Hot answers tagged des
9
Within the DES block cipher itself, the XOR operation is used at two different places:
On the input of S-boxes, XOR-ing 48 bits per round: 48 bits from a subkey (extracted from the 56-bit key), and 48 bits that are the output of expansion E. The 48-bit result forms the eight 6-bit inputs of the S-boxes.
On the output of S-boxes, XOR-ing 32 bits per round: ...
8
The main difference is that with two 56 bit keys the maximal security level is 112 bit, and thus an attack that has a cost of $2^{112}$ operations is no attack, whereas for three 56 bit keys the maximal security level is 168 bits, and an attack that has a cost of $2^{112}$ operations counts as an attack.
This means that two-key 3DES is still a bit weaker ...
6
Main drawbacks of DES are:
Small key space (56 bits).
Small blocks (64 bits).
Terrible performance in software, due to all the bit-juggling.
Relative weaknesses with regards to linear and differential cryptanalysis.
Changing any of these will imply heavy changes, not little tweaking. Doing that while maintaining or increasing security is no mere feat... ...
6
I'm not going to look up the DES key schedule such, but the connection between your two sequences
$$( a_i ) = ( 1, 2, 4, 6, 8, 10, 12, 14, 15, 17, 19, 21, 23, 25, 27, 28 )$$
and
$$( b_i ) = ( 1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1 )$$
is obvious: $b_i = a_i - a_{i - 1}$ (with $a_0$ taken to be 0).
That is, the second sequence gives the ...
5
As for the salt, it is a two-character string chosen from the set of printable characters leading to an effective 12-bit entropy.
The fact that every eigth bit of the password is dropped is due to the DES itself: from the 64-bit key, only 56 bits are actually used, thereby dropping 8 of the bits of the key. So, you just input the 64-bit password as the ...
5
The question has morphed over time. I am answering the following.
So to be sure, with DES, only when you encrypt something twice with a weak key. You get the back the original plaintext?
That is correct as that is the definition of a DES weak key, a key for which encryption and decryption have the same effect.
So when using DES in OFB mode with a ...
3
Using the -k option, you can specify a password. Passwords are not really encryption keys, so OpenSSL uses a key derivation process to turn the password into an encryption key. It turns out by default OpenSSL uses a salt in that derivation process (which is why you see Salted in the output).
If the salt changes, the encryption key changes. If the encryption ...
3
Let us denote by $x = x_5x_4x_3x_2x_1x_0$, where $x_i \in \{0, 1\}$ the input of a DES S-box and by $y = y_3y_2y_1y_0$, with $y_i \in \{0, 1\}$ its output. Basically, $\mathrm{NS}_5(16, 15) = 12$ means that for S-box #5, the relation $x_4 = y_3 \oplus y_2 \oplus y_1 \oplus y_0$, where $y = S_5(x)$, holds with probability $\frac{\mathrm{NS}_5(16, 5)}{64} = ...
2
The question is actually about Simplified DES (rather than Simple DES as stated), which is an extremely simplified mock-up of Single DES.
From reading the definition of S-DES in the reference used in the question, we get that:
The S-boxes operate as follows. The first and fourth input bits are treated as a 2-bit number that specify a row of the S-box, ...
2
Yes.
For DES, it looks like the composition $T_i\circ T_j^{-1}\circ T_k$ is normally not a $T_l$ with another key $l$.
I do not have a proof of this fact.
So DES is not a pure system, and otherwise triple-DES would not more secure than DES itself.
In a comment, fgrieu linked a proof for a weaker fact, that DES is not a group (by Campbell and Wiener).
...
1
A quick follow up, there is a problem with using DES in OFB mode when you are not using the full feedback register.
The generated keystream will become cyclic with on average a period of the order $2^{32}$ instead of $2^{64}$.
See
(R.R. Jueneman, “Analysis of certain aspects of Output Feedback Mode,” Advances in Cryptology, Proceedings Crypto’82, D. ...
1
You are right that a brute force attack on DES requires a single plaintext/ciphertext pair; another plaintext/ciphertext pair is useful to confirm the result once found (and rule out a false positive).
No, $DES_{k_1}(x) = DES_{k_2}(x)$ does not imply $k_1=k_2$; the same holds with high probability for any ideal cipher, modeled as a key-dependent family of ...
1
A Karnaugh map is definitely the way to go to understand the concept and derive the equations in a few binary variables.
If you want to do this for a larger numbers of variables, look into Quine-McCluskey. There's a sizable introductory literature in logic design that is relevant to this.
In general, since algebraic cryptanalysis involves doing a lot of ...
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