Note: In the following post OTP = one-time-password
I am working on a project wherein I need send one way messages between two end points. There is already a shared secret via a side channel between the two end points. No user is not involved at either of the end points. Due to some constraints I don't have control whether or not encryption will be used when the message is actually send, so I have to assume the message is going over plain text. The reader can assume the messages as some sort of "action".
The packet carrying the message also contains HMAC of the message calculated using the shared key. I need to prevent replay attack. From wikipedia, I see the options being: OTP, nonce and time stamp.
I can only assume that the clocks between two end points are roughly synchronized (both end points have some reliable time source). I'm thinking of adding some kind of nonce when computing the HMAC. I want to ask whether the option I selected below would be correct for nonce. I want to use some standard technique. I do understand that nonce cannot be repeated.
My first question is that whether certain OTP values can be considered as nonce (the wikipedia categories make them appear distinct).
I found the TOTP algorithm which can calculate a one time password using a shared key and timestamp. I want to ask whether TOTP can be used as nonce.
TC = floor((unixtime(now) − unixtime(T0)) / TS), TOTP = HOTP(SecretKey, TC), TOTP-Value = TOTP mod 10d, where d is the desired number of digits of the one-time password.
TC will be stored in an unsigned 64 bit variable. Since HOTP is a hash function (assume SHA-256), TOTP won't repeat (until the unsigned 64 bit TC reaches its max value and wraps around). My second question is whether TOTP can be used as nonce when calculating HMAC ?