Re: [hybi] Handshake was: The WebSocket protocol issues.

[Scott Ferguson <[e-mail]>]

Maciej Stachowiak wrote:
> On Sep 29, 2010, at 3:28 AM, Greg Wilkins wrote:
>
>   
>> On 29 September 2010 18:56, Maciej Stachowiak <[e-mail]> wrote:
>>     
>>> I suspect -76 is not strong enough, but it is slightly stronger in some
>>> specific ways. First, by spreading the data across multiple header fields
>>> and incorporating whitespace, it makes it slightly trickier to do injection
>>> attacks.
>>>       
>> Why is hex encoding simpler to inject than the space/char injected
>> decimal encoding?
>>     
>
> Because you can't inject whitespace into the resource name in the request line.
>   

Maciej, you're missing the entire point of the hash.  The main purpose 
of a hash/digest is to detect modification or replacement of a message 
as in an injection attack. That's what a digest does.

If the hijacker modifies the nonce, changing c-nonce to h-nonce through 
injection, the server will return H(h-nonce, "WebSocket"). The browser 
will then compare the returned hash to its internally-calculated 
H(c-nonce, "WebSocket"), which will not collide because of the hash 
function, detect the injection and drop the connection. That's why you 
have a hash.

In other words, the hash operation protects against the injection 
attacks you're talking about, and it does so in a clean and 
well-understood fashion without any need to play header syntax games. 
Creating multiple nonces does not increase the strength of the digest, 
and the division operation in -76 *weakens* the digest because it 
increases the chance that H(h-nonce, ...) will collide with H(c-nonce, ...).

-- Scott