edjx/cashier

A self-service CA for OpenSSH

Cashier

• Cashier
  • How it works
• Installing
  • Docker
• Requirements
  • Server
  • Client
• Configuration
  • server
    • database
  • auth
    • Provider-specific options
  • ssh
  • aws
  • vault
• Usage
  • Using cashier client
  • Configuring SSH
  • Revoking certificates
• Future Work
• Contributing

Cashier is a SSH Certificate Authority (CA).

OpenSSH supports authentication using SSH certificates. Certificates contain a public key, identity information and are signed with a standard SSH key.

Unlike ssh keys, certificates can contain additional information:

• Which user(s) may use the certificate
• When the certificate is valid from
• When the certificate expires
• Permissions

Other benefits of certificates:

• Unlike keys certificates don't need to be distributed to every machine - the sshd just needs to trust the key that signed the certificate.
• This also works for host keys - machines can get new (signed) host certs which clients can authenticate. No more blindly typing "yes".
• Certificates can be revoked.

See also the CERTIFICATES section of ssh-keygen(1)

How it works

The user wishes to ssh to a production machine.

They run a command which opens the CA site (e.g. https://sshca.exampleorg.com) in a browser and they login.

The CA displays a token which the user copies.

The user provides the token to the client. The client generates a new ssh key-pair.

The client sends the ssh public key to the CA along with the token.

The CA verifies the token and signs the public key with the signing key and returns the signed certificate.

The client receives the certificate and loads it and the private key into the ssh agent.

The user can now ssh to the production machine, and continue to ssh to any machine that trusts the CA signing key until the certificate is revoked or expires or is removed from the agent.

Installing

Stable versions can be obtained from the release page. Releases contain both static and dynamically linked executables. Statically linked executables do not have sqlite support.

Note that installing using standard Go tools is possible, but the master branch should be considered unstable.

The server requires a configuration file (sample config).

See the configuration section for more detail.

Docker

A docker image is available. Example usage:

docker run -it --rm -p 10000:10000 --name cashier -v ${PWD}:/cashier nsheridan/cashier

Requirements

Server

Go 1.10 or 1.11, though it may work with earlier versions.

Client

• Go 1.10 or 1.11 or later, though it may work with earlier versions.
• OpenSSH 5.6 or newer.
• A working SSH agent (note that the GPG agent does not handle certificates)

Note: Cashier has only been tested on macOS and Linux.

Configuration

Configuration is divided into different sections: server, auth, ssh, and aws.

A note on files:

For any option that takes a file path as a parameter (e.g. SSH signing key, TLS key, TLS cert), the path can be one of:

• A relative or absolute filesystem path e.g. /data/ssh_signing_key, tls/server.key.
• An AWS S3 bucket + object path starting with /s3/ e.g. /s3/my-bucket/ssh_signing_key. You should add an aws config as needed.
• A Google GCS bucket + object path starting with /gcs/ e.g. /gcs/my-bucket/ssh_signing_key.
• A Vault path + key starting with /vault/ e.g. /vault/secret/cashier/ssh_signing_key. You should add a vault config as needed.

Exception to this: the http_logfile option ONLY writes to local files.

server

• use_tls : boolean. If this is set then either tls_key and tls_cert are required, or letsencrypt_servername is required.
• tls_key : string. Path to the TLS key. See the note on files above.
• tls_cert : string. Path to the TLS cert. See the note on files above.
• letsencrypt_servername: string. If set will request a certificate from LetsEncrypt. This should match the expected FQDN of the server.
• letsencrypt_cachedir: string. Directory to cache the LetsEncrypt certificate. See the note on files above.
• address : string. IP address to listen on. If unset the server listens on all addresses.
• port : int. Port to listen on.
• user : string. User to which the server drops privileges to. Note Dropping privileges might not work as expected as some threads may retain their privileges due to the limitations of the Go runtime.
• cookie_secret: string. Authentication key for the session cookie. This can be a secret stored in a vault using the form /vault/path/key e.g. /vault/secret/cashier/cookie_secret.
• csrf_secret: string. Authentication key for CSRF protection. This can be a secret stored in a vault using the form /vault/path/key e.g. /vault/secret/cashier/csrf_secret.
• http_logfile: string. Path to the HTTP request log. Logs are written in the Common Log Format. The only valid destination for logs is a local file path.
• require_reason: bool. Require the client to provide a reason when requesting a certificate. Defaults to false.
• database: See below.

database

The database is used to record issued certificates for audit and revocation purposes.

• type : string. One of mysql, sqlite or mem.
• address : string. (mysql only) Hostname and optional port of the database server.
• username : string. Database username.
• password : string. Database password. This can be a secret stored in a vault using the form /vault/path/key e.g. /vault/secret/cashier/mysql_password.
• filename : string. (sqlite only). Path to sqlite database.
• dbname: string (mysql only). Name of database to use.

Examples:

server {
  database {
    type = "mysql"
    address = "my-db-host.corp"
    username = "user"
    password = "passwd"
    dbname = "cashier_production"
  }

  database {
    type = "mem"
  }

  database {
    type = "sqlite"
    filename = "/data/cashier.db"
  }
}

Cashierd will not create the database for you - you need to create this. On startup cashierd will execute any schema changes. Obviously you should setup a role user for running in prodution.

auth

• provider : string. Name of the oauth provider. Valid providers are currently "google", "github" and "gitlab".
• oauth_client_id : string. Oauth Client ID. This can be a secret stored in a vault using the form /vault/path/key e.g. /vault/secret/cashier/oauth_client_id.
• oauth_client_secret : string. Oauth secret. This can be a secret stored in a vault using the form /vault/path/key e.g. /vault/secret/cashier/oauth_client_secret.
• oauth_callback_url : string. URL that the Oauth provider will redirect to after user authorisation. The path is hardcoded to "/auth/callback" in the source.
• provider_opts : object. Additional options for the provider.
• users_whitelist : array of strings. Optional list of whitelisted usernames. If missing, all users of your current domain/organization are allowed to authenticate against cashierd. For Google auth a user is an email address. For GitHub auth a user is a GitHub username.

Provider-specific options

Oauth providers can support provider-specific options - e.g. to ensure organization membership. Options are set in the provider_opts hash.

Example:

auth {
  provider = "google"
  provider_opts {
    domain = "example.com"
  }
}

Supported options:

Provider	Option	Notes
Github	organization	If this is unset then you must whitelist individual users using users_whitelist. The oauth client and secrets should be issued by the specified organization.
Gitlab	allusers	Allow all valid users to get signed keys. Only allowed if siteurl set.
Gitlab	group	If allusers and this are unset then you must whitelist individual users using users_whitelist. Otherwise the user must be a member of this group.
Gitlab	siteurl	Optional. The url of the Gitlab site. Default: https://gitlab.com/
Google	domain	If this is unset then you must whitelist individual email addresses using users_whitelist.
Microsoft	groups	Comma separated list of valid groups.
Microsoft	tenant	The domain name of the Office 365 account.

ssh

• signing_key: string. Path to the certificate signing ssh private key. Use ssh-keygen to create the key and store it somewhere safe. See also the note on files above.
• additional_principals: array of string. By default certificates will have one principal set - the username portion of the requester's email address. If additional_principals is set, these will be added to the certificate e.g. if your production machines use shared user accounts.
• max_age: string. If set the server will not issue certificates with an expiration value longer than this, regardless of what the client requests. Must be a valid Go time.Duration string.
• permissions: array of string. Specify the actions the certificate can perform. See the -O option to ssh-keygen(1) for a complete list. e.g. permissions = ["permit-pty", "permit-port-forwarding", force-command=/bin/ls", "source-address=192.168.0.0/24"]

aws

AWS configuration is only needed for accessing signing keys stored on S3, and isn't totally necessary even then.
The S3 client can be configured using any of the usual AWS-SDK means - environment variables, IAM roles etc.
It's strongly recommended that signing keys stored on S3 be locked down to specific IAM roles and encrypted using KMS.

• region: string. AWS region the bucket resides in, e.g. us-east-1.
• access_key: string. AWS Access Key ID. This can be a secret stored in a vault using the form /vault/path/key e.g. /vault/secret/cashier/aws_access_key.
• secret_key: string. AWS Secret Key. This can be a secret stored in a vault using the form /vault/path/key e.g. /vault/secret/cashier/aws_secret_key.

vault

Vault support is currently a work-in-progress.

• address: string. URL to the vault server.
• token: string. Auth token for the vault.

Usage

Cashier comes in two parts, a cli and a server.
The server is configured using a HCL configuration file - example.

For the server you need the following:

• A new ssh private key. Generate one using ssh-keygen - e.g. ssh-keygen -f ssh_ca - this is your CA signing key. At this time Cashier supports RSA, ECDSA and Ed25519 keys. Important This key should be kept safe - ANY ssh key signed with this key will be able to access your machines.
• OAuth (Google or GitHub) credentials. You may also need to set the callback URL when creating these.

Using cashier client

Once the server is up and running you'll need to configure your client.
The client is configured using either a HCL configuration file - example - or command-line flags.

• --ca CA server (default "http://localhost:10000").
• --config Path to config file (default "~/.cashier.conf").
• --key_size Key size. Ignored for ed25519 keys (default 2048).
• --key_type Type of private key to generate - rsa, ecdsa or ed25519 (default "rsa").
• --key_file_prefix Prefix for filename for SSH keys and cert (optional, no default). The public key is put in a file with id_<id>.pub appended to it; the public cert file in a file with id_<id>-cert.pub appended to it. The private key is stored in a file with id_<id> appended to it. is taken from the id stored on the server.
• --validity Key validity (default 24h).

Running the cashier cli tool will open a browser window at the configured CA address. The CA will redirect to the auth provider for authorisation, and redirect back to the CA where the access token will printed.
Copy the access token. In the terminal where you ran the cashier cli paste the token at the prompt.
The client will then generate a new ssh key-pair and send the public part to the server (along with the access token).
Once signed the client will install the key and signed certificate in your ssh agent. When the certificate expires it will be removed automatically from the agent.

If you set key_file_prefix then the public key and public cert will be written to the files that start with key_file_prefix and end with .pub and -cert.pub respectively.

In your ssh_config you can load these for a given host with the IdentityFile and CertificateFile. However prior to OpenSSH version 7.2p1 the latter option didn't exist. In that case you could specify ~/.ssh/some-identity as your IdentityFile and OpenSSH would look in ~/.ssh/some-identity.pub and ~/.ssh/some-identity-cert.pub.

Starting with 7.2p1 the two options exist in the ssh_config and you'll need to use the full paths to them. Note that like these ssh_config options, the key_file_prefix supports tilde expansion.

Configuring SSH

The ssh client needs no special configuration, just a running ssh-agent.
The ssh server needs to trust the public part of the CA signing key. Add something like the following to your sshd_config:

TrustedUserCAKeys /etc/ssh/ca.pub

where /etc/ssh/ca.pub contains the public part of your signing key.

If you wish to use certificate revocation you need to set the RevokedKeys option in sshd_config - see the next section.

Revoking certificates

When a certificate is signed a record is kept in the configured database. You can view issued certs at http(s)://<ca url>/admin/certs and also revoke them.
The revocation list is served at http(s)://<ca url>/revoked. To use it your sshd_config must have RevokedKeys set:

RevokedKeys /etc/ssh/revoked_keys

See the RevokedKeys option in the sshd_config man page for more.
Keeping the revoked list up to date can be done with a cron job like:

*/10 * * * * * curl -s -o /etc/ssh/revoked_keys https://sshca.example.com/revoked

Remember that the revoked_keys file must exist and must be readable by the sshd or else all ssh authentication will fail.

Future Work

• Host certificates - only user certificates are supported at present.

Contributing

Pull requests are welcome but forking Go repos can be a pain. This is a good guide to forking and creating pull requests for Go projects.
Dependencies are vendored with govendor.