Abstract. A new stream cipher, Grain, is proposed. The design targets hardware environments where gate count, power consumption and memory is very limited. It is based on two shift registers and a nonlinear output function. The cipher has the additional feature that the speed can be increased at the expense of extra hardware. The key size is 80 bits and no attack faster than exhaustive key search has been identified. The hardware complexity and throughput compares favourably to other hardware oriented stream ciphers like E0 and A5/1. MotivationWhen designing a cryptographic primitive there are many different properties that have to be addressed. These include e.g. speed, security and simplicity. Comparing several ciphers, it is likely that one is faster on a 32 bit processor, another is faster on an 8 bit processor and yet another one is faster in hardware. The simplicity of the design is another factor that has to be taken into account, but while the software implementation can be very simple, the hardware implementation might be quite complex.There is a need for cryptographic primitives that have very low hardware complexity. An RFID tag is a typical example of a product where the amount of memory and power is very limited. These are microchips capable of transmitting an identifying sequence upon a request from a reader. Forging an RFID tag can have devastating consequences if the tag is used e.g. in electronic payments and hence, there is a need for cryptographic primitives implemented in these tags. Today, a hardware implementation of e.g. AES on an RFID tag is not feasible due to the large number of gates needed. Grain is a stream cipher primitive that is designed to be very easy and small to implement in hardware.Many stream ciphers are based on linear feedback shift registers (LFSR), not only for the good statistical properties of the sequences they produce, but also for the simplicity and speed of their hardware implementation. Several recent LFSR based stream cipher proposals, see e.g. [6,7] and their predecessors, are based on word oriented LFSRs. This allows them to be efficiently implemented in software

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A new stream cipher, Grain-128, is proposed. The design is very small in hardware and it targets environments with very limited resources in gate count, power consumption, and chip area. Grain-128 supports key size of 128 bits and IV size of 96 bits. The design is very simple and based on two shift registers, one linear and one nonlinear, and an output function.