- [How it works](#how-it-works)
- [Quick start](#quick-start)
- [Installation using Go tools](#installation-using-go-tools)
- [Using docker](#using-docker)
- [Provider-specific options](#provider-specific-options)
- [Using cashier](#using-cashier)
- [Configuring SSH](#configuring-ssh)
- [Revoking certificates](#revoking-certificates)
- [Future Work](#future-work)
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
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](http://man.openbsd.org/OpenBSD-current/man1/ssh-keygen.1#CERTIFICATES) of `ssh-keygen(1)`
## How it works
The user wishes to ssh to a production machine.
They visit the CA site (e.g. https://sshca.exampleorg.com) in a browser and authenticate.
The site shows a page with a token which the user copies.
The user runs a local command which generates a new ssh key-pair in memory and requests the token from the user.
The token is sent to the CA along with the ssh public key.
The CA verifies the token and signs the public key with the signing key and returns the signed certificate.
The command on the user's machine receives the certificate and loads it and the previously generated 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.
# Quick start
## Installation using Go tools
1. Use the Go tools to install cashier. The binaries `cashierd` and `cashier` will be installed in your $GOPATH.
go get -u github.com/nsheridan/cashier/cmd/cashier
go get -u github.com/nsheridan/cashier/cmd/cashierd
2. Create a signing key with `ssh-keygen` and a [cashierd.conf](example-server.conf)
3. Run the cashier server with `cashierd` and the cli with `cashier`.
## Using docker
1. Create a signing key with `ssh-keygen` and a [cashierd.conf](example-server.conf)
docker run -it --rm -p 10000:10000 --name cashier -v $(pwd):/cashier nsheridan/cashier
Go 1.5 (with `GO15VENDOREXPERIMENT` set) or later. May work with earlier versions.
OpenSSH 5.6 or newer.
A working SSH agent.
I have only tested this on Linux & OSX.
Configuration is divided into different sections: `server`, `auth`, `ssh`, and `aws`.
- `use_tls` : boolean. If set `tls_key` and `tls_cert` are required.
- `tls_key` : string. Path to the TLS key.
- `tls_cert` : string. Path to the TLS cert.
- `address` : string. IP address to listen on. If unset the server listens on all addresses.
- `port` : int. Port to listen on.
- `cookie_secret`: string. Authentication key for the session cookie.
- `http_logfile`: string. Path to the HTTP request log. Logs are written in the [Common Log Format](https://en.wikipedia.org/wiki/Common_Log_Format). If not set logs are written to stderr.
- `datastore`: string. Datastore connection string. See [Datastore](#datastore).
Datastores contain a record of issued certificates for audit and revocation purposes. The connection string is of the form `engine:username:password:host[:port]`.
Supported database providers: `mysql`, `mongo`, `sqlite` and `mem`.
`mem` is an in-memory database intended for testing and takes no additional config options.
`mysql` is the MySQL database and accepts `username`, `password` and `host` arguments. Only `username` and `host` arguments are required. `port` is assumed to be 3306 unless otherwise specified.
`mongo` is MongoDB and accepts `username`, `password` and `host` arguments. All arguments are optional and multiple hosts can be specified using comma-separated values: `mongo:dbuser:dbpasswd:host1,host2`.
`sqlite` is the SQLite database and accepts a `path` argument.
If no datastore is specified the `mem` store is used.
datastore = "mem" # use the in-memory database.
datastore = "mysql:root::localhost" # mysql running on localhost with the user 'root' and no password.
datastore = "mysql:cashier:PaSsWoRd:mydbprovider.example.com:5150" # mysql running on a remote host on port 5150
datastore = "mongo:cashier:PaSsWoRd:mydbprovider.example.com:27018" # mongo running on a remote host on port 27018
datastore = "mongo:cashier:PaSsWoRd:server1.example.com:27018,server2.example.com:27018" # mongo running on multiple servers on port 27018
datastore = "sqlite:/data/certs.db"
Prior to using MySQL, MongoDB or SQLite datastores you need to create the database and tables using the [dbinit tool](cmd/dbinit/dbinit.go).
Note that dbinit has no support for replica sets.
- `provider` : string. Name of the oauth provider. At present the only valid value is "google".
- `oauth_client_id` : string. Oauth Client ID.
- `oauth_client_secret` : string. Oauth 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.
provider = "google"
domain = "example.com"
| Provider | Option | Notes |
| Google | domain | If this is unset then any gmail user can obtain a token. |
| Github | organization | If this is unset then any GitHub user can obtain a token. The oauth client and secrets should be issued by the specified organization. |
- `signing_key`: string. Path to the signing ssh private key you created earlier. This can be a S3 or GCS path using `/s3/<bucket>/<path/to/key>` or `/gcs/<bucket>/<path/to/key>` as appropriate. For S3 you should add an [aws](#aws) config as needed.
- `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`](https://golang.org/pkg/time/#ParseDuration) string.
- `permissions`: array of string. Actions the certificate can perform. See the [`-O` option to `ssh-keygen(1)`](http://man.openbsd.org/OpenBSD-current/man1/ssh-keygen.1) for a complete list.
AWS configuration is only needed for accessing signing keys stored on S3, and isn't required even then.
The S3 client can be configured using any of [the usual AWS-SDK means](https://github.com/aws/aws-sdk-go/wiki/configuring-sdk) - 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.
- `secret_key`: string. AWS Secret Key.
Cashier comes in two parts, a [cli](cmd/cashier) and a [server](cmd/cashierd).
The server is configured using a HCL configuration file - [example](example-server.conf).
For the server you need the following:
- A new ssh private key. Generate one in the usual way using `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.
- Google OAuth credentials which you can generate at the [Google Developers Console](https://console.developers.google.com). You also need to set the callback URL here.
## Using cashier
Once the server is up and running you'll need to configure your client.
The client is configured using either a [HCL](https://github.com/hashicorp/hcl) configuration file - [example](example-client.conf) - or command-line flags.
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.
## 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:
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 datastore. 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:
See the [`RevokedKeys` option in the sshd_config man page](http://man.openbsd.org/OpenBSD-current/man5/sshd_config) 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.