SC4 is a secure communications system specifically designed for (relatively) easy auditability by way of a ruthless commitment to simplicity. SC4 provides the functional equivalent of PGP from the end-user's point of view, but implements it in two order of magnitude less code. The cryptographic core of SC4 is Daniel J. Bernstein's TweetNaCl library (<800 LOC). On top of this we have a variety of UI implementations ranging from 1000 to 5000 LOC. One of these has completed a formal audit to date.