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+# HMAC based One Time Password in OP-TEE
+[HMAC] based One Time Passwords or shortly just 'HOTP' has been around for many
+years and was initially defined in [RFC4226] back in 2005. Since then it has
+been a popular choice for doing [two factor authentication]. With the
+implementation here we are showing how one could leverage OP-TEE for generating
+such HMAC based One Time Passwords in a secure manner.
+
+## Client (OP-TEE) / Server solution
+The most common way of using HOTP is in a client/server setup, where the client
+needs to authenticate itself to be able to get access to some resources on the
+server. In those cases the server will ask for an One Time Password, the client
+will generate that and send it over to the server and if the server is OK with
+the password it will grant access to the client.
+
+Technically how it is working is that the server and the client needs to agree
+on shared key ('`K`') and also start from the same counter ('`C`'). How that is
+done in practice is another topic, but RFC4226 has some discussion about it. You
+should at least have a secure channel between the client and the server when
+sharing the key, but even better would be if you could establish a secure
+channel all the way down to the TEE (currently we have TCP/UDP support in
+OP-TEE, but not TLS).
+
+When both the server and the client knows about and use the same key and
+counter they can start doing client authentication using HOTP. In short what
+happens is that both the client and the server computes the same HOTP and the
+server compares the result of both computations (which should be the same to
+grant access). How that could work can be seen in the sequence diagram below.
+
+In the current implementation we have OP-TEE acting as a client and the server
+is a remote service running somewhere else. There is no server implemented, but
+that should be pretty easy to add in a real scenario. The important thing here
+is to be able to register the shared key in the TEE and to get HOTP values from
+the TEE on request.
+
+Since the current implementation works as a client we do not need to think about
+implementing the look-ahead synchronization window ('`s`') nor do we have to
+think about adding throttling (which prevents/slows down brute force attacks).
+
+#### Sequence diagram - Client / Server
+![Client Server based HOTP using OP-TEE](img/sequence_diagram_01.png?raw=true "Client Server based HOTP using OP-TEE")
+
+##  Client / Server (OP-TEE)?
+Even though the current implementation works as a HOTP client, there is nothing
+saying that the implementation cannot be updated to also work as the validating
+server. One could for example have a simple device (a [security token] only
+generating one time passwords) and use the TEE as a validating service to open
+up other secure services.
+
+[HMAC]: https://en.wikipedia.org/wiki/Hash-based_message_authentication_code
+<!--- The link below to mscgen.js.org should be updated when regenerating the image -->
+[link to sequence diagram]: https://mscgen.js.org/?lang=xu&msc=msc%20%7B%0A%20%20wordwraparcs%3Doff%2C%0A%20%20hscale%3D%220.95%22%2C%0A%20%20watermark%3D%22HOTP%20OP-TEE%20%22%3B%0A%0A%20%20tee%20%5Blabel%3D%22TEE%20%2F%20TA%22%2C%20linecolor%3D%22darkgreen%22%2C%20textcolor%3D%22white%22%2C%20textbgcolor%3D%22darkgreen%22%2C%20arclinecolor%3D%22darkgreen%22%2C%20arctextcolor%3D%22darkgreen%22%5D%2C%0A%20%20client%20%5Blabel%3D%22Client%22%2C%20linecolor%3D%22darkgreen%22%2C%20textcolor%3D%22white%22%2C%20textbgcolor%3D%22darkgreen%22%2C%20arclinecolor%3D%22darkgreen%22%2C%20arctextcolor%3D%22darkgreen%22%5D%2C%0A%20%20server%20%5Blabel%3D%22Server%22%2C%20linecolor%3D%22%233a5795%22%2C%20textcolor%3D%22white%22%2C%20textbgcolor%3D%22%233a5795%22%2C%20arclinecolor%3D%22%233a5795%22%2C%20arctextcolor%3D%22%233a5795%22%5D%3B%0A%20%20%0A%20%20client%20note%20client%20%5Blabel%3D%22Shared%20key%20needs%5Cnto%20be%20handled%5Cnusing%20secure%5Cnchannels%20(TLS%2FSSL)%22%5D%3B%0A%20%20client%20%3C%3D%3E%20server%20%5Blabel%3D%22Agree%20on%20shared%20key%22%5D%3B%0A%20%20client%20%3D%3E%20tee%20%5Blabel%3D%22Store%20shared%20key%22%5D%3B%0A%20%20client%20%3D%3E%20server%20%5Blabel%3D%22Login%22%5D%3B%0A%20%20server%20%3D%3E%20client%20%5Blabel%3D%22Request%20HOTP%22%5D%3B%0A%20%20client%20%3D%3E%20tee%20%5Blabel%3D%22Get%20HOTP%20from%20TEE%22%5D%3B%0A%20%20tee%20%3E%3E%20tee%20%5Blabel%3D%22Calulate%20HOTP%22%5D%3B%0A%20%20tee%20%3E%3E%20client%20%5Blabel%3D%22HOPT%20value%22%5D%3B%0A%20%20client%20%3E%3E%20server%20%5Blabel%3D%22Send%20HTOP%20value%22%5D%3B%0A%20%20server%20%3E%3E%20server%20%5Blabel%3D%22Calulate%20HOTP%20locally%22%5D%3B%0A%20%20client%20alt%20server%20%5Blabel%3D%22Client%20HOTP%20%3D%3D%20Server%20HOTP%3F%22%2C%20linecolor%3D%22grey%22%2C%20textbgcolor%3D%22white%22%5D%20%7B%0A%20%20%09%0A%20%20%20%20---%20%5Blabel%3D%22Yes%22%2C%20linecolor%3Dgrey%2C%20textbgcolor%3Dwhite%5D%3B%0A%20%20%20%20server%20%3E%3E%20client%20%5Blabel%3D%22Grant%20access%22%5D%3B%0A%20%20%20%20%0A%20%20%20%20---%20%5Blabel%3D%22No%22%2C%20linecolor%3Dgrey%2C%20textbgcolor%3Dwhite%5D%3B%0A%20%20%20%20server%20%3E%3E%20client%20%5Blabel%3D%22Access%20denied%22%5D%3B%0A%20%20%7D%3B%0A%7D
+[RFC4226]: https://www.ietf.org/rfc/rfc4226.txt
+[security token]: https://en.wikipedia.org/wiki/Security_token
+[two factor authentication]: https://en.wikipedia.org/wiki/Multi-factor_authentication
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