Re: [secdir] secdir review of draft-ietf-alto-protocol

["Dan Harkins" <[e-mail]>]

  Hi RIchard,

On Sun, February 9, 2014 11:03 pm, Richard Alimi wrote:
> On Sun, Feb 2, 2014 at 7:08 PM, Dan Harkins <[e-mail]> wrote:
>
>>
>> On Sun, February 2, 2014 11:33 am, Jeffrey Hutzelman wrote:
>> > On Sat, 2014-02-01 at 10:54 -0800, Dan Harkins wrote:
>> >
>> >>  Also, given those
>> >>      restrictions and the fact that a tag just has to be less than
>> >>      or equal to 64 octets, the probability of identical tags being
>> >>      used is not zero. I think the probability of the tag from
>> >>      example 11.3.1.7 is 0.5 to collide with one of just 460
>> >>      other Network Maps.
>> >>
>> >>      I suggest requiring a tag to be 64 octets. That will make
>> >>      even money probability of collision among nearly 3000
>> >>      other Network Maps, which is safer.
>> >
>> > OK, maybe I'm confused and reading out of context here.  But I once
>> had
>> > someone tell me I needed to change my 5-character username because
>> they
>> > were requiring all usernames to be at least 6 characters, _in order to
>> > increase the number of possible usernames_.  That is, they claimed
>> they
>> > were increasing the size of a namespace by eliminating possible names.
>>
>>   Well that's a hair brained policy, but username selection is not a
>> good
>> analogy. I was at a company that had no strict requirements on a
>> username
>> so there should have been a near infinite size of the namespace. But we
>> had
>> a collision when the company had less than 10 employees because there
>> was another "dan" at the company.
>>
>> > The point is, if a tag is required to be exactly 64 octets, you get
>> > 0x5e^64 possible tags.  But if it is required to be up to 64 octets,
>> you
>> > get Sum(i=0..64) 0x5e^i possible tags, which is strictly greater than
>> > 0x5e^64.  So, requiring a tag to be 64 octets _reduces_ the number of
>> > possible tags, thereby increasing the chance of collision.
>>
>>   That would be the case if all tags in the Sum(i=1..64) 0x5e^i tagspace
>> were equally probable of being chosen. Which implies implementations
>> choosing a random tag length for each tag generated in addition to a
>> random tag selection scheme for the randomly chosen length. I suspect,
>> though, that in practice the tag length will be fixed for a particular
>> implementation and the tag selection scheme will not necessarily be
>> random. So the herd mentality, plus the proliferation of one or two
>> companies' ALTO servers, will result in a severely reduced size of the
>> effective tagspace and the increased possibility of collisions.
>>
>>   A tag generated as SHA256(NetworkMap) represented in 64 hex
>> characters would basically guarantee you'd never have a collision.
>> Saying, "it can be anything you want as long as it's less than 64
>> octets" would not.
>>
>
> Should I interpret your comment to say that we should to require
> particular
> mechanisms for generating version tags, or be more explicit about
> suggesting mechanisms that have a low collision probability?

  Yes, I think you should. Suggestions on how to ensure a low
probability of collision would be helpful.

> To help steer readers towards better implementation practices, we'll
> change
> the examples to use hashes in the version tags.

  That's a great idea. So people who implement ALTO and check
the example will use hashes themselves and that will help ensure
a low probability of collision.

> Thank you again for the review!

  You're very welcome!

  regards,

  Dan.