Convert C++ function to C#

Edit I have a suspicion that your code actually does a MD5 digest of the input data. See below for a snippet based on that assumption.

Translation steps

A few hints that should work well¹

Q: tmp[0] = input[i].toAscii();

bytes[] ascii = ASCIIEncoding.GetBytes(input);
tmp[0] = ascii[i];

Q: t = a + (int)(strtol(tmp, NULL, 16));

t = a + int.Parse(string.Format("{0}{1}", tmp[0], tmp[1]),
               System.Globalization.NumberStyles.HexNumber);

Q: v = (int)(strtol(input.mid(t, 2).toLocal8Bit(), NULL, 16));

No clue about the toLocal8bit, would need to read Qt documentation...

Q: snprintf(tmp, 2, "%x", (v * m) % 0x10);

{
    string tmptext = ((v*m % 16)).ToString("X2");
    tmp[0] = tmptext[0];
    tmp[1] = tmptext[1];
}

What if ... it's just MD5?

You could try this directly to see whether it achieves what you need:

using System;

public string FDigest(string input)
{
   MD5 md5 = System.Security.Cryptography.MD5.Create();
   byte[] ascii = System.Text.Encoding.ASCII.GetBytes (input);
   byte[] hash  = md5.ComputeHash (ascii);

   // Convert the byte array to hexadecimal string
   StringBuilder sb = new StringBuilder();
   for (int i = 0; i < hash.Length; i++)
       sb.Append (hash[i].ToString ("X2")); // "x2" for lowercase
   return sb.ToString();
}

¹ explicitly not optimized, intended as quick hints; optimize as necessary

A few more hints:

t is a two byte buffer and you only ever write to the first byte, leaving a trailing nul. So t is always a string of exactly one character, and you're processing a hex number one character at a time. So I think

tmp[0] = input[i].toAscii();
t = a + (int)(strtol(tmp, NULL, 16));

this is roughly int t = a + Convert.ToInt32(input.substring(i, 1), 16); - take one digit from input and add its hex value to a which you've looked up from a table. (I'm assuming that the toAscii is simply to map the QString character which is already a hex digit into ASCII for strtol, so if you have a string of hex digits already this is OK.)

Next

v = (int)(strtol(input.mid(t, 2).toLocal8Bit(), NULL, 16));

this means look up two characters from input from offset t, i.e. input.substring(t, 2), then convert these to a hex integer again. v = Convert.ToInt32(input.substring(t, 2), 16); Now, as it happens, I think you'll only actually use the second digit here anyway since the calculation is (v * a) % 0x10, but hey. If again we're working with a QString of hex digits then toLocal8Bit ought to be the same conversion as toAscii - I'm not clear why your code has two different functions here.

Finally convert these values to a single digit in tmp, then append that to b

snprintf(tmp, 2, "%x", (v * m) % 0x10);
b += tmp;

(2 is the length of the buffer, and since we need a trailing nul only 1 is ever written) i.e.

int digit = (v * m) % 0x10;
b += digit.ToString("x");

should do. I'd personally write the mod 16 as a logical and, & 0xf, since it's intended to strip the value down to a single digit.

Note also that in your code i is never set - I guess that's a loop or something you omitted for brevity?

So, in summary

int t = a + Convert.ToInt32(input.substring(i, 1), 16);
int v = Convert.ToInt32(input.substring(t, 2), 16);
int nextDigit = (v * m) & 0xf;
b += nextDigit.ToString("x");