Flip for identity on auth account

flips/20220518-authaccount-identity.md

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+# Title of FLIP
+
+| Status        | (Proposed / Rejected / Accepted / Implemented)       |
+:-------------- |:---------------------------------------------------- |
+| **FLIP #**    | [NNN](https://github.com/onflow/flow/pull/NNN) (update when you have PR #) |
+| **Author(s)** | Bjate Stien Karlsen (bjarte@find.xyz)         |
+| **Sponsor**   | AN Expert (core-contributor@example.org)      |
+| **Updated**   | 2022-05-18                                    |
+
+## Objective
+
+One of the most asked questions regarding access control in Cadence is: "How do I know what user called this method?". In Solidity the answer is `msg.sender`. 
+However, [this pattern has several problems](https://docs.onflow.org/cadence/msg-sender/) and it is not recommended.
+
+One of the reasons it is not recommended is that there is no secure way of doing that.  This FLIP aims to create a way to do it securely.
+
+*Note: This FLIP does not cover __when__ you should use this instead of other patterns for identifying the sender, it only provides a way to do it safely  _if_ you need to do it.
+
+## Motivation
+
+Add a secure to identify if  a particular auth account signed a transaction. 
+
+## User Benefit
+
+It makes it possible to use patterns that are common to Solidity developers in Flow.
+
+## Design Proposal
+
+The design proposal and a implementation is provided in the following PR
+https://github.com/onflow/cadence/pull/1363/files
+
+### Drawbacks
+
+Currently the documentation states that this pattern is not a best practice. Should this documentation be changed?
+
+
+### Best Practices
+
+It makes it possible to do something that it is documented is not best practice. Should this documentation be changed?
+
+### Tutorials and Examples
+
+See PR for examples and changed docs
+
+### User Impact
+
+A user will be able to call `authAccount.identity` and get a token representing the ability to access to an account, without it being obtainable any other way, i.e. it cannot be forged, as there is no constructor function for the  `AuthAccount.Identity` type.
+
+
+## Related Issues
+
+https://github.com/onflow/cadence/pull/1363/files
+
+## Prior Art
+
+Solidity provides the `msg.sender` field which allows identifying who signed the currently running transaction.