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5

This is a good question, but I would consider hardcoding a known good group. There does not seem to be an advantage to letting the server decide if you can afford to use high enough parameter values. The SRP paper lists the following checks: "n is a large safe prime" (this is your first three points) "g is a primitive root of GF(n)" (your next point) "A >...

3

In the introduction of the Logjam paper, it is stated that After a week-long precomputation for a specified 512-bit group, we can compute arbitrary discrete logs in that group in about a minute. So it seems that what it actually does is attack the discrete logarithm problem, so any discrete-logarithm-based system which uses a common prime should ...

3

RFC 2945 By Tom Wu the SRP inventor uses x = H(s, H(I, ":", p)) where I is the username demonstrating that can do anything you like to the stretch the password such as prefixing the username then hashing it. So stretching the user entered password before putting it into function using PBKDF2 would increase the time taken for a dictionary attack with no ...

2

As mentioned in the comments this is a standard problem not unique to SRP and not really about the cryptography of SRP. So this question would probably be better posted on 'security.stackexchange.com' as it is more of a generic problem. Fundamentally setting a shared secret like a regular password or a password verifier over a public network has its ...

2

The property of SRP is that: If the attacker is a passive listener to an exchange between a client and a server, he learns nothing about the shared password If the attacker is a participant in the exchange with either a client or a server with the password (or modifies the exchange between the client and the server), the attacker learns nothing except for ...

2

Yes, it's okay. This is actually mentioned in passing in the SRP 6 design paper. Previous versions used a random $u$ where an attacker who saw (or could predict) it before revealing $A$ could compute $A = g^a v^{-u}$ and use this to effectively cancel out the long term secret. With $u$ derived from a hash, even if the attacker saw $B$, the dependence of $u$ ...

2

I know PBKDF2 is essentially "useless" against anyone with a GPU rig and I have read that bcrypt is "useless" to anyone with an FPGA setup. Neither is useless. Newer alternatives like scrypt and the eventual PHC winner make better use of the defenders' resources, but even a thousand iterations of PBKDF2 is useful, compared to doing nothing. If you assume ...

1

From the Security Considerations -section of the RFC: Even if the host's password database were publicly revealed, the attacker would still need an expensive dictionary search to obtain any passwords. The exponential computation required to validate a guess in this case is much more time-consuming than the hash currently used by ...

1

I don't see how the server can directly learn $x$; what a server would be able to is perform a single exchange with the client (recording the initial parts of the protocol), and then go through a dictionary, and test various passwords (that is, various possible $x$ values), and realize when he finds the right one. The idea here would be to select a prime ...

1

According to the Stanford page mutual authentication can be provided if both sides keep their secrets secret. Thanks mikeazo.

1

This question both describes the SRP-Z variant and mentions that the patent should expire in three years. I think the patent in question is this one, but I'm not sure: US 6539479. If it is, it seems to actually have expired earlier this year due to non-payment of fees. Looking it up on the USPTO PAIR says: Status: Patent Expired Due to NonPayment of ...

1

Nimbus or Thinbus are two popular SRP implementations in Java and Javascript compute the proof of password as you describe H(A|B|S) where you can supply your own hash functions such as SHA1 or SHA256 or stronger. The actual protocol design document states H(H(N) xor H(g), H(I), s, A, B, K) where K=H(S) so that expands to H(H(N) xor H(g)|H(I)|s|A|B|H(S)). ...

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