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BarsMonster
Asked: 2010-07-15 02:39:50 +0800 CST

OpenSSH : Key-based authorization, maximum key length

  • 772

I'am using Putty on windows with key-based authentication to access some of mine servers.

It works totally fine with ~3700-bit key, but with ~17000-bit key it thinks for like 20 seconds on client-side and then just says "Access denied" and asks for a password.

Is there any key length limit or timeout in OpenSSH for key-based authentication?

I understand that using such large keys have not much practical sence, especially when looking at these 20 seconds of calculation, just trying to solve any problems I face :-)...

authentication ssh rsa

4 Answers

  1. Best Answer

    At one point I looked into the source of OpenSSL for Diffie-Hellman keys, and found there was an "arbitrary" 10K limit on the size of DH keys. I changed the source for a test, and found that worked. I wrote a bug to the authors, and they replied back that it was design intent to prevent DoS by using massive keys.

    Wouldn't surprise me to see something similar in OpenSSH.

    • 10

  2. There is no maximum key size or timeout defined in the protocol (or at least none that you'd be hitting), but an implementation might not support such long keys. A 20-second processing time with the private key doesn't sound high for a 17kbit RSA key. Then the server might not want to spend too much computing power on an unauthenticated user: refusing very large keys is a protection against DoS attacks.

    Currently 2048 bits is considered reasonable for an RSA key; 4096 bits is higher than necessary but usually supported; beyond this you shouldn't be surprised if some programs to reject the key.

    • 6

  3. Were you able to generate that size of key on the intended target system? You may be running into a limit to what is supported. Rather current Centos system of mine supports a 16k maximum which seems sufficient for massive keys. You should see the maximum if you try to go above it with ssh-keygen as shown below.

    [nathan@omni ~]# ssh-keygen -t rsa -b 32768
key bits exceeds maximum 16384
    • 4

  4. The openssh Server has a LoginGraceTime setting. From the man page:

    The server disconnects after this time if the user has not suc-
cessfully logged in.  If the value is 0, there is no time limit.
The default is 120 seconds.

    This could be a limit that you are hitting if it is set to 20 seconds.

    Wild guess: It could also be that putty itself has this limit, thinking that if the client side processing of the public key authentication takes that long, something is wrong.

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