When DES was originally developed, the NSA changed the s-boxes. For decades people thought that their changes introduced a backdoor but then it was discovered that their changes actually strengthened DES against differential cryptoanalysis.

The NSA is well known for it's placement of backdoors into algorithms. They kinda did this with DES when they reduced it's key strength but Dual_EC_DBRG is the most famous contemporary example of this.

Anyway, my question is... are there any other examples of the NSA strengthening non-classified cryptographic algorithms?

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    $\begingroup$ I guess SHA-0 to SHA-1 would probably apply here $\endgroup$ – Richie Frame Jul 17 '18 at 4:19
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    $\begingroup$ Reducing the keysize of DES is more of a frontdoor than a backdoor, since they make no attempt to hide that fact. Also it's not really that the NSA changed the keysize so much as they told everyone that they keysize must be 56 bits or less. IBM simply went along with it. $\endgroup$ – forest Jul 17 '18 at 10:43
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    $\begingroup$ @forest: it appears that the NSA wanted 48-bit DES, IBM wanted 64-bit or more, and 56-bit was a compromise. See this for what I could gather on that. $\endgroup$ – fgrieu Sep 5 '18 at 17:29

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