2

I generated the keypair in Computer 1.

And move the public key to the Computer2(server) and make it in authorized_keys.

And move the private key to Computer3(client) and use ssh-add to add it.

Why I can directly login to server without offering a public key? What's the real workflow of ssh key authorization?

2

The client (Computer 3) does have access to the public key

You are perhaps confusing a private key file with a private key.

When you generate a public/private key pair, most implementations will create a private key file which contains BOTH the private and public key. Many implementations also write public key to a file separately for convenience.

According RFC 4252 Section 7 the public key is supplied by the client during authentication. Therefore your client MUST have it available.

With openssh's ssh-keygen you can extract the public key from your private key file:

ssh-keygen -y -f ~/.ssh/id_rsa

The authentication mechanism

https://tools.ietf.org/html/rfc4252#section-7

Before the client tries to login, it may first check to see what would be acceptable. This check can include sending public keys matching its available private keys and thus allows the server to indicate which public/private key to use.

... the signing operation involves some expensive computation. To avoid unnecessary processing and user interaction, the following message is provided for querying whether authentication using the "publickey" method would be acceptable.

  byte      SSH_MSG_USERAUTH_REQUEST
  string    user name in ISO-10646 UTF-8 encoding [RFC3629]
  string    service name in US-ASCII
  string    "publickey"
  boolean   FALSE
  string    public key algorithm name
  string    public key blob

Then it attempts to login

To perform actual authentication, the client MAY then send a
signature generated using the private key. The client MAY send the
signature directly without first verifying whether the key is
acceptable. The signature is sent using the following packet:

  byte      SSH_MSG_USERAUTH_REQUEST
  string    user name
  string    service name
  string    "publickey"
  boolean   TRUE
  string    public key algorithm name
  string    public key to be used for authentication
  string    signature

Notice that this includes both the public key and a signature generated with the private key. The public key is helpful to the server SSH where it has many "authorized keys"; the server doesn't have to test the signature against each one.

Unlike some similar algorithms, SSH doesn't use a challenge-response. That is it doesn't use a four step (1 client initiate, 2 server challenge, 3 client sign, 4 server verifies) it performs a two step:

  1. Client signs the session identifier (Ie: a hash produced by the earlier DH Key Exchange)
  2. The server then verifies that:
    • The the specified public key is acceptable (in the user's authorized keys)
    • Decrypting the signature with the specified public key, produces the session identifier

Why are some people confused about this?

The technique of public / private key authentication doesn't need the client to hold the public key. The client just needs to write a signature with the private key. The server just needs to check the signature with the matching public key.

However SSH allows a client to have many private keys and the server to have many authorized keys for a user. If a client had 10 keys and the server accepted 10 keys but only one pair of them matched, the client would have to send 10 signatures and the server would have to check each against 10 keys (100 checks in all). This is computationally expensive. SSH instead can handle the same situation with just a single signature check.

2

Your public key is only used to verify the private key is valid.

When you ssh from the local machine and you point to the private key for authentication, the remote machine creates a challenge. Your local machine then sends the challenge back using your private key to encrypt it. The server will then decrypt the message using your public key and as long as the challenge is valid it will allow connection.
You should never share your private key

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  • But in rfc4252 there is no random data sent to client and client should sign a signature with specific public key. – Anon Apr 27 at 18:47
  • "An authentication request MAY result in a further exchange of messages." – Jesse_b Apr 27 at 18:50
  • "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." – Jesse_b Apr 27 at 18:51
  • Where does it say the client should sign with a public key? – Jesse_b Apr 27 at 18:53
  • "This method works by having the client send a signature created with the private key of the client host, which the server checks with that host's public key." – Jesse_b Apr 27 at 18:54

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