How do I create an MD5 Hash of a string in Cocoa?

Question

I know SHA-1 is preferred, but this project requires I use MD5.

#include 

- (NSString*) MD5Hasher: (NSString*) query {
    NSData* hash         


        

Answer 1

cdespinosa and irsk have already shown you your actual problem, so let me go through your GDB transcript:

(gdb) p digest
$1 = (unsigned char *) 0xa06310e4 "\0206b\260/\336\316^\021\b\a/9\310\225\204"

You've printed digest as a C string. You can see here that this string is raw bytes; hence all the octal escapes (e.g., \020, \225) and the couple of punctuation characters (/ and ^). It is not the printable ASCII hexadecimal representation you were expecting. You're lucky that there were no zero bytes in it; otherwise, you would not have printed the entire hash.

(gdb) po final
Cannot access memory at address 0x0

final is nil. This makes sense, as your string above isn't valid UTF-8; again, it's just raw data bytes. stringWithUTF8String: requires a UTF-8-encoded text string; you didn't give it one, so it returned nil.

For passing raw data around, you'd use NSData. In this case, I think you want the hex representation, so you'll need to create that yourself the way irsk showed you.

Finally, consider how lucky you are that your input didn't hash to a valid UTF-8 string. If it had, you wouldn't have noticed this problem. You may want to construct a unit test for this hash method with this input.

(gdb) po digest

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0xb0623630
0x98531ed7 in objc_msgSend ()

Your program crashed (specific problem: “bad access”, “invalid address”) in objc_msgSend. This is because digest either is not a Cocoa/CF object at all or was one but was freed. In this case, it's because digest is not a Cocoa object; it is a C array of bytes, as shown by your p digest line above.

Remember, Objective-C is a superset of C. All of C exists unchanged in it. That means there are C arrays (e.g., char []) and Cocoa's NSArrays side by side. Moreover, since NSArray comes from the Cocoa framework, not the Objective-C language, there's no way to make NSArray objects interchangeable with C arrays: You can't use the subscript operator on Cocoa arrays, and you can't send Objective-C messages to C arrays.