Docker Hub API v2 token authentication issue - authentication

Question

Currently, I'm working on a very light version of Docker containers orchestrator and I have to be able to get image digest from public Docker Hub registry. I want to use Docker Registry API v2 for this purposes.
I'm trying to get authorization token using following API call:
curl https://auth.docker.io/token?service=index.docker.io&scope=repository:alpine:pull
... and I get a response like following:
{"token":"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.eyJhY2Nlc3MiOltdLCJhdWQiOiJpbmRleC5kb2NrZXIuaW8iLCJleHAiOjE0NzQ1NDcyODMsImlhdCI6MTQ3NDU0Njk4MywiaXNzIjoiYXV0aC5kb2NrZXIuaW8iLCJqdGkiOiIzVTdCazF4dm9CLVA0MGJWVF9JSiIsIm5iZiI6MTQ3NDU0Njk4Mywic3ViIjoiIn0.516fS692WGHNi5Sc44iB8OFSLairrM6n1zNvVo5KXAxlsxsP4rE7VDmW5d0YqvTYeKfZAYIi9yEptx4xJlQ6DA"}
It seems that everything is okay and I'm trying to use obtained token for my next API call:
curl -i -H "Authorization: Bearer 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.eyJhY2Nlc3MiOltdLCJhdWQiOiJpbmRleC5kb2NrZXIuaW8iLCJleHAiOjE0NzQ1NDcyODMsImlhdCI6MTQ3NDU0Njk4MywiaXNzIjoiYXV0aC5kb2NrZXIuaW8iLCJqdGkiOiIzVTdCazF4dm9CLVA0MGJWVF9JSiIsIm5iZiI6MTQ3NDU0Njk4Mywic3ViIjoiIn0.516fS692WGHNi5Sc44iB8OFSLairrM6n1zNvVo5KXAxlsxsP4rE7VDmW5d0YqvTYeKfZAYIi9yEptx4xJlQ6DA" https://index.docker.io/v2/alpine/manifests/latest
... and here what I've got:
HTTP/1.1 401 Unauthorized
Content-Type: application/json; charset=utf-8
Docker-Distribution-Api-Version: registry/2.0
Www-Authenticate: Bearer realm="https://auth.docker.io/token",service="registry.docker.io",scope="repository:alpine:pull",error="invalid_token"
Date: Thu, 22 Sep 2016 12:27:54 GMT
Content-Length: 138
Strict-Transport-Security: max-age=31536000
{"errors":[{"code":"UNAUTHORIZED","message":"authentication required","detail":[{"Type":"repository","Name":"alpine","Action":"pull"}]}]}
Does anybody have ideas why token aouthentication fails for such simple flow?

Answer 1

The response in the Www-Authenticate header is trying to tell you what you need, although it could be more helpful.
First thing: the service you want is registry.docker.io.
Second thing: the repository name in the scope is incomplete. Official repositories only have a single-part name like alpine for the CLI, but in the registry it's properly called library/alpine.
So your token request should be:
curl "https://auth.docker.io/token?service=registry.docker.io&scope=repository:library/alpine:pull"
And then your image request is:
curl --header "Authorization: Bearer eyJh..." https://index.docker.io/v2/library/alpine/manifests/latest
And then you'll get the response.