MD5 hashes from string don't match up

MD5 hashes from string don't match up

I need to convert a password to an MD5 hash and compare it to the MD5 hash
I get from the webservice. If these two match, the user can login.
Let's say the user's password is 'cat'. If I make a hash out of that I get
d077f244def8a70e5ea758bd8352fcd8. This equals the hash I receive from the
webservice. Now I know the code for creating the MD5 is correct.
However, when the password is 'çàt', I get this hash:
727f8e931135b44b37eb147c8a7a56af. The webservice gives me back this hash:
f571ca52b4e3c5a6f49675deaea89cac.
So that's the problem. For some reason these 2 hashes of the same word are
different, didn't know such a thing was even possible. It only happens
when there's an accent or some other weird character in the password
though, so I think it has something to do with encoding.
I tried using NSISOLatin1StringEncoding like this:
NSString *string = [NSString stringWithFormat:@"çàt"];
char converted[([string length] + 1)];
[string getCString:converted maxLength:([string length] + 1) encoding:
NSISOLatin1StringEncoding];
NSString *converted_str = [NSString stringWithCString:converted
encoding:NSISOLatin1StringEncoding];
Found here: How to convert utf8 encoding to iso 8859-1 encoding?
But that doesn't give me the correct hash either.
I tried all the possible NSString encodings but none of em returned the
right hash.
So what can I do about this. Is there something wrong with the code for
converting the encoding? Or maybe it doesn't have anything to do with
encoding (since trying other encodings isn't working)?