A Pre-Authentication Mechanism for SSH

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Table of Contents

1. Introduction

Devices running SSH are frequently exposed on the Internet, either because of operational considerations or through misconfiguration, making them vulnerable to the constant 3-degree background radiation of scanning and probing attacks that pervade the Internet. This document describes a simple pre-authentication mechanism that limits these attacks with minimal changes to SSH implementations and no changes to the SSH protocol itself.¶

2. Background

This section covers the background and threat model for the SSH pre-authentication process and mechanism.¶

3. Description

The pre-authentication mechanism for SSH takes the existing exchange of client and server ID strings and adds a simple challenge/response to them, preventing the exchange of any SSH handshake messages, in other words any actual SSH protocol messages, unless the pre-authentication succeeds. It does this by adding a random challenge in the Comment field of the server's SSH ID, with the client responding with the response in the comment field of its SSH ID. The server challenge in the comment field is denoted with 'C=<challenge>' and the client response with 'R=<response>'. These MUST be the first values in the Comment field, with any further entries that follow separated by either a comma or a space.¶

The challenge is a 64-bit server-generated nonce which is then base64-encoded to create a text string suitable for use in the Comment field. This encoded form, and the base64-encoded response from the client, are sent without any base64 padding characters '=' at the end.¶

The response to the challenge is an HMAC-SHA256 of the challenge, with the MAC value truncated to 64 bits and base64-encoded in the same manner as the server's challenge. The server challenge is MAC'd in base64 form as sent, without decoding back to binary form.¶

Computing the response to the challenge requires a shared secret 'preAuthSecret' between the client and server. This SHOULD NOT be the same as any user password or other authentication value that might be used for authentication but should be created or generated independently and only used for pre-authentication. In the situation where more than a single user or account exists on the device, the preAuthSecret functions in a manner similar to a WiFi password, with the preAuthSecret granting access to the SSH server and subsequent user authentication granting access to whatever sits behind the server.¶

The HMAC key is calculated as:¶

    key = SHA256( string    challenge
                  string    preAuthSecret )

The response is then computed as a truncated HMAC:¶

    rawResponse = HMAC-SHA256( key, challenge )
    response = base64( rawRespone[ 0...7 ]

In other words the response is the base64 encoding (without adding base64 padding) of the first 64 bits of the HMAC value.¶

4. Test Vectors

Test vectors to be added when the format is finalised.¶

5. Contributors/Acknowledgements

6. IANA Considerations

7. Security Considerations

As the introduction points out, using this pre-authentication mechanism for SSH is not intended to be all things to all people but to address a specific problem, stopping scanning and probing attacks of SSH-enabled devices at the gates. Conceptually it works like a WiFi password, granting initial access to the SSH server while subsequent user authentication grants access to whatever sits behind the server. Its primary usage scenario is embedded devices with few users, most commonly only one. Use with public SSH systems with large numbers of users is optionally possible, although unlike embedded devices these are expected to have up-to-date software and proper mitigations in place.¶

The use of a separate preAuthSecret is the lesser of two evils, the other option being to reuse existing authentication information like a password, after due cryptographic processing, for the pre-authentication. This makes the pre-authentication process transparent without requiring the management of additional keying material, since more than two decades of SSH compromise history have taught us that many users are not good at managing such keying material. However with a simple gatekeeper pre-authentication mechanism of the kind described here it appears to be impossible to implement it in a manner that doesn't open it up to an offline dictionary attack by an on-path attacker who, even in the presence of countermeasures like a severely truncated MAC that leads to false positives, can intercept many challenge/response pairs over time and use those to get to true positives.¶

An additional benefit of using a distinct preAuthSecret is that it makes enabling and use of pre-authentication explicit. A downside of using a distinct preAuthSecret is that it requires explicit configuration actions to enable and use pre-authentication.¶

It is recommended that implementations perform rate-limiting on pre-authentication attempts, throttling back responses if too many pre-authentication failures occur in a given time interval. To further confound attackers, servers may in addition opt to continue with an emulated handshake if the pre-authentication fails, eventually failing anyway or dropping the attacker into a tarpit.¶

The server nonce generation process is described explicitly rather than in general terms like "a random string" to avoid implementations using things like the server name as a nonce value, or more generally just hardcoding something into the otherwise static server ID.¶

Following Grigg's Law, "There is only one mode and that is secure", the pre-authentication mechanism hardcodes use of SHA256, the de facto universal standard hash in SSH implementations. Since the security property required of the hash function is preimage resistance rather than collision resistance, and even beyond that the ability to find one specific preimage rather than any valid preimage, almost any hash function would suffice; SHA256 is chosen because of its universal acceptance and use.¶

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