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i got my YubiKey 4 today and first tried it to use KeePass with OATH-HOTP (OtpKeyProv plugin). My Configuration was 3 OTPs with look-ahead count = 0. It was not working that good because sometimes the OtpKeyProv plugin did not recognize my input when i pressed the button too fast. So i changed to 6 OTPs with look-ahead count = 12. But with that it was not "that comfortable" anymore because i needed to press the button 6 times and had to wait a few seconds between the pressed to be sure the plugin recognizes the input.

So i tried the challenge-response method with the KeeChallenge Plugin. it just worked flawless. does anything speak against using the challenge-response method with the KeeChallenge Plugin or is it safe to use? i know the OtpKeyProv plugin is more secure but does that make that much of a difference?

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depending how how the plugin does it internally this can be quite an intresting way of adding security.

essentially this thing misuses HOTP big time, by calculating the next however-many OTPs needed and forms them together with the master pass to encrypt the database.

but of course when you have lookahead active (and you really should, especially on a nano) it generates a few more sequences of keys in case you incresed the counter accidentially.

The idea behind it isnt bad, although obviously keepass needs to store the seed (shared secret) in the database to allow for re-encrypting it.

the keechallenge plugin works on the same premise: hashing a shared secret with some kind of challenge (the counter in HOTP), but this plugin is actually safer from a few standpoints.

1) it provides a 160 bit output of HMAC-SHA1 instead of "just" of log2(10^(6*x)) bits (x is the number of consecutive OTPs you select, 8 consecutive OTPs would you get just to 159 and a half bits of length)

Simply saying, you need to tap MUCH less for the same security Level, and while the OTP plugin could probably be configured to use a ton of OTPs for even higher security, with modifications the Challenge-Response plugin could also run multiple challenges throwing the number of bits through the roof, with again just an eighth of the needed taps (and no concern of mis-tapping and not getting the proper sequence).

2) as the challenge is provided from the database, we dont need to worry about accidentially touching the key or lookaheads, which means we dont need to lower the security by allowing multiple second factor solutions to get in.

Simply said number 2 means the Yubi only has the Secret and the DB has what could be called the "state" of the challenge, while in TOTP, the Yubi has both the state and the secret needed to decrypt the DB and the state changes everytime you touch the key, meaning you would need to account for multiple states.

although obviously the shared secret still needs to be stored in one way or another inside the database (unless you don't change the challenge at all, but that's reckless).

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You should be aware that any mechanism to implement 2factor with KeePass is not fundamentally adding any true security. Any attacker who gets a hold of your database will simply not install the OTPKeyProv plugin, and will not be required to use the second factor.

OTP 2-factor provides authentication, not encryption, and only works in scenarios where the attacker does not have direct access to the password vault prior to authentication, such as with online services like LastPass. With KeepPass, the attacker must have access to the database during authentication, and as such can simply download the database and ignore the authentication piece.

More simply: Using OtpKeyProv does not add any security to your master key and is mostly security theatre.

EDIT: I stand corrected on some fronts-- based upon the plugin author's responses, the plugin is designed to add an additional secret key to the master password and protect it using HOTPs. Precisely how that is done is unclear but it sounds plausible. However, it should be noted that this is not true 2-factor, and is more like factor-and-a-half; the main factor (encryption key) is being strengthened by a weaker key that is itself weakly guarded. So it is additional security, but not as much as true 2-factor offers in authentication systems.

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    It is worth noting the discussion on the keepass forum which states this post is "largely incorrect because the OTPKeyProv plugin does provide a true second factor called the Secret Key" and "If an attacker bypassed the OTPKeyProv plugin the complete Master Key would still be needed (e.g. the Master Password AND the Secret Key) to access the database." – Andy Brown Feb 28 '16 at 13:23
  • The OTP - Plugin will just harden the encryption by a short secret. This doesnt change any major part of the answer despite that it maybe adds some more length to your password. The main answer remains: An attacker doesnt need to use that plugin and he doesnt need to get the OTP if he can just bruteforce the whole thing (master+secret). Thats why OTP should be mainly used for authentification, not encryption. – James Cameron Mar 30 '16 at 10:18
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    Sorry that is just plain wrong. It depends on what you try to achieve. Using a second factor is NEVER about harding the encryption it's about making the authentication more secure e.g. to harden it against keyloggers. So the hole point of this response is nonsense. And as @AndyBrown stated you can not simply bypass the second factor by "not using the plugin". It will not open. So you will have to BF the KeePass AES256 encryption. As of today: good luck with that. If you find this usefull please upvote me. Once I have 125 points I can downvote that crap answer above. – masi Apr 1 '16 at 9:26
  • This answer does not address the question whatsoever. The question states “does anything speak against using the challenge-response method with the KeeChallenge Plugin or is it safe to use?” while this answer does not even mention KeeChallenge at all. Instead, it focuses entirely on dubious claims about OTPKeyProv. – Socob Jan 16 '17 at 4:22
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    @masi I downvoted for you. – A. Hersean Jan 24 '17 at 13:46

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