This Objective-C category is rewritten from Erik Doernenburg’s Foundation extensions: NSData(MIME). You can find his code in EDFrameworks’ EDMessage Framework. You can use it to convert hex value that hold by NSData and convert it into binary value.

NSData+Hex.h

#import <Foundation/Foundation.h>

@interface NSData (NSData_Hex)

• (NSData*) hex2raw; @end

NSData+Hex.m

#import “NSData+Hex.h”

static int asciitable[128] = { 99,99,99,99, 99,99,99,99, 99,99,99,99, 99,99,99,99, 99,99,99,99, 99,99,99,99, 99,99,99,99, 99,99,99,99, 99,99,99,99, 99,99,99,99, 99,99,99,99, 99,99,99,99, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 99,99, 99,99,99,99, // 0..9 99,10,11,12, 13,14,15,99, 99,99,99,99, 99,99,99,99, // A..F 99,99,99,99, 99,99,99,99, 99,99,99,99, 99,99,99,99, 99,10,11,12, 13,14,15,99, 99,99,99,99, 99,99,99,99, // a..f 99,99,99,99, 99,99,99,99, 99,99,99,99, 99,99,99,99 };

@implementation NSData (NSData_Hex)

• (NSData*) hex2raw { // Based on Erik Doernenburg’s NSData+MIME.m const char *source, *endOfSource; NSMutableData *decodedData; char *dest;

  source = [self bytes]; endOfSource = source + [self length]; decodedData = [NSMutableData dataWithLength:[self length]]; dest = [decodedData mutableBytes];

  while (source < endOfSource) { if (isxdigit(source) && isxdigit((source+1))) { dest++ = asciitable[(int)source] * 16 + asciitable[(int)*(source+1)]; source += 2;
  } else return Nil; }

  [decodedData setLength:(unsigned int)((void *)dest - [decodedData mutableBytes])];

  return decodedData; } @end

Reference: http://hollowout.blogspot.com/2009/01/convert-hex-value-to-binary-data.html