# Tag Info

2

An "encryption scheme" defines the encryption/decryption of data. A "message transmission scheme" is about securing transmission and defines both "privacy" and "authenticity" between a sender and a receiver. Since you haven't asked about the definition of CCA-secure (encryption) schemes and since you've been given this as an exercise, I won't mention ...

0

Assuming that Alice and Bob have exchanged keys before, when Alice receives the message [R]B, she knows that B has sent that message at some point. Therefore she knows (at most) that B has participated in the protocol. She doesn't know that Bob has ever sent the message [R]B to her, maybe he sent it to Charlie and Charlie or Mallory resent it. There is no ...

1

AES-128 uses the full set $\{0, 1\}^{128}$ as keyspace, and for each key the blockcipher is defined for each input block in $\{0, 1\}^{128}$. The same goes for AES-256, but it uses a 256-bit keyspace (but still a 128-bit block). So the answer to 1 is yes. For 2, we have this equation: $$AES_K(AES_K^{-1}(x)) = x$$ We can decrypt both sides: ...

2

I'll expand my comments into a full answer. Start by examining that you know the value of $r^3$ - this is just $F(r)$. You know you can express $F(r+1)$ and $F(r+2)$ symbolically. You can do this for any term in the key sequence, but it will be useful to write down $F(r+3)$. Forget any constant terms in the computations because they can be added in at ...

5

The modulo operator keeps the result of the addition of $M$ and $K$ within the set $Z$. For example, if $m$ is 10, $M$ is 6 and $K$ is 5, $M + K$ would be 11 which is no longer in the set $Z$. Taking 11 mod 10 results in 1 which is in the set $Z$. As a help towards answering the question whether scheme $M + K$ mod $m$ is perfectly secure, when $m$ is 26 ...

3

Background: An infinite sequence $c_0,c_1,c_2,\dots$ is generated by a (linear feedback shift register (LFSR) with) polynomial $f(x) = \sum_{i=0}^n f_i x^i$ if for any $j$, $$\sum_{i=0}^n c_{j+i} f_{n-i} = 0 \text.$$ We can consider the sequence as a power series $\sum_{i=0}^{\infty} c_i x^i$. If we multiply this power series with the polynomial $f(x)$, we ...

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