On 13.11.2013 19:21, Dave Cridland wrote: > On 13 Nov 2013 17:01, "Fedor Brunner" <[email protected]> wrote: >> There is good comparison website for key sizes recommendations: > http://www.keylength.com/en/compare/ >> Enter the year until when your system should be protected and see the > Discrete Logarithm Group column. > Yes, that site is very nice. > >> The scenario I thinking of is "record now and decrypt later", the 1024 > bit DH could protect your message for next year, but if the attacker makes > a copy of your conversation now, he can later (for example in 5-10 years > with much stronger hardware) break DH easily. There is communication which > should be protected even for long time, for example: business strategies, > client-lawyer communication, patent information. > > Yes, I agree that some considerations might raise it, but I think the case > for making PFS last as long as the assymmetric identity algorithm is pretty > weak in general. To decrypt all communications using 1024-bit DH over a > year is likely to be vastly bigger than for one conversation; the same > isn't true for RSA, for example, where you could solve the private key once. > > It is, I agree, the obvious attack point for a single conversation, but > you're still talking in terms of vast computational resources for all the > traffic. Bear in mind that if we had used 768-bit DH two years ago in PFS, > I'd still have only got as far as two of your sessions - I'd have to be > pretty good on my targetting to get the information I wanted at that rate. For detailed description of various attack scenarios with calculations please read
ECRYPT II Yearly Report on Algorithms and Keysizes (2011-2012) http://www.ecrypt.eu.org/documents/D.SPA.20.pdf The 1024 bit length for DH used in older versions of software, is a remnant of US export regulations. This regulations in 1999 permitted the export of software programs using maximum 56-bit data encryption and maximum 1024-bit key exchange. https://tools.ietf.org/html/draft-ietf-tls-56-bit-ciphersuites-01
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