# AES and One Time Passwords

I have used AES to secure my short term wireless network(1 day lifetime). The same symmetric key is used for securing entire network traffic. is it possible to improve security with onetime password scheme? ie, Again AES encryption,but with different 128 bit keys each time.

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There isn't necessarily. Your question needs some clarification as to what you're asking, because at the moment its not very clear. How are you using AES, what is a session, etc etc... – figlesquidge May 2 '14 at 7:46
Yes, I agree with figlesquidge. You should treat any answer here with caution until you describe the protocols and how you are using them. Currently we don't even know how the key is actually used. – Maarten Bodewes May 3 '14 at 17:10

Think about it this way.. with AES, you only generate one key, and use that for all encryptions(of the day).., however with OTP a single key won't work and you'll need to have (say) $n$ keys to send $n$ packets securely..

Now, again, you might suggest using a seed with a PRNG to create a $128*n$ bit key for all $n$ packets.. but then here's the problem: You need to maintain states .. the $i^{th}$ packet will only make sense when decrypted with $i^{th}$ key, i.e the string from $128.(i-1):128.i$, so suppose now you send a message, and somehow, say due to racing conditions in connection, your $(j+1)^{th}$ message reaches before $j^{th}$ message, your server won't be able to decrypt it ..

Also, even simply, PRNGs are key-ed functions (the seed IS the key) .. so you'll need to set that up anyway :)

You can probably come up with many more attacks here.. OTPs, as called, are for ONE TIME USE!! two time's a over kill :D ..

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Instead of OTP, can i use simple key regeneration? – Aswini May 3 '14 at 5:51
Instead of OTP, can i use simple key regeneration? ie,128 bit key based on md5 will be generated periodically and will be used as symmetric key for AES. Here the current key can be used as the input for the next key generation. Is this possible? – Aswini May 3 '14 at 6:03
well, I don't thing OTPs are a substitute to key generation .. OTPs are just $\xor(s_i, m_i)$ – Subhayan May 3 '14 at 8:32
ok, can i use MD5 output(128 bit) as the symmetric key for AES? – Aswini May 3 '14 at 14:05