Token Authentication Implementation

Docker Registry v2 Bearer token specification

This specification covers the docker/distribution implementation of the v2 Registry’s authentication schema. Specifically, it describes the JSON Web Token schema that docker/distribution has adopted to implement the client-opaque Bearer token issued by an authentication service and understood by the registry.

This document borrows heavily from the JSON Web Token Draft Spec

Getting a Bearer Token

For this example, the client makes an HTTP GET request to the following URL:

https://auth.docker.io/token?service=registry.docker.io&scope=repository:samalba/my-app:pull,push

The token server should first attempt to authenticate the client using any authentication credentials provided with the request. As of Docker 1.8, the registry client in the Docker Engine only supports Basic Authentication to these token servers. If an attempt to authenticate to the token server fails, the token server should return a 401 Unauthorized response indicating that the provided credentials are invalid.

Whether the token server requires authentication is up to the policy of that access control provider. Some requests may require authentication to determine access (such as pushing or pulling a private repository) while others may not (such as pulling from a public repository).

After authenticating the client (which may simply be an anonymous client if no attempt was made to authenticate), the token server must next query its access control list to determine whether the client has the requested scope. In this example request, if I have authenticated as user jlhawn, the token server will determine what access I have to the repository samalba/my-app hosted by the entity registry.docker.io.

Once the token server has determined what access the client has to the resources requested in the scope parameter, it will take the intersection of the set of requested actions on each resource and the set of actions that the client has in fact been granted. If the client only has a subset of the requested access it must not be considered an error as it is not the responsibility of the token server to indicate authorization errors as part of this workflow.

Continuing with the example request, the token server will find that the client’s set of granted access to the repository is [pull, push] which when intersected with the requested access [pull, push] yields an equal set. If the granted access set was found only to be [pull] then the intersected set would only be [pull]. If the client has no access to the repository then the intersected set would be empty, [].

It is this intersected set of access which is placed in the returned token.

The server will now construct a JSON Web Token to sign and return. A JSON Web Token has 3 main parts:

1. Headers

   The header of a JSON Web Token is a standard JOSE header. The “typ” field will be “JWT” and it will also contain the “alg” which identifies the signing algorithm used to produce the signature. It also must have a “kid” field, representing the ID of the key which was used to sign the token.

   The “kid” field has to be in a libtrust fingerprint compatible format. Such a format can be generated by following steps:

   1. Take the DER encoded public key which the JWT token was signed against.

   2. Create a SHA256 hash out of it and truncate to 240bits.

   3. Split the result into 12 base32 encoded groups with : as delimiter.

   Here is an example JOSE Header for a JSON Web Token (formatted with whitespace for readability):

   {
       "typ": "JWT",
       "alg": "ES256",
       "kid": "PYYO:TEWU:V7JH:26JV:AQTZ:LJC3:SXVJ:XGHA:34F2:2LAQ:ZRMK:Z7Q6"
   }
   

   It specifies that this object is going to be a JSON Web token signed using the key with the given ID using the Elliptic Curve signature algorithm using a SHA256 hash.

2. Claim Set

   The Claim Set is a JSON struct containing these standard registered claim name fields:

   iss (Issuer)

   The issuer of the token, typically the fqdn of the authorization server.

   sub (Subject)

   The subject of the token; the name or id of the client which requested it. This should be empty (`""`) if the client did not authenticate.

   aud (Audience)

   The intended audience of the token; the name or id of the service which will verify the token to authorize the client/subject.

   exp (Expiration)

   The token should only be considered valid up to this specified date and time.

   nbf (Not Before)

   The token should not be considered valid before this specified date and time.

   iat (Issued At)

   Specifies the date and time which the Authorization server generated this token.

   jti (JWT ID)

   A unique identifier for this token. Can be used by the intended audience to prevent replays of the token.

   The Claim Set will also contain a private claim name unique to this authorization server specification:

   access

   An array of access entry objects with the following fields:

   type

   The type of resource hosted by the service.

   name

   The name of the resource of the given type hosted by the service.

   actions

   An array of strings which give the actions authorized on this resource.

   Here is an example of such a JWT Claim Set (formatted with whitespace for readability):

   {
       "iss": "auth.docker.com",
       "sub": "jlhawn",
       "aud": "registry.docker.com",
       "exp": 1415387315,
       "nbf": 1415387015,
       "iat": 1415387015,
       "jti": "tYJCO1c6cnyy7kAn0c7rKPgbV1H1bFws",
       "access": [
           {
               "type": "repository",
               "name": "samalba/my-app",
               "actions": [
                   "pull",
                   "push"
               ]
           }
       ]
   }
   

3. Signature

   The authorization server will produce a JOSE header and Claim Set with no extraneous whitespace, i.e., the JOSE Header from above would be

   {"typ":"JWT","alg":"ES256","kid":"PYYO:TEWU:V7JH:26JV:AQTZ:LJC3:SXVJ:XGHA:34F2:2LAQ:ZRMK:Z7Q6"}
   

   and the Claim Set from above would be

   {"iss":"auth.docker.com","sub":"jlhawn","aud":"registry.docker.com","exp":1415387315,"nbf":1415387015,"iat":1415387015,"jti":"tYJCO1c6cnyy7kAn0c7rKPgbV1H1bFws","access":[{"type":"repository","name":"samalba/my-app","actions":["push","pull"]}]}
   

   The utf-8 representation of this JOSE header and Claim Set are then url-safe base64 encoded (sans trailing ‘=’ buffer), producing:

   eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiIsImtpZCI6IlBZWU86VEVXVTpWN0pIOjI2SlY6QVFUWjpMSkMzOlNYVko6WEdIQTozNEYyOjJMQVE6WlJNSzpaN1E2In0
   

   for the JOSE Header and

   eyJpc3MiOiJhdXRoLmRvY2tlci5jb20iLCJzdWIiOiJqbGhhd24iLCJhdWQiOiJyZWdpc3RyeS5kb2NrZXIuY29tIiwiZXhwIjoxNDE1Mzg3MzE1LCJuYmYiOjE0MTUzODcwMTUsImlhdCI6MTQxNTM4NzAxNSwianRpIjoidFlKQ08xYzZjbnl5N2tBbjBjN3JLUGdiVjFIMWJGd3MiLCJhY2Nlc3MiOlt7InR5cGUiOiJyZXBvc2l0b3J5IiwibmFtZSI6InNhbWFsYmEvbXktYXBwIiwiYWN0aW9ucyI6WyJwdXNoIl19XX0
   

   for the Claim Set. These two are concatenated using a ‘.’ character, yielding the string:

   eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiIsImtpZCI6IlBZWU86VEVXVTpWN0pIOjI2SlY6QVFUWjpMSkMzOlNYVko6WEdIQTozNEYyOjJMQVE6WlJNSzpaN1E2In0.eyJpc3MiOiJhdXRoLmRvY2tlci5jb20iLCJzdWIiOiJqbGhhd24iLCJhdWQiOiJyZWdpc3RyeS5kb2NrZXIuY29tIiwiZXhwIjoxNDE1Mzg3MzE1LCJuYmYiOjE0MTUzODcwMTUsImlhdCI6MTQxNTM4NzAxNSwianRpIjoidFlKQ08xYzZjbnl5N2tBbjBjN3JLUGdiVjFIMWJGd3MiLCJhY2Nlc3MiOlt7InR5cGUiOiJyZXBvc2l0b3J5IiwibmFtZSI6InNhbWFsYmEvbXktYXBwIiwiYWN0aW9ucyI6WyJwdXNoIl19XX0
   

   This is then used as the payload to a the ES256 signature algorithm specified in the JOSE header and specified fully in Section 3.4 of the JSON Web Algorithms (JWA) draft specification

   This example signature will use the following ECDSA key for the server:

   {
       "kty": "EC",
       "crv": "P-256",
       "kid": "PYYO:TEWU:V7JH:26JV:AQTZ:LJC3:SXVJ:XGHA:34F2:2LAQ:ZRMK:Z7Q6",
       "d": "R7OnbfMaD5J2jl7GeE8ESo7CnHSBm_1N2k9IXYFrKJA",
       "x": "m7zUpx3b-zmVE5cymSs64POG9QcyEpJaYCD82-549_Q",
       "y": "dU3biz8sZ_8GPB-odm8Wxz3lNDr1xcAQQPQaOcr1fmc"
   }
   

   A resulting signature of the above payload using this key is:

   QhflHPfbd6eVF4lM9bwYpFZIV0PfikbyXuLx959ykRTBpe3CYnzs6YBK8FToVb5R47920PVLrh8zuLzdCr9t3w
   

   Concatenating all of these together with a . character gives the resulting JWT:

   eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiIsImtpZCI6IlBZWU86VEVXVTpWN0pIOjI2SlY6QVFUWjpMSkMzOlNYVko6WEdIQTozNEYyOjJMQVE6WlJNSzpaN1E2In0.eyJpc3MiOiJhdXRoLmRvY2tlci5jb20iLCJzdWIiOiJqbGhhd24iLCJhdWQiOiJyZWdpc3RyeS5kb2NrZXIuY29tIiwiZXhwIjoxNDE1Mzg3MzE1LCJuYmYiOjE0MTUzODcwMTUsImlhdCI6MTQxNTM4NzAxNSwianRpIjoidFlKQ08xYzZjbnl5N2tBbjBjN3JLUGdiVjFIMWJGd3MiLCJhY2Nlc3MiOlt7InR5cGUiOiJyZXBvc2l0b3J5IiwibmFtZSI6InNhbWFsYmEvbXktYXBwIiwiYWN0aW9ucyI6WyJwdXNoIl19XX0.QhflHPfbd6eVF4lM9bwYpFZIV0PfikbyXuLx959ykRTBpe3CYnzs6YBK8FToVb5R47920PVLrh8zuLzdCr9t3w
   

This can now be placed in an HTTP response and returned to the client to use to authenticate to the audience service:

HTTP/1.1 200 OK
Content-Type: application/json

{"token": "eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiIsImtpZCI6IlBZWU86VEVXVTpWN0pIOjI2SlY6QVFUWjpMSkMzOlNYVko6WEdIQTozNEYyOjJMQVE6WlJNSzpaN1E2In0.eyJpc3MiOiJhdXRoLmRvY2tlci5jb20iLCJzdWIiOiJqbGhhd24iLCJhdWQiOiJyZWdpc3RyeS5kb2NrZXIuY29tIiwiZXhwIjoxNDE1Mzg3MzE1LCJuYmYiOjE0MTUzODcwMTUsImlhdCI6MTQxNTM4NzAxNSwianRpIjoidFlKQ08xYzZjbnl5N2tBbjBjN3JLUGdiVjFIMWJGd3MiLCJhY2Nlc3MiOlt7InR5cGUiOiJyZXBvc2l0b3J5IiwibmFtZSI6InNhbWFsYmEvbXktYXBwIiwiYWN0aW9ucyI6WyJwdXNoIl19XX0.QhflHPfbd6eVF4lM9bwYpFZIV0PfikbyXuLx959ykRTBpe3CYnzs6YBK8FToVb5R47920PVLrh8zuLzdCr9t3w"}

Using the signed token

Once the client has a token, it will try the registry request again with the token placed in the HTTP Authorization header like so:

Authorization: Bearer eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiIsImtpZCI6IkJWM0Q6MkFWWjpVQjVaOktJQVA6SU5QTDo1RU42Ok40SjQ6Nk1XTzpEUktFOkJWUUs6M0ZKTDpQT1RMIn0.eyJpc3MiOiJhdXRoLmRvY2tlci5jb20iLCJzdWIiOiJCQ0NZOk9VNlo6UUVKNTpXTjJDOjJBVkM6WTdZRDpBM0xZOjQ1VVc6NE9HRDpLQUxMOkNOSjU6NUlVTCIsImF1ZCI6InJlZ2lzdHJ5LmRvY2tlci5jb20iLCJleHAiOjE0MTUzODczMTUsIm5iZiI6MTQxNTM4NzAxNSwiaWF0IjoxNDE1Mzg3MDE1LCJqdGkiOiJ0WUpDTzFjNmNueXk3a0FuMGM3cktQZ2JWMUgxYkZ3cyIsInNjb3BlIjoiamxoYXduOnJlcG9zaXRvcnk6c2FtYWxiYS9teS1hcHA6cHVzaCxwdWxsIGpsaGF3bjpuYW1lc3BhY2U6c2FtYWxiYTpwdWxsIn0.Y3zZSwaZPqy4y9oRBVRImZyv3m_S9XDHF1tWwN7mL52C_IiA73SJkWVNsvNqpJIn5h7A2F8biv_S2ppQ1lgkbw

This is also described in Section 2.1 of RFC 6750: The OAuth 2.0 Authorization Framework: Bearer Token Usage

Verifying the token

The registry must now verify the token presented by the user by inspecting the claim set within. The registry will:

• Ensure that the issuer (iss claim) is an authority it trusts.
• Ensure that the registry identifies as the audience (aud claim).
• Check that the current time is between the nbf and exp claim times.
• If enforcing single-use tokens, check that the JWT ID (jti claim) value has not been seen before.
  • To enforce this, the registry may keep a record of jtis it has seen for up to the exp time of the token to prevent token replays.
• Check the access claim value and use the identified resources and the list of actions authorized to determine whether the token grants the required level of access for the operation the client is attempting to perform.
• Verify that the signature of the token is valid.

If any of these requirements are not met, the registry will return a 403 Forbidden response to indicate that the token is invalid.

Note: it is only at this point in the workflow that an authorization error may occur. The token server should not return errors when the user does not have the requested authorization. Instead, the returned token should indicate whatever of the requested scope the client does have (the intersection of requested and granted access). If the token does not supply proper authorization then the registry will return the appropriate error.

At no point in this process should the registry need to call back to the authorization server. The registry only needs to be supplied with the trusted public keys to verify the token signatures.