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What exactly is an SSH key and how is it used in SSH client and server products ?

asked Feb 11 '14 at 19:24

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Roman ♦♦
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Secure Shell (SSH) keys are based on the principle of public key cryptography.

In a nutshell:

A pair of encryption keys are generated which are mathematically linked to each other: a private key and a public key. The public key is used to encrypt a message whereas the private key is used to decrypt it. This provides a very nice property which allows anyone having access to the public key to encrypt a message that can only be decrypted by someone with access to the private key.

The inverse of this property is used for creating digital signatures in which a sender uses the private key to digitally sign a message and a receiver uses the corresponding public key (which has been verified via other channels to belong to the sender) to verify the signature. The receiver can then be confident that the message was indeed from someone with access to the private key. This forms the basis of public key authentication.

The SSH protocol uses keys for authentication in two ways:

  • Authenticating users

This is where the SSH server verifies the identity of the user that is connecting. SSH offers various methods of authenticating users. Password authentication is the most common one but public key authentication is another. When using public key authentication the SSH server checks if the user trying to authenticate to an account that has an authorized public key is in possession of the corresponding private key. Keys used for this purpose are referred to as SSH user keys.

  • Authenticating servers

This is where the SSH client verifies the identity of the SSH server. Allows users connecting to an SSH server to verify that this is in fact the same server they connected to the last time or that this is indeed the server that it claims to be (basically to prevent man in the middle attacks). Keys used for this purpose are referred to as SSH host keys.

When referring to SSH keys the speaker could be referring to either of these (user keys or host keys). Most often than not (unless you’re a system administrator) when someone talks about SSH keys they’re generally referring to user keys.

Note on security of keys

While the public key is meant to be shared (in the case of SSH keys copied to the SSH server under a particular location) the private key is, as it’s name implies, meant to be kept private as anyone having access to the private key would be able to decrypt and sign messages on the owner’s behalf. Some general guidelines for protecting a private key are ensuring secure file system permissions (when the key is stored on disk) and generally not copying it from the system where it was generated. It’s also possible (and highly recommended) to encrypt SSH keys with a passphrase. Doing so will prevent someone who managed to get a copy of the private key of using it without having to also know the passphrase.

One additional note regarding SSH keys is that using SSH keys in user authentication actually provides better protection (than for example password authentication) against man-in-the-middle attacks in the case where the attacker has compromised the SSH server. This has to do with the way session identifiers are calculated when using public key authentication.

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answered Feb 11 '14 at 19:29

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Roman ♦♦
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edited Feb 12 '14 at 09:16

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Asked: Feb 11 '14 at 19:24

Seen: 3,530 times

Last updated: Feb 12 '14 at 09:16

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