Henno Brandsma
  • Member for 10 years, 5 months
  • Last seen this week
How does the frequency work with Vigenère Cipher?
1 votes

So in your cipher text, the characters $c_1, c_7, c_{13}, \ldots, c_{1+6i}, \ldots$ should roughly obey the frequency statistics of a permuted or shifted standard English frequency. In the most ...

View answer
Why do we need to compute message digest of a message first while signing it?
1 votes

Alongside the other arguments: signing (with RSA) means exponentiating it modulo $N$ (with power $d$, the secret exponent). So anything you sign that way must be of bitsize smaller than $N$, and ...

View answer
What type of encryption is this?
1 votes

It's base64 encoded binary data, after decoding it (e.g. using the base64 command line tool in Linux or MacOS) we see it consists of 544 bytes of random looking data. As this is a multiple of 16, we ...

View answer
RSA: least significant bit position in key
1 votes

Because (if $C=P^e$ where $P$ is the plain text): $2^e C= (2P)^c \pmod{N}$ so multiplying the ciphertext by $2^e$ modifies the plain text by a multiplication of $2$, which is a bitshift to the left of ...

View answer
What to pad DES key with?
Accepted answer
0 votes

The '0b' representation removes leading 0's. The actual value (as a byte) of l (lowercase letter L) as a binary 8bit byte is 01101100 and likewise for the others (they are lower ASCII, so the highest ...

View answer
What is k in RSA?
2 votes

It's a translation of $ed \equiv 1 \pmod{\phi(N)}$, namely that $e$ and $d$ are each other's inverse modulo $\phi(N)$, which says that $ed-1$ must be an integer multiple of $\phi(N)$ (recall that $a \...

View answer
A simple misunderstanding of SSL/TLS
1 votes

DH in the classic form can be vulnerable to MITM, but the form used in TLS (SSL is deprecated, and hardly used any more, TLS is the more secure successor) is that the server sends its DH-contribution (...

View answer
How are the four 8-bit permutation tables of Present cipher calculated?
Accepted answer
2 votes

In short: Trace back where bits come from in every byte in the output state (8 bytes) of the SP_network. In one round, after we have xored the round key bits, we have 8 bytes (a 64 bit word, with ...

View answer
Hill cipher -- obtain matrix key
1 votes

The key is 4 long, so should be a $2 \times 2$ matrix. In members of $\mathbb{Z}_{26}$ CFDG becomes $2, 5, 3, 6$ in the usual A becomes $0$, Z becomes 25 encoding. After some experimenting I found ...

View answer
Hill Cipher Plaintext Attack: Dealing with Non-uniqueness of Matrix Inverses?
Accepted answer
1 votes

More standard is that we multiply a column with a plain text vector to get the cipher text vector so the known plain text equation (the first two pairs) yield $$KP=C = K\begin{bmatrix}5 & 8\\17 &...

View answer
Problem while decrypting Hill cipher
Accepted answer
2 votes

These modular equations are not uniquely solvable: $$\begin{bmatrix}7&2\\ 10& 20\end{bmatrix}, \begin{bmatrix}7&2\\ 23& 7\end{bmatrix}, \begin{bmatrix}20&15\\ 10& 20\end{...

View answer
Hill Cipher how to encrypt and decrypt when both "I" and "J" are in plaintext
1 votes

"KEYWORD" is a weird format for a Hill cipher, aren't you confused with the Playfair cipher? There you work with a 5x5 matrix where I and J are often conflated into I (as 26 is one too big) and the ...

View answer
Hill Cipher with unequal matrix
1 votes

It's indeed standard to add pre-agreed padding characters at the end to make the plain text a multiple of $n$ when we use an $n \times n$ encryption matrix. So your encoded plain text could be $$\...

View answer
What is the Knowledge Of Secret Key Assumption (KOSK)
1 votes

A zero-knowledge proof for the secret key is a probabilistic challenge-response algorithm where the prover has probability 1 to get it right knowing the secret key, and some chance $p < 1$ if he ...

View answer
Rijndaelcipher mix column query
0 votes

You should consider the field $\textrm{GF}(2^8)$ as a "given", a way to multiply and add bytes (the addition is just XOR, the multiply could be given as a table of 256 by 256 entries; you'll see we ...

View answer
Efficient attack on RSA with known padding scheme and public key
1 votes

It might help to describe the padding arithmetically: first shift $p$ (the pin) $2048$ bits to the left, so compute $2^{2048}\cdot p$. To replace the trailing $0$'s by $1$'s we add $2^{2048}-1$. So ...

View answer
IVs in WEP encryption
0 votes

It must be sent with the message in order to be decipherable to the legitimate decipherer. The message sent has the IV as the first part. The shared key $K$ is known, so the "packet key" $K'$ is $\...

View answer
Discrepancy in modular encryption
1 votes

This is a toy system that is used in didactic contexts as an introduction to RSA, in a way. We have some prime $p$ (we are going to do the power operations modulo this prime). If $m$ is the message (...

View answer
Blowfish encryption with different keys produces same ciphertext
3 votes

It looks like the library is treating the string as the key to Blowfish, which has a veriable key size; the way the keysetup is done (with a cyclical use of the key bytes, see more details on the ...

View answer
Difference between when select x from $\mathbb{Z}_{p-1}$ and $\mathbb{Z}_p$ in discrete logarithm Problem?
1 votes

Because $\mathbb{Z}/{p\mathbb{Z}}$ when used for El-Gamal is used in its multiplicative form: we have a generator $g$ such that all powers of $g$ cycle through $\{1,2,\ldots,p-1\}$ (so $0$ is excluded)...

View answer
Why is padding the plaintext with a random string before encryption worse than OAEP / PKCS#7?
3 votes

The padding used for RSA is not the PKCS #5/#7 padding (as you seem to suggest in your own answer), but the Wikipedia entry seems to refer to PKCS #1 v1.5 (RFC2313)) which uses a padding 00 || BT || ...

View answer
Hash algorithm for user accounts passwords in macOS 10.12, 10.13, 10.14
3 votes

As @kelalaka said, the answer is still valid as described in this question, so a PBKDF2-SHA512 hash with a large salt and iteration count in the 100000's, usually.

View answer
AEAD using AES-CBC and MAC: how to order the input to the MAC
Accepted answer
2 votes

This part of the standard (at the end) explains why the length of the authenticated data is at the end, namely to ensure a unique input to the MAC for all pair of ciphertext and authenticated data: ...

View answer
How many keys in a Hybrid Cryptosystem?
1 votes

In a typical hybrid system (PGP, S-MIME etc.), a message contains both an encrypted key $k_s$ for the symmetrical system, encrypted by the recipient’s public key. A new $k_s$ is generated per message....

View answer
RSA How to select a value K such that $e d = 1 + k \varphi(N)$ holds
Accepted answer
1 votes

You apply the extended Euclidean algorithm to $e$ and $\phi(N)$ (which have to have gcd equal to $1$) and we get $x,y \in \mathbb{Z}$ such that $$xe + y\phi(N) = 1$$ The $x$ (taken modulo $\phi(N)$, ...

View answer
In modular arithmetic, why is $\left(g^{k_1} \bmod n\right)^{k_2} \bmod n = \left(g^{k_1}\right)^{k_2} \bmod n$?
0 votes

You're just taking two succesive operations, namely to the power $k_1$ and then to the power $k_2$ in the ring of integers modulo $n$, in both cases. You are just confused by the different notations ...

View answer
Diffie-Hellman key exchange: Why is $(g^{k_1} \bmod n )^{k_2} \bmod n \equiv (g^{k_2} \mod n)^{k_1} \bmod n$
1 votes

It's a standard fact in abstract algebra: The statement is just $(g^{k_1})^{k_2} =(g^{k_2})^{k_1}$ in the ring $R=\mathbb{Z}/n\mathbb{Z}$, with $k_1, k_2 \in \mathbb{N}$. This statement is valid in ...

View answer
Question about DES S-boxes from Encryption to Decryption
2 votes

The $S$-boxes are used in the round function of a Feistel cipher, so they will be exactly the same for encryption and decryption. We're not using a SP-network here like AES, where we need the ...

View answer
AES-128 as compression function in Merkle-Damgard construction
3 votes

This short thesis describes some of the basic ways in which to use the Merkle-Damgård construction based on block ciphers. The construction that is commonly used in MD5 and related constructions is in ...

View answer
Base64 encoding of strings
5 votes

The whole purpose of base64 encoding is to take arbitary byte values and transform them into a limited subset of ASCII characters so that they can be "safely" sent (older transmission protocols could ...

View answer
1
2 3 4 5