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Diving into the history and evolution of the Message-Digest algorithm by Ronald Rivest, I have been able to track back papers from MD6 down to MD2. Yet, somehow I can not seem to be able to find any papers, specifications or even the slightest indications of MD1.

As the numbering (MD2, MD4, MD5, MD6) seems to be a constant, it would be logic to expect that MD1 did exist at some point in the past. Since I can not find anything related, I'm reaching out for help...

Has MD1 ever been published or was MD2 indeed the first Message-Digest algorithm that Ronald Rivest showed to the world? And if MD1 has been published, where can I find that publication (or a copy of it)?

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    $\begingroup$ The talk page on Wikipedia has some information, if you haven't seen it: en.wikipedia.org/wiki/Talk%3AMD4. It suggests MD1 as such never existed, but was instead just MD (and was never published) and that MD3 was a failed experiment; apparently there exists a specification somewhere (I cannot find the referenced document 1335: MD2, MD4, MD5, SHA and other hash functions. M.J.B. Robshaw). The best way to know would be to send the author an email, though :p $\endgroup$
    – Thomas
    Aug 31 '13 at 10:54
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Since I have not received any reply from Mr. Rivest's office after bugging them with a total of four emails in four weeks, I have no other option than to give up on hoping I ever receive a reply from his office.

After spending 6 weeks hunting down information all over the internet (and not receiving any reply to my emails), I am currently suspecting that MD1 has indeed never been published. I base this on the fact that another algorithm — MD3 — never made it beyond the experimental status either… due to problems with its algorithm. It could well be MD1 faced similar issues and also never made it beyond the lab. Another reason for suspecting this is that all RFCs were published in April 1992. If there had been any "beyond lab" MD1, I'm pretty sure it would have been published together with the other MD papers. My best guess is that MD2 was born out of failed MD1 experiments.

Yet, my suspicions might also be completely wrong. As said: I was not able to find any papers or hard facts in relation to MD1. Should you ever find reliable information about MD1, please post it as an answer. I will gladly unaccept my own answer to accept any answer that's able to present verifiable information and/or a publication/paper/book/whatever related to MD1.

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    $\begingroup$ Bart Preneel in his 1993 PhD thesis writes "R. Rivest of RSA Data Security Inc. has designed a series of hash functions, that were named MD for “message digest” followed by a number. MD1 is a proprietary algorithm. MD2 [175] was suggested in 1990, and was recommended to replace BMAC [193]. MD3 was never published, and it seems to have been abandoned by its designer." Since I suppose Bart as an insider should be well informed, I suppose that the algorithm exists but has never been made public. $\endgroup$
    – DrLecter
    Dec 12 '13 at 19:58
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    $\begingroup$ @DrLecter Seems part of my suspicions just got confirmed, while the other part got a tiny slap in the face. ;) Thanks for the additional info! (Nice paper btw.) $\endgroup$
    – e-sushi
    Dec 12 '13 at 20:44

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