This paper presents xmx, a new symmetric block cipher optimized for public-key libraries and microcontrollers with arithmetic coprocessors. xmx has no S-boxes and uses only modular multiplications and xors. The complete scheme can be described by a couple of compact formulae that o#er several interesting time-space trade-o#s (number of rounds/key-size for constant security). In practice, xmx appears to be tiny and fast: 136 code bytes and a 121 kilo-bits/second throughput on a Siemens SLE44CR80s smart-card (5 MHz oscillator). 1