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+++ b/mod_http_upload_external/README.markdown	Thu Oct 13 18:58:53 2016 +0100
@@ -0,0 +1,108 @@
+---
+description: HTTP File Upload (external service)
+labels: 'Stage-Alpha'
+---
+
+Introduction
+============
+
+This module implements [XEP-0363], which lets clients upload files
+over HTTP to an external web server.
+
+This module generates URLs that are signed using a HMAC. Any web service that can authenticate
+these URLs can be used. There is a PHP implementation available
+[here](https://hg.prosody.im/prosody-modules/raw-file/tip/mod_http_upload_external/share.php). To
+implement your own service compatible with this module, check out the implementation notes below (and if you
+publish your implementation - let us know!).
+
+Configuration
+=============
+
+Add `"http_upload_external"` to modules_enabled in your global section, or under the host(s) you wish
+to use it on.
+
+External URL
+------------
+
+You need to provide the path to the external service. Ensure it ends with '/'.
+
+For example, to use the PHP implementation linked above, you might set it to:
+
+``` {.lua}
+http_upload_external_base_url = "https://your.example.com/path/to/share.php/"
+```
+
+Secret
+------
+
+Set a long and unpredictable string as your secret. This is so the upload service can verify that
+the upload comes from mod_http_upload_external, and random strangers can't upload to your server.
+
+``` {.lua}
+http_upload_external_secret = "this is a secret string!"
+```
+
+You need to set exactly the same secret string in your external service.
+
+Limits
+------
+
+A maximum file size can be set by:
+
+``` {.lua}
+http_upload_external_file_size_limit = 123 -- bytes
+```
+
+Default is 100MB (100\*1024\*1024).
+
+Compatibility
+=============
+
+Works with Prosody 0.9.x and later.
+
+Implementation
+==============
+
+To implement your own external service that is compatible with this module, you need to expose a
+simple API that allows the HTTP GET, HEAD and PUT methods on arbitrary URLs located on your service.
+
+For example, if http_upload_external_base_url is set to `https://example.com/upload/` then your service
+might receive the following requests:
+
+Upload a new file:
+
+```
+PUT https://example.com/upload/foo/bar.jpg?v=49e9309ff543ace93d25be90635ba8e9965c4f23fc885b2d86c947a5d59e55b2
+```
+
+Recipient checks the file size and other headers:
+
+```
+HEAD https://example.com/upload/foo/bar.jpg
+```
+
+Recipient downloads the file:
+
+```
+GET https://example.com/upload/foo/bar.jpg
+```
+
+The only tricky logic is in validation of the PUT request. Firstly, don't overwrite existing files (return 409 Conflict).
+
+Then you need to validate the auth token. This will be in the URL query parameter 'v'. If it is absent, fail with 403 Forbidden.
+
+Calculate the expected auth token by reading the value of the Content-Length header of the PUT request. E.g. for a 1MB file
+will have a Content-Length of '1048576'. Append this to the uploaded file name, separated by a space (0x20) character.
+
+For the above example, you would end up with the following string: "foo/bar.jpg 1048576"
+
+The auth token is a SHA256 HMAC of this string, using the configured secret as the key. E.g.
+
+```
+calculated_auth_token = hmac_sha256("foo/bar.jpg 1048576", "secret string")
+```
+
+If this is not equal to the 'v' parameter provided in the upload URL, reject the upload with 403 Forbidden.
+
+Note: your language/environment may provide a function for doing a constant-time comparison of these, to guard against
+timing attacks that may be used to discover the secret key.