# Tag Info

## Hot answers tagged block-cipher

17

People found MARS to be clunky and overly complex, leading to more effort for implementation and optimization, and also a less clear overall security picture. Assessments of "security" are, in fact, extremely subjective, because they rely on speculations about unknown future cryptanalytic attack, empiric traditions (e.g. "more rounds" = "more security"), ...

12

The reason why you see that is because Camellia is the highest-preference cipher in NSS (Chrome and Firefox). Servers that support Camellia and use the client-preferred cipher suite will use Camellia. NSS's rationale for this ordering is: National ciphers such as Camellia are listed before international ciphers such as AES and RC4 to allow servers ...

11

I'm just curious to know why the 128-bit version become the standard[.] That question is easy to respond. In the section Minimum Acceptability Requirements of Request for Candidate Algorithm Nominations for the AES, it says: The candidate algorithm shall be capable of supporting key-block combinations with sizes of 128-128, 192-128, and 256-128 ...

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: ...

9

If you look at the CBC diagram, you'll see that having a fixed IV is equivalent to having the first ciphertext block become the IV. If your cipher is a good pseudorandom permutation, then what you are doing does work, if and only if all timestamps are unique such that the "new IV" is unique and unpredictable. And in fact, if you do not use the ...

9

A block cipher is an invertible transformation that maps an $n$ bit block of bits to an $n$ bit block of bits, under the control of a key (and where $n=128$ in the case of AES) Now, we most often need to do things other than mapping blocks of $n$ bits; how we do that is using the block cipher within a Mode of Operation. A mode of operation is just a way to ...

8

Take $C_2$ and pick any $k_2$. Then decrypt using $k_2$ so that $M_2 = AES_{k_2}^{-1}(C_2)$. Now obviously we have $AES_{k_2}(M_2) = C_2 = C_1$. This extends to any blockcipher, because blockciphers are specifically designed to be reversible. In the comments you asked about the scenario where $M_2$ is also fixed. This is as hard as breaking AES. Consider ...

8

At the time of the competition (I can talk about it, I was there), there was a lot of discussion and various people showed arguments. However, there was never an official, publicly known "board of scores" with totals and definite rules, as the pictures you show seem to purport. It is possible that the NIST people did make something similar internally, but ...

7

Eli Biham examined this, along with a long series of other "internal chaining" modes in the paper "Cryptanalysis of Multiple Modes of Operation" in AsiaCrypt 1994. His conclusion was that there were ways of attacking such modes that were strictly easier than the standard "external chaining" mode that is now commonly used; his paper is the chief reason that ...

7

The most common way to transmit an initialization vector is, indeed, to prepend it immediately before the ciphertext. When you look at the original ciphermodes the first used IVs (CBC, CFB, OFB), the IV actually does function as a 'previous ciphertext block' for the very first actual ciphertext block; placing it immediately in front of the very first ...

7

There is only one main difference between PKCS#5 and PKCS#7 padding is the block size. PKCS#5 padding is only defined for 8-byte block sizes. PKCS#7 padding would work for any block size from 1 to 255 bytes. This is the definition of PKCS#5 padding (6.2): The padding string PS shall consist of 8 - (||M|| mod 8) octets all having value 8 - (||M|| mod ...

7

Yes, they can be described as a multivariate polynomial over $GF(2)$ (or over $GF(2^8)$). See algebraic cryptanalysis. This expression does not seem to help cryptanalyze AES, so far as we know, but it can be done. For an example of how to write AES in this way, see the following paper: A simple algebraic representation of Rijndael. Niels Ferguson, ...

6

This is not a "block cipher" because a block cipher is a key-dependent permutation of the space of blocks of a given size. Here, you handle data by blocks, but the "encryption" part is done by XORing with a value $H(k+n)$ which depends on the key $k$ and on the "block number" $n$. So you do not have one permutation (for a given key), but a lot of them. ...

6

The question makes a number of statements that are incorrect. It is not correct that a fixed point is guaranteed to exist. It is not correct that if you hold the plaintext constant and vary the key, then a fixed point is guaranteed to exist. Moreover, the existence of fixed points has only an extremely tenuous connection to security. Assume $E$ is a ...

6

In their 2012 paper "The Security of Ciphertext Stealing", Phillip Rogaway, Mark Wooding and Haibin Zhang prove that all the NIST-approved ciphertext stealing modes provide the same level of security as ordinary CBC mode, i.e. ciphertext indistinguishability under a chosen-plaintext attack. To quote their abstract: "Abstract. We prove the security of ...

5

Most vulnerabilities in block ciphers are related to key security. Successful attacks have not been practical against anything except smaller keysizes than 256 bits or fewer rounds of encryption. Since there are no variables to be selected for AES except the S-box and the P-box, the Holy Grail is key management. Lateral attacks against AES rely on bad ...

5

A block cipher is pretty much a substitution cipher. So let's look at a simple alphabetic substitution cipher. There are 26 different plaintexts and 26 different ciphertext. The cipher is a permutation of these 26 values. But that does not mean there are 26 different permutations, it means that there are $26! \approx 2^{88.4}$ different permutations, which ...

5

Biclique cryptanalysis is the current best known attack on AES. It reduces the security of AES-256 from $2^{256}$ to $2^{254.4}$. Related key attacks are not practical attacks as they should never occur in the wild. they are symptomatic of a poor implementation, and contrary to the recommended use of AES. The best known theoretical attack is Grover's ...

5

Well, whether $AES'$ is as secure as $AES$ depends on the length of $k_1, k_2$. If they are both 128 bit, then what you effectively have is a standard 128-bit AES, except that prior to round 6, you replace the running key with an independent key (and you tweaked the last round, but that's cryptographically harmless). Now, it is never a good idea to do ...

5

Analyzable in this case means "simple to study". If your cipher consists of a small function that mixes a few things together, and then you repeat that often then your cipher is more easily understood and more easily fit into previous research knowledge than a overclomplicated large design. This is the case with Simon and Speck. Being easy to analyze is a ...

5

The exact information leaked depends on the mode of operation that is in use. The simplest case is ECB, where a duplicate ciphertext block means that the corresponding plaintext blocks are also equal. For the CBC mode, when two ciphertext block are equal, i.e. $C_i=C_j$, we know that there was equality before applying the block cipher. This in turn ...

5

A 6-byte nonce can expect to receive a collision 0.01% of the time after around 240,000 nonce generations (based on a birthday attack). After 100 such rotations (a little under 17 years, based on your 2-month rotation policy), that comes out to a likelihood of just under 1% of experiencing a collision. On the surface, to me that seems like a reasonable ...

5

Don't believe every claim ever made in any paper ever written, particularly when the paper provides little or no justification for the claim; not everything you read reflects the cryptographic consensus. This is particularly true for a paper written in 2002, which is a time our understanding of authenticated encryption and security engineering was still in ...

5

A block cipher is (or tries to be) a pseudorandom permutation on a given space. Let $\mathcal{M}$ be the set of $n$-bit blocks for a given $n$. There are $2^n$ possible block values, and a permutation on $\mathcal{M}$ sends each block value to another value. There are $2^n!$ such permutations. A block cipher is a mapping from key values (in a given key space ...

5

A mode of operation is an explicit method by which we use a block cipher (eg AES) to do more than just encrypt one block of data. For example, it may allow us to encrypt multiple blocks of data (eg ECB,CBC etc), provide us with some authenticated encryption (eg GCM) or a method for encrypting disc storage (eg XTS). Rijndael,DES etc are block ciphers. That ...

5

Did you try Wikipedia? DES consists of 16 rounds of the form: $$L_{i+1} = R_{i}, \quad R_{i+1} = L_i \oplus F(R_i, K_i),$$ which are identical except for the round subkeys $K_i$. (The last round is slightly different, in that the half-blocks $L$ and $R$ are not swapped as they are after all other rounds, but that makes no cryptanalytic difference.) The ...

4

A block cipher is any cipher that works(encrypts, decrypts) on data in blocks (more than just one bit or one byte). A stream cipher is a cipher that works on the data one bit or byte at a time. For example, DES is a block cipher because it works on the data in 64 bit chunks. This means that when you want to encrypt something with DES, you must first break ...

4

This will probably be OK. It does have some non-trivial side effects/caveats: The effective key length is reduced to 86 bits. Only the low 22 bits of each of the 4 key words will matter, so only 88 bits of the key material are relevant. Then, there are known equivalent-key properties of TEA that further reduce the effective key length to 86 bits. A ...

4

Many block ciphers are defined by specifying a round and then running that specification multiple times. For example, in AES, a round consists of the operations SubBytes, ShiftRows, MixColumns, AddRoundKey. That is one round and, to get AES, you run that multiple times (plus some setup and some post-processing). Thus a round is defined by each cipher and ...

4

Yes, of course you need to know the mode of operation in order to decrypt. On the other hand, the mode of operation isn't usually explicitly transmitted in the ciphertext. Using the same key with different modes of operation may cause unexpected weaknesses. For example, consider one message encrypted with CFB mode with IV=2 (which is perfectly secure) and ...

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