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On the French wikipedia page about DES it says that the original DES algorithm from IBM used 112 bit keys.

Why did they reduce this to 56 bits?

Edit: ok NSA convinced IBM but today it seems like a mistake. So did they have objective arguments or just impose their will?

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    Maybe the french Wikipedia is just wrong? The english Wikipedia states that they discussed between 48, 56 and 64 bits. en.wikipedia.org/wiki/…
    – Lukas
    May 9, 2016 at 11:54
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    As I recall, Lucifer used 112 key bits. DES (which was based on Lucifer) never had keys that long, however.
    – user
    May 9, 2016 at 12:30

2 Answers 2

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The NSA convinced IBM that 56 bits was "enough":

But whereas Lucifer had a key that was 112 bits long, the DES key was shortened to 56 bits at the request of the National Security Agency.

from Practical UNIX & Internet Security

In the development of DES, NSA convinced IBM that a reduced key size was sufficient

from Data Encryption Standard - Wikipedia

The NSA made two changes to DES: It tweaked the algorithm, and it cut the key size by more than half.

from The Legacy of DES - Bruce Schneier

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    By convinced, you mean that they already at the time found a way to decrypt it by reducing the key to 56 bits and hide it to everybody or you mean convinced convinced ?
    – Mxsky
    May 9, 2016 at 14:09
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    @Mxsky: More like "Hey guys, anything beyond 56 bits we can currently not break, and you know we don't like that, sooo about that party you had last weak..."
    – PlasmaHH
    May 9, 2016 at 15:03
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    IBM wanted 64 bits, NSA wanted 48 bits. The compromised on 56 bits. It is plausible that the NSA could crack 48 bit keys in a reasonable time in 1977, but I don't think it is likely that the NSA could crack 56 bit keys. EFF cracked a 56 bit DES key in 56 hours in 1998. Using Moore's law to go back to 1977 (i.e. halving the number of work every two years), that would mean that a 46-bit key in 1977 would also take 56 hours to crack. a 48-bit key would then take 9 days, a 56-bit key would take 6 years.
    – Sjoerd
    May 9, 2016 at 15:15
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    @Sjoerd You underestimate the difference in resources. EFF built Deep Crack for less than $250,000 in 1998 ($375,000 today). NSA's budget is $11 billion a year. Diffie and Hellman sketched out a DES machine in "Exhaustive Cryptanalysis of the NBS Data Encryption Standard" for $20 million in 1977 ($80 million today). NSA plausibly could have decrypted DES from day 1. May 9, 2016 at 15:58
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    @MattNordhoff What was NSA's budget in 1977? I don't know exactly for that year but based on previous budgets I could tell you it was no where close to $11 billion. And the NSA's focus in the 70s was on satellite surveillance which likely ate into most of their budget.
    – Bacon Brad
    May 9, 2016 at 18:13
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Key size was reduced to 56 bits because IBM wanted to fit LUCIFER on a single chip. LUCIFER then became DES.

Because of the promising results produced by the LUCIFER project, IBM embarked on an effort to develop a marketable commercial encryption product that ideally could be implemented on a single chip. The effort was headed by Walter Tuchman and Carl Meyer,and it involved not only IBM researchers but also outside consultants and technical advice from the National Security Agency (NSA). The outcome of this effort was a refined version of LUCIFER that was more resistant to cryptanalysis but that had a reduced key size of 56 bits, in order to fit on a single chip.

Stallings, W. Cryptography and network security, 5th ed.. p.78

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  • Hmm... I frist heard the NSA story 30+ years ago. I wonder if your reference is correct. This answer has references for the NSA story that I've heard. Alas, we may never know the truth. May 10, 2016 at 4:35
  • Well... 30+ years ago people were worried that NSA interfered to make DES weaker, but that conspiracy theory has been proven wrong long ago. May 10, 2016 at 14:17
  • Interesting. Can you add some references? I consider Bruce Schneier to be a pretty solid reference (see above answer). It wouldn't be unheard of for them to do this as, thanks to Snowden, we know that they messed with RSA algorithms. BTW, I didn't downvote this answer as you have a good reference. I view it as a different opinion and not a wrong answer. May 10, 2016 at 14:35
  • @NeilSmithline references to what else exactly? May 10, 2016 at 15:34
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    Let us continue this discussion in chat. May 10, 2016 at 16:01

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