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I am currently maintaining a few different identities each associated with different gpg keys (for personal, academic, professional correspondence etc., whether or not this is a good idea is probably a matter for another question). For convenience, I would like to use the same (strong) password across these three keys. I think this is OK for the following reasons:

  1. These keys are all subkeys of master keys that are kept offline in a secure location. They can all be revoked quickly if necessary.

  2. All these subkeys are stored together (I need all of them on all of my devices). As a result, if one is compromised all of them are compromised, and I would revoke all of them (and use a different password for the next generation of keys).

Having said this I am no expert. Enlightenment one way or the other would be much appreciated.

Edit:

OK password reuse has some obvious implications. For instance if one is compromised, they are all compromised. As per the comments, more interesting to see would be if there are non intuitive implications. For instance: Is the password easier to crack if there are 3 keys using vs just the one.

  • Reusing a password increases the chances of being compromised. Also, what if you fail to identify that one subkey has been compromised? You will fail to protect the others. Some easy solution, for example, is to use a strong password with some variation for each key (if that variation is not that predictable). – lepe May 23 '16 at 1:22
  • @lepe While using a variation on the password might be slightly better, but I don't think it would mitigate this. For the variation to be easily memorable, it could only introduce a few bits of additional entropy. An attacker would probably attempt variations on the password for the other keys anyways. – maxf130 May 23 '16 at 7:31
  • It would be interesting to know if there are non-obvious implications, for instance concerning the cryptographic strength. – Arminius May 24 '16 at 10:28
  • @Arminus right. The fact that password reuse is problematic is not surprising. But is it easier to crack the password if you have three different keys vs just the one? – maxf130 May 24 '16 at 11:05
  • You’ve already stated the problem which is you’re protected 3 keys with the same password. If the password is compromised, you have 3 keys to revoke instead of 1. Additionally, they could determine that those 3 keys are from the same person which would compromise your separate IDs. Moving along to mitigation. You need 3 keys for 3 IDs. The only thing that can really improve security here is to use 3 different passwords so that if a compromise of 1 is only that 1 and doesn’t link to your other IDs. – Paraplastic2 May 24 '16 at 19:15
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Generally, in terms of security, reusing passwords is not a good idea.

Mind that the revocation is not the only problem if you get compromised. An attacker with access to your previously encrypted messages, by having the key and password, is gonna be able to decrypt them. So, having different passwords makes the attacker's job harder.

Also, making this setup (keeping the keys in the same machines) is not necessarily a good idea. If you can avoid having one of your keys in a machine, do it. Make your attack surface as smaller as possible.

  • I agree with keeping keys off of a machine if possible. However since I write and receive emails on all of these machines (which I want to sign), and have encrypted documents that I want to access that is unfortunately not an option. As far as I can see my current setup is no different (in terms of security) to just using the same gpg key with multiple identities. By having different keys, I have the ability to revoke individual ones when I leave the company I am working for, or my academic identity becomes irrelevant. – maxf130 May 23 '16 at 7:34

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