This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
The big unknown in any encryption scheme based on RSA is whether or
not there is an efficient way to factor huge numbers, or if there is
some backdoor algorithm that can break the code without solving the
factoring problem. Even if no such algorithm exists, it is still
believed that RSA is the weakest link in the PGP chain.