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I am targeting a security level of 128 bits for GnuPG, so I am using SHA-256 and AES-128. Now I need to choose between RSA-3072 and RSA-4096.

According to multiple sources RSA-3072 security level is very close to 128 bits. Based on this statement, choosing RSA-4096 over RSA-3072 makes no sense. Public keys are significantly bigger, signing/verifying speed is twice slower, and the extra security it adds is useless; digest and symmetric encryption algorithms are 128 bits secure after all.

The only issue I am aware of is a possible reduction on compatibility.

Do all GnuPG implementations (including hardware devices such as smart cards) that support RSA-4096 also support RSA-3072 too? Is it better to go with RSA-4096 for maximal compatibility or can I safely use RSA-3072?

  • This is pretty close to being off-topic as an opinion question. The only definitive answer is "Check the specs of the smart cards you plan to use". – Mike Ounsworth Aug 9 '18 at 16:20
  • @MikeOunsworth Smart cards is only a punctuation. Question is about usage and general compatibility of a security tool, so in my opinion on-topic here. – user3368561 Aug 9 '18 at 16:23
  • So either you're looking for people's opinions about general compatibility with all existing software ("Off topic: opinion-based"), or a giant list of which software supports RSA-3072 ("Off topic: too broad"). Your core question is good, but if you could give some concrete details about which software you are worried about compatibility with, that would make the question easier to answer. – Mike Ounsworth Aug 9 '18 at 16:31
  • @MikeOunsworth My question is about GnuPG as states the first line. – user3368561 Aug 9 '18 at 16:33
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    Compatibility isn't based on opinion, it's completely based on fact. Either a device supports some algorithm, or it doesn't. If someone happens to like it doesn't have anything to do with it. (Classifying incomplete support into supported/buggy but usable/unsupported/etc might be based on opinion, though.) On the other hand, listing all devices that support X may very well be way too broad, and such lists will get out of date in a year or two. – ilkkachu Aug 10 '18 at 8:53

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