Context : Anti-proton and positron Galactic cosmic ray spectra are among the key targets for indirect detection of dark matter .
The boost factors , corresponding to an enhancement of the signal , and linked to the clumpiness properties of the dark matter distribution , have been taken as high as thousands in the past .
The dramatic impact of these boost factors for indirect detection of antiparticles , for instance with the PAMELA satellite or the coming AMS-02 experiment , asks for their detailed calculation .

Aims : We take into account the state-of-the-art results of high resolution N-body dark matter simulations to calculate the most likely energy dependent boost factors , which are linked to the cosmic ray propagation properties , for anti-protons and positrons .
The results from extreme , but still possible , configurations of the clumpy dark matter component are also discussed .

Methods : Starting from the mass and space distributions of sub-halos , the anti-proton and positron propagators are used to calculate the mean value and the variance of the boost factor for the primary fluxes .
We take advantage of the statistical method introduced in Lavalle et al .
( 2007 ) and cross-check the results with Monte Carlo computations .

Results : By spanning some extreme configurations of sub-halo and propagation properties , we find that the average contribution of the clumps is negligible compared to that of the smooth dark matter component .
Dark matter clumps do not lead to enhancement of the signals , unless they are taken with some extreme ( unexpected ) properties .
This result is independent of the nature of the self-annihilating dark matter candidate considered , and provides precise estimates of the theoretical and the statistical uncertainties of the antimatter flux from sub-halos .

Conclusions : Spectral distortions can still be expected in antimatter flux measurements , but scenarios invoking large and even mild clumpiness boost factors are strongly disfavoured by our analysis .
Some very extreme configurations could still lead to large enhancements , e.g .
( i ) very small clumps with masses \lesssim 10 ^ { -6 } { M _ { \odot } } following a M ^ { - \alpha } mass distribution with \alpha \gtrsim 2 , highly concentrated with internal r ^ { - \beta } profiles with \beta \gtrsim 1.5 , and spatially distributed according to the smooth component ; or ( ii ) a big sub-halo of mass \gtrsim 10 ^ { 7 } { M _ { \odot } } within a distance of \lesssim 1 kpc from the Earth .
However , they are very unlikely from either theoretical or statistical arguments .