iSCSI authentication requirements

["Bernard Aboba" <bernard_aboba@hotmail.com>]

In order to move forward on selecting an alternative mandatory iSCSI login 
authentication method, it is important to understand what the requirements 
are. I would like to suggest that the following requirements are essential:

a. Mutual authentication
b. Pre-shared key support with sufficient key size (e.g. 128 bits)
c. Resistance to man-in-the-middle attack

On the other hand, I would argue that the following requirements are *not* 
important:

d. Resistance to hijacking
e. Dictionary attack resistance
f. Support for certificate authentication

Goals

Mutual authentication is important so that not only can the iSCSI Target 
authenticate the Initiator, but also the Initiator can authenticate the 
Target. The ability to detect a rogue Target is important, especially since 
iSCSI can be used for booting and rogue Targets could fools Initiators into 
making use of bogus data. The ability of the Target to authenticate the 
Initiator is important so that the Target can control access.

Pre-shared key support is important since this is likely to be the most 
common use of iSCSI login authentication. The pre-shared key should be 
unique to the two parties, and not suceptible to man-in-the-middle attack, 
as opposed to the Group Pre-Shared key that is so widely implemented within 
IPsec VPN clients, and that enables man-in-the-middle vulnerabilties. 
Sufficient entropy is required to avoid brute-force attacks.

Non-goals

iSCSI login authentication can be used with or without IPsec. When IPsec is 
not used, the iSCSI connection can be hijacked, but this is not something 
that login authentication can protect against.

One of the reasons that SRP was chosen was its resistant to dictionary 
attack when weak secrets are used. However, it is not clear that this is 
useful functionality for iSCSI login authentication.

Mounting iSCSI volumes is inherently a machine activity, since access to 
that volume, when mounted, is determined by the operating system and its 
access controls rather than security services within the wire protocol.

As a result, the credentials used for iSCSI login may be machine 
credentials, which can be assumed to be pre-shared keys with significant 
entropy, rather than a user password.

The once scenario in which a user password might be relevant is mounting an 
iSCSI volume via a storage service provider. However, this is exactly the 
scenario in which IPsec protection of iSCSI would be most likely. Therefore, 
I would claim that dictionary attack resistance is not important here 
either.

If certificate authentication is possible and desired, this can be provided 
within IKE Main Mode. As a result, certificate-based authentication is not 
required within iSCSI login.




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