1---
2title: "Token Authentication Implementation"
3description: "Describe the reference implementation of the Docker Registry v2 authentication schema"
4keywords: registry, on-prem, images, tags, repository, distribution, JWT authentication, advanced
5---
6
7# Docker Registry v2 Bearer token specification
8
9This specification covers the `docker/distribution` implementation of the
10v2 Registry's authentication schema. Specifically, it describes the JSON
11Web Token schema that `docker/distribution` has adopted to implement the
12client-opaque Bearer token issued by an authentication service and
13understood by the registry.
14
15This document borrows heavily from the [JSON Web Token Draft Spec](https://tools.ietf.org/html/draft-ietf-oauth-json-web-token-32)
16
17## Getting a Bearer Token
18
19For this example, the client makes an HTTP GET request to the following URL:
20
21```
22https://auth.docker.io/token?service=registry.docker.io&scope=repository:samalba/my-app:pull,push
23```
24
25The token server should first attempt to authenticate the client using any
26authentication credentials provided with the request. As of Docker 1.8, the
27registry client in the Docker Engine only supports Basic Authentication to
28these token servers. If an attempt to authenticate to the token server fails,
29the token server should return a `401 Unauthorized` response indicating that
30the provided credentials are invalid.
31
32Whether the token server requires authentication is up to the policy of that
33access control provider. Some requests may require authentication to determine
34access (such as pushing or pulling a private repository) while others may not
35(such as pulling from a public repository).
36
37After authenticating the client (which may simply be an anonymous client if
38no attempt was made to authenticate), the token server must next query its
39access control list to determine whether the client has the requested scope. In
40this example request, if I have authenticated as user `jlhawn`, the token
41server will determine what access I have to the repository `samalba/my-app`
42hosted by the entity `registry.docker.io`.
43
44Once the token server has determined what access the client has to the
45resources requested in the `scope` parameter, it will take the intersection of
46the set of requested actions on each resource and the set of actions that the
47client has in fact been granted. If the client only has a subset of the
48requested access **it must not be considered an error** as it is not the
49responsibility of the token server to indicate authorization errors as part of
50this workflow.
51
52Continuing with the example request, the token server will find that the
53client's set of granted access to the repository is `[pull, push]` which when
54intersected with the requested access `[pull, push]` yields an equal set. If
55the granted access set was found only to be `[pull]` then the intersected set
56would only be `[pull]`. If the client has no access to the repository then the
57intersected set would be empty, `[]`.
58
59It is this intersected set of access which is placed in the returned token.
60
61The server will now construct a JSON Web Token to sign and return. A JSON Web
62Token has 3 main parts:
63
641. Headers
65
66 The header of a JSON Web Token is a standard JOSE header. The "typ" field
67 will be "JWT" and it will also contain the "alg" which identifies the
68 signing algorithm used to produce the signature. It also must have a "kid"
69 field, representing the ID of the key which was used to sign the token.
70
71 The "kid" field has to be in a libtrust fingerprint compatible format.
72 Such a format can be generated by following steps:
73
74 1. Take the DER encoded public key which the JWT token was signed against.
75
76 2. Create a SHA256 hash out of it and truncate to 240bits.
77
78 3. Split the result into 12 base32 encoded groups with `:` as delimiter.
79
80 Here is an example JOSE Header for a JSON Web Token (formatted with
81 whitespace for readability):
82
83 ```
84 {
85 "typ": "JWT",
86 "alg": "ES256",
87 "kid": "PYYO:TEWU:V7JH:26JV:AQTZ:LJC3:SXVJ:XGHA:34F2:2LAQ:ZRMK:Z7Q6"
88 }
89 ```
90
91 It specifies that this object is going to be a JSON Web token signed using
92 the key with the given ID using the Elliptic Curve signature algorithm
93 using a SHA256 hash.
94
952. Claim Set
96
97 The Claim Set is a JSON struct containing these standard registered claim
98 name fields:
99
100 <dl>
101 <dt>
102 <code>iss</code> (Issuer)
103 </dt>
104 <dd>
105 The issuer of the token, typically the fqdn of the authorization
106 server.
107 </dd>
108 <dt>
109 <code>sub</code> (Subject)
110 </dt>
111 <dd>
112 The subject of the token; the name or id of the client which
113 requested it. This should be empty (`""`) if the client did not
114 authenticate.
115 </dd>
116 <dt>
117 <code>aud</code> (Audience)
118 </dt>
119 <dd>
120 The intended audience of the token; the name or id of the service
121 which will verify the token to authorize the client/subject.
122 </dd>
123 <dt>
124 <code>exp</code> (Expiration)
125 </dt>
126 <dd>
127 The token should only be considered valid up to this specified date
128 and time.
129 </dd>
130 <dt>
131 <code>nbf</code> (Not Before)
132 </dt>
133 <dd>
134 The token should not be considered valid before this specified date
135 and time.
136 </dd>
137 <dt>
138 <code>iat</code> (Issued At)
139 </dt>
140 <dd>
141 Specifies the date and time which the Authorization server
142 generated this token.
143 </dd>
144 <dt>
145 <code>jti</code> (JWT ID)
146 </dt>
147 <dd>
148 A unique identifier for this token. Can be used by the intended
149 audience to prevent replays of the token.
150 </dd>
151 </dl>
152
153 The Claim Set will also contain a private claim name unique to this
154 authorization server specification:
155
156 <dl>
157 <dt>
158 <code>access</code>
159 </dt>
160 <dd>
161 An array of access entry objects with the following fields:
162
163 <dl>
164 <dt>
165 <code>type</code>
166 </dt>
167 <dd>
168 The type of resource hosted by the service.
169 </dd>
170 <dt>
171 <code>name</code>
172 </dt>
173 <dd>
174 The name of the resource of the given type hosted by the
175 service.
176 </dd>
177 <dt>
178 <code>actions</code>
179 </dt>
180 <dd>
181 An array of strings which give the actions authorized on
182 this resource.
183 </dd>
184 </dl>
185 </dd>
186 </dl>
187
188 Here is an example of such a JWT Claim Set (formatted with whitespace for
189 readability):
190
191 ```
192 {
193 "iss": "auth.docker.com",
194 "sub": "jlhawn",
195 "aud": "registry.docker.com",
196 "exp": 1415387315,
197 "nbf": 1415387015,
198 "iat": 1415387015,
199 "jti": "tYJCO1c6cnyy7kAn0c7rKPgbV1H1bFws",
200 "access": [
201 {
202 "type": "repository",
203 "name": "samalba/my-app",
204 "actions": [
205 "pull",
206 "push"
207 ]
208 }
209 ]
210 }
211 ```
212
2133. Signature
214
215 The authorization server will produce a JOSE header and Claim Set with no
216 extraneous whitespace, i.e., the JOSE Header from above would be
217
218 ```
219 {"typ":"JWT","alg":"ES256","kid":"PYYO:TEWU:V7JH:26JV:AQTZ:LJC3:SXVJ:XGHA:34F2:2LAQ:ZRMK:Z7Q6"}
220 ```
221
222 and the Claim Set from above would be
223
224 ```
225 {"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"]}]}
226 ```
227
228 The utf-8 representation of this JOSE header and Claim Set are then
229 url-safe base64 encoded (sans trailing '=' buffer), producing:
230
231 ```
232 eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiIsImtpZCI6IlBZWU86VEVXVTpWN0pIOjI2SlY6QVFUWjpMSkMzOlNYVko6WEdIQTozNEYyOjJMQVE6WlJNSzpaN1E2In0
233 ```
234
235 for the JOSE Header and
236
237 ```
238 eyJpc3MiOiJhdXRoLmRvY2tlci5jb20iLCJzdWIiOiJqbGhhd24iLCJhdWQiOiJyZWdpc3RyeS5kb2NrZXIuY29tIiwiZXhwIjoxNDE1Mzg3MzE1LCJuYmYiOjE0MTUzODcwMTUsImlhdCI6MTQxNTM4NzAxNSwianRpIjoidFlKQ08xYzZjbnl5N2tBbjBjN3JLUGdiVjFIMWJGd3MiLCJhY2Nlc3MiOlt7InR5cGUiOiJyZXBvc2l0b3J5IiwibmFtZSI6InNhbWFsYmEvbXktYXBwIiwiYWN0aW9ucyI6WyJwdXNoIl19XX0
239 ```
240
241 for the Claim Set. These two are concatenated using a '.' character,
242 yielding the string:
243
244 ```
245 eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiIsImtpZCI6IlBZWU86VEVXVTpWN0pIOjI2SlY6QVFUWjpMSkMzOlNYVko6WEdIQTozNEYyOjJMQVE6WlJNSzpaN1E2In0.eyJpc3MiOiJhdXRoLmRvY2tlci5jb20iLCJzdWIiOiJqbGhhd24iLCJhdWQiOiJyZWdpc3RyeS5kb2NrZXIuY29tIiwiZXhwIjoxNDE1Mzg3MzE1LCJuYmYiOjE0MTUzODcwMTUsImlhdCI6MTQxNTM4NzAxNSwianRpIjoidFlKQ08xYzZjbnl5N2tBbjBjN3JLUGdiVjFIMWJGd3MiLCJhY2Nlc3MiOlt7InR5cGUiOiJyZXBvc2l0b3J5IiwibmFtZSI6InNhbWFsYmEvbXktYXBwIiwiYWN0aW9ucyI6WyJwdXNoIl19XX0
246 ```
247
248 This is then used as the payload to a the `ES256` signature algorithm
249 specified in the JOSE header and specified fully in [Section 3.4 of the JSON Web Algorithms (JWA)
250 draft specification](https://tools.ietf.org/html/draft-ietf-jose-json-web-algorithms-38#section-3.4)
251
252 This example signature will use the following ECDSA key for the server:
253
254 ```
255 {
256 "kty": "EC",
257 "crv": "P-256",
258 "kid": "PYYO:TEWU:V7JH:26JV:AQTZ:LJC3:SXVJ:XGHA:34F2:2LAQ:ZRMK:Z7Q6",
259 "d": "R7OnbfMaD5J2jl7GeE8ESo7CnHSBm_1N2k9IXYFrKJA",
260 "x": "m7zUpx3b-zmVE5cymSs64POG9QcyEpJaYCD82-549_Q",
261 "y": "dU3biz8sZ_8GPB-odm8Wxz3lNDr1xcAQQPQaOcr1fmc"
262 }
263 ```
264
265 A resulting signature of the above payload using this key is:
266
267 ```
268 QhflHPfbd6eVF4lM9bwYpFZIV0PfikbyXuLx959ykRTBpe3CYnzs6YBK8FToVb5R47920PVLrh8zuLzdCr9t3w
269 ```
270
271 Concatenating all of these together with a `.` character gives the
272 resulting JWT:
273
274 ```
275 eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiIsImtpZCI6IlBZWU86VEVXVTpWN0pIOjI2SlY6QVFUWjpMSkMzOlNYVko6WEdIQTozNEYyOjJMQVE6WlJNSzpaN1E2In0.eyJpc3MiOiJhdXRoLmRvY2tlci5jb20iLCJzdWIiOiJqbGhhd24iLCJhdWQiOiJyZWdpc3RyeS5kb2NrZXIuY29tIiwiZXhwIjoxNDE1Mzg3MzE1LCJuYmYiOjE0MTUzODcwMTUsImlhdCI6MTQxNTM4NzAxNSwianRpIjoidFlKQ08xYzZjbnl5N2tBbjBjN3JLUGdiVjFIMWJGd3MiLCJhY2Nlc3MiOlt7InR5cGUiOiJyZXBvc2l0b3J5IiwibmFtZSI6InNhbWFsYmEvbXktYXBwIiwiYWN0aW9ucyI6WyJwdXNoIl19XX0.QhflHPfbd6eVF4lM9bwYpFZIV0PfikbyXuLx959ykRTBpe3CYnzs6YBK8FToVb5R47920PVLrh8zuLzdCr9t3w
276 ```
277
278This can now be placed in an HTTP response and returned to the client to use to
279authenticate to the audience service:
280
281
282```
283HTTP/1.1 200 OK
284Content-Type: application/json
285
286{"token": "eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiIsImtpZCI6IlBZWU86VEVXVTpWN0pIOjI2SlY6QVFUWjpMSkMzOlNYVko6WEdIQTozNEYyOjJMQVE6WlJNSzpaN1E2In0.eyJpc3MiOiJhdXRoLmRvY2tlci5jb20iLCJzdWIiOiJqbGhhd24iLCJhdWQiOiJyZWdpc3RyeS5kb2NrZXIuY29tIiwiZXhwIjoxNDE1Mzg3MzE1LCJuYmYiOjE0MTUzODcwMTUsImlhdCI6MTQxNTM4NzAxNSwianRpIjoidFlKQ08xYzZjbnl5N2tBbjBjN3JLUGdiVjFIMWJGd3MiLCJhY2Nlc3MiOlt7InR5cGUiOiJyZXBvc2l0b3J5IiwibmFtZSI6InNhbWFsYmEvbXktYXBwIiwiYWN0aW9ucyI6WyJwdXNoIl19XX0.QhflHPfbd6eVF4lM9bwYpFZIV0PfikbyXuLx959ykRTBpe3CYnzs6YBK8FToVb5R47920PVLrh8zuLzdCr9t3w"}
287```
288
289## Using the signed token
290
291Once the client has a token, it will try the registry request again with the
292token placed in the HTTP `Authorization` header like so:
293
294```
295Authorization: Bearer eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiIsImtpZCI6IkJWM0Q6MkFWWjpVQjVaOktJQVA6SU5QTDo1RU42Ok40SjQ6Nk1XTzpEUktFOkJWUUs6M0ZKTDpQT1RMIn0.eyJpc3MiOiJhdXRoLmRvY2tlci5jb20iLCJzdWIiOiJCQ0NZOk9VNlo6UUVKNTpXTjJDOjJBVkM6WTdZRDpBM0xZOjQ1VVc6NE9HRDpLQUxMOkNOSjU6NUlVTCIsImF1ZCI6InJlZ2lzdHJ5LmRvY2tlci5jb20iLCJleHAiOjE0MTUzODczMTUsIm5iZiI6MTQxNTM4NzAxNSwiaWF0IjoxNDE1Mzg3MDE1LCJqdGkiOiJ0WUpDTzFjNmNueXk3a0FuMGM3cktQZ2JWMUgxYkZ3cyIsInNjb3BlIjoiamxoYXduOnJlcG9zaXRvcnk6c2FtYWxiYS9teS1hcHA6cHVzaCxwdWxsIGpsaGF3bjpuYW1lc3BhY2U6c2FtYWxiYTpwdWxsIn0.Y3zZSwaZPqy4y9oRBVRImZyv3m_S9XDHF1tWwN7mL52C_IiA73SJkWVNsvNqpJIn5h7A2F8biv_S2ppQ1lgkbw
296```
297
298This is also described in [Section 2.1 of RFC 6750: The OAuth 2.0 Authorization Framework: Bearer Token Usage](https://tools.ietf.org/html/rfc6750#section-2.1)
299
300## Verifying the token
301
302The registry must now verify the token presented by the user by inspecting the
303claim set within. The registry will:
304
305- Ensure that the issuer (`iss` claim) is an authority it trusts.
306- Ensure that the registry identifies as the audience (`aud` claim).
307- Check that the current time is between the `nbf` and `exp` claim times.
308- If enforcing single-use tokens, check that the JWT ID (`jti` claim) value has
309 not been seen before.
310 - To enforce this, the registry may keep a record of `jti`s it has seen for
311 up to the `exp` time of the token to prevent token replays.
312- Check the `access` claim value and use the identified resources and the list
313 of actions authorized to determine whether the token grants the required
314 level of access for the operation the client is attempting to perform.
315- Verify that the signature of the token is valid.
316
317If any of these requirements are not met, the registry will return a
318`403 Forbidden` response to indicate that the token is invalid.
319
320**Note**: it is only at this point in the workflow that an authorization error
321may occur. The token server should *not* return errors when the user does not
322have the requested authorization. Instead, the returned token should indicate
323whatever of the requested scope the client does have (the intersection of
324requested and granted access). If the token does not supply proper
325authorization then the registry will return the appropriate error.
326
327At no point in this process should the registry need to call back to the
328authorization server. The registry only needs to be supplied with the trusted
329public keys to verify the token signatures.
View as plain text