33 votes
Accepted

Why is double encryption that's equivalent to single encryption no better than single encryption?

This is simply saying that if a cryptosystem has a functional composition that is $$ h_{k}(x) = f_{k_1}(g_{k_2}(x)) $$ then you can find a key for single encryption that works as the double encryption....
kelalaka's user avatar
  • 48.5k
13 votes
Accepted

Can double-encrypting be easier to break then either algorithm on its own?

Can double-encrypting (with either the same or separate algorithms) weaken security? If you do not assume that the algorithms and keys are independent, then it certainly can. The example of ROT13 ...
otus's user avatar
  • 32.1k
9 votes

Can double-encrypting be easier to break then either algorithm on its own?

The answers and comments here are good, but I think that it's worth tidying it all up a bit. The question is broad, and this is exactly expressed in the answers. There are multiple questions here. ...
Yehuda Lindell's user avatar
8 votes
Accepted

Is AES a group?

See the paper DES is not a group by Campbell and Wiener. TL;DR There are computational proofs that DES is not a group. The point is to carry out the types of computations that established DES is not a ...
kodlu's user avatar
  • 22.5k
7 votes

Is a 4DES or 5DES system possible?

There is a very interesting paper that relates to this exact question (but you wouldn't guess it from the title). The paper is titled Efficient Dissection of Composite Problems, with Applications to ...
Yehuda Lindell's user avatar
6 votes
Accepted

Is there a system where multiple people can encrypt a message and then decrypt it in any order?

Yes, systems that allow this have a name: commutative encryption. In practice, there are two varieties: If A, B, C just xor in a keystream, it all commutes. Of course, anyone seeing the ...
poncho's user avatar
  • 147k
6 votes

Is an invulnerable code possible (including brute force attack)?

No, If we assume that the mythical computer can brute force the multiple AES encryptions and there are many ciphertexts available which are encrypted under the same key and their corresponding ...
kelalaka's user avatar
  • 48.5k
5 votes
Accepted

Multisignature or Multi-password File

This is precisely the domain of secret sharing, of which there are various popular schemes like Shamir's secret-sharing scheme, and there are many widely available implementations of it in various ...
Squeamish Ossifrage's user avatar
5 votes

Encrypting messages for multiple recipients

Yes, this is possible. The most natural option is to look at identity-based encryption (I point to the wikipedia page, as it gather some links to various schemes), such as the Boneh-Franklin IBE. An ...
Geoffroy Couteau's user avatar
5 votes
Accepted

Order of multiple encryption algorithms

Yes, in case of VeraCrypt there is a difference, but it is negligible in practice. First we need to consider how VeraCrypt actually performs the cascading of the encryption algorithms which is (...
SEJPM's user avatar
  • 46k
5 votes

Does the double-hash H(H(x)) have greater collision probability than H(x)?

I'm not sure what the question here is, but obviously applying the hash function twice can never decrease the number/probability of collision as all collisions in the first invocation are maintained. ...
Meir Maor's user avatar
  • 11.8k
5 votes

Double encryption - One Time Pad

The answer is we cannot improve the security of the one-time pad in this manner. Intuitively the reason is that the double one-time pad is just a less efficient one time pad. The security of the ...
Marc Ilunga's user avatar
  • 3,188
5 votes

Encrypt different inputs with different keys to obtain the same output

Is there a standardized way to achieve this? That's actually a fairly common request - here's the problem - there's no way to do it without making the ciphertext as long as the two plaintext messages ...
poncho's user avatar
  • 147k
4 votes

Is there any way to have a coded string such that it can be decoded in multiple ways?

Maybe this example is only remotely related to the question, but anyway: VeraCrypt application for encrypting computer's disks has an ability to offer part of the disk capacity to reach the plausible ...
MarianD's user avatar
  • 219
4 votes
Accepted

"Double Encryption" using the same cipher?

When I think of encryption and doubling it, I believe the strength of encryption, in the case of AES, as being tied to the number of rounds through the algorithm. I don't believe that's the best way ...
poncho's user avatar
  • 147k
3 votes

What 'exactly' are AES dual ciphers?

$\newcommand{\compose}{\mathbin{\circ}}\DeclareMathOperator{\AES}{AES}\DeclareMathOperator{\Round}{Round}\DeclareMathOperator{\Final}{Final}\DeclareMathOperator{\KeyExpansion}{KeyExpansion}\...
Squeamish Ossifrage's user avatar
3 votes
Accepted

Is this scheme multi-message secure?

It is not entirely clear what you mean by "multi-message indistinguishable". I'm going to assume the weakest possible interpretation I can think of. So consider the scenario where the adversary is ...
Maeher's user avatar
  • 6,818
3 votes
Accepted

Multi party anonymous key distribution

OpenPGP supports an approximation to what you seek, albeit with at best weak privacy guarantees: To transmit the message $m$, Alice picks a session key $k$ and sends $$E_{f_1}(k) \mathbin\| E_{f_2}(k) ...
Squeamish Ossifrage's user avatar
3 votes
Accepted

Require x of y possible passwords to decrypt

This is a well studied problem in cryptography; what you're looking for is known as a Secret Sharing Scheme. This is a scheme where we take a master secret, and from that generate $N$ shares, and ...
poncho's user avatar
  • 147k
3 votes
Accepted

Repeated NONCE in CTR mode

This would be better than simply CTR if your nonces do collide, but it is still not very good. Suppose you use the same IV for two messages $m_0||m_1||...m_n$ and $m_0'||m_1'||...m_n'$ where all the $...
otus's user avatar
  • 32.1k
3 votes

Multiple Encryption - One-Time Pad

No. Here is a breakdown of definitions: One Time Pad encryption refers to one bit of key material is used to encrypt at most one bit of output. The "One Time" part is critical. Using any bit twice ...
Future Security's user avatar
3 votes
Accepted

How can Alice enable Bob to look-up values in a private map

Many (most?) leading PSI protocols can be easily adapted to provide this functionality. It is sometimes called "PSI with associated data" or "PSI with data transfer". For example, ...
Mikero's user avatar
  • 13.2k
3 votes

Multiple encryption using the same key

I have a program that uses a custom algorithm to encrypt a message This algorithm is called a cipher. There are plenty well known ciphers that are considered secure. If you have a "custom cipher" ...
Maarten Bodewes's user avatar
  • 92.6k
3 votes

Is an invulnerable code possible (including brute force attack)?

When you cascade two ciphers with independent keys, $E_{k_1,k_2}\colon x \mapsto \operatorname{AES}_{k_2}(\operatorname{AES}_{k_1}(x))$, the intelligent adversary will use a parallel collision search ...
Squeamish Ossifrage's user avatar
3 votes
Accepted

Is multiple encryption using a block cipher mode of operation that use only encryption processes vulnerable to Meet-in-the-middle attacks?

Consider double encryption with the CTR mode; The first layer outputs a stream $o$ of the encryption of the counter block per NIST definition of the CTR document. $$o_i = \operatorname{AES-CTR}(k_1,...
kelalaka's user avatar
  • 48.5k
2 votes
Accepted

How would multiple uses of a vigenère cipher be decrypted on paper (classic)?

My impression is that it would at most take a longer ciphertext to be decrypted. Imagine first the scenario where the different keys have the same length: $K_1=hello$ $K_2=world$ In that scenario, ...
Sergio A. Figueroa's user avatar
2 votes
Accepted

Is there any way to have a coded string such that it can be decoded in multiple ways?

The One-Time Pad can be made to do this. With plaintext1 ("test1") and plaintext2 ("test2") given, we can chose (perhaps, randomly) any one of ciphertext (encoded string), pad1 ("key1"), pad2 ("key2");...
fgrieu's user avatar
  • 141k
2 votes

Is there any way to have a coded string such that it can be decoded in multiple ways?

Another relevant term is "deniable encryption". See the answer to the question one ciphertext multiple keys multiple messages, from which I quote: Deniable encryption makes it impossible to prove ...
kodlu's user avatar
  • 22.5k
2 votes

Is there any way to have a coded string such that it can be decoded in multiple ways?

I'll try to lay out a scenario that I think you are describing. You have a publisher that is sending out one message that goes out to two people, and you want these two people to be able to get ...
daniel's user avatar
  • 912
2 votes

Is it worth using custom encoding on top of standard encryption?

Absolutely not as it's a violation of Kerckhoffs' principle and relies on security through obscurity which is rather hard to justify. Both have been widely discussed on this site. You have to ...
Paul Uszak's user avatar
  • 15.4k

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