How can I restore ssh on my mac?

Question

I've broken ssh passwordless logins on MacA and MacB on my LAN. (Catalina and Monterey.) I wanted to write a simple, low-paranoia, no-understanding-required recipe for setting up passwordless ssh for the several users on two local Macs. To start from scratch, I deleted ~/.ssh from each user.

I gleaned from the many different articles I found that running ssh-keygen (no args) on MacA and accepting the defaults, and then copying the resulting ~/.ssh to a user on MacB would be sufficient to allow me to ssh in as that user, as well as to then ssh back in to MacA.

(One Linux-oriented source mentioned using ssh-copy-id to copy the public/private keys to MacB. I found this in /usr/bin and tried it, but all it did was create a file in .ssh/ called authenticated_hosts -- it did not copy id_rsa or id_rsa.pub. So I abandoned that.)

As a last-ditch effort I used homebrew to install openSSL, but nothing changed.

At any rate, once I had duplicate ~/.ssh on both users, with the public and private key files in both and the permissions verified (700 on .ssh, 600 on id_rsa, and 644 on id_rsa.pub), I tried running ssh user@macb.local from MacA, but I'm always prompted for a password.

I'd appreciate a hand. Here is the verbose output:

OpenSSH_9.0p1, OpenSSL 1.1.1n  15 Mar 2022
debug1: Reading configuration data /opt/homebrew/etc/ssh/ssh_config
debug1: Connecting to retsina.local [fe80::b79e:86e0:dfcc%en0] port 22.
debug1: Connection established.
debug1: identity file /Users/chap/.ssh/id_rsa type 0
debug1: identity file /Users/chap/.ssh/id_rsa-cert type -1
debug1: identity file /Users/chap/.ssh/id_ecdsa type -1
debug1: identity file /Users/chap/.ssh/id_ecdsa-cert type -1
debug1: identity file /Users/chap/.ssh/id_ecdsa_sk type -1
debug1: identity file /Users/chap/.ssh/id_ecdsa_sk-cert type -1
debug1: identity file /Users/chap/.ssh/id_ed25519 type -1
debug1: identity file /Users/chap/.ssh/id_ed25519-cert type -1
debug1: identity file /Users/chap/.ssh/id_ed25519_sk type -1
debug1: identity file /Users/chap/.ssh/id_ed25519_sk-cert type -1
debug1: identity file /Users/chap/.ssh/id_xmss type -1
debug1: identity file /Users/chap/.ssh/id_xmss-cert type -1
debug1: identity file /Users/chap/.ssh/id_dsa type -1
debug1: identity file /Users/chap/.ssh/id_dsa-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_9.0
debug1: Remote protocol version 2.0, remote software version OpenSSH_8.1
debug1: compat_banner: match: OpenSSH_8.1 pat OpenSSH* compat 0x04000000
debug1: Authenticating to retsina.local:22 as 'chap'
debug1: load_hostkeys: fopen /Users/chap/.ssh/known_hosts2: No such file or directory
debug1: load_hostkeys: fopen /opt/homebrew/etc/ssh/ssh_known_hosts: No such file or directory
debug1: load_hostkeys: fopen /opt/homebrew/etc/ssh/ssh_known_hosts2: No such file or directory
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ssh-ed25519
debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: SSH2_MSG_KEX_ECDH_REPLY received
debug1: Server host key: ssh-ed25519 SHA256:LPhM+eeposmTyzQiRXnh92AUZr/9MIrMgiW3VjLHCQM
debug1: load_hostkeys: fopen /Users/chap/.ssh/known_hosts2: No such file or directory
debug1: load_hostkeys: fopen /opt/homebrew/etc/ssh/ssh_known_hosts: No such file or directory
debug1: load_hostkeys: fopen /opt/homebrew/etc/ssh/ssh_known_hosts2: No such file or directory
debug1: Host 'retsina.local' is known and matches the ED25519 host key.
debug1: Found key in /Users/chap/.ssh/known_hosts:1
debug1: rekey out after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey in after 134217728 blocks
debug1: get_agent_identities: ssh_fetch_identitylist: agent contains no identities
debug1: Will attempt key: /Users/chap/.ssh/id_rsa RSA SHA256:sEwBJnORX9BWQ5itF6CoX36w7jcl6E0Lcnkunvh4ew8
debug1: Will attempt key: /Users/chap/.ssh/id_ecdsa
debug1: Will attempt key: /Users/chap/.ssh/id_ecdsa_sk
debug1: Will attempt key: /Users/chap/.ssh/id_ed25519
debug1: Will attempt key: /Users/chap/.ssh/id_ed25519_sk
debug1: Will attempt key: /Users/chap/.ssh/id_xmss
debug1: Will attempt key: /Users/chap/.ssh/id_dsa
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<ssh-ed25519,ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521>
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Next authentication method: publickey
debug1: Offering public key: /Users/chap/.ssh/id_rsa RSA SHA256: [HIDDEN]
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Trying private key: /Users/chap/.ssh/id_ecdsa
debug1: Trying private key: /Users/chap/.ssh/id_ecdsa_sk
debug1: Trying private key: /Users/chap/.ssh/id_ed25519
debug1: Trying private key: /Users/chap/.ssh/id_ed25519_sk
debug1: Trying private key: /Users/chap/.ssh/id_xmss
debug1: Trying private key: /Users/chap/.ssh/id_dsa
debug1: Next authentication method: keyboard-interactive
(chap@retsina.local) Password:

Answer 1

If I understand what you did, the problem is that the public key file in each of the ~/.ssh directories is named something like id_rsa.pub -- that means that's a key that's just sitting there, not one that's allowed to be used to authenticate to that account. You need to copy (or move) that key into a file named ~/.ssh/authorized_keys or ~/.ssh/authorized_keys2 in order to authorize it for login.

(I think this is what ssh-copy-id did. Did you delete the file it created afterward?)

Try running cat ~/.ssh/id_rsa.pub >>~/.ssh/authorized_keys on each of the computers, then try sshing back and forth and see if it works.

Note: as you pointed out in a comment, it's more common (and probably more secure) to use separate keypairs for each computer. That is, generate a keypair on each of the computers, then use ssh-copy-id to install each one's public key in the other's ~/.ssh/authorized_keys file. This way you could also authorize MacA's key to other computers, without implicitly authorizing MacB's key.