Even today when you use hmac-sha2-256 you have to first decrypt the packet and then get the packet length. The reason might have been to encrypt the packet length to thwart traffic analysis. I am not an expert on this and I can only guess why the initial SSH developers did this. The older non-ETM MACs like hmac-md5 first computed the MAC on the unencrypted SSH payload and then encrypted the message. The cryptographic doom principle and the SSH -etm MACs Whether SSH negotiates an ETM or non-ETM MAC has a rather big implication for our traffic analysis. This represents a break from the older SSH which used “MAC then Encrypt”. # New kid on the non-ETM MACs - ETM stands for “Encrypt then MAC”. The comments are added by me ssh -Q cipherģdes-cbc blowfish-cbc cast128-cbc arcfour arcfour128 arcfour256Īes128-cbc aes192-cbc aes256-cbc ctr mode AES - popular Type ssh -Q cipher to get a list of supported ciphers by your client. In the real world, both directions use the same ciphers and MACs even though the SSH protocol itself does not mandate it.Ĭiphers are used to encrypt your payload. This is followed by a key exchange usually Diffie Hellman. The client and the server first exchange packets and agree on the MAC and Cipher algorithms.If you capture packets using a tool like Wireshark, this is what a SSH record would look like.
Each packet is encrypted using a Cipher and authenticated using a MAC. The SSH protocol offers both encryption and message integrity. For login detection, we use the Terminal Capabilties Exchange, there are only a handful of terminal types so the message is predictable. The approach is to use knowledge of the ciphers and MAC used in SSH and calculate the SSH message lengths on the wire. TLDR Use traffic analysis to detect successful login , keystrokes, and Tunnels – reverse and forward. Lets dive a little deeper to see if we can get a accurate answer. You could observe anywhere between 28 and 96 bytes of SSH payload depending on the type of secure channel negotiated. What do you see on the wire when you press a key in a SSH terminal? In this article, we are looking to use passive traffic analysis to detect various SSH events like login, keypress, and presence of SSH tunnels. Secure Shell (SSH) is a ubiquitous protocol used everywhere for logins, file transfers, and to execute remote commands.
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