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36

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 ...


35

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). ...


34

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 ...


23

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 ...


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 ...


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 ...


21

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 ...


20

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 - ...


19

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 ...


17

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 ...


15

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 implement a 1→1 ...


15

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 ...


15

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 ...


15

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 ...


14

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 ...


14

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 ...


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

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 ...


13

First, I'll assume we're talking about encrypting/decrypting exactly 128 bits of data, i.e. the block size of AES. Otherwise, you'll need to specify a mode of operation — and if your data's length isn't a multiple of the block size, well, that'll be more difficult to deal with. So, I'll assume we're working with a single block. (If you are using a mode ...


13

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 ...


12

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 ...


12

Because OCB is patented. And there are other good solutions for authenticated encryption that aren't patented. This makes them more suitable, in most situations. I can recommend, e.g., EAX, GCM, or CWC. EAX and GCM have been used in some standards, and AES-GCM has been standardized. For pointers where you can learn more, read Wikipedia. And try using ...


12

If you are using an AES library that has not undergone the FIPS validation process, then you are not FIPS compliant (or, at least, your use of AES is not). FIPS compliant means more than "we use algorithms that FIPS likes", it means "having passed the FIPS certification process"; that is how NIST defines it. Sorry, but NIST is quite strict about this; if ...


12

AES is deemed secure because: Its building blocks and design principles are fully specified. It was selected as part of an open competition. It has sustained 15 years of attempted cryptanalysis from many smart people, in a high-exposure situation, and it came out relatively unscathed. Another reason, which is not as good but felt important by many ...


12

AES has fewer rounds than Serpent so AES should be faster. The number of rounds by itself is meaningless. Some ciphers have a few complex rounds and others have many simple rounds. See my answer to Why does SHA-1 have 80 rounds? for a related explanation. There is no speed decrease with bigger key size in Serpent while there is in AES. The ...


11

AES in general does not specify that it should return a bad padding message. In fact, AES in general says nothing about padding. Padding schemes are external to AES. Therefore, the message you are getting is .Net specific. That said, be careful with these messages, as they can lead to a padding oracle attack.


11

The paper Enabling Standardized Cryptography on Ultra-Constrained 4-bit Microcontrollers (page 255) describes such an implementation.


11

Splitting a key does not reduce the key strength at all. Simply generate two random 128-bit strings and give one to each party. Encrypt the data with the exclusive OR of the two random strings. Each string alone gives no information whatsoever about the final key, assuming your random number generator is sound. No party has any advantage.


11

Having taken The Design of Rijndael from the library just yesterday, I had a look on this problem, too. Fixee wrote in a comment: However, my question is not so much about security implications, but rather "how does omissions of MixColumns make the inverse cipher similar to the cipher?" and "how does this help in implementing the cipher?" The ...



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