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....
- 47.6k
14
votes
Accepted
Affine Cipher over an Affine Cipher
If you combine two affine ciphers, you obtain one affine cipher. Say the first cipher is $e_1(x) = a_1x+b_1$ and the second is $e_2(x) = a_2x+b_2$. Then $e_1(e_2(x)) = a_1(a_2x+b_2)+b_1 = (a_1a_2)x+(...
- 8,082
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 ...
- 32k
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. ...
- 27.6k
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 ...
- 27.6k
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 ...
- 145k
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 ...
- 47.6k
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 ...
- 19.1k
5
votes
Can Grover's Algorithm be combined with a meet-in-the-middle attack?
You could be able to reduce the space required for a meet-in-the-middle attack, if you follow a similar idea as the application of Grover's algorithm on collisions. Suppose you have two layers of $n$-...
- 32k
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 (...
- 45.7k
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 ...
- 47.9k
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.
...
- 11.7k
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 ...
- 2,858
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 ...
- 145k
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 ...
- 219
4
votes
Accepted
Is super encryption (aka “multiple encryption”) always good?
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 ...
- 13k
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 ...
- 145k
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 ...
- 6,613
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) ...
- 47.9k
3
votes
Accepted
Combining a block cipher with a (pseudo) OTP
There's quite a few things wrong with this. For starters, a block cipher does not imply authentication. Common block cipher modes such as CTR, CBC, and (god forbid) ECB provide nothing but pure ...
- 2,379
3
votes
Accepted
Can Grover's Algorithm be combined with a meet-in-the-middle attack?
The combination of Grover algorithm and man in the middle attack is the main subject of a paper (arXiv:1410.1434) published last year by Marc Kaplan (Full disclosure: Marc is a friend of mine.)
In ...
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 ...
- 145k
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 $...
- 32k
3
votes
Adding dummy bytes to ciphertext
If you're using a real encryption scheme then no it cannot weaken or strengthen the system because the encryption scheme's security is supposed to be independent the actual data being encrypted. The ...
- 7,438
3
votes
What 'exactly' are AES dual ciphers?
$\newcommand{\compose}{\mathbin{\circ}}\DeclareMathOperator{\AES}{AES}\DeclareMathOperator{\Round}{Round}\DeclareMathOperator{\Final}{Final}\DeclareMathOperator{\KeyExpansion}{KeyExpansion}\...
- 47.9k
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 ...
- 3,283
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, ...
- 12k
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" ...
- 91.8k
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 ...
- 47.9k
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,...
- 47.6k
Only top scored, non community-wiki answers of a minimum length are eligible