[fdgkwbbv] Extensible error correcting code

Design an error correcting code which appends not a fixed-size checksum (block) but an infinite stream.  Truncate the stream at any point depending on how much error correction you want.

Even more flexible would be if the length of the input message is not fixed.

Easiest is simply to repeat the message over and over.  Can we do better?  Maybe we want to guard against periodic errors where the period happens to be the same as the message length.