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Questions tagged [block-cipher]

A block cipher is an encryption algorithm which encrypts fixed-size blocks of plaintext to same-sized blocks of ciphertext. For good ciphers every bit of the ciphertext block depends on every bit of the plaintext block and every bit of the key.

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How many number of sequences (sample size) should be selected for randomness testing of bock cipher with 256 bits key and block length?

I am working on randomness testing of block cipher. I have gone through the NIST two papers of randomness testing of AES candidate algorithm, links are given below https://csrc.nist.gov/csrc/media/...
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Why is ShiftRow important in AES?

I came across a comment about AES 128 without ShiftRow. I want to use this weakness to perform an attack. So far, I get that It possible to divide in 4 independent blocks of 32 bits. So we can launch ...
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How to crack sha256 used as streamcipher with known or guessable block

Using SHA-256 as a stream cipher appears to bear cryptographic weaknesses, however, I am not quite sure how to implement them in decryption. Assume that I have an encryption function using 64-byte ...
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Why not use a random permutation as a block cipher?

The purpose of a block cipher is to act like a random permutation, and indeed a common security definition is one in which the block cipher is taken to be indistinguishable from a random permutation (...
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What is the difference between AES-CCM8 mode and AES-CCM mode?

I know AES-GCM and AES-CCM but what is the difference between AES-CCM8 mode and AES-CCM mode ? Is it the length of the cipher?
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How do different types of ciphers relate to each other?

I understand that there are symmetric and asymmetric (public-key) cyphers. The first have the same key used for encryption and decryption, while the second use a public key for encryption and a ...
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Post-quantum security of multiple-encryption with CTR mode of operation while keeping the IVs secret

I received an answer in one of my questions saying that multiple-encryption with CTR mode of operation is vulnerable to a sort of meet-in-the-middle attack if the IVs are public. The same user said ...
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Which cipher is more secure TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA or TLS_RSA_WITH_AES_256_GCM_SHA384?

TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA supports forward secrecy but it doesn't use GCM mode and use SHA1 TLS_RSA_WITH_AES_256_GCM_SHA384 uses GCM mode and SHA2 but it doesn't support forward secrecy. ...
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What would be the consequences of using a IV smaller than the block size in CBC encryption?

Here I use a Threefish kernel module with CBC mode. But there is no hash function that can supply a IV with Threefish block size (1024-bits), so I use a 64-bit random IV inserted in the dm-crypt with ...
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CBC MAC forgery

$\text{CBC-MAC}$ is defined as follows: $\text{CBC-MAC}_k() = IV $ $\text{CBC-MAC}_k(m_1, . . . , m_n) = E_k(m_n \oplus \text{CBC-MAC}_k(m_1, . . . , m_{n−1}))$ My lecture notes say that this is ...
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In Feistel block ciphers, does using a weak but iterated encryption function result in a cryptographically strong cipher? [duplicate]

There is a Feistel block cipher based on Blowfish called Kaweichel. In one of its papers, there is this affirmation: For the construction of the round function one choses usually parallel ...
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Show that block cipher with CTR is a good PRNG

Assume that block cipher scheme $(KeyGen, Enc, Dec)$ is CPA-secure. Show that $CTR_{Enc_k(0^n)}$ is a good PRNG. Counter mode works as follows: $Y_i = Enc_k(IV + f(i))$ $C_i = Y_i \bigoplus P_i = Y_i \...
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What kind of assumptions usually go into the design of block ciphers?

What are some standard assumptions made in showing the security of a block cipher? For example, is it commonly assumed that $P\not=NP$? To this end, are there any block ciphers whose security does ...
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Using a PRP $\pi: \{0,1\}^{16} \to \{0,1\}^{16}$ to construct an ideal cipher

Say we have an ideal 16-bit PRP. Since it appears that a permutation with a small domain can be used to turn it into a PRF, which can then be used in a Feistel network. On the surface, it makes it ...
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Does inserting an entry of 32/64-bits into a Feistel S-box would consume the same cycles per byte as inserting a single byte?

Blowfish splits a 32-bit word into 4 bytes and insert each byte as an entry in a S-box. Let's suppose I do the same but with an entire word, 32 or 64-bits. MARS block cipher does the same with 32-bit ...
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Security of Blowfish with secret S-boxes

There are similar questions about secret S-box(es), but none of them about secret Blowfish (or any other canonical Feistel cipher) S-Boxes, only about secret AES S-Box. Let's suppose that instead of ...
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If converting a SPN Block Cipher to a version with larger block size, should its S-boxes be enlarged too?

Let's suppose I want to convert a Substitution-permutation network block cipher like Kuznyechik to a version with a larger block size. Should its S-boxes be enlarged too in order to achieve similar ...
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16-bit entry in a S-Box of a SPN block cipher: Would it have the same speed/performance than a 8-bit entry?

I learned that in Substitution–permutation block ciphers, the maximum size of S-Box set to receive an entry from a data is the max numeric count of the data, for example, if the entry is 8-bits the S-...
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Is it possible to use words of bits as entries in S-Boxes bigger than 4/8 bits and achive similar security/speed in SPN block ciphers?

Blowfish splits a 32-bit word into 4 sets of 8-bits (1 byte) and use them as entries in its S-Boxes. Kuznyechick splits two 64-bits words in many nibbles (4-bits chunks), use them as entries in its S-...
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In SPN block cipher, is each bit of the ciphertext dependent on each bit of the S-boxes set?

In substitution–permutation network block ciphers like AES, when encrypting a plaintext into a ciphertext, will each bit of the ciphertext be associated to each bit of the S-boxes set in the cipher? I ...
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Encrypting with one key and decrypting with a related key

Given $n$-bit block cipher $E$ (and its inverse $E^{-1}$), define block cipher $E^\prime_k(m) = E_k(E_{f(k)}^{-1}(m))$ where $k,f(k) \in \{0,1\}^n$ and $\forall k:f(k) \ne k$. Under the ideal block ...
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Can the security margin of any Feistel block cipher be multiplied if I just multipy the number of rounds and use different random subkeys each round?

Kaweichel is a port of Blowfish for 64-bit architectures, its key size accepted is 1920-bytes, instead of 16 rounds it has 32 rounds, the double of Blowfish. ICE is a feistel block cipher that has a ...
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If keeping the S-Boxes secret in SPN ciphers increase they security, does using one different S-Box set per round in them increases even more? [duplicate]

I read in this paper that keeping the S-boxes secret in AES raise its security from 128-256-bits to 1812-1940-bits. Let's suppose I use one different S-Boxes set per single round. /\ Does this ...
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Does the security margin of Kalyna block cipher double per for additional 4 rounds? Would be possible to create a version of it with larger key sizes?

I believe this question should be directed to the authors of Kalyna block cipher, but I think they cannot answer me because of the war they are facing in their country actually. I know that AES uses ...
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What would be the disadvantages of very large block sizes in block ciphers?

Let's suppose someone creates a block cipher with 8192-bytes of the block size (65536-bits) or maybe 16384-bytes of the block size (131072-bits). What would be its disadvantages over a block cipher ...
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OFB mode discussion

In OFB mode, I understand that a bit flip in $c_i$ for $i > 0$ only causes a bit flip in message block $m_i$. However, how is it possible for a bit flip in $c_0$ (i.e, in IV) will result in all the ...
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Does random IVs in CTR block cipher mode combined with XOR allow to encrypt number of blocks up to the same numeric count of IV?

I read about CTR block cipher mode in Wikipedia: "If the IV/nonce is random, then they can be combined with the counter using any invertible operation (concatenation, addition, or XOR) to produce ...
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How to calculate/generate the inverse S-box of Kuznyechik block cipher?

Let's suppose I want to modify Kuznyechik block cipher by choosing a random S-box (taken from /dev/random for example). How can I calculate/generate the inverse S-box? Does anyone know the formula or ...
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Replacing a block cipher's key schedule with a stream cipher

Take Rijndael (AES) of block size 128 and reformulate it with a tweakable number of rounds and no key schedule. Suppose we obtain: Rijndael128(r, k) where r is ...
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The different bounds of PRP/PRF switching lemma

The PRP/PRF switching lemma is usually denoted as follows: I understand the proof of this version of the bound $\frac{q(q-1)}{2^{n+1}}$ and the game-playing technique behind it. However, I came ...
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Security analysis of North Korean block cipher Jipsam1

The cipher is a modification to AES and is described on this website: The only difference between Jipsam1 and AES-256 is the S-box. Whereas in AES the S-box is public and constant, namely $$ \begin{...
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8 bit block cipher in CTR mode for practice - counter is 32 bits

I'm given a very simple 8 bit block cipher in which to encrypt plaintext x with k k one performs $ x \oplus k $. I am given IV = 0101 and a CTR of value CCCCCCCC (36bits). My understanding is that ...
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How are keys shared safely for block / stream encryption?

I've picked up a grad level computer security class. I read a brief overview on block and stream encryption. I understand and have used PGP in the past and it make since to share your public key. ...
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Cracking a key using a combination of partial keys

As part of a cryptanalytic attack I've carried out, I used a divide-and-conquer strategy to divide the secret key $k$ into several equal-sized partial keys $k = {k_1, k_2, ...}$. The attack works ...
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Objective block cipher round function measures of security

One issue that may arise when attempting to evaluate the security of a round function for a block cipher is that the analysis of the round function does not treat the round key space and the message ...
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What is benefit of using substitution before permutation or permutation before substitution in a block cipher?

I am trying to understand the difference between the usage of permutation and substitution in a SP Network. Normally, in ciphers except DES, substitution appears to be done before permutation. But ...
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How to understand the "Authenticity" and "Privacy" of CCM-mode encryption scheme?

CCM mode refers to CTR+ CBC-MAC encryption mode. Based on this paper, the adversary's advantages against the authenticity of CCM is: Eq(1) Authenticity: it should be infeasible for an adversary to ...
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Is F' = G(F)) a secure PRF given F and G are secure? [closed]

If we have a secure PRF 𝐹(𝑘, 𝑥) and a PRG G where 𝐺 : 𝒴 → 𝒴 × 𝒴 is a secure PRG. Is the PRF F'(k, x) = = 𝐺(𝐹(𝑘, 𝑥)) also secure?
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Have the automorphism groups of block ciphers like the different version of the AES or DES been calculated?

Suppose that $F:K\times X\rightarrow X$ is a function. If $k\in K$, then let $F_{k}:X\rightarrow X$ be the mapping defined by letting $F_{k}(x)=F(k,x)$ for each $x\in X$. Then we shall call $F$ a ...
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Can RSA (theoretically) be used as a block cipher?

Assume AES gets a devastating attack making it unusable. Is it possible to use RSA in a secure way that can encrypt data in bulk? I can't see how RSA (assuming OAEP padding) can be used in normal ...
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Is secure of RC6 cipher consider as weak secure? / how to increase safety in RC6 cipher?

I saw a paper said RC6 cipher is weak and weakness cause of using 'SINGLE CLASS OF WEAK KEYS' What is that mean? and could we solve this problem ? Thank you.
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Ideal cipher vs Ideal encryption scheme

Ideal cipher is a random permutation for every key in its key space. And, ideal encryption scheme is the one which has perfect secrecy/indistinguishability. For an encryption scheme, random ...
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Time and space complexity analyzation of meet and middle attack of triple DES

How can we analyze the time and space complexity of meet and middle attack on a Triple-DES?
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Encrypting a random block with XTS and XORing into the plaintext: Would this encipherment scheme be secure?

I had an idea to make XTS mode parallelizable at CPU level. Let's suppose I take two AES-256 keys for XTS and encrypt a 512-byte block with this block cipher mode and XOR the output into the plaintext....
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What percentage of the keyspace should be brute-forced to find an equivalent key if the block cipher has?

I have been reading the Hasty Pudding Cipher specification and the author make clear that the cipher has some equivalent keys if the keyspace is larger than 8192-bits: ...
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How does dm-crypt in Linux make non-parallelizable encryption modes have all the 512-byte blocks different each other?

dm-crypt in Linux uses 512-byte blocks (disk sectors) having sub-blocks of encrypted plaintext for block cipher modes that are non-parallelizable: "With the usual modes in cryptsetup (CBC, ESSIV, ...
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Is multiple encryption using CBC mode of operation susceptible to Meet-in-the-middle attacks?

I read it once on a page (I don't remember the link and I coundn't find it) that said about a cascade of AES with two 256-bits keys and that it provides 384-bits of security. Maybe not 512-bits of ...
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Substitution-Permutation Network (SPN) Example

I came in contact with an algorithm that deals with SPN in an example, first I'd like to give a definition of what it is: Let $l, m, $ and $N$ be positive integers, let $\pi_s: \{0,1\}^l \to \{0,1\}^...
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Is multiple encryption using a block cipher mode of operation that use only encryption processes vulnerable to Meet-in-the-middle attacks?

Some block cipher modes of operation use only encryption processes, such as CFB, OFB and CTR. If doing multiple encryptions using them, will these encipherment schemes be vulnerable to Meet-in-the-...
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How to encrypt affine cipher in block?

Assuming the alphabet of capital letters, if an affine linear cipher $v\to Av+b \bmod 26$ with block length $3$ maps plaintext ENCRYPTAGAIN to ciphertext ...

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