the newsgroup!

"different value (for the same input array) than the OpenSSL MD5 algorith"
.Net implementation will has the same output if we give the same input for
the MD5 as OpenSSL. I suspect the cause why you donot obtain the same
result is the input data. The char data type in .Net has two bytes, not  
the same to the char type in C/C++. This is becaues .Net runttime has the
support to Unicode naturally so that each char has two bytes.

I have the sample code below to illustrate this for you:
C++ code for using OpenSSL.
//-----------
#include <stdio.h>
#include <openssl/md5.h>

int _tmain(int argc, _TCHAR* argv[])
{
    MD5_CTX    ctx,ctx1;

    unsigned char final[MD5_DIGEST_LENGTH];
    MD5_Init(&ctx);

          //input: one hundred char array, each char has the letter 'A'
    char pw[100];
    for( int i=0; i<100; i++)
        pw[i] = 'A';

    MD5_Update(&ctx, (char *)pw, 100);
    MD5_Final(final, &ctx);

    int count = 0;
    for( int i=0; i<MD5_DIGEST_LENGTH; i++)
    {
        printf( "%x", final[i] );
        if( (++count)%2 == 0 )
            printf( " " );
    }

    return 0;
}
output:
8adc 5937 e635 f6c9 af64 6fb 2356 fae

//-----------

C# code
//-----------
using System;
using System.Security.Cryptography;

namespace ConsoleApp
{
   class Program
   {
       static void Main(string[] args)
       {

           char[] myarray = new char[100];            

//the same input as above
           byte[] byteStream = new byte[100];
           for (int i = 0; i < byteStream.Length; i++)
               byteStream[i] = (byte)'A';

           MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
           byte[] result = md5.ComputeHash(byteStream);
           Console.WriteLine("---------------------");
           string output = string.Empty;
           int count = 0;    
           foreach( byte b in result )                
           {                
               output += string.Format("{0:x}",b);
               if ((++count) % 2 == 0)
                   output += " ";                
           }
           Console.WriteLine(output);
       }
   }
}
output:
8adc 5937 e635 f6c9 af64 6fb 2356 fae

//-----------

From the two, we can find the same result. So this is to say, we have the
same input and obtain the same output no matter whether we are using
OpenSSL or .Net MD5CryptoServiceProvider.

So for your .Net code, I'd suggest please ensure the input data is passed
in as byte[] data type which should have the same binary layout as in C/C++
language.

Please feel free to let me know if you have any further question on this
issue.

Have a nice day!

Best Regards,
Wei-Dong XU
Microsoft Support
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It is my pleasure to be of any assistance.