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I've got some practical questions about SRP when the shared secret must be established online.

  1. Is there a way to establish the shared secret online without the use of HTTPS?
  2. If you've already used HTTPS to establish the shared secret, why bother with TLS-SRP at all? Meaning, what's the advantage of switching to TLS-SRP if you already trust HTTPS?
  3. https://security.stackexchange.com/a/2531/5002 claims that "SRP stores the password on the server in a non-recoverable [...] way." What prevents an attacker from figuring out the user's password using a brute-force attack in the same way we attack databases that store password hashes?
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SRP is for situations where you already have a shared secret between client and server, e.g. as part of a "pairing" procedure which would occur naturally in your context (for instance, SRP has been proposed for Bluetooth peripherals: one device has a screen and displays a code, the other device has a keypad on which the code is entered). For a "fully online" situation where there never is any occasion to obtain such a shared secret, SRP has little advantage. Security has to start somewhere.

In SRP, what the server stores is indeed a sort-of hash of the password -- only "sort-of" because the stored value must also fulfil some mathematical structure so that it can be used in the protocol. That value cannot be immediately used to authenticate as a client, but, like any hashed password, it is indeed susceptible to brute force. The magic of SRP is that observing data packets on the wire is not sufficient to apply a dictionary attack on the password; however, what the server stores allows such an attack (and, in a very general way, this is unavoidable).

In TLS/SRP as currently specified, the value stored on the server is computed as:

    x = SHA1(s | SHA1(I | ":" | P))
    v = g^x % N

where s is the salt, I the user name, P the password, and v is that which is stored (x is not stored). g and N are SRP parameters (public). If the attacker can obtain the value v, then he can "try passwords" by doing the computation above.

This is a salted hash (that's good), but it is also a quite fast hash (that's bad). SRP could be coupled with a good password hashing function (e.g. bcrypt), with the following caveats:

  • There is no specified or de facto standard for that. But both client and server must agree on the particular.
  • If slow hashing is used, then, when a connection is performed, it must occur on the client, not on the server. So this will run at the speed of the client.
  • Excellent answer, as always. Thank you! Out of curiosity, how would you replace the default hashing function with bcrypt? Would the underlying algorithm have to change or would you simply pass the output of bcrypt as the password in the existing algorithm? – Gili May 27 '14 at 14:55

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