guacamole-auth-json

guacamole-auth-json is an authentication extension for Apache Guacamole which authenticates users using JSON which has been signed using HMAC/SHA-256 and encrypted with 128-bit AES in CBC mode. This JSON contains all information describing the user being authenticated, as well as any connections they have access to.

Configuring Guacamole to accept encrypted JSON

To verify and decrypt the received signed and encrypted JSON, a secret key must be generated which will be shared by both the Guacamole server and systems that will generate the JSON data. As guacamole-auth-json uses 128-bit AES, this key must be 128 bits.

An easy way of generating such a key is to echo a passphrase through the "md5sum" utility. This is the technique OpenSSL itself uses to generate 128-bit keys from passphrases. For example:

$ echo -n "ThisIsATest" | md5sum
4c0b569e4c96df157eee1b65dd0e4d41  -

The generated key must then be saved within guacamole.properties as the full 32-digit hex value using the json-secret-key property:

json-secret-key: 4c0b569e4c96df157eee1b65dd0e4d41

JSON format

The general format of the JSON (prior to being encrypted, signed, and sent to Guacamole), is as follows:

{

    "username" : "arbitraryUsername",
    "expires" : TIMESTAMP,
    "connections" : {

        "Connection Name" : {
            "protocol" : "PROTOCOL",
            "parameters" : {
                "name1" : "value1",
                "name2" : "value2",
                ...
            }
        },

        ...

    }

}

where TIMESTAMP is a standard UNIX epoch timestamp with millisecond resolution (the number of milliseconds since midnight of January 1, 1970 UTC) and PROTOCOL is the internal name of any of Guacamole's supported protocols, such as vnc, rdp, or ssh.

The JSON will cease to be accepted as valid after the server time passes the timestamp. If no timestamp is specified, the data will not expire.

The top-level JSON object which must be submitted to Guacamole has the following properties:

Property name	Type	Description
username	string	The unique username of the user authenticated by the JSON. If the user is anonymous, this should be the empty string ("").
expires	number	The absolute time after which the JSON should no longer be accepted, even if the signature is valid, as a standard UNIX epoch timestamp with millisecond resolution (the number of milliseconds since midnight of January 1, 1970 UTC).
connections	object	The set of connections which should be exposed to the user by their corresponding, unique names. If no connections will be exposed to the user, this can simply be an empty object ({}).

Each normal connection defined within each submitted JSON object has the following properties:

Property name	Type	Description
id	string	An optional opaque value which uniquely identifies this connection across all other connections which may be active at any given time. This property is only required if you wish to allow the connection to be shared or shadowed.
protocol	string	The internal name of a supported protocol, such as vnc, rdp, or ssh.
parameters	object	An object representing the connection parameter name/value pairs to apply to the connection, as documented in the Guacamole manual.

Connections which share or shadow other connections use a join property instead of a protocol property, where join contains the value of the id property of the connection being joined:

Property name	Type	Description
id	string	An optional opaque value which uniquely identifies this connection across all other connections which may be active at any given time. This property is only required if you wish to allow the connection to be shared or shadowed. (Yes, a connection which shadows another connection may itself be shadowed.)
join	string	The opaque ID given within the id property of the connection being joined (shared / shadowed).
parameters	object	An object representing the connection parameter name/value pairs to apply to the connection, as documented in the Guacamole manual. Most of the connection configuration is inherited from the connection being joined. In general, the only property relevant to joining connections is read-only.

If a connection is configured to join another connection, that connection will only be usable if the connection being joined is currently active. If two connections are established having the same id value, only the last connection will be joinable using the given id.

Generating encrypted JSON

To authenticate a user with the above JSON format, the JSON must be both signed and encrypted using the same 128-bit secret key specified with the json-secret-key within guacamole.properties:

1. Generate JSON in the format described above

2. Sign the JSON using the secret key (the same 128-bit key stored within guacamole.properties with the json-secret-key property) with HMAC/SHA-256. Prepend the binary result of the signing process to the plaintext JSON that was signed.

3. Encrypt the result of (2) above using AES in CBC mode, with the initial vector (IV) set to all zero bytes.

4. Encode the encrypted result using base64.

5. POST the encrypted result to the /api/tokens REST endpoint as the value of an HTTP parameter named data (or include it in the URL of any Guacamole page as a query parameter named data).

   For example, if Guacamole is running on localhost at /guacamole, and BASE64_RESULT is the result of the above process, the equivalent run of the "curl" utility would be:

   $ curl --data-urlencode "data=BASE64_RESULT" http://localhost:8080/guacamole/api/tokens
   

   NOTE: Be sure to URL-encode the base64-encoded result prior to POSTing it to /api/tokens or including it in the URL. Base64 can contain both "+" and "=" characters, which have special meaning within URLs.

If the data is invalid in any way, if the signature does not match, if decryption or signature verification fails, or if the submitted data has expired, the REST service will return an invalid credentials error and fail without user-visible explanation. Details describing the error that occurred will be in the Tomcat logs, however.

Reference implementation

The source includes a shell script, doc/encrypt-json.sh, which uses the OpenSSL command-line utility to encrypt and sign JSON in the manner that guacamole-auth-json requires. It is thoroughly commented and should work well as a reference implementation, for testing, and as a point of comparison for development. The script is run as:

$ ./encrypt-json.sh HEX_ENCRYPTION_KEY file-to-sign-and-encrypt.json

For example, if you have a file called auth.json containing the following:

{
    "username" : "test",
    "expires" : "1446323765000",
    "connections" : {
        "My Connection" : {
            "protocol" : "rdp",
            "parameters" : {
                "hostname" : "10.10.209.63",
                "port" : "3389"
            }
        },
        "My OTHER Connection" : {
            "protocol" : "rdp",
            "parameters" : {
                "hostname" : "10.10.209.64",
                "port" : "3389"
            }
        }
    }
}

and you run:

$ ./encrypt-json.sh 4C0B569E4C96DF157EEE1B65DD0E4D41 auth.json

You will receive the following output:
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The resulting base64 data above, if submitted using the data parameter to Guacamole, will authenticate a user and grant them access to the connections described in the JSON (at least until the expires timestamp is reached, at which point the JSON will no longer be accepted).