Why do we use a different key to sign than to encrypt

[Jeffrey Walton]

On Tue, Mar 1, 2011 at 8:13 AM, Guy Halford-Thompson <guy at cach.me> wrote:
> Not GPG specific, but I was wondering if someone could point me in the
> direction of some resources that explain why we use different keys to
> sign and encrypt (for cases where the same key _could_ do both e.g.
> RSA).  I cant seem to pick anything up on google.
Key separation and management. See Handbook of Applied Cryptography,
Chapter 13 (http://www.cacr.math.uwaterloo.ca/hac/).

Jeff