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Jun
4
awarded  Scholar
Jun
4
comment Encrypt-then-MAC paradigm
Great answer that is really helpful to me. Thank you. Is there a mode of operation for AES you can recommend?
Jun
4
accepted Encrypt-then-MAC paradigm
Jun
4
comment Encrypt-then-MAC paradigm
I know that HMAC itself is based on twice the application of a hash function, thus you might wonder, why I asked question 1. My reasoning is when I have something like HMAC(some || concatenaed || fields || ciphertext), it might be possible for an attacker to shift the boundary between "fields" and the ciphertext, because ciphertexts usually don't have a fixed length.
Jun
4
awarded  Supporter
Jun
4
asked Encrypt-then-MAC paradigm
Dec
21
comment Efficient Symmetric Mutual Entity Authentication Protocol
I just used the word "server" to make clear that it's the verifier that receives the authentication token and grants/denies access. I cannot rely on the availability of a cryptographically secure time on the verifier's system nor can I presume both the client and the verifier have a reliable online connection to synchronize their clocks. Also I want to point out that with my scheme the authentication token changes for every new access attempt. Isn't there another light-weight protocol for access control that isn't based on timestamps but offers strong mutual entity authentication?
Dec
21
comment Efficient Symmetric Mutual Entity Authentication Protocol
I cannot use Kerberos since I don't have synchronized clocks. Also a dedicated Kerberos server is not an option and Kerberos not really that efficient. @PaĆ­loEbermann: Let's say you'd use a smartcard as hotel room key where $X$ is the smartcard and $S$ the door of your room. As soon as $X$ has authenticated to $S$, access is granted and the door is opened. Now under the condition that the door is offline I also want to track which door has been opened by that particular smartcard key - i.e. to collect all access (attempts) in a dedicated database.
Dec
20
awarded  Student
Dec
20
comment Efficient Symmetric Mutual Entity Authentication Protocol
Thanks for your answer @PaĆ­loEbermann. If $X$ wanted to have access to server $S$, $X$ would need to provide $S$ a cryptographic evidence about his claimed identity so that $S$ knows whether to grant access or not. But I also want to have that the server $S$ proves his identity to $X$ so that the client itself knows it is accessing the right resource. I don't need to exchange any further information after access has been granted - thus I don't need to agree upon a symmetric encryption key which is used for efficient message encryption. But I want it to be as secure and efficient as possible.
Dec
20
awarded  Editor
Dec
20
revised Efficient Symmetric Mutual Entity Authentication Protocol
added 19 characters in body
Dec
20
asked Efficient Symmetric Mutual Entity Authentication Protocol