Non-random x values is probably relatively benign, since all information about the polynomial is in the f(x) values.
You can choose any x as long as the coefficients are random. I don't quite see how randomizing x achieves anything, as you have to provide entire points (x, f(x)) as fragments, therefor x is public information. Maybe you were trying to say that the x values should be chosen at random, instead of say, as part of a sequence? I don't think that's relevant at all for SSS, since all operation are performed on a finite field.
Is there any way I can see on my 3-of-5 backup if it was made before or after that update was made? I am of course going to move my funds, but doing so before a new 3-of-5 backup is in place is also a risk.
The coefficients are 32bytes, therefor your backup would have 5 lines (1 header line + 4 * (16bytes of data + 2bytes of checksum) lines) per fragment. If the fragment only has 1 header and 2 data lines, it's using implicit [1,...,N] for x.
Thus you only need N/2 sheets to recover the key !!!. The seed can be recovered from a single sheet of what was supposed to be a 2-of-M backup, or from two sheets of a 3-of-M or 4-of-M backup, etc etc. Shocked
It's far worse than that. Look at how the coefficients are constructed, they're hashes of the previous coefficient. Whatever fragment the attacker gets access to, he will be able to compute all following coefficients just by hashing x once.
Read this again and look at the code again. In the first scheme, anyone with access to a fragment that carries the first coefficient can compute
*ALL* all of the coefficients. The single point he has with the fragment is then enough to reveal the secret.
This is slightly worsened by the fact that since there are more fragments than coefficients, certain coefficients reoccur on several fragments. Now people being people, when they distribute fragments they tend keep the first few for themselves, by happenstance. If the coefficients were distributed in order with fragments, this very human yet entirely irrelevant quirk would actually save these users from total disaster. Again, the order of coefficients matter since they are derived consecutively from one another