How can I find a list of MACs, Ciphers, and KexAlgorithms that my openssh client supports?

Question

Is there a way to make ssh output what MACs, Ciphers, and KexAlgorithms that it supports?

I'd like to find out dynamically instead of having to look at the source.

Answer 1

Relevant OpenSSH man page: https://man.openbsd.org/ssh#Q

• Ciphers: ssh -Q cipher
• MACs: ssh -Q mac
• KexAlgorithms: ssh -Q kex
• PubkeyAcceptedKeyTypes: ssh -Q key

Answer 2

You can also remotely probe a ssh server for its supported ciphers with recent nmap versions:

nmap --script ssh2-enum-algos -sV -p <port> <host>

And there is an online service called sshcheck.com as well (and a pretty large number of similar scanner projects as I just found out).

Answer 3

Just a quick tip that if you want to compare 2 servers you can use @eckes method like this:

$ sdiff -bW <(nmap --script ssh2-enum-algos -sV -p 22 192.168.1.107) <(nmap --script ssh2-enum-algos -sV -p 22 192.168.1.10)

Starting Nmap 6.47 ( http://nmap.org ) at 2018-01-22 22:35 ES   Starting Nmap 6.47 ( http://nmap.org ) at 2018-01-22 22:35 ES
Nmap scan report for skinner.bubba.net (192.168.1.107)        | Nmap scan report for mulder.bubba.net (192.168.1.10)
Host is up (0.0037s latency).                                 | Host is up (0.0031s latency).
PORT   STATE SERVICE VERSION                                    PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 4.3 (protocol 2.0)               | 22/tcp open  ssh     OpenSSH 5.3 (protocol 2.0)
| ssh2-enum-algos:                                              | ssh2-enum-algos:
|   kex_algorithms: (3)                                       | |   kex_algorithms: (4)
                                                              > |       diffie-hellman-group-exchange-sha256
|       diffie-hellman-group-exchange-sha1                      |       diffie-hellman-group-exchange-sha1
|       diffie-hellman-group14-sha1                             |       diffie-hellman-group14-sha1
|       diffie-hellman-group1-sha1                              |       diffie-hellman-group1-sha1
|   server_host_key_algorithms: (2)                             |   server_host_key_algorithms: (2)
|       ssh-rsa                                                 |       ssh-rsa
|       ssh-dss                                                 |       ssh-dss
|   encryption_algorithms: (13)                                 |   encryption_algorithms: (13)
|       aes128-ctr                                              |       aes128-ctr
|       aes192-ctr                                              |       aes192-ctr
|       aes256-ctr                                              |       aes256-ctr
|       arcfour256                                              |       arcfour256
|       arcfour128                                              |       arcfour128
|       aes128-cbc                                              |       aes128-cbc
|       3des-cbc                                                |       3des-cbc
|       blowfish-cbc                                            |       blowfish-cbc
|       cast128-cbc                                             |       cast128-cbc
|       aes192-cbc                                              |       aes192-cbc
|       aes256-cbc                                              |       aes256-cbc
|       arcfour                                                 |       arcfour
|       [e-mail]                             |       [e-mail]
|   mac_algorithms: (6)                                       | |   mac_algorithms: (9)
|       hmac-md5                                                |       hmac-md5
|       hmac-sha1                                               |       hmac-sha1
                                                              > |       [e-mail]
                                                              > |       hmac-sha2-256
                                                              > |       hmac-sha2-512
|       hmac-ripemd160                                          |       hmac-ripemd160
|       [e-mail]                              |       [e-mail]
|       hmac-sha1-96                                            |       hmac-sha1-96
|       hmac-md5-96                                             |       hmac-md5-96
|   compression_algorithms: (2)                                 |   compression_algorithms: (2)
|       none                                                    |       none
|_      [e-mail]                                        |_      [e-mail]

Service detection performed. Please report any incorrect resu   Service detection performed. Please report any incorrect resu
Nmap done: 1 IP address (1 host up) scanned in 0.19 seconds   | Nmap done: 1 IP address (1 host up) scanned in 0.22 seconds

In the above I'm showing a side-by-side diff of a CentOS 5.x and 6.x server.

$ ssh root@192.168.1.107 cat /etc/redhat-release
CentOS release 5.11 (Final)
$ ssh root@192.168.1.10 cat /etc/redhat-release
CentOS release 6.8 (Final)

The output shows you that you have 4 additional lines in the CentOS 6.x server vs. 5.x.

Reading the output

There's 1 additional kex_algorithm:

• diffie-hellman-group-exchange-sha256

3 additional mac_algorithms:

• [e-mail]
• hmac-sha2-256
• hmac-sha2-512

Answer 4

Some old versions of OpenSSH do not support the -Q option, but this works for any ssh and it has the benefit of showing both client and server options, without the need for any third party tools like nmap:

ssh -vv username@servername

Scan the output to see what ciphers, KEX algos, and MACs are supported...

• by your client: "local client KEXINIT proposal"
• by the server: "peer server KEXINIT proposal"

...
debug2: local client KEXINIT proposal
debug2: KEX algorithms: curve25519-sha256,[e-mail],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,ext-info-c
debug2: host key algorithms: [e-mail],[e-mail],[e-mail],ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,[e-mail],[e-mail],[e-mail],[e-mail],ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
debug2: ciphers ctos: [e-mail],aes128-ctr,aes192-ctr,aes256-ctr,[e-mail],[e-mail]
debug2: ciphers stoc: [e-mail],aes128-ctr,aes192-ctr,aes256-ctr,[e-mail],[e-mail]
debug2: MACs ctos: [e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],hmac-sha2-256,hmac-sha2-512,hmac-sha1
debug2: MACs stoc: [e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],hmac-sha2-256,hmac-sha2-512,hmac-sha1
debug2: compression ctos: none,[e-mail],zlib
debug2: compression stoc: none,[e-mail],zlib
...
debug2: peer server KEXINIT proposal
debug2: KEX algorithms: curve25519-sha256,[e-mail],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1
debug2: host key algorithms: rsa-sha2-512,rsa-sha2-256,ssh-rsa,ecdsa-sha2-nistp256,ssh-ed25519
debug2: ciphers ctos: [e-mail],aes128-ctr,aes192-ctr,aes256-ctr,[e-mail],[e-mail]
debug2: ciphers stoc: [e-mail],aes128-ctr,aes192-ctr,aes256-ctr,[e-mail],[e-mail]
debug2: MACs ctos: [e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],hmac-sha2-256,hmac-sha2-512,hmac-sha1
debug2: MACs stoc: [e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],hmac-sha2-256,hmac-sha2-512,hmac-sha1
debug2: compression ctos: none,[e-mail]
debug2: compression stoc: none,[e-mail]
...