I'm working (finishing paper) on an algorithm compatible with deflate/gzip that is safe to use before encryption (i.e. it is guaranteed to not leak random secr

I'm working (finishing paper) on an algorithm compatible with deflate/gzip that is safe to use before encryption (i.e. it is guaranteed to not leak random secrets cookies). It's a bit more complex than your suggestion - matching common substrings longer than key length would still be vulnerable as substring boundary may still fall inside secret.