[saag] The use of AES-192 and AES-256 in Secure RTP

[David McGrew <mcgrew at cisco.com>]

Hello,

here's some additional text that explains the "hedge" strategy.   
Perhaps it's worth adding to the draft.  Comments welcome.

David

The main motivation for AES-192 and AES-256 is to provide alternative  
ciphers to AES-128 that can be adopted in event that unforeseen  
advances in cryptanalysis significantly erode the security level of  
AES-128.  The main purpose of this specification is to provide  
algorithm agility to SRTP, to allow that protocol to easily adopt the  
alternative ciphers if the need arises in the future.  It MUST NOT be  
interpreted as discouraging the use of AES-128.  Implementers MAY  
support the alternative ciphers in advance of any need to replace  
AES-128, in order to facilitate a potential future 'hot swap'  
replacement, but those implementations MUST be prepared to  
interoperate with implementations that do not support the alternative  
ciphers.