Re: Searching for a special challenge&response algorithm

Jan Peter Stotz wrote:
> Carlos Moreno schrieb:
>
>>>I am searching for a special challenge & response algorithm which allows to
>>>prevent (or better slow down) denial of service attacks. Until now I did
>>>not succeed in finding a suitable algorithm, but may be I only used the
>>>wrong keywords.
>
>>What's the DoS you're referring to??
>
> Sorry, DoS is the wrong description. I had more something similar to an SSH
> brute force attack in mind.

Ok. A tiny nitpick: SSH login attempts do not constitute a brute
force attack -- that falls more in the category of a dictionary
attack (they're not trying all possible names with all possible
passwords -- they're only trying a list of the commonly used
combinations).

Ok, but then -- a C-R implies that you're simply trusting a remote
user as soon as they prove to you that they know the right password;
so, there's nothing that the protocol or the implementation can do
about it; if the passwords *can be guessed*, then the remote
attacker will be able to log in.

The solution is: make the passwords non-guessable. Restrict the
frequency with which logins may be attempted (with configurable
exceptions, so that "privileged" IP addresses can always attempt
the login).

Youcan also do what people do with SSH: do not use password
authentication at all -- use public-key authentication; the
server has a list of authorized public-keys, then sends a
randomly generated challenge and request the client to decrypt
it with the client's [matching] private key. In a sense, this
is similar to giving the client a password that is a randomly
generated sequence of 256 bits, and use it as part of a standard
C-R authentication; the advantage of the public-key approach
is that if the server is temporarily compromised and the
passwords file is stolen, that information is useless, since
they're only public keys, and hence not useful to impersonate
those users.

HTH,

Carlos

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