Ciphers, MACs or KeX algorithms differ from "sshd -T" to what is provided by current crypto policy level

Environment

• Red Hat Enterprise Server (RHEL) 8
• openssh-server

Issue

• While comparing the output of sshd -T with the Ciphers, MACs or Key Exchange Algorithms from the current crypto policy configured, some inconsistencies can be noted:

        # sshd -T | grep -i ciphers
        ciphers chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com
        
        # cat /etc/crypto-policies/back-ends/opensshserver.config | cut -f 2 -d \' | cut -f 1 -d " "
        -oCiphers=aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes256-ctr,aes256-cbc,aes128-gcm@openssh.com,aes128-ctr,aes128-cbc
  
        # update-crypto-policies --show
        DEFAULT
  

• aes192-ctr should not be available for the DEFAULT level.

Resolution

• Manually load crypto policy file as extra argument for sshd -T, placing the contents of CRYPTO_POLICY='' at the end of the command line:

        # sshd -T `sed -e s/CRYPTO_POLICY\=//g -e s/\'//g  /etc/crypto-policies/back-ends/opensshserver.config`
  

• Loading /etc/crypto-policies/back-ends/opensshserver.config is enough to provide a visually accurate output that matches the currently configured crypto policy. Examples:

        # update-crypto-policies --show
        DEFAULT
  
        # sshd -T `sed -e s/CRYPTO_POLICY\=//g -e s/\'//g  /etc/crypto-policies/back-ends/opensshserver.config`| grep cipher
        ciphers aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes256-ctr,aes256-cbc,aes128-gcm@openssh.com,aes128-ctr,aes128-cbc
  
        # update-crypto-policies --set FUTURE
        Setting system policy to FUTURE
        Note: System-wide crypto policies are applied on application start-up.
        It is recommended to restart the system for the change of policies
        to fully take place.
  
        # sshd -T `sed -e s/CRYPTO_POLICY\=//g -e s/\'//g  /etc/crypto-policies/back-ends/opensshserver.config`| grep cipher
        ciphers aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes256-ctr
  

Root Cause

This difference between the output provided by sshd -T and crypto policies is expected, because the upstream openssh project is not developed to support Red Hat provided Strong crypto defaults in RHEL8. Crypto policies are applied to sshd.service unit as an EnvironmentFile during it's start.

        # systemctl cat sshd.service
        # /usr/lib/systemd/system/sshd.service
        [Unit]
        Description=OpenSSH server daemon
        Documentation=man:sshd(8) man:sshd_config(5)
        After=network.target sshd-keygen.target
        Wants=sshd-keygen.target
        
        [Service]
        Type=notify
        EnvironmentFile=-/etc/crypto-policies/back-ends/opensshserver.config     <<<---HERE
        EnvironmentFile=-/etc/sysconfig/sshd
        ExecStart=/usr/sbin/sshd -D $OPTIONS $CRYPTO_POLICY
        ExecReload=/bin/kill -HUP $MAINPID
        KillMode=process
        Restart=on-failure
        RestartSec=42s
        
        [Install]
        WantedBy=multi-user.target

/etc/crypto-policies/back-ends/opensshserver.config is a symbolic link to the current configured crypto policy level, and will be loaded directly by sshd service.

        # ls -l /etc/crypto-policies/back-ends/opensshserver.config
        lrwxrwxrwx. 1 root root 52 Feb  4 13:57 /etc/crypto-policies/back-ends/opensshserver.config -> /usr/share/crypto-policies/DEFAULT/opensshserver.txt

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