Kashaph (a hebrew term meaning mutter) is a long extinct, enchanted language used by the Inconnu to communicate secretly, and also as a method of identifying one another. It cannot be decoded by mundane means, like modern cryptography. It has no written form.
Kashaph can only be accessed via a specific ritual that imprints the language in the target's brain. This is used as one of the first initiations of a new member of the sect.
- Rites of the Blood, p.100