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Libelle is a German cipher system, developed by the Federal Office for Information Security. The algorithm is not published, in an attempt to make cryptanalysis more difficult (following the principle of security through obscurity and against Kerckhoffs's principle). In order to keep the algorithm secret, it is only distributed as a microchip named Pluto and not as a piece of software.
It is possible to have a secret cipher algorithm while still reaping the benefits of public cryptography research: one way is to make a non-weakening change to a public algorithm, like changing the Nothing up my sleeve numbers, or chaining the public cipher with an unrelated secret cipher.
A BSI specification document about Pluto gives these numbers: 64 bit block size, 160 bit key size. Supported modes: Electronic Codebook (ECB), Cipher Block Chaining (CBC), Cipher Feedback (CFB)/Output Feedback (OFB) with 1,8,16,32 or 64 bits.
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