NSString <-> NSData 형변환

NSString to NSData

NSString* str = @"test String";
NSData* data = [str dataUsingEncoding:NSUTF8StringEncoding];

NSData to NSString

NSString *str = [NSString stringWithUTF8String:[data bytes]];
NSString *str = [NSString stringWithFormat:@"%.*s", [data length], [data bytes]];