# Tag Info

4

Yes. The keys are indeed used in a linear manner. In particular, they are used in $E$-$D$-$E$ mode: encrypt using first 56 bits as key, decrypt using next 56 bits as key and then again encrypt using final 56 bits. This way its possible to use triple DES (which is officially called TDEA) for the DES, 2-DES and 3-DES variations. The first would use ...

4

You are quite correct. A PRP in counter mode is, in fact, distinguishable from a random sequence if you approach the "birthday bound". We get around this by never generating that much output at once. With a 128 bit block cipher, an output of $2^{40}$ bytes (which is a lot of output) gives us a distinguishing advantage of about $2^{-56}$ (the probability ...

4

The reference for this is NIST SP800-38A, especially its appendix B. Basically we consider the IV a binary value of the width of the block cipher (64-bit for DES, 128-bit for AES), and add 1 to that, except for one detail: there is no carry at some application-specified rank, defining the maximum number of blocks that can be enciphered with a single IV; if ...

3

In CTR, you can use any operation which has a full cycle through the space of the IV with the counter. You could use the plus operator like the example: $69dda8455c7dd4254bf353b773304eec + 1 = 69dda8455c7dd4254bf353b773304eed$ To calculate the next value, just again add 1. You could also use a increasing counter and xor it with the original IV: ...

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