[messaging] Sphinx symmetric crypto questions

[Ian Goldberg]

On Thu, Nov 12, 2015 at 11:13:32AM +0100, Jeff Burdges wrote:
> On Wed, 2015-11-11 at 19:06 -0500, Ian Goldberg wrote:
> > What Sphinx needs from Lioness is a "large block" block cipher.  You
> > can
> > implement that however you like, but Lioness was a straightforward
> > construction.
> 
> Ahh, these large block cyphers are relatively rare I take it.  There
> are plenty of block cyphers where the forward and reverse directions
> might not both be secure, right?  I'd imagine these should be avoided. 

I'm not sure I follow.  Can you give an example of a block cipher where
one direction is secure, but the reverse is not?  Wouldn't that be more
of a trapdoor one-way function (i.e. a public key primitive)?