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Security/Guidelines/OpenSSH: Difference between revisions
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UsePrivilegeSeparation sandbox | UsePrivilegeSeparation sandbox | ||
</source> | </source> | ||
File: <code>/etc/ssh/moduli</code> | |||
All Diffie-Hellman moduli in use should be at least 3072-bit-long (they are used for <code>diffie-hellman-group-exchange-sha256</code>) as per our [[Security/Guidelines/Key_Management]] recommendations. See also <code>man moduli</code>. | |||
To deactivate short modules in two commands: <code>awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.tmp && mv /etc/ssh/moduli.tmp /etc/ssh/moduli</code> | |||
=== '''Intermediate''' (OpenSSH 5.3) === | === '''Intermediate''' (OpenSSH 5.3) === | ||
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PermitRootLogin No | PermitRootLogin No | ||
</source> | </source> | ||
File: <code>/etc/ssh/moduli</code> | |||
All Diffie-Hellman moduli in use should be at least 2048-bit-long. From the structure of <code>moduli</code> files, this means the fifth field of all lines in this file should be greater than or equal to 2047. | |||
To deactivate weak moduli in two commands: <code>awk '{if ($5 < 2047) {print ("#", $0)} else print $0}' /etc/ssh/moduli > /etc/ssh/moduli.tmp; mv /etc/ssh/moduli.tmp /etc/ssh/moduli</code> | |||
=== '''Multi-Factor Authentication''' (OpenSSH 6.3+) === | === '''Multi-Factor Authentication''' (OpenSSH 6.3+) === | ||
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* NIST curves (<code>ecdh-sha2-nistp512,ecdh-sha2-nistp384,ecdh-sha2-nistp256</code>) are listed for compatibility, but the use of <code>curve25519</code> is [https://safecurves.cr.yp.to/ generally preferred]. | * NIST curves (<code>ecdh-sha2-nistp512,ecdh-sha2-nistp384,ecdh-sha2-nistp256</code>) are listed for compatibility, but the use of <code>curve25519</code> is [https://safecurves.cr.yp.to/ generally preferred]. | ||
* SSH protocol 2 supports [https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange DH] and [https://en.wikipedia.org/wiki/Elliptic_curve_Diffie%E2%80%93Hellman ECDH] key-exchange as well as [https://en.wikipedia.org/wiki/Forward_secrecy forward secrecy]. | * SSH protocol 2 supports [https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange DH] and [https://en.wikipedia.org/wiki/Elliptic_curve_Diffie%E2%80%93Hellman ECDH] key-exchange as well as [https://en.wikipedia.org/wiki/Forward_secrecy forward secrecy]. Regarding group sizes, please refer to [[Security/Guidelines/Key_Management]]. | ||
The various algorithms supported by a particular OpenSSH version can be listed with the following commands: | The various algorithms supported by a particular OpenSSH version can be listed with the following commands: | ||
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Existing keys are generally stored in <code>~/.ssh/</code> (Linux/OSX) or <code>%APPDATA%</code> (Windows). | Existing keys are generally stored in <code>~/.ssh/</code> (Linux/OSX) or <code>%APPDATA%</code> (Windows). | ||
Look for <code>id_{rsa,ed25519,ecdsa,dsa}</code>, | Look for <code>id_{rsa,ed25519,ecdsa,dsa}, identity, IdentityFile, *.pem</code>, and other <code>identity</code> files. | ||
==== Display SSH keys information ==== | ==== Display SSH keys information ==== | ||
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* OpenSSH server keys (<code>/etc/ssh/ssh_host_*key</code>) | * OpenSSH server keys (<code>/etc/ssh/ssh_host_*key</code>) | ||
* Client keys (<code>~/.ssh/id_{rsa,dsa,ecdsa,ed25519}</code> and <code>~/.ssh/identity</code>). | * Client keys (<code>~/.ssh/id_{rsa,dsa,ecdsa,ed25519}</code> and <code>~/.ssh/identity</code>). | ||
== Client key size and login latency == | == Client key size and login latency == | ||
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* [http://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/src/usr.bin/ssh/PROTOCOL.certkeys?rev=1.9&content-type=text/plain PROTOCOL.certkeys] | * [http://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/src/usr.bin/ssh/PROTOCOL.certkeys?rev=1.9&content-type=text/plain PROTOCOL.certkeys] | ||
* [https://wiki.gnupg.org/rfc4880bis rfc44880bis from GnuPG] | * [https://wiki.gnupg.org/rfc4880bis rfc44880bis from GnuPG] | ||
* [https://weakdh.org/ Weak Diffie-Hellman and the Logjam Attack] | |||
* [https://jbeekman.nl/blog/2015/05/ssh-logjam/ On OpenSSH and Logjam, by Jethro Beekman] | |||