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

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

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.

  Dan.