Ssh – Is it possible to use one SSH key per account for each account on a given remote server

Alice can give Bob her public key, and Bob can add a line to his own .ssh/authorized_keys file that will allow Alice to start a ssh session (as Bob) on Bob's workstation foo. Using the command= option, Bob can restrict Alice's key to not grant an interactive shell, but a nested ssh connection to baz as Bob, and using a key locally accessible to Bob on bar.

command="ssh -I .ssh/id_rsa bob@baz:22" ssh-rsa AAA...== alice@whatever

Optionally, you can include further constraint options (no-port-forwarding, from=, etc. - see sshd man page section on AUTHORIZED_KEYS FILE FORMAT).

When Alice runs ssh bob@bar and authenticates with her private key, she will be connected through bar to a ssh session on baz, without having any control over the intermediate session on Bob's workstation.

Note that from the perspective of baz (logging, security audit), there is no distinction between this tunneled connection initiated by Alice, and a "normal" connection by Bob from his own workstation. This may be what you asked for, but not what you want.

For making the protected key available to Alice for her session, you can explicitly set the environment variable SSH_AUTH_SOCK to be the path that Bob uses for his session (change the authorized_keys entry to command="SSH_AUTH_SOCK=/path/to/bobs/agent_socket ssh bob@baz:22" ssh-rsa AAA...) To avoid having to update the path when it changes (logoff/login typically), Bob can run a dedicated agent on Alice's behalf with an explicit path specified (ssh-agent -a ~/.ssh/agent_for_alice and add only the specific key with SSH_AUTH_SOCK=~/.ssh/agent_for_alice ssh-add ~/.ssh/id_rsa and entering the passphrase.