Threshold cryptosystems use algebraic properties such as homomorphisms in order to allow several parties to jointly evaluate a cryptographic primitive. Several cryptographic primitives, however, avoid -by definition- the use of algebraic properties, or otherwise their security is compromised; this is the case, for instance, of block ciphers, pseudo-random functions, and pseudo-random permutations. Is it then impossible to construct a threshold cryptosystem in order to share the computation of a block cipher ?