Convert between UIImage and Base64 string

Does anyone know how to convert a UIImage to a Base64 string, and then reverse it?

I have the below code; the original image before encoding is good, but I only get a blank image after I encode and decode it.

NSData *imageData = UIImagePNGRepresentation(viewImage);

NSString *b64EncStr = [self encode: imageData];

NSString *base64String = [self encodeBase64:imageData];