@implementation NSString (StringToHexData)

//
// Decodes an NSString containing hex encoded bytes into an NSData object

• (NSData *) stringToHexData
  {
  int len = [self length] / 2; // Target length
  unsigned char *buf = malloc(len)
  unsigned char *whole_byte = buf;
  char byte_chars[3] = {'\0','\0','\0'};

  int i;
  for (i=0; i < [self length] / 2; i++) {
  byte_chars[0] = [self characterAtIndex:i2];
  byte_chars[1] = [self characterAtIndex:i2+1];
  *whole_byte = strtol(byte_chars, NULL, 16);
  whole_byte++;
  }

  NSData *data = [NSData dataWithBytes:buf length:len];
  free( buf );
  return data;
  }

@end
@implementation NSData (DataToHexString)

• (NSString *) dataToHexString
  {
  NSUInteger len = [self length];
  char * chars = (char *)[self bytes];
  NSMutableString * hexString = [[NSMutableString alloc] init];

for(NSUInteger i = 0; i < len; i++ )
[hexString appendString:[NSString stringWithFormat:@"%0.2hhx", >chars[i]]];

return hexString;
}
@end