Cryptanalysis of the Stream Cipher DECIM by ypl44131

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									Cryptanalysis of the Stream
      Cipher DECIM
    Hongjun Wu and Bart Preneel
    Katholieke Universiteit Leuven
            ESAT/COSIC
                      Overview
1.   Introduction to DECIM
2.   Key Recovery Attack (on Initialization)
3.   Distinguishing Attack
4.   Conclusion




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       Description of DECIM (1)
submission to the eStream
80-bit key, 64 or 80-bit IV
hardware efficient stream cipher (profile II)

Main features

1. ABSG decimation algorithm
   (similar to the self-shrinking generator, 25% more efficient)
2. Buffer for constant output rate
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     Description of DECIM (2)
Keystream generation




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       Description of DECIM (3)
DECIM consists of
 192-bit regularly clocked LFSR (14 taps)
 two filtering functions (different tap positions)



  ABSG decimation
      split the sequence into the form
      if i = 0,output the bit b; otherwise, output the inverse of b
  32-bit Buffer
      for every 4/3 input bits, only one output bit
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      Description of DECIM (4)
Key/IV setup
   192 steps
    each step -- the non-linear feedback
                  a permutation on 7 LFSR bits




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     Key Recovery Attack (1)
Overview of the Attack

    The permutations are used to update the LFSR

   => 54.5 bits in the LFSR are not updated during
      the key/IV setup

   => key recovered with 220 random IVs,
      the first 2 keystream bytes,
      negligible computations
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       Key Recovery Attack (2)
Two permutations operate on 7 elements
     (st+5, st+31,st+59,st+100,st+144,st+177,st+186)

If the output of ABSG is 1, the first permutation
is used; otherwise, the second is used




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       Key Recovery Attack (3)
Using permutation to update FSR is bad

    If no permutation, then every bit in the FSR
    is updated once every 192 steps

    But with the permutation on the FSR, the bit
    positions are changed, some bits would be updated
    more than once while some bits not updated!

    => no matter how to design the permutation
      the updating would not be uniform for all the bits
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      Key Recovery Attack (4)
The key-dependent selection of permutations does not
hide the intrinsic weakness of the permutation

=>in average 54.5 bits in the LFSR are not updated




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       Key Recovery Attack (5)
To recover the key, we need to trace each key bit to see
how that key bit is updated during those 192 steps
in the initialization

=> very tedious
   use computer program to trace those key bits




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          Key Recovery Attack (6)
One example – recovering K21

s21 = K21 \/ IV21
s21 is not updated and it becomes s192+6 with prob 1/27

s192+6 used in the generation of the first keystream bit z0
if s192+6 is 0, then z0=0 with prob. 56/128
if s192+6 is 1, then z0=0 with prob. 72/128

if K21 = 1, the distribution of z0 independent of IV21
if K21 = 0, the distribution of z0 affected by IV21
=> Being used to identify K21 with about 218.5 random IVs
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      Distinguishing Attack (1)
Overview of the Attack

   The filtering functions are not 1-resilient
   ABSG could not hide the non-randomness

   => any two adjacent bits are equal with 0.5+2-9
      message being recovered if encrypted 218 times


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       Distinguishing Attack (2)
Bias from the filtering function



If two inputs share one common bit, the two outputs bits
are equal with prob. 65/128




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        Distinguishing Attack (3)
Bias passing through the ABSG decimation and buffer

Deal with the bits with relations not affected significantly by
the ABSG decimation algorithm
i.e., the bits with small distance


For these three pairs of bits, passing through the ABSG
decimation and buffer does not reduce the bias too much
(about 8 to 32 times)
But the analysis is too complicated (details ignored here)
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       Distinguishing Attack (4)
Any two adjacent keystream bits are equal with
probability 0.5+2-9

The bias is large enough for the broadcast attack

If a message if encrypted by DECIM for 218 times, then
the message could be recovered


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                      DECIM v2
Initialization
       Permutation removed
       768 steps

Keystream generation
     one LFSR + one filtering function + ABSG + buffer
     1-resillient filtering function

Greatly simplified comparing to the original version


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               Conclusion
Using permutation to update FSR is undesirable

Try to design Boolean function conservatively
(high resilience, ….)




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Thank you!

  Q&A



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