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SSH agent for TPM

ssh-tpm-agent is a ssh-agent compatible agent that allows keys to be created by the Trusted Platform Module (TPM) for authentication towards ssh servers.

TPM sealed keys are private keys created inside the Trusted Platform Module (TPM) and sealed in .tpm suffixed files. They are bound to the hardware they where produced on and can't be transferred to other machines.

This allows one to utilize a native client instead of having to side load existing PKCS11 libraries into the ssh-agent and/or ssh client.

Features

  • A working ssh-agent.
  • Keys created on the TPM, sealed outside of it.
  • PIN support.
  • TPM session encryption.

Experimental

The key format and technical details might change between iterations. Consider this agent experimental.

Instead of utilizing the TPM directly, you can use --swtpm or export SSH_TPM_AGENT_SWTPM=1 to create a identity backed by swtpm which will be stored under /var/tmp/ssh-tpm-agent.

Note that swtpm provides no security properties and should only be used for testing.

Installation

The simplest way of installing this plugin is by running the follow go command.

go install github.com/foxboron/ssh-tpm-agent/cmd/...@latest

Alternatively download the pre-built binaries.

Usage

# Allow the user to interact with the TPM infrastructure
$ sudo usermod -aG tss $USER
# Refresh the newly granted group permissions *in the current shell*
# NOTE: Restart logout and log back in for this to be fully applied
$ newgrp tss
# Create key
$ ssh-tpm-keygen
Generating a sealed public/private ecdsa key pair.
Enter file in which to save the key (/home/fox/.ssh/id_ecdsa):
Enter pin (empty for no pin):
Enter same pin again:
Your identification has been saved in /home/fox/.ssh/id_ecdsa.tpm
Your public key has been saved in /home/fox/.ssh/id_ecdsa.pub
The key fingerprint is:
SHA256:NCMJJ2La+q5tGcngQUQvEOJP3gPH8bMP98wJOEMV564
The key's randomart image is the color of television, tuned to a dead channel.

$ cat /home/fox/.ssh/id_ecdsa.pub
ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBOTOsMXyjTc1wiQSKhRiNhKFsHJNLzLk2r4foXPLQYKR0tuXIBMTQuMmc7OiTgNMvIjMrcb9adgGdT3s+GkNi1g=

# Using the socket
$ ssh-tpm-agent -l /var/tmp/tpm.sock

$ export SSH_AUTH_SOCK="/var/tmp/tpm.sock" ssh git@github.com

ssh-config

It is possible to use the public keys created by ssh-tpm-keygen inside ssh configurations.

The below example uses ssh-tpm-agent and also passes the public key to ensure not all identities are leaked from the agent.

Host example.com
    IdentityAgent $SSH_AUTH_SOCK

Host *
    IdentityAgent /var/tmp/tpm.sock
    IdentityFile ~/.ssh/id_ecdsa.pub

License

Licensed under the MIT license. See LICENSE or http://opensource.org/licenses/MIT