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From "Alon Bar-Lev (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SSHD-618) Support restricting server host key algorithms
Date Fri, 18 Dec 2015 20:23:46 GMT

    [ https://issues.apache.org/jira/browse/SSHD-618?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15064704#comment-15064704

Alon Bar-Lev commented on SSHD-618:

Hi Lyor,

If you outline a desired solution I will be able to provide an implementation for review.


> Support restricting server host key algorithms
> ----------------------------------------------
>                 Key: SSHD-618
>                 URL: https://issues.apache.org/jira/browse/SSHD-618
>             Project: MINA SSHD
>          Issue Type: Improvement
>    Affects Versions: 1.0.0, 1.1.0
>            Reporter: Alon Bar-Lev
> In current implementation the signature factories effects all algorithms that can be
used during a connection. There is no way of limiting only sever host key algorithm to be
able to request a specific server key. This is required in order to connect to pre-approved
server using weaker key.
> It should be possible as in rfc4253 "Algorithm Negotiation" we have two fields one for
available algorithms, and another for requesting a specific set of server keys which is subset
of the available algorithms.
>       name-list    kex_algorithms
>       name-list    server_host_key_algorithms
> In rfc4252 we have "Public Key Authentication Method:
> "publickey"" "Public key algorithms are defined in the transport layer specification".
So client public key types are subset of kex_algorithms.
> As far as I understand if we set kex algorithms of rsa and nistp256
> and force host key algorithms of rsa, we should be able to force
> server to use weaker algorithm while client can use any of rsa and
> nistp256.
> To prove that I hacked the AbstractSession with:
>      protected byte[] sendKexInit() throws IOException {
> -        String resolvedAlgorithms = resolveAvailableSignaturesProposal();
> +        //String resolvedAlgorithms = resolveAvailableSignaturesProposal();
> +        //String resolvedAlgorithms = "ssh-rsa";
> +        String resolvedAlgorithms = "ecdsa-sha2-nistp256";
> If I force ssh-rsa I receive ssh-rsa sever key as expected.
> If I force ecdsa-sha2-nistp256 I receive ecdsa-sha2-nistp256 server
> key as expected while can authenticate using client ssh-rsa key, this
> means that server and client are indeed detached.
> Adding an option to specify a list of server host key type like "ssh-rsa" or "ecdsa-sha2-nistp256"
will be nice as once having a pre-approved server keys, we can enforce them easily without
transformation/guessing signature algorithm.

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