Cipher or unknown language - historical perspective

[Jorge_Stolfi]

It is unlikely that any community in Europe that still used an unwritten language had no sight of writing from other languages. When the time came for them to adopt a writing system they would have adapted an existing alphabet.

That is not quite correct.  You are not allowed to view links. Register or Login to view., You are not allowed to view links. Register or Login to view., and You are not allowed to view links. Register or Login to view. are three alphabets from ~400 CE  that were not adaptations of other alphabets.  Their creators got the concept of alphabet from other languages, like Latin and Greek, but developed their own set of symbols without trying to imitate the letters from those or any other alphabets.  Ogham seems to be a bit earlier; it clearly was inspired by other alphabetic scripts, but the letter shapes were not borrowed from any other alphabet.

All the best, --jorge