My Google search got me to this: http://avrcryptolib.das-labor.org/trac, includes RSA-1024 which is an asymmetric encryption, although the library doesn't seem to be very fast (according to this document http://www.lix.polytechnique.fr/hipercom/SmartObjectSecurity/slides/Jari.pdf). This numbers are for the ATmega based Arduinos, the Due should be factors faster.