[Swan-dev] how should a proposal (SA's crytpo suite) be selected

[Paul Wouters]

Note Hugh had one comment regarding the "stop reading when you found
an acceptable proposal to return". It could be that the unread remainder
of the proposal/transforms are badly formed. It could be argued that
we should return NO_PROPOSAL_CHOSEN or INVALID_SYNTAX.

for example, the case where the initiator sends proposals numbered #1,
#2, #4. Technically, we should reject everything. Your proposed solution
would allow us to use #1.

I understood both views. I tempted to lean towards being okay to ignore
and have unparsed parts of a proposal tree. But there is some security
impact - for example it could allow an attack to insert arbitrary length
data into the packet for some cryptographic attack. Limiting the
proposals to only valid syntax would prevent that.

So perhaps possibly the best solution is to confirm the proposal list as
valid, but return the first mutually agreed match?

Paul