GHASH is used essentially only in AES-GCM. The tag was introduced in late 2018 and applies to only five questions that were all asked within the span of one week. In contrast, there are over seventy tagged and over two hundred tagged . Do we really need three different tags , , and ?


No. We should merge all three into .

  • $\begingroup$ If/when you make meta questions/answers like this, can you please also include the alternative option for people to vote on? The up votes on this don't mean much if there is no opposing answer. Posting options on meta questions doesn't necessarily mean you advocate the option. If you're really worried about that, then you can include a disclaimer in the comments that says something to the effect of "I posted this as an option, I do not necessarily endorse it as the correct solution". $\endgroup$ – Ella Rose Sep 16 '19 at 15:04
  • $\begingroup$ I figured I'd leave it to someone else to argue why the tags are meaningfully distinct, but OK, I'll do that next time. $\endgroup$ – Squeamish Ossifrage Sep 16 '19 at 15:54
  • $\begingroup$ Or you can make the other target a community wiki answer and / or note that the answer is just the alternative voting target. $\endgroup$ – SEJPM Sep 16 '19 at 16:14
  • $\begingroup$ Please don't pre-fill such “poll” answers. We should choose the solution that makes the most sense, not the solution that gets the highest score. A proposal with no reasoning is worthless. Anyway you've also left it to someone else to argue why the tags are synonyms, so why not offer all the options (merge gcm and ghash but keep aes-gcm separate, merge gcm and aes-gcm but keep ghash separate, merge gcm into steganography, etc.). $\endgroup$ – Gilles 'SO- stop being evil' Sep 17 '19 at 20:45

Looking quickly at the use of , I see mostly or only questions that are about GCM, where the block cipher itself is secondary or irrelevant. Furthermore, AES-GCM derives its properties from AES and GCM; I can't think of a question that is specifically about AES-GCM and not about one or both of AES-GCM. So the tag should not exist, most questions currently tagged should have the tag instead, but maybe a few should also have the tag .

GHASH is only used in the context of GCM in practice, but in principle it could be used independently (and arguably it is when you use GCM with an empty plaintext). Furthermore GHASH is a somewhat specific aspect of GCM. Looking at its usage, I see some distinction between and questions: most questions are about using GCM as a black box, whereas questions are about the design of GHASH. Therefore I think should stay, and should be applied to a few more questions (which can happen organically).

Therefore I propose the following actions:

  1. Collectively review questions to add the tag where it is warranted.
  2. Once this is done, merge into , keeping it as a synonym so that it doesn't reappear as a distinct tag.
  • $\begingroup$ I see some benefit of keeping ghash separate, because if you're just interested in ghash, then choosing gcm will give you all the other questions and answers as well (and ghash may be mentioned in many of them, rendering searches more or less useless as well). I guess we can always add the gcm tag to those Q/A's. $\endgroup$ – Maarten Bodewes Sep 20 '19 at 10:59

Yes, we really need three different tags.


  • ghash could conceivably be used/discussed outside of the context of aes-gcm
  • gcm could conceivably be discussed in conjunction with a cipher other than AES
  • $\begingroup$ Perhaps aes and aes-gcm could be merged, and ghash be kept separate? $\endgroup$ – forest Sep 17 '19 at 5:31
  • $\begingroup$ @forest Feel free to make that suggestion a candidate answer for users to vote on $\endgroup$ – Ella Rose Sep 17 '19 at 14:05

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