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https://issues.apache.org/jira/browse/SSHD-1216?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17429226#comment-17429226
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Thomas Wolf edited comment on SSHD-1216 at 10/15/21, 10:27 AM:
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Yes, it can be done in application code; the library provides all the support
needed. (Note that you should send the EXT_INFO message only if the client had
requested it by sending ext-info-c earlier in its KEX proposal, though.)
However, a server that does rsa-sha2-512 or rsa-sha2-256 SHOULD implement
server-sig-algs. An Apache MINA sshd server by default does SHA-2 RSA
signatures, so it should by default also implement server-sig-algs, and thus it
should be in the library.
was (Author: wolft):
Yes, it can be done in application code; the library provides all the support
needed. (Note that you should send the EXT_INFO message only if the client had
requested it by sending ext-info-c earlier in its KEX proposal, though.)
However, a server that does rsa-sha2-512 or rsa-sha2-256 SHOULD implement
server-sig-algs. An Apache MINA sshd by default does SHA-2 RSA signatures, so
it should by default also implement server-sig-algs, and thus it should be in
the library.
> Implement RFC 8332 server-sig-algs on the server
> ------------------------------------------------
>
> Key: SSHD-1216
> URL: https://issues.apache.org/jira/browse/SSHD-1216
> Project: MINA SSHD
> Issue Type: Improvement
> Reporter: Ben Humphreys
> Priority: Major
>
> In the recently released [OpenSSH
> 8.8|https://www.openssh.com/txt/release-8.8] for RSA keys the public key
> signature algorithm that depends on SHA-1 has been disabled by default:
> {quote}This release disables RSA signatures using the SHA-1 hash algorithm
> 2by default. This change has been made as the SHA-1 hash algorithm is
> cryptographically broken, and it is possible to create chosen-prefix 4hash
> collisions for <USD$50K [1]
> {quote}
> As a result OpenSSH 8.8 clients are unable to authenticate with Mina SSHD
> servers with RSA based keys (it is however possible to reenable ssh-rsa).
> OpenSSH since 7.2 does however support RFC 8332 RSA/SHA-256/512 signatures,
> indeed the release notes go on to say:
> {quote}
> For most users, this change should be invisible and there is no need to
> replace ssh-rsa keys. OpenSSH has supported RFC8332 RSA/SHA-256/512
> signatures since release 7.2 and existing ssh-rsa keys will automatically use
> the stronger algorithm where possible.
> {quote}
> It appears Mina SSHD partly implements support for RFC 8332, indeed the
> client code appears to support it (see SSHD-1141). However the server appears
> to lack full support because it doesn't full implement the"server-sig-algs"
> extension.
> The basic framework for supporting this seems to be present, specifically
> {{AbstractKexFactoryManager.setKexExtensionHandler()}} could perhaps permit
> such a "server-sig-algs" extension.
>
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