NSString <---->NSData

// 原始的字符串
NSString *string = @"Message";
    
// 将字符串编码为二进制数据
NSData *data = [string dataUsingEncoding:NSUTF8StringEncoding];
    
// 将二进制数据编码为Base64字符串:TWVzc2FnZQ==
NSString *base64EncodedString = [data base64EncodedStringWithOptions:NSDataBase64Encoding64CharacterLineLength];
    
// 将Base64字符串解码为二进制数据
NSData *decodedData = [[NSData alloc] initWithBase64EncodedString:base64EncodedString options:NSDataBase64DecodingIgnoreUnknownCharacters];
    
// 将二进制数据解码为字符串:Message
NSString *newstring = [[NSString alloc] initWithData:decodedData encoding:NSUTF8StringEncoding];