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The sum of PRPs is a secure PRF. That paper gives as a security bound for a sum of two independent PRPs $q^3/2^{2n-1}$, where $q$ is the number of queries and $n$ the block size (i.e. 128 for AES). That means that your construction, correctly used, is more secure than a single PRP, for which the bound is $q^2/2^n$. If you wanted to give an adversary an ...


4

No, this is safe. In fact, if you show a way of distinguish the stream $AES_{k_1}(C) \oplus AES_{k_2}(C)$ from a random stream with fewer than $2^{64}$ outputs, you have just demonstrated a way of distinguishing AES from a random permutation. Here is how this works: suppose we are given Oracle assess to a permutation $P$, which might be $AES_{k_1}$ for ...


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The once part inside of the nonce in CTR mode means effectively "once for this particular key". If you use a fresh key for each message (e.g. by encrypting it using public-key crypto or similar), you can use the same nonce for all the messages (or a size-zero nonce). The important part is that the combination of nonce and ctr-value (i.e. what is input into ...


1

Some cryptographers feel that the ultimate goal for an encryption scheme is semantic security or, even better, perfect security. An encryption scheme that supports de-duplication also allows the backup server -- and the attacker, who we assume steals every backup tape that the server sysadmins send to the off-site backup location -- to detect which parts ...



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