Understanding SSH Key Pairs

In every SSH/SFTP connection there are four keys (or two key-pairs) involved. This article explains a difference between them and what keys an SFTP client user needs to care about.

The SSH employs a public key cryptography. A public-key cryptography, also known as asymmetric cryptography, is a class of cryptographic algorithms which requires two separate keys, one of which is secret (or private) and one of which is public.1 Together they are known as a key-pair. In SSH, the public key cryptography is used in both directions (client to server and server to client), so two key pairs are used. One key pair is known as a host key, the other as a user key.

User Private Key
User Public Key
Host Private Key
Host Public Key

User Private Key

A user private key is key that is kept secret by the SSH user on his client machine. The user must never reveal the private key to anyone, including the server (server administrator) not to compromise his/hers identity.

To protect the private key, it should be generated locally on a user’s machine (e.g. using PuTTYgen) and stored encrypted by a passphrase. The passphrase should be long enough (that’s why it’s called passphrase, not password) to withstand a brute-force attack for a reasonably long time, in case an attacker obtains the private key file.

Different file formats are used to store private keys. WinSCP supports PuTTY format, .ppk.

Note that there’s always a user key-pair involved, even when password authentication method is used, when no key-pair seemingly exists. In this case an ad-hoc generated key-pair is used.

User Public Key

A user public key is a counterpart to user private key. They are generated at the same time. The user public key can be safely revealed to anyone, without compromising user identity. To authorize the user on a server, the user public key is registered on the server. In the most widespread SSH server implementation, the OpenSSH, file ~/.ssh/authorized_keys is used for that.

Host Private Key

A host private key is generated when the SSH server is set up. It is safely stored in a location that should be accessible by server administrator only. The user connecting to the SSH server does not need to care about host private key in general.

Host Public Key

A host public key is a counterpart to user private key. They are generated at the same time. The host public key can be safely revealed to anyone, without compromising host identity. To authorize the host/server to the user/client, the user should be provided with host public key in advance. The client application typically prompts the user with host public key on the first connection to allow the user to verify the key. The host public key is then saved and verified automatically on further connections. The client application should warn the user, if the host key changes.