Nextpertise a journal of interesting technical ideas . . .

SSH Crypto

by Brent Stewart on Tuesday, Dec 29, 2020

Cleaning up Crypto

A previous article - SSH Admin - went through understanding who was logging into a Linux server using SSH or SFTP. To continue that thought, let’s suppose that we are required to make sure that only cypher suites recommended in the CIS benchmarks are in use on a server. Before we disable old options, we need to check and make sure that no one is using them!

Understanding local crypto

From a client, we can see which cipher elements are supported. Each of these commands outputs a range of protocols. When connecting to a server, the client transmits protocols that it supports and the server reciprocates. They then agree to use the first option from the client’s list that is supported on the server (or the connection fails). The table below lists commands used to see the protocols supported on a client. The examples were chosen because they were well known and establish context, and not as a recommendation.

Table: SSH options
Element Command Example options
Cipher ssh -Q cipher 3des-cbc, aes256-cbc
MAC ssh -Q mac hmac-md5, hmac-sh2-256
Key ssh -Q key ssh-rsa, ecdsa-sha2-nistp256
Kex ssh -Q kex diffie-hellman-group1-sha1, curve25519-sha256

Setting up an SSH connection goes through some basic phases:

Understanding remote clients crypto

It’s surprising that there isn’t a command to show which cipher suites are in use by particular clients. To build a tool, I went into /etc/ssh/sshd_config and set the logging level to grab everything.

# Logging
SyslogFacility AUTH
LogLevel DEBUG3

This can then be reviewed using journalctl -u ssh to display entries related to the sshd unit. I noticed that the relevant lines were at DEBUG1 level and that each sequence completed with the “password accepted” line. Based on this pattern, I wrote a utility in Python to create a report.

Dec 28 15:56:44 pop-os sshd[701591]: debug1: kex: algorithm: curve25519-sha256 [preauth]
Dec 28 15:56:44 pop-os sshd[701591]: debug1: kex: host key algorithm: ecdsa-sha2-nistp256 [preauth]
Dec 28 15:56:44 pop-os sshd[701591]: debug1: kex: client->server cipher: MAC: compression: none [preauth]
Dec 28 15:56:44 pop-os sshd[701591]: debug1: kex: server->client cipher: MAC: compression: none [preauth]
Dec 28 15:56:46 pop-os sshd[701591]: debug1: PAM: password authentication accepted for pop

ssh-crypto is a Python3 program to read ssh debugging and identify who has logged in and what settings were used. It expects a file name, which is a text file that contains ssh logging output. First make sure that sshd is logging at least at DEBUG1. Restart the SSH service for the new logging setting to take effect.

sudo service sshd restart

Keep in mind that the crypto fields won’t be populated for logins before the loggin change takes effect. To create the text file for analysis, export from journalctl.

journalctl -u ssh > ~/ssh.txt

Again, __ssh-crypto assumes that the system has Python3, uses Systemd, has debugging setup.


 pop  pop-os  ~  $  ~/git/ssh-crypto/ ~/ssh.txt
| # |       User        |       IP       |     Algorithm      |        Host        |            Cipher            |
|  0|pop                |    |undefined           |undefined           |undefined                     |
|  1|pop                |   |undefined           |undefined           |undefined                     |
|  2|pop                |   |undefined           |undefined           |undefined                     |
|  3|pop                |   |undefined           |undefined           |undefined                     |
|  4|pop                |   |undefined           |undefined           |undefined                     |
|  5|pop                |   |curve25519-sha256   |ecdsa-sha2-nistp256 | |
|  6|pop                |   |curve25519-sha256   |ecdsa-sha2-nistp256 | |

Removing Weak Ciphers

Per the CIS Ubuntu 20.04 Standard (5.2.12), FIPS compliant ciphers include aes256-ctr, aes192-ctr, aes128-ctr. FIPS compliant MACs include hmac-sha2-256 and 512. FIPS allows a pretty broad range of key exchange protocols, including ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521, diffie-hellman-group-exchange-sha256, diffie-hellman-group16-sha512, diffie-hellman-group18-sha512, and diffie-hellman-group14-sha256. To limit the server to only accept these options, edit /etc/ssh/sshd_config. Here are the ones I’ve chosen to support.

KexAlgorithms curve25519-sha256,,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256

Using ssh-crypto will allow review of recent client connections and unused ciphers can be weeded out. After communicating the change to users, specific recalcitrant users can be identified for follow-up with the utility before ultimately removing the old protocols.

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