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This article describes a synchronization algorithm for an output feedback mode (OFM) encryption device which does not allow cycle repetition from its initialization vectors (IVs).
English (United States)
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Non-Cyclic Initialization Vector Generation
This article describes a synchronization algorithm for an output feedback
mode (OFM) encryption device which does not allow cycle repetition from its
initialization vectors (IVs).
Given a pseudo random number as a seed (IV(1)) and using this
value as the initial data and key inputs into an encryption engine,
there will be predicted an initialization vector (IV(2)). A certain
period or number of iterations (N) exist when feeding the previous
initialization vector (IV(i-1)) back into both the data and key
inputs of an encryption engine where:
IV(1+N = IV(1).
This indicates the beginning of the previous cycle. This period may be long
or short, but when it repeats, the strength of any permutations generated from
these initialization vectors (IV(N+i)) become weak.
The figure illustrates a system where this effect can be detected, causing the
initialization vector to be reseeded, thus eliminating the cyclic characteristics of
The time of day (TOD) register 1 is a sequentially increasing counter based
on a Unit of time. It is used as the initial seed to the seed register 2 and
encryption engine 3. Later, when repetition is detected, the TOD register 1 is
also used as a new seed and the new input to the encryption engine 3.
Normally the IV(i-1) register 4 is used as both key and data
inputs to the encryption engine 3. However, if the beginning of the
previous cycle is detected:
IV(i-1) = seed, then the time-of-da...