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:

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

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:

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",
                "ignore-cert": "true",
                "recording-path": "/recordings",
                "recording-name": "My-Connection-${GUAC_USERNAME}-${GUAC_DATE}-${GUAC_TIME}"
            }
        },
        "My OTHER Connection" : {
            "protocol" : "rdp",
            "parameters" : {
                "hostname" : "10.10.209.64",
                "port" : "3389",
                "ignore-cert": "true",
                "recording-path": "/recordings",
                "recording-name": "My-OTHER-Connection-${GUAC_USERNAME}-${GUAC_DATE}-${GUAC_TIME}"
            }
        }
    }
}

and you run:

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

You will receive the following output:

A2Pf5Kpmm97I2DT1PifIrfU6q3yzoGcIbNXEd60WNangT8DAVjAl6luaqwhBJnCK
uqcf9ZZlRo3uDxTHvUM3eq1YvdghL0GbosOn8Mn38j2ydOMk+Cd15a8ggb4/ddt/
yIBK4DxrN7MNbouZ091KYtXC6m20E6sGzLy676BlMSg1cmsENRIihOynsSLSCvo0
diif6H7T+ZuIqF7B5SW+adGfMaHlfknlIvSpLGHhrIP4aMYE/ZU2vYNg8ez27sCS
wDBWu5lERtfCYFyU4ysjRU5Hyov+yKa+O7jcRYpw3N+fHbCg7/dxVNW07qNOKssv
pzUciGvDPUCPpa02WmPJNEBowwQireO1952/MNAI77cW2UepbljD/bwOiZl2THJz
LrENo7K5acimBa+EjWEesgn7lx/WTCF3zxR6TH1CWrQM8Et1aUK1Nf8K11xEQbTy
klyaNtCmTfyahRZ/fUPxDNrdJVpPOSELkf7RJO5tOdK/FFIFIbze3ZUyXgRq+pHY
owpgOmudDBTBlxhXiONdutRI/RZbFM/7GBMdmI8AR/401OCV3nsI4jLhukjMXH3V
f3pQg+xKMhi/QExHhDk8VTNYk7GurK4vgehn7HQ0oSGh8pGcmxB6W43cz+hyn6VQ
On6i90cSnIhRO8SysZt332LwJCDm7I+lBLaI8NVHU6bnAY1Axx5oH3YTKc4qzHls
HEAFYLkD6aHMvHkF3b798CMravjxiJV3m7hsXDbaFN6AFhn8GIkMRRrjuevfZ+q9
enWN14s24vt5OVg69DljzALobUNKUXFx69SR8EpSBvUcKq8s/OgbDpFvKbwsDY57
HGT4T0CuRIA0TGUI075uerKBNApVhuBA1BmWJIrI4JXw5MuX6pdBe+MYccO3vfo+
/frazj8rHdkDa/IbueMbvq+1ozV2+UuxrbaTrV2i4jSRgd74U0QzOh9e8Q0i7vOi
l3hnIfOfg+v1oULmZmJSeiAYWxeGvPptp+n7rNFqHGM=

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).