Ronald Rivest designed MD2, MD4, MD5, and MD6, but what about MD3? Was it skipped? Or perhaps kept private and never publicly released?

(edit: found a related question)

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    $\begingroup$ @kelalaka don't be so sure, in Bruce Schneier's Applied Cryptography, Second Edition: Protocols, Algorthms, and Source Code in C (cloth) on line 595 we find MD3 is yet another hash function designed by Ron Rivest. It had several flaws and never really made it out of the laboratory, although a description was recently published in [1335]. - stating that a description of MD3 was published at some point, don't know what "1335" means tho, its certainly not page 1335, because the last page is 1027 ( mrajacse.files.wordpress.com/2012/01/… ) $\endgroup$
    – hanshenrik
    Commented Jan 22 at 18:43
  • $\begingroup$ Good find. I missed. $\endgroup$
    – kelalaka
    Commented Jan 22 at 18:46
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    $\begingroup$ @hanshenrik: [1335] is a reference number - if you look at the reference section, it points to the paper given by kodlu's answer... $\endgroup$
    – poncho
    Commented Jan 22 at 19:19

1 Answer 1


This is the referenced article in Schneier:

  1. M.J.B. Robshaw, “MD2, MD4, MD5, SHA, and Other Hash Functions,” Technical Report TR–101, Version 3.0, RSA Laboratories, Jul 1994.

I couldn't find a soft copy at a quick first look.

Here is a related question and answer here which is about MD1 but a comment below it states:

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.

Edit: Related later report by Robshaw with similar title is online here https://networkdls.com/Articles/bulletn4.pdf and refers to his earlier TR-101, the string MD3 does not appear in this document.

  • $\begingroup$ If you have a hard copy, could you summarize what it says about MD3? $\endgroup$
    – poncho
    Commented Jan 22 at 19:15
  • $\begingroup$ No, I'm afraid I don't have a copy. I was on the lookout for a soft-copy. $\endgroup$
    – kodlu
    Commented Jan 22 at 19:16

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