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41

The really simple explanation for the difference between the two is this: ECB (electronic code book) is basically raw cipher. For each block of input, you encrypt the block and get some output. The problem with this transform is that any resident properties of the plaintext might well show up in the ciphertext – possibly not as clearly – that's what blocks ...


38

The initial and final permutation have no influence on security (they are unkeyed and can be undone by anybody). The usual explanation is that they make implementation easier in some contexts, namely a hardware circuit which receives data over a 8-bit bus: it can accumulate the bits into eight shift registers, which is more efficient (in terms of circuit ...


38

There are a variety of reasons why AES is more widely used: AES is a standard. AES has been vetted by cryptanalysts more extensively than Camellia. As a result, we can have greater confidence in the security of AES than in Camellia. Therefore, on the merits, there may be good reasons to choose AES over Camellia. AES is a government standard (FIPS). ...


37

For practical purposes, 128-bit keys are sufficient to ensure security. The larger key sizes exist mostly to satisfy some US military regulations which call for the existence of several distinct "security levels", regardless of whether breaking the lowest level is already far beyond existing technology. The larger key sizes imply some CPU overhead (+20% for ...


25

The difference between the PKCS#5 and PKCS#7 padding mechanisms is the block size; PKCS#5 padding is 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) as defined in the RFC: The padding string PS shall consist of 8 - (||M|| mod 8) octets all having value 8 - ...


24

As a bonus feature, AES has hardware support in Intel processors which implement the AES instruction set, with AMD support coming soon in their Bulldozer based processors. The AES instructions set consists of six instructions. Four instructions, namely AESENC, AESENCLAST, AESDEC, AESDECLAST, are provided for data encryption and decryption (the ...


23

A known-plaintext attack (i.e. knowing a pair of corresponding plaintext and ciphertext) always allows a brute-force attack on a cipher: Simply try all keys, decrypt the ciphertext and see if it matches the plaintext. This always works for every cipher, and will give you the matching key. (For very short plaintext-ciphertext pairs, you might get multiple ...


23

Applied Cryptography is book which is becoming, say, not-so-recent. NSA has quite a lot of budget, but not an infinite amount, and there are other organization, in particular big private corporation, which also have impressive means. Google or Apple, for instance, are companies with R&D activity in the area of cryptography, and who are able to ...


22

Assume that 1 evaluation of {DES, AES} takes 10 operations, and we can perform $10^{15}$ operations per second. Trivially, that means we can evaluate $10^{14}$, or about $2^{46.5}$ {DES, AES} encryptions per second. This is a simplistic view: we are ignoring here the cost of testing whether we found the correct key, and the key schedule cost. So on our ...


22

Why shouldn't I use ECB encryption? The main reason not to use ECB mode encryption is that it's not semantically secure — that is, merely observing ECB-encrypted ciphertext can leak information about the plaintext (even beyond its length, which all encryption schemes accepting arbitrarily long plaintexts will leak to some extent). Specifically, the ...


21

The actual encryption algorithm is almost the same between all variants of AES. They all take a 128-bit block and apply a sequence of identical "rounds", each of which consists of some linear and non-linear shuffling steps. Between the rounds, a round key is applied (by XOR), also before the first and after the last round. The differences are: The longer ...


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"), ...


16

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 ...


15

Basically it's analysis of a cryptographic cypher by the means of finding a relationship between the difference in the input data and the output data. Ideally, the slightest difference in input data (cleartext), even a single bit, should produce a completely different cypthertext. However, if the cypher is not well-designed, a correlation between the two ...


15

Well, the exact reason for an IV varies a bit between different modes that use IV. At a high level, what the IV does is act as a randomizer, so that each encrypted message appears to be encrypted to a random pattern, even if those messages are similar. In general, IVs disguise when you encrypt the same message twice (and more generally, when two messages ...


15

Many cryptographic algorithms are expressed as iterative algorithms. E.g., when encrypting a message with a block cipher in CBC mode, each message "block" is first XORed with the previous encrypted block, and the result of the XOR is then encrypted. The first block has no "previous block" hence we must supply a conventional alternate "zero-th block" which we ...


15

Not only we can turn block ciphers into hash functions, but we do. The usual hash functions (MD5, SHA-1, SHA-256...) use the Merkle-Damgård construction which relies on a block cipher E. A running state r is initialized to a conventional value. Then the input data is split into a number of chunks, each chunk being used as key for the block cipher: r is ...


15

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 ...


14

In complete honesty: if you have to ask this question, it's overwhelmingly unlikely that you have actually succeeded in breaking the security of AES. At best, you may have discovered a well-known attack against misuse of particular block cipher modes; for instance, plaintext recovery with a chosen-ciphertext attack against ECB, or blind manipulation of the ...


14

You should not use ECB mode because it will encrypt identical message blocks (i.e., the amount of data encrypted in each invocation of the block-cipher) to identical ciphertext blocks. This is a problem because it will reveal if the same messages blocks are encrypted multiple times. Wikipedia has a very nice illustration of this problem.


13

Well, to start off with, IVs have different security properties than keys. With keys (as you are well aware), you need to hide them from anyone in the middle; if someone did learn your keys, then he could read all your traffic. IVs are not like this; instead, we don't mind if someone in the middle learns what the IV is; as long as he doesn't know the key, ...


13

ECB and CBC are only about encryption. Most situations which call for encryption also need, at some point, integrity checks (ignoring the threat of active attackers is a common mistake). There are combined modes which do encryption and integrity simultaneously; see EAX and GCM (see also OCB, but this one has a few lingering patent issues; assuming that ...


13

What you're looking for can be done using existing schemes for format preserving encryption (FPE). In general, FPE schemes convert an existing strong algorithm like AES into a block cipher that operates on a set of any size. For instance, FPE can encrypt 15 digit integers to other 15 digit integers (eg for credit card numbers, one of the common reasons for ...


13

Let a "block cipher" be defined with a fixed S-box $S$ (i.e. a permutation of some space) and a key $K$ (same size than a block), such that the encryption of a block $M$ is $C = S[P\oplus K]$. Everybody knows $S$ and can apply and invert it (that's a "S-box", not a "key" -- if the S-box is "key dependent" then the S-box is itself a block cipher in its own ...


12

If you look closely at the definition of authenticated encryption modes, you will see they all are, actually, the combination of symmetric encryption and a MAC. Using traditional encryption and an independent MAC has a few tricky points, none of them being unsolvable: The encryption mode will use a key, and the MAC will also use a key; using the same key ...


12

This approach, at a high level, is actually fairly common; many stream ciphers operate on this very principle. For instance, Salsa20 uses what is effectively a hash function (a PRF) to convert a secret input (that includes a counter) into the keystream which is XORed with the plaintext. However, this kind of function can be much faster than a secure ...


12

If a block cipher is linear with respect to some field, then, given a few known plaintext-ciphertext pairs, it is possible to recover the key using a simple Gaussian elimination. This clearly contradicts the security properties one expects from a secure block cipher.


12

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 ...


12

The security of that approach is equivalent to that of normal CBC. Your scheme with first plaintext block $IV^\prime$ is clearly identical to normal CBC with $IV=AES(IV^\prime)$. Since a block cipher is a permutation over a block, a uniformly random first plaintext block will lead to a uniformly random IV for normal CBC. A ciphertext produced with your ...



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