Cryptographie indéchiffrable (subtitle: basée sur de nouvelles combinaisons rationelles) is a French book on cryptography written by Émile Victor Théodore Myszkowski (a retired French colonel) and published in 1902.
His book described a cipher that the author had invented and claimed (incorrectly) was "undecipherable" (i.e. secure against unauthorised attempts to read it).
It was based on a form of repeated-key transposition.
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