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Can someone explain why, in remote access scenarios, IKEv2 EAP password-based user auth combined with IKEv2 PUBKEY gateway auth is preferable over mixed IKEv2 PSK/PUBKEY auth using the same password on the user side and the same certificate on the gateway side?

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Is it because IKEv2 with PSK is vulernable to active PSK sniffing attacks? But strong PSKs are resistant to MITM attacks. –  niklr Aug 21 '12 at 15:47
    
No, the PSK is exchanged using a secure key agreement mechanism. –  Polynomial Aug 21 '12 at 15:51

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up vote 2 down vote accepted

There are a few reasons why EAP is preferred over PSK:

  • PSK requires the use of a (relatively) low-entropy password, which is (relatively) easy to bruteforce.
  • PSK requires the passphrase to be stored on the server, which may result in key theft.
  • EAP allows clients to authenticate using different credentials in each direction, so a server might authenticate itself to the client with a completely type of credential than the client uses to authenticate itself to the server.
  • EAP allows clients to authenticate using their own digital certificate, which is much more secure than a password. The user just has to unlock their certificate (or certificate store) using their password on the local device.
  • IKEv2 supports EAP-only authentication, which theoretically allows any form of add-on authentication mechanism to be used.
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is the passphrase stored in cleartext on the server or just the hash of it? what do you mean with add-on authentication mechanism? –  niklr Aug 21 '12 at 16:07
    
does a length limit for PSK exist or why does it REQUIRES the use of a realtively low entropy password? –  niklr Aug 21 '12 at 16:10
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@Moo I'm not exactly sure on the specifics. I would imagine that you could hash the password on the server side, but I'm not exactly sure what hash functions / key derivation functions are supported. There's no length limit that I'm aware of on the PSK, but the point is that the entropy of a password is much lower than that of a certificate. To gain 256 bits of entropy, you need 43 perfectly random upper/lower alphanumeric characters to represent that randomness as a password. Rather difficult to remember! –  Polynomial Aug 21 '12 at 17:37
    
The bolt-on authentication mechanisms are anything you like. It's designed such that you can implement your own authentication tokens, as long as both your client and server applications support it. –  Polynomial Aug 21 '12 at 17:38

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