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Generic Low Overhead Message Exchange (GLOME)

GLOME Login is a challenge-response authentication mechanism. It resembles one-time authorization codes (aka OTPs) but is different from [HOTP] and [TOTP] in the following ways:

  • It is stateless (unlike [HOTP]).
  • It does not depend on time (unlike [TOTP]).
  • It does not require predefined secret sharing (unlike [HOTP] and [TOTP]).

These properties make it a good choice for low dependency environments (e.g., devices with no persistent storage a real-time clock). It can be also useful for managing access to a large fleet of hosts where synchronising state or sharing predefined secrets can be a challenge.

GLOME Login can be easily integrated with existing systems through PAM (libglome) or through the login(1) wrapper (glome-login).

GLOME Login protocol is is built on top of the Generic Low Overhead Message Exchange (GLOME) protocol.

[TOTP]: https://www.rfc-editor.org/rfc/rfc6238 [HOTP]: https://www.rfc-editor.org/rfc/rfc4226

How does it work?

Let's imagine the following scenario:

Alice is a system engineer who got paged to investigate an unresponsive machine that happens to be located far away. She calls Bob, a datacenter technican with physical access to the machine.

Alice is authorized to access the machine but has no connectivity. Bob faces the opposite problem, he can access the machine's serial port but does not have credentials to log in.

Alice is able to use GLOME Login to grant Bob one-time access to the machine. First, Bob connects to the machine over serial port and types root on the login prompt. He is then provided with a challenge that he forwards to Alice. The challenge contains information about the identity of accessed host and the requested action (i.e., root shell access). Alice verifies that the request is legitimate (e.g., the accessed host is indeed the one she's trying to diagnose), and uses the glome CLI to generate an authorization code. She forwards that authorization code to Bob who provides it as a challenge response.

The authorization succeeds and Bob is able to run diagnostic commands and share the results with Alice.

Getting started

Installation on the client host

These steps should be followed on the host you are planning to use to generate authorization codes (e.g., a laptop).

  1. Follow build to build the glome CLI binary.
  2. Generate a key pair using the glome command. Note that if the glome command is not in your $PATH, you might need to provide a full path to the binary.
$ glome genkey | tee glome-private.key | glome pubkey | tee glome-public.key | xxd -c 32 -p
4242424242424242424242424242424242424242424242424242424242424242

The output of that command is the approver public key that will be used to configure the target host.

Installation on the target host

  1. Follow instructions to configure your host to use PAM module (recommended) or glome-login.
  2. Edit the configuration file (by default located at /etc/glome/config) and replace the key value with the approver public key generated in the previous section.
$ cat /etc/glome/config
key=4242424242424242424242424242424242424242424242424242424242424242
key-version=1

Usage

Try to log in to the target host. You should see the prompt with the challenge:

GLOME: v1/AU7U7GiFDG-ITgOh8K_ND9u41S3S-joGp7MAdhIp_rQt/myhost/shell/root/
Password:

Use the glome CLI on the client host to obtain an authorization code:

$ glome --key glome-private.key login
v1/AU7U7GiFDG-ITgOh8K_ND9u41S3S-joGp7MAdhIp_rQt/myhost/shell/root/Tm90aGluZyB0byBzZWUgaGVyZSwgbW92ZSBhbG9uZy4K

Provide the generated authcode as a response to the challenge.

Repository

This repository consists of a number of components of the GLOME ecosystem.

Documentation:

Core libraries:

Binaries:

  • glome Command-line interface for GLOME
  • glome-login Replacement of login(1) implementing GLOME Login protocol

Building

Building the GLOME library requires

  • Compiler conforming to C99 (e.g. gcc, clang)
  • Meson >=0.49.2
  • OpenSSL headers >=1.1.1
  • glib-2.0 (for glome-login as well as tests)
  • libpam (for PAM module)

Alternatively, on systems with Nix, you can simply run nix-shell in the root directory of this repository.

Instructions

GLOME is built using Meson. First, initialize the Meson build directory. You only have to do this once per Meson configuration.

$ meson build

NOTE: You can customize the installation target by passing the --prefix flag.

Build the shared library libglome.so and the command line utility glome inside the build root ./build.

$ ninja -C build

Now run the tests.

$ meson test -C build

Install both the binary and the library into the configured prefix (the default prefix is /usr/local/, which will require admin privileges).

$ meson install -C build

Disclaimer

This is not an officially supported Google product.