Re: [TLS] TLS and middleboxes again

[Yoav Nir <[e-mail]>]

On Aug 26, 2011, at 11:04 PM, Yaron Sheffer wrote:

> Hi Uri,
> 
> two reasons for the middlebox to use the MAC (which may or may not be 
> "justifiable" to you) are:
> 
> - Many security applications add a textual signature similar to "This 
> mail was scanned by The Best Antivirus".
> - If something bad is detected in the traffic, the middlebox might want 
> to issue an HTTP redirect, e.g. to display a copy of the company's 
> security policy.
> 
> And I like Nikos' idea of having a different MAC for middlebox-generated 
> traffic. Although sharing the middlebox-specific MAC with the individual 
> middlebox AND with the server, while not sharing it with other 
> middleboxes along the path (yes the draft assumes there could be 
> several) would be a challenge.

Only if you require the other middleboxes to verify the MAC. If the MAC for the middlebox is derived from the master secret (for example, with an extractor), the server can automatically calculate it. But once you've begun to make changes, you likely have to re-MAC all subsequent records, because it's likely that the modified content takes a different amount of records, and then the sequence number count is off.