Good day, I'm a Computer Science Honours student doing a thesis in which I want to add a new feature to OpenSSH (specifically, version 7.9). I've read the authentication protocol for SSH (RFC 4252) several times but I'm having trouble figuring out exactly how the SSH client communicates with the SSH server in OpenSSH during public key authentication. According to the PROTOCOL document in the OpenSSH package, there are no deviations from the authentication protocol spelled out in the document I mentioned before. I am hoping there is someone on this mailing list that can enlighten me on some of my questions. The second paragraph in Section 7 (Public Key Authentication Method: "publickey") of RFC 4252 says: "With this method, the possession of a private key serves as authentication. This method works by sending a signature created with a private key of the user. The server MUST check that the key is a valid authenticator for the user, and MUST check that the signature is valid. If both hold, the authentication request MUST be accepted; otherwise, it MUST be rejected. Note that the server MAY require additional authentications after successful authentication." I understand that the SSH authentication protocol is just a guideline that provides a higher level explanation of what should happen during authentication. Is there anyone here who can provide me with a detailed description of how authentication occurs once both the ssh client and the server start communicating? (A link or copy of a document describing this would be fine.) I can find high-level descriptions of the protocol in RFC 4252, and slightly more detailed explanations in postings such as https://security.stackexchange.com/questions/9366/ssh-public-private-key-pair/9389, although there are variations between postings I find on-line, which hinders my understanding of the details. On the other hand, I can obviously read through the code to find out exactly what OpenSSH is doing. However, making the jump from the very high-level descriptions to the code is proving difficult, and I was hoping to get some insight by finding out some more details from sources which are authoritative (hopefully people on this mailing list). For example, the PROTOCOL file says "The server MUST check that the key is a valid authenticator for the user". *Exactly how*, in OpenSSH, does the server check that the key is a valid authenticator? It also says "The server MUST check that the signature is valid..." but it does not say exactly what data is signed. That is, what specific data is hashed (presumably) by which hash function? My main issue in understanding the protocol is that I would like a more in-depth and clear explanation of what exactly is going on behind the scenes in OpenSSH. There are many sources on forums, but they all have varying answers that never say exactly the same thing. This understanding would greatly assist me in my thesis project. Thank you for your time, Vistassja Williams _______________________________________________ openssh-unix-dev mailing list openssh-unix-dev@xxxxxxxxxxx https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev
