How much will we be able to trust archived digital materials? If I find a set of 100 year old handwritten letters, it's unlikely that anyone has tampered with t

How much will we be able to trust archived digital materials? If I find a set of 100 year old handwritten letters, it's unlikely that anyone has tampered with them to change the message, although possible with a lot of effort. Digital messages could be compromised undetectably by some archiver. Is it still "primary" if it has been downloaded from the internet, converted between formats, copied across various storage media...