5 ms·

> nobody outside of the design staff understood how the algorithm was weak And, as I mention, pointing that out was a contribution of the paper. > or (AFAIK)

• matthewdgreen 5y ago
  > And, as I mention, pointing that out was a contribution of the paper. Maybe I didn't make it clear. The open question prior to this paper was not "precisely how did the algorithm implement a specific level of security", the question was: what is that specific level of security? This was totally unknown and not specified by the designers. Notice that the specification doesn't define the desired security, in the same way that it defines, say, the key size. It just handwaves towards 'should be exportable'. I can't find a copy of the requirements document anymore, but the quote given in the spec doesn't specify anything more than that statement. >I think this 40 bit limit is well documented for other encryption algorithms. I couldn't find it in any (old) US regulation text though after a cursory search. In the United States (note: GEA-1 was not a US standard) some expedited licenses were granted to systems that used effective 40 bit keys. In practice (for symmetric ciphers) this usually meant RC2 and RC4 with explicitly truncated keys. GEA-1 does not have a 40-bit key size -- a point I made in the previous post. It has a 64-bit key size. Nowhere does anyone say "the requirement for this design is effective 40-bit security": they don't say anything at all. It could have had 24 bit security, 40 bit security, 56 bit security or even 64-bit security. ETA: Moreover, there is more to this result than 40-bit effective keysize. A critical aspect of this result is that the attackers are able to recover keys using 65 bits of known keystream. The uncompromised GPRS algorithms require several hundred (or over 1000) bits. Note that these known plaintext requirements are somewhat orthogonal to keysize: capturing 65 bits of known keystream is possible in protocols like GPRS/IP due to the existence of structured, predictable packet headers -- as the authors point out. Capturing >1000 bits may not be feasible at all. That's really significant and interesting, and not the result one would expect if the design goal was simply "effective 40-bit key size". One has to wonder if "ability to perform passive decryption using small amounts of known plaintext" is also included in the (missing) security design requirements document. I bet it isn't.
  • thaumasiotes 5y ago
    > In the United States (note: GEA-1 was not a US standard) some expedited licenses were granted to systems that used effective 40 bit keys. In practice (for symmetric ciphers) this usually meant RC2 and RC4 with explicitly truncated keys. GEA-1 does not have a 40-bit key size -- a point I made in the previous post. It has a 64-bit key size. Nowhere does anyone say "the requirement for this design is effective 40-bit security": they don't say anything at all. It could have had 24 bit security, 40 bit security, 56 bit security or even 64-bit security. Really? How would you get 64-bit security from a 40-bit key?
    • matthewdgreen 5y ago
      GEA-1 doesn’t have a 40-bit key.