crypto/rsa: check for minimal PKCS#1 v1.5 padding.

The PKCS#1 spec requires that the PS padding in an RSA message be at
least 8 bytes long. We were not previously checking this. This isn't
important in the most common situation (session key encryption), but
the impact is unclear in other cases.

This change enforces the specified minimum size.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/9222045
4 files changed