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51

AES is an algorithm which is split into several internal rounds, and each round needs a specific 128-bit subkey (and an extra subkey is needed at the end). In an ideal world, the 11/13/15 subkeys would be generated from a strong, cryptographically secure PRNG, itself seeded with "the" key. However, this world is not ideal, and the subkeys are generated ...


46

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 - ...


44

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 ...


44

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). ...


43

In the "ideal cipher" model, the block cipher is a permutation of the space of input blocks, chosen uniformly among all such permutations. A plaintext that gets encrypted to itself is a fixed point for the permutation; it is expected that about 63.21% of all permutations have at least one fixed point (a permutation with no fixed point is called a derangement)...


30

Blum-Blum-Shub is a stream cipher: given a short key, it produces an effectively unlimited-length stream of pseudorandom bits. Other well-known examples of stream ciphers include AES-CTR and RC4. Blum-Blum-Shub gets mentioned a lot by non-expert cryptographers. I suspect this is because it comes with a "proof" of security, which sounds like a wonderful ...


28

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 ...


27

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 ...


27

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 ...


27

I wouldn't assume that the NSA has cracked AES ciphers. I would assume that most crypto systems that use AES have implementation flaws that the NSA exploits when they feel it is worth it. In any case, when the only possible way a state can know something is by breaking a cipher, it's difficult for them to use that information; doing so would reveal that ...


25

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 ...


24

According to 7-Zip, Use ZipCrypto, if you want to get archive compatible with most of the ZIP archivers. AES-256 provides stronger encryption, but now AES-256 is supported only by 7-Zip, WinZip and some other ZIP archivers. So really there is some balance to be played with. Do you require better security at the sacrifice of compatibility or more ...


24

No. AES-256 is not weaker than AES-128. Absolutely not. And I disagree with the the advice that you should avoid AES-256. The attack against AES-256 is a related-key attack, which is irrelevant to most real-world uses of AES-256. Related-key attacks only become relevant if you use the block cipher improperly, which is not something that you ought to be ...


23

Caveat: as very rightly pointed in that other answer, using a fixed/no IV does make some attacks less difficult. I wish the following answer would have been less affirmative. I have accordingly made adjustments in italic. There's no imperious need for an IV when unique keys are used. When each key is used only to encipher a single message, it is reasonably ...


21

I wrote a rather lengthy answer on another site a few days ago. Bottom-line is that CTR appears to be the "safest" choice, but that does not mean safe. The block cipher mode is only part of the overall protocol. Every mode has its quirks and requires some extra systems in order to use it properly; but in the case of CTR, the design of these extra systems is ...


21

It's meaningless nonsense. I would be inclined to avoid spending any money with these people. If you scroll down on this page, you'll find a table labelled key size vs. time to crack, according to which their $2 \times 256$ bit encryption takes $3.31 \times 10^{112}$ years to crack, making it (apparently) superior to ordinary $256$-bit encryption (which can ...


19

An interesting thing about some modern standardized ciphers, like AES, is that the government is "eating its own dogfood" by using them internally. (AES 192 and 256 are approved for top-secret data.) Back in the day (up through the 90s), U.S. government internal encryption standards was not closely aligned with public sector cryptography, and we largely had ...


18

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 bits. A ...


18

Well, AES is not a Feistel cipher because it's a substitution-permutation network instead. If I were taking a test that asked me why AES was not a Feistel cipher, this would be my argument: namely, that the structure of substitution-permutation networks is fundamentally different from that of Feistel networks. (Here one could elaborate on invertibility and ...


18

CBC does not perform authentication This property makes it less suitable for places where authentication is required, basically any transport protocol. TLS uses CBC, but by default performs authentication over the plain text instead of the ciphertext, which opened up a host of attacks. CBC can be used here, but it is error prone and may require an ...


18

In short: You must authenticate the IV. Which particular attacks apply if you don't depends on the block cipher mode; I will give two common examples. In CTR mode, an attacker who fiddles with the IV can forge authenticated messages, but the content of the corresponding plaintext is beyond his control (since he doesn't know the key). Depending on the ...


17

Short version: It is quite likely that a large proportion of the keys have fixed points, but I don't have any idea on how to find them. Long version: A stochastic argument There are $2^{128}!$ permuations of 128-bit blocks, and of these, $!2^{128}$ (this is the subfactorial) are fixpoint-free. It is known that $\lim_{N\to\infty}\frac{!N}{N!} = \frac 1e \...


17

The paper explains why. Preventing the OS from interrupting the AES computation is part of Bernstein's proposed method of defense against cache-based timing attacks. Let me sketch the argument for you: The early part of the paper explains that if the time is variable, then it introduces a risk of timing attacks. Sections 3-6 demonstrate that such an ...


17

Blowfish has strong points regarding speed because bulk encryption (and decryption) reduce to an alternation of: a 8->32-bit table lookup, and one or two 32-bit operations (addition or XOR). That structure is very well suited to 32-bit CPUs with a short pipeline and a fast cache of at least 4 kByte; and is well suited for a straight C implementation, which ...


16

Generally speaking, a lookup-table can be implemented in constant time by doing it as if it was a hardware circuit. Consider a multiplexer: this is a circuit which accepts three inputs $a$, $b$ and $c$, and yields one output $d$ which is equal to $a$ if $c = 0$, to $b$ otherwise (I am talking about single-bit values here). A multiplexer can be used to ...


16

The most efficient related-key attacks on AES-256 and resulting weaknesses AES-256-based hash functions are summarized in my PhD thesis. Though collision and preimage attacks on hash functions are out of reach yet, the components of these functions still expose some properties that are not expected of good hash functions or random oracles. Getting to the ...


16

We can't implement "AES 512 key size" because AES is defined for key sizes $k\in\{128,192,256\}$ bits only; much like we can't make a bicycle with 3 wheels. I see no reason why we would want to define an AES variant with 512-bit key size (since AES-128 is safe enough for anything foreseeable most current applications except those that require huge security ...


16

TL;DR No, the approach is not secure. Use a standard like CMAC instead. Or even better, check your AES accelerator module to see if it supports any AEAD modes of encryption like GCM, CCM, EAX. Long Version In order for a message authentication code (MAC) to be secure, an adversary with oracle access to the MAC (basically this means the adversary can send ...


16

Even if the 32 characters are completely random, they won't contain non-printable characters. Actually, there are only about 107 printable characters in ASCII (out of 256 values for a full byte) and that even includes the space character. So if all the printable characters are used, it would result to a security level of about $log_2(107^{32}) = 215$ bits, ...


16

There are two important differences between AES-128 and AES-256: AES-128 has 10 rounds, AES-256 has 14 The key expansion process (that is, how they generate subkeys) is different If your AES-128 encryption hardware just takes a plaintext block and a 128 bit key, and produces a ciphertext block, well, no, there's not much you can do. In this case, the ...



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