# Tag Info

26

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 ...

14

With CBC (Cipher block chaining) mode, before encryption, each block is XOR-ed with the ciphertext of the previous block, to randomize the input to the block cipher (and avoid encrypting the same block twice with the same key, as this would give the same output, and tell the attacker something about the plaintext). As the first block has no previous block, ...

14

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 2 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 ...

13

The algorithm (now reasonably clear) is reminiscent of a block cipher in CFB mode, with $random$ as the IV (which can be public), $secret$ as the key, and MD5 used as keystream generator instead of the block cipher. Decryption works as in CFB: $$M_1 = C_1 \oplus \operatorname{MD5}( secret||random )$$ M_n = C_n \oplus \operatorname{MD5}( secret||C_{n-1} ...

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

The initialization vector is a property of the mode of operation (aka "chaining mode"), not of the block cipher itself. A block cipher does only one thing, which is mapping blocks (block size depends on the cipher, 64-bit for DES, 128-bit for AES) unto other blocks. The chaining mode is what says how input data should be transformed into block values, and ...

12

The crucial difference between plain encryption and authenticated encryption (AE) is that AE additionally provides authenticity, while plain encryption provides only confidentiality. Let's investigate in detail these two notions. In the further text, we assume $K$ to be a secret key, which is known to authorized parties, but unknown to attackers. Goals ...

11

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 ...

11

After reading the paper How to Break XML Encryption (thanks to Krzysztof for the link), here are my two cents. This attack relies on the fact that a CBC-ciphertext C = (IV, C1, ... Cd) can be decomposed into pairs of (IV, C1), (C1, C2), (C2, C3), ... (C(d-1), Cd), each of which is also a valid CBC ciphertext for the same key, relating to the corresponding ...

11

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 ...

10

Neither. It means that an attacker can decrypt all messages that have been encrypted using this standard. The attack is a padding oracle attack. That means that, if the attacker has a ciphertext they want to decrypt, they can send several variations of the ciphertext to the server. By analyzing the server's responses (e.g., error messages returned), it ...

10

While you do operate block-by-block when generating the pseudorandom stream, the actual encryption step (i.e., the XOR) is bitwise, and therefore does not require the message to be padded. For example, the message "Hello" will be processed as follows (pseudocode): byte stream[16] = AES(Key, Nonce); byte plaintext[5] = "Hello"; byte ciphertext[5]; for i ...

10

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 ...

10

There are some serious problems with this design that would preclude it from being standardized, so it probably does not have a name. The 2 visibly main flaws are as follows: If the plaintext follows a pattern similar to the block counter, the block cipher inputs may repeat, exposing information about the plaintext (exact same issue as reuse of nonce, but ...

9

Each mode of operation has its own IV requirements. Some need uniform, unpredictable randomness. Other are equally happy with just uniqueness. CBC is well-known for its need of an IV chosen randomly and uniformly among the possible IV values, and such that an attacker who can choose the text to encrypt may not predict the IV value before submitting the said ...

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 ...

8

SHA-256(SHA-256(x)) was proposed by Ferguson and Schneier in their excellent book "Practical Cryptography" (later updated by Ferguson, Schneier, and Kohno and renamed "Cryptography Engineering") as a way to make SHA-256 invulnerable to "length-extension" attack. They called it "SHA-256d". We started using SHA-256d for everything when we launched the ...

8

The flaw in CBC which the recent BEAST attack exploits occurs when the attacker can choose part of the encrypted message while knowing the IV which will be used. In the case of SSL/TLS, data is split into successive records, each record being "a message" in its own right. The attacker produces some data, observes the corresponding record, and knows that the ...

8

Assuming that you can indeed guarantee that the keys will never be reused, both schemes should be secure. The only requirement for the nonce in CTR mode is that it must be unique (and, if used directly as the initial counter value, not equal to any intermediate counter value used in the past or in the future). If you're only encrypting one message with a ...

8

Free space and used space look exactly the same to someone who only sees one version of the ciphertext. First, the basic idea of a secure block cipher is that you learn nothing about the plaintext block simply by observing the ciphertext block. You may be able to learn something about the plaintext from the surrounding context, such as by collecting more ...

8

It's not clear from your decryption what the algorithm is used for. But you should be aware that while at first glance it provides privacy : it's a weird mode CFB with md5 used as a block cipher ; it doesn't provide authenticity. A simple bit flip of the ciphertext will result in the corresponding bit being flipped in the plaintext and such a bit flip ...

7

The paper you cite (Deterministic Authenticated-Encryption...) gives quite a bit of useful information (but I'm assuming you already knew that). It looks like a pretty good read (I'll let you know if that assumption holds after I finish it). For why simpler constructions (CBC/CTR with a MAC or even AEX mode) don't satisfy (emphasis added): A key-wrap ...

7

Given only what you've said, and assuming the keys are created and stored in a strong manner, using a different key to encrypt database entries mitigates the problem of ECB mode. Namely that identical plaintext, when encrypted with the same key always outputs the same ciphertext. No security is gained by switching to CBC mode (assuming you can easily store ...

7

You say that a random IV "would also be unique", but really that is the crux of the problem. The problem with counter mode is that it is secure unless the same counter is used twice; if it is, it is likely that an attacker will be able to recover both plaintext messages. This contrasts with CBC mode, which if you repeat an IV, it has the relatively benign ...

7

A reason to use CBC (or CFB) over CTR and OFB could be that they are a bit more misuse-resistant: If you use CBC with a repeated initialization vector, a (read-only) attacker only can get the fact that the plaintexts are equal up to some block, and not much more (and from the first different block the rest is different). With CTR and OFB, a repeated ...

7

Well, with CFB mode, the encryption process is "take the most recent ciphertext block, pass it through the block cipher, and then exclusive-or that with the plaintext block to generate the next ciphertext block". As for the IV, that's used as "the most recent ciphertext block" when encrypting the first plaintext block (where you don't have a most recent ...

7

Thomas is correct; there's no attack on CFB mode if you can predict the IV; NIST is just being cautious. With CBC, the value of the first encrypted block $C_0 = E_k( IV \oplus P_0)$, where $IV$ is the IV used for that packet, $P_0$ is the value of the first plaintext block, and $E_k$ is the evaluation of the block cipher. If an attacker can predict the ...

7

XTS vs. Undiffused CBC. The issue here is malleability. Both XTS and CBC prevent an attacker from learning information about encrypted data. However, neither one completely succeeds in preventing an attacker from tampering with encrypted data. However, it's arguably easier to tamper with an (undiffused) CBC ciphertext than it is to tamper with an XTS ...

7

If you could use the same IV, then yes, you would need to rewrite everything after the modified block. But you shouldn't do that; every time the contents change, you should generate a new IV, which would require the whole file to be rewritten. Otherwise an attacker can learn more information than it should about how the file changed (precisely by checking ...

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