Discussions of the weakness of the U.S. Data Encryption Standard frequently cite a letter by Martin Hellman and Whitfield Diffie to the National Bureau of Standards that reported stated:

Whit Diffie and I have become concerned that the proposed data encryption standard, while probably secure against commercial assault, may be extremely vulnerable to attack by an intelligence organization" (letter to NBS, October 22, 1975).

I've been trying to find a scan of the original letter, but all I can find is the above quotation, which seems to have been faithfully repeated in many publications (for example, here).

Interestingly, this book attributes the letter solely to Martin Hellman.

Where can I find the original letter?

  • $\begingroup$ The letter may not have been published. It may be possible to get it published by asking either of the authors or making a freedom of information request to the NBS / its successor (which is NIST I think?). $\endgroup$
    – SEJPM
    Jan 29, 2021 at 15:50
  • $\begingroup$ If the letter was not published, then how come so many authors have quoted it over and over? $\endgroup$
    – vy32
    Jan 29, 2021 at 16:32
  • 1
    $\begingroup$ @vy32 Because the letter became historically significant. At the start of the "Crypto Wars," Diffie and Hellman had the guts to say that intelligence organizations would probably be a threat because "the cipher may have a crude form of trap door..." Creating trap doors later turned into a growth industry, evidently. $\endgroup$
    – Patriot
    Jan 30, 2021 at 8:15
  • $\begingroup$ @Patriot you misunderstand me. I didn't ask why do so many people quote it, I ask how. When citing, it's appropriate to review the original reference, and not just blindly repeat the citations of others. That's a basic of good scholarship. $\endgroup$
    – vy32
    Jan 30, 2021 at 13:26

1 Answer 1


This letter, among other letters of the time, can be found in the Stanford digital repository of Hellman's papers.

This October 22 1975 letter, in particular, is here.

  • $\begingroup$ This repository is fab. Thank you so much for pointing me at it! $\endgroup$
    – vy32
    Jan 30, 2021 at 13:27

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