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+Authentication guide - SAML
+===
+
+The basic procedure is the same as the case of OneLogin which is mentioned in [OneLogin-Guide](./saml-onelogin.md). If you want to match your IdP, you can use more configurations as below.
+
+* If your IdP accepts metadata XML of the service provider to ease configuraion, use this url to download metadata XML.
+    * {{your-serverurl}}/auth/saml/metadata
+    * _Note: If not accessable from IdP, download to local once and upload to IdP._
+* Change the value of `issuer`, `identifierFormat` to match your IdP.
+  * `issuer`: A unique id to identify the application to the IdP, which is the base URL of your HackMD as default
+  * `identifierFormat`: A format of unique id to identify the user of IdP, which is the format based on email address as default. It is recommend that you use as below.
+    * urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress (default)
+    * urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified
+  * config.json:
+    ````javascript
+    {
+      "production": {
+        "saml": {
+          /* omitted */
+          "issuer": "myhackmd"
+          "identifierFormat": "urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified"
+        }
+      }
+    }
+    ````
+  * environment variables
+    ````
+    HMD_SAML_ISSUER=myhackmd
+    HMD_SAML_IDENTIFIERFORMAT=urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified
+    ````
+
+* Change mapping of attribute names to customize the displaying user name and email address to match your IdP.
+  * `attribute`: A dictionary to map attribute names
+  * `attribute.id`: A primary key of user table for your HackMD
+  * `attribute.username`: Attribute name of displaying user name on HackMD
+  * `attribute.email`: Attribute name of email address, which will be also used for Gravatar
+    * _Note: Default value of all attributes is NameID of SAML response, which is email address if `identifierFormat` is default._
+  * config.json:
+    ````javascript
+    {
+      "production": {
+        "saml": {
+          /* omitted */
+          "attribute": {
+            "id": "sAMAccountName",
+            "username": "displayName",
+            "email": "mail"
+          }
+        }
+      }
+    }
+    ````
+  * environment variables
+    ````
+    HMD_SAML_ATTRIBUTE_ID=sAMAccountName
+    HMD_SAML_ATTRIBUTE_USERNAME=nickName
+    HMD_SAML_ATTRIBUTE_EMAIL=mail
+    ````
+
+* If you want to controll permission by group membership, add group attribute name and required group (allowed) or external group (not allowed).
+  * `groupAttribute`: An attribute name of group membership
+  * `requiredGroups`: Group names array for allowed access to HackMD. Use vertical bar to separate for environment variables.
+  * `externalGroups`: Group names array for not allowed access to HackMD. Use vertical bar to separate for environment variables.
+    * _Note: Evaluates `externalGroups` first_
+  * config.json:
+    ````javascript
+    {
+      "production": {
+        "saml": {
+          /* omitted */
+          "groupAttribute": "memberOf",
+          "requiredGroups": [ "hackmd-users", "board-members" ],
+          "externalGroups": [ "temporary-staff" ]
+        }
+      }
+    }
+    ````
+  * environment variables
+    ````
+    HMD_SAML_GROUPATTRIBUTE=memberOf
+    HMD_SAML_REQUIREDGROUPS=hackmd-users|board-members
+    HMD_SAML_EXTERNALGROUPS=temporary-staff
+    ````