Re: [tcpm] "The SYN trick"

[Brian Weis <[e-mail]>]

Hi Eric,

I like the simplicity of this approach. But using the ISNs to  
generate keys requires that the ISN values be generated from good  
random numbers (to avoid the generation of predicable keys). I think  
most systems today do happen to use a good RNG for generating the  
ISN, but there's no guarantee.

This implies to me that ISN generation must be a service of the TCP- 
AO module, else the TCP-AO key generation process will not be  
guaranteed to generate good quality K_connection keys. I don't know  
if that's a requirement that can be made in the TCP-AO draft though.

Thanks,
Brian

On Mar 11, 2008, at 2:36 PM, Eric Rescorla wrote:

> I'd suggested using the ISNs as an implicit diversifier for
> a single master shared key.
>
> E.g.,
>
> K_connection = HMAC(K_master, ISN_i, ISN_r)
>
> As Joe points out if you use ISNs as an implicit diversifier
> for a shared connection key. Obviously, you can't use the
> ISN sound in the SYN/ACK to key the initial SYN. The natural
> thing to do here is:
>
> - For the initial SYN use K_connection = HMAC(K_master, ISN_i, 0)
> - For subsequent packets use K_connection = HMAC(K_master, ISN_i,  
> ISN_r)
>
> -Ekr
>
> _______________________________________________
> tcpm mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/tcpm

-- 
Brian Weis
Advanced Security Development, Security Technology Group, Cisco Systems
Telephone: [phone]
Email: [e-mail]

_______________________________________________
tcpm mailing list
[e-mail]
https://www.ietf.org/mailman/listinfo/tcpm