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Shannon is a synchronous stream cipher
designed for a secret key that may be up to 256 bits in length. The cipher
outputs the key stream in 32-bit blocks. Shannon is a software-oriented
cipher based on simple 32-bit operations (operations on data are restricted
to XOR, OR and fixed rotations. Consequently, Shannon is at home in many
computing environments, from simple hardware implementations through smart
cards to large computers. Source code for Shannon is freely available and
use of this source code, or independent implementations, is allowed free for
any purpose. Shannon is a back-to-basics design incorporating lessons
learned from a variety of sources. From members of the SOBER family of
stream ciphers, it gets its basic shift register structure. Helix introduced
the hybrid stream cipher directly incorporating message authentication.
Trivium showed how a simple nonlinear feedback structure could compound
rapidly to provide security, Scream first taught the value of keeping the
nonlinearity in the cipher state. SHA-256, in its resistance to the attacks
against earlier hash functions[15], demonstrates the importance of
propagating differentials forward for message authentication codes. Finally,
many aspects of the design have been influenced by the theory of Golomb
Rulers[9] (also often known as Full Positive Difference Sets). The use of
only extremely primitive operations and no tables follows work by
Bernstein[1] on timing attacks related to table lookups.
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