Summary - IV field in DES-CBC mode

[Paul_Lambert-P15452@email.mot.com]

Options for shorter IV for IPSP DES-CBC modes of operation:

!   1) PAD with zeros ...
!   2) PAD with a fixed (non-zero) pattern
!!  3) use the SAID also (to get to 8-bytes)
    4) PAD with SAID determined bits (secret to outside observer)
    5) expand (with MD5) to 8-bytes
X   6) use part of IP address
!!  7) duplicating the 32 bits in each half of the IV

Option 4 adds yet another secret to negotiate.  IVs do not need to be secret.

We do need to consider high speed implementations of IPSP.  Option 5 (using MD5 
to create the extra 32 bits of IV) would create significant extra processing.  

Option 6 will not work.

This leaves options 1, 2, 3 and 7.

I am partial to option 3, using the SAID as part of the IV.  Some DES 
implementations expect the IV to be "in front" of the encrypted data.  

Option 7 also seems quite simple...



Paul

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