. 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 offer several interesting time-space trade-offs (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 Introduction Since efficiency and flexibility are probably the most appreciated design criteria, block ciphers were traditionally optimized for either software (typically SAFER ) or hardware (DES ) implementation. More recently, autonomous agents and object-oriented technologies motivated the design of particularly tiny codes (such as TEA , 189 bytes on a 68HC05) and algorithms adapted to particular programming languages such as PERL. Surprisingl..