T-function

In cryptography, a T-function is a bijective mapping that updates every bit of the state in a way that can be described as ${\displaystyle x_{i}'=x_{i}+f(x_{0},\cdots ,x_{i-1})}$, or in simple words an update function in which each bit of the state is updated by a linear combination of the same bit and a function of a subset of its less significant bits. If every single less significant bit is included in the update of every bit in the state, such a T-function is called triangular. Thanks to their bijectivity (no collisions, therefore no entropy loss) regardless of the used Boolean functions and regardless of the selection of inputs (as long as they all come from one side of the output bit), T-functions are now widely used in cryptography to construct block ciphers, stream ciphers, PRNGs and hash functions. T-functions were first proposed in 2002 by A. Klimov and A. Shamir in their paper "A New Class of Invertible Mappings". Ciphers such as TSC-1, TSC-3, TSC-4, ABC, Mir-1 and VEST are built with different types of T-functions.

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operations. However, triangular T-functions remain highly inefficient in hardware.

T-functions do not have any restrictions on the types and the widths of the update functions used for each bit. Subsequent transposition of the output bits and iteration of the T-function also do not affect bijectivity. This freedom allows the designer to choose the update functions or S-boxes that satisfy all other cryptographic criteria and even choose arbitrary or key-dependent update functions (see family keying).

Hardware-efficient lightweight T-functions with identical widths of all the update functions for each bit of the state can thus be easily constructed. The core accumulators of VEST ciphers are a good example of such reasonably light-weight T-functions that are balanced out after 2 rounds by the transposition layer making all the 2-round feedback functions of roughly the same width and losing the "T-function" bias of depending only on the less significant bits of the state.

• Klimov, Alexander; Shamir, Adi (2002). "A New Class of Invertible Mappings" (PDF). Cryptographic Hardware and Embedded Systems - CHES 2002. Lecture Notes in Computer Science. Vol. 2523.
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• Klimov, Alexander; Shamir, Adi (2003). "Cryptographic Applications of T-Functions". Selected Areas in Cryptography. Lecture Notes in Computer Science. Vol. 3006. Springer-Verlag. pp. 248–261.
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• Klimov, Alexander; Shamir, Adi (2004). "New Cryptographic Primitives Based on Multiword T-Functions". Fast Software Encryption. Lecture Notes in Computer Science. Vol. 3017. Springer-Verlag. pp. 1–15.
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• Daum, Magnus (2005). "Narrow T-Functions". Fast Software Encryption. Lecture Notes in Computer Science. Vol. 3557. Springer-Verlag. pp. 50–67.
  ISBN 978-3-540-26541-2
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• Hong, Jin; Lee, Dong Hoon; Yeom, Yongjin; Han, Daewan (2005). "A New Class of Single Cycle T-Functions". Fast Software Encryption. Lecture Notes in Computer Science. Vol. 3557. Springer-Verlag. pp. 68–82.
  ISBN 978-3-540-26541-2
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• Klimov, Alexander; Shamir, Adi (2005). "New Applications of T-Functions in Block Ciphers and Hash Functions". Fast Software Encryption. Lecture Notes in Computer Science. Vol. 3557. Springer-Verlag. pp. 18–31.
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