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4

Here's a better solution, using a Merkle tree. Suppose the string $S$ is $n$ letters long. Build a binary tree with $n$ leaves, with each leaf corresponding to one letter in the string. Then, each node corresponds to a substring: the $i$th leaf corresponds to the $i$th letter in the string, and an internal node $x$ corresponds to the substring obtained by ...

3

Does it negatively affect security to calculate a hash value of the ciphertext before MAC calculation? Like exchanging step 2. with this: HMAC-SHA256(SHA256(ciphertext)). Technically, yes, but not significantly. In order to attack the scheme you propose, the attacker would have to be able to do at least one of two things: (1) Find an attack on the ...

2

Regarding GCM mode and the uniqueness of the nonce, it should be noted that EAX mode and OCB mode also require unique nonces. One potential problem EAX mode has, which neither GCM or CCM have, is that it is hard to implement it in such way that you can guarantee that the probability of nonce collisions is zero; only that it is acceptably low. OCB mode has ...

1

Applying the hash function assures that only the next password in sequence will be valid. There is no reason to store the previous state, or the client's original secret. Once a valid key is used, the server stores the hash of it (which is the same algorithm used to generate the list of hashes in the first place.) This puts the burden of storage on the ...

1

In entity authentication, the parties do not necessarily exchange a session key, but authenticate to each other by providing some proof related to their identites. For instance, if one party signs a challenge given by its partner, this party proves its identity. Implicit key authentication merely guarantees that the partner (with whom you are talking right ...

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