# Tag Info

## Hot answers tagged modes-of-operation

13

Contrary to what Stephen says, you absolutely can compute the tag in parallel. Here's how it works; the tag computation is essentially "assemble the AAD, data, the length field and $Encr(Nonce)$ into a series of values $x_n, x_{n-1}, x_{n-2}, ..., x_0$", and then "compute the polynomial $x_nh^n + x_{n-1}h^{n-1} + x_{n-2}h^{n-2} + ... + x_0h^0$ This ...

6

SIV is a mode specially designed for this purpose. SIV-AES would be a good choice, but it has the same issues as AES-wrap; not many implementations. If you use a GCM you should make sure that the IV is unique (if your plaintext is ever not random you would otherwise be in problems). As for the password based key derivation function: yes, PBKDF2 is good, ...

5

The GCM flowchart on Wikipedia and my intuition support the notion that some of the GCM work can be done in parallel. At the very least you can do each $E_k(ctr)$ operation in parallel, but it doesn't look like you should be able to parallelize the authentication, as each $mult_H$ requires the output of a previous call as its input. Edit: poncho explains why ...

5

I know that some of them are pretty hard to crack, but since they are so commonly known is it even practical to consider using something like that as an encryption method considering the algorithms for encryption and decryption are commonly known (from a security perspective)? In fact, this is exactly what we want. Schneier's law Anyone, from the ...

4

If you have an error in a cipher text block you can generally represent this as: $$C'=C\oplus\Delta$$ Now if you try to decrypt this block using the previous ciphertext block $IV$ as IV you get $P'=IV\oplus D_K(C\oplus\Delta)$ which is completely unrelated to $P=IV\oplus D_K(C)$ assuming the block cipher acts as a pseudo-random permutation. As the input ...

4

What is this method/algorithm/construction called? Dunno; this is a new one on me. Is it as secure as CBC implemented the normal way? Should be. Modeled as an abstract 'take plaintext, output ciphertext' model, this method (with a random last ciphertext bits) has precisely the same ciphertext output distribution as CBC mode (with a random IV), ...

4

There are two well-known Encryption modes, that can construct a $mn$-bit tweakable blockciphers from a $n$-bit blockcipher ($n=64$ for DES) with $1\le m\le n$. The older one is CMC, being not parallelizable. It was superseeded by Encrypt-Mix-Encrypt (EME), which is parallelizable. The basic idea of the two algorithms is to encrypt each block of input data ...

4

Actually, Maarten isn't quite correct; in most cases, the counter doesn't have to be updated in constant time (because it's not secret); however in one case it does: GCM with an IV size that's not 12 bytes. The reason the counter needs to be secret in this case is not because how it is used, but how it is generated. It is initialized to ...

4

Since you are deriving the key from a password, there is generally not a security advantage to using multiple encryption in the way you described. The entropy of key material generated is less than the maximum security provided by AES, which means an attack on the password will be more effective than a generic key recovery attack on the cipher. A ...

4

One of the basic security requirements of a block cipher mode of operation is that it is indistinguishable under chosen plaintext attack (IND-CPA). Essentially, this means that, if an attacker chooses two messages $m_A$ and $m_B$ and the defender randomly returns either $\text{Encrypt}(K, m_A)$ or $\text{Encrypt}(K, m_B)$ (with $K$ kept secret from the ...

3

In addition to the tweakable enciphering schemes in the comments, I'll leave this reference here: https://eprint.iacr.org/2009/356.pdf It essentially shows (in the ideal cipher model) that using an n-bit block cipher in a three-round Feistel construction gives you a 2n-bit block cipher.

3

Your mode is essentially equivalent to CFB mode, except that: you've reversed the order of the blocks in the message, and you're using the block cipher in the opposite direction than usual. Neither of those differences should have any direct security implications (since all standard block ciphers have the same security properties in both directions), ...

3

Forget OFB mode. You should use CTR (counter) mode. It has the best bounds, and is parallelizable. This means that when you are using the AES-NI instruction set, encrypt with CTR is about 7 times faster than CBC, OFB etc. If you encrypt in OpenSSL you will get this performance. For a good thorough analysis and comparison of modes of operation, see ...

3

The standard for full-disk encryption (FDE) is XTS mode or ESSIV-AES-CBC. XTS tweaks each block within each sector differently (and hence avoids ECB's problems) and is considered the best choice available at the moment. ESSIV-AES-CBC works by using AES-CBC with the IV being the hash of the sector index. The problem with this mode is that you can flip bits ...

3

Better is a subjective term. However for the choice between ECB and CBC, the choice should be CBC for almost all situations. Although ECB and CBC are modes of operation of a block cipher, you could also turn this way of thinking around and see the block cipher as a configuration option for the mode of operation. The mode of operation has a big influence on ...

3

Block ciphers map bit strings of fixed length to other bit strings of the same length. Hence, using only the block cipher primitive, you can't encrypt more than one block (typically 16 bytes), which is of course undesirable. The straight-forward (but bad!) way around this limitation would be to split up the message into chunks of block length and ...

3

The schemas from the relevant Wikipedia page really explain it all: As you see in the decryption schema, the IV is used for a single XOR that yields the first plaintext block; it is obvious that the IV impacts only that block. When encrypting, though, modifying the IV alters the first ciphertext block, then the second ciphertext block, and so on. The ...

3

According to Handbook of Applied Cryptography (15.3.2, ii), ANSI X9.9 (which SEJPM mentioned in the comments but I have no access to) defined CFB-MAC only as a compatible alternative to CBC-MAC: The X9.9 MAC algorithm may be implemented using either the cipher-block chaining (CBC) or 64-bit cipher feedback (CFB-64) mode, initialized to produce the same ...

2

Look at how the keys $K_1$ and $K_2$ are used in CMAC (pdf, Section 6.2): If $M_n^*$ is a complete block, let $M_n = K_1 \oplus M_n^*$; else, let $M_n = K_2 \oplus (M_n^*||10^j)$, where $j = nb-Mlen-1$. They are combined with message blocks using XOR. So they must be equal in length to the block size, not the key size (if different), of the ...

2

PCBC is provably secure for confidentiality, assuming you use a random IV like with CBC. The attacks you mention are all on the integrity rather than confidentiality of PCBC. No, you probably cannot construct secure authentication with PCBC and an unkeyed hash. For that you should instead use an actual MAC. While PCBC propagates errors, it only propagates ...

2

No, the IGE encryption cannot be parallelized. Also the decryption of IGE/ABC is serial. The input to the block cipher for encryption is the ciphertext of the previous block xor'd with the plaintext (and the result is then xor'd with the previous block plaintext). For decryption, you have to XOR the ciphertext with the plaintext of the previous block ...

2

The output of the block cipher is used as the new key, and also passed to the "output block" function, which is referenced in the NIST document as $B^m_R$. The purpose of the IV $R$ and the function $B^m_R$ is to reduce the output to a smaller size in a manner that hides the true output of $f$. Too large an output allows key recovery. The output of this ...

2

Yes, there are modes of operation that achieve the property that you are describing. For example, the Propagating Cipher Block Chaining (PCBC) mode of operation: This mode is similar to CBC but the output for each block is propagated to the input of the next one, so a small error will propagate indefinitely, both for encryption and decryption. There may ...

2

The answer by Paŭlo is completely wrong. There is a simple way to convert a stream cipher into a block cipher. In pact, it works for any PRF, regardless of if it is reversible or not. This presentation perfectly covers how to use a PRG such as a stream cipher to construct a block cipher. Basically, you use the stream cipher in the following manner: Take ...

2

This [Carter-Wegman] MAC is not, in general, secure in the quantum setting This is true; however we need to ask "what is this setting, and is it a realistic one?" This setting is one where the adversary can ask queries that are composed of a superposition of quantum states, and the oracle returns the superposition of the answers. In other words, the ...

2

I have a problem with OFB mode, because I have heard that it is stronger than CFB. On the contrary I would say that CFB is stronger. OFB means encrypting the IV again and again to produce the keystream. If you end up in a cycle, the keystream will start repeating itself. (This should not be a practical weakness, but why chance it?) CFB is more like ...

2

XTS has been designed for disk encryption, where an attacker typically has access to the disk only a single time (when they steal/confiscate the device). When an attacker sees several ciphertexts encrypted using the same key, they can tell which blocks differ between the versions, but not the content of the blocks. Compare this with CTR mode, which leaks ...

2

I've been thinking a little bit about it, and now I think it is possible, but you have to consider the generalization of CFB in ISO 10116 (I don't have access to the ISO 10116 standard, so I will assume that the description by Rogaway is correct). The generalization of CFB from the ISO standard seems to have two main changes: The feedback block (FB), of ...

2

In general, this is a bad idea. I won't give you any concrete attacks, but will try to explain why you shouldn't do this. In general, if you want to encrypt a key then you should do it using a secure mode for this purpose. (One is the SIV mode of operation. Another is just to use GCM or CCM.) First, I want to stress that you should always use an ...

1

I think it is now valid to answer this question as this course is likely to be over. First observer how CTR-mode works: $C_1=E_K(IV)\oplus P_1$ As you can see, there's a linear relation between the plaintext and the ciphertext. You now use $C_1$ (observed) and $P_1$ (known). You want to make $C'_1$ decrypt to $P_1'$. To obtain this you first construct ...

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