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Questions tagged [encryption]

Encryption is the process of transforming plaintext using a cipher into ciphertext to make it unreadable to anyone except those possessing the key. Decryption is the process of transforming that ciphertext back into plaintext, using the key.

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424 votes
13 answers

Should we MAC-then-encrypt or encrypt-then-MAC?

Most of the time, when some data must be encrypted, it must also be protected with a MAC, because encryption protects only against passive attackers. There are some nifty encryption modes which ...
Thomas Pornin's user avatar
189 votes
23 answers

Time Capsule cryptography?

Does there exist any cryptographic algorithm which encrypts data in such a way that it can only be decrypted after a certain period of time? The only idea that I can think of, is something like this: ...
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173 votes
2 answers

What is the main difference between a key, an IV and a nonce?

What are the main differences between a nonce, a key and an IV? Without any doubt the key should be kept secret. But what about the nonce and the IV? What's the main difference between them and their ...
curious's user avatar
  • 6,190
163 votes
10 answers

Why is writing your own encryption discouraged?

Say I want to write an encryption algorithm to communicate between me and my friend for this private use. How is that bad? E.g. I can take the word Hello and ...
Pierte's user avatar
  • 1,537
152 votes
7 answers

Should we sign-then-encrypt, or encrypt-then-sign?

Frequently, we want to send messages that are (a) encrypted, so passive attackers can't discover the plaintext of the message, and (b) signed with a private-key digital signature, so active attackers ...
David Cary's user avatar
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148 votes
5 answers

What is a cryptographic "salt"?

I'm a beginner to cryptography and looking to understand in very simple terms what a cryptographic "salt" is, when I might need to use it, and why I should or should not use it. Can I get a ...
Bhavik Ambani's user avatar
104 votes
3 answers

If WhatsApp cannot read our message, how can the media forwarding happen in an instant?

WhatsApp says even the photos shared on its platform are end-to-end encrypted. When WhatsApp says encrypted I assume the data is encrypted in my device and then sent across to the recipient. When we ...
0xAB1E's user avatar
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99 votes
1 answer

What is the difference between PKCS#5 padding and PKCS#7 padding

One runtime platform provides an API that supplies PKCS#5 padding for block cipher modes such as ECB and CBC. These modes have been defined for the triple DES, AES and Blowfish block ciphers. The ...
Maarten Bodewes's user avatar
  • 93.2k
92 votes
2 answers

What is the difference between CBC and GCM mode?

I am trying to learn more about GCM mode and how it differs from CBC. I already know that GCM provides a MAC, which is used for message authentication. From what I have read and from the code snippets ...
Bob Bryan's user avatar
  • 1,283
91 votes
5 answers

Is AES-256 weaker than 192 and 128 bit versions?

From a paper via Schneier on Security's Another AES Attack (emphasis mine): In the case of AES-128, there is no known attack which is faster than the 2128 complexity of exhaustive search. However, ...
quantumSoup's user avatar
  • 1,021
90 votes
4 answers

What are the practical differences between 256-bit, 192-bit, and 128-bit AES encryption?

AES has several different variants: AES-128 AES-192 AES-256 But why would someone prefer use one over another?
foobarfuzzbizz's user avatar
89 votes
10 answers

In end-to-end encryption, doesn't the server need to be trusted?

Applications like WhatsApp use end to end encryption. WhatsApp says that only the users share a specific key and no third party can view the messages. But I do not understand how the two users agree ...
AV94's user avatar
  • 1,009
87 votes
5 answers

How secure is AES-256?

The cipher AES-256 is used among other places in SSL/TLS across the Internet. It's considered among the top ciphers. In theory it's not crackable since the combinations of keys are massive. Although ...
Gustav's user avatar
  • 1,065
86 votes
11 answers

Is modern encryption needlessly complicated?

RSA, DES, AES, etc., all use (relatively) complicated mathematics to encrypt some message with some key. For each of these methods, there have been several documented vulnerabilities found over the ...
Ozzah's user avatar
  • 977
83 votes
9 answers

Should I use ECB or CBC encryption mode for my block cipher?

Can someone tell me which mode out of ECB and CBC is better, and how to decide which mode to use? Are there any other modes which are better?
midhunhk's user avatar
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77 votes
8 answers

Layman's explanation of encryption backdoors

In the media, I sometimes read about "backdoors" in encryption algorithms. I'd like to understand what such a backdoor actually consists of. Is it: a) a hidden weakness in the math formulas ...
user avatar
77 votes
1 answer

Can you explain Bleichenbacher's CCA attack on PKCS#1 v1.5?

I've studied that the Bleichenbacher's CCA attack on PKCS#1 v1.5. is a base to many versions of attacks in the area. I'm trying to understand that attack, but every explanation I saw starts with the ...
Bush's user avatar
  • 2,140
76 votes
4 answers

How come Public key cryptography wasn't discovered earlier?

I became interested in crypto lately and read about symmetric and public key crypto algorithms. I understand how crucial the discoveries of the 1970s like RSA, DES and DH were in advancing the ...
pls no's user avatar
  • 919
72 votes
4 answers

How can I use asymmetric encryption, such as RSA, to encrypt an arbitrary length of plaintext?

RSA is not designed to be used on long blocks of plaintext like a block cipher, but I need to use it to send a large (encrypted) message. How can I do this?
foobarfuzzbizz's user avatar
70 votes
3 answers

Why shouldn't I use ECB encryption?

I'm using Java to generate encrypted strings, and I get this warning at build time: ECB encryption mode should not be used So I'm wondering why I shouldn't use ECB and what I can use instead?
Rogue's user avatar
  • 826
61 votes
4 answers

Difference between stream cipher and block cipher

I read that A typical stream cipher encrypts plaintext one byte at a time, although a stream cipher may be designed to operate on one bit at a time or on units larger than a byte at a time. (...
Ravindra Bagale's user avatar
61 votes
3 answers

Hashing or encrypting twice to increase security?

Over on the bitcoin forums I asked why the bitcoin client computes SHA-256(SHA-256(x)) as its cryptographic hash for a variety of purposes. The leading theory--since the bitcoin author has disappeared-...
maaku's user avatar
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59 votes
2 answers

Is the software that uses PGP broken, or is it PGP itself?

PGP is all over the news (even on TV) and there seems to be a lot of confusion about it. For the time being, people face articles like Attention PGP users: new vulnerabilities require you to take ...
e-sushi's user avatar
  • 17.9k
58 votes
5 answers

What are the chances that AES-256 encryption is cracked?

I'm currently building a web application and would like to encrypt all data on the back-end. I was thinking of using the AES-256 encryption but wasn't sure how safe it was. I did that math and felt ...
Jacob Henning's user avatar
58 votes
3 answers

RSA encryption with private key and decryption with a public key

When using the RSA cryptosystem, does it still work if you instead encrypt with the private key and decrypt with the public key? What about in the case of using RSA for sender authentication?
Humam Shbib's user avatar
57 votes
3 answers

Do these new insights into prime numbers affect encryption security?

Quanta Magazine reports: Two mathematicians have uncovered a simple, previously unnoticed property of prime numbers [...]. Prime numbers, it seems, have decided preferences about the final digits of ...
user's user avatar
  • 513
57 votes
2 answers


Recently I started studying Elliptic Curve Cryptography and I just loved it. I want to transfer some big data (like 3KB), What is the best method, ECDSA, ECIES, or ECDH (and why)? I am confused, how ...
user3160055's user avatar
54 votes
7 answers

One Encryption, Many Decryption Keys

I would like to share access to encrypted data among many recipients. I do not know the recipients ahead of time (when encrypting the data). Once the data is encrypted, I do not have access to the ...
Justin Bailey's user avatar
51 votes
5 answers

How to find modulus from a RSA public key?

I am studying the RSA cryptosystem. The public key consists of $(n, e)$, the modulus (product of two large primes), and the encryption exponent. I want to separate the modulus $n$ and exponent $e$. A ...
user3001408's user avatar
49 votes
1 answer

AES256-GCM - can someone explain how to use it securely (ruby)

I am looking into using AES256-GCM for encrypting some database fields. I know that for AES256-CBC, I need to generate a new IV for each encrypt, but I can use the same key. The IV can be openly ...
breakingbreadmuffins's user avatar
46 votes
5 answers

Is there a secure cryptosystem that can be performed mentally?

I, myself, do not plan on getting into a situation where I would be unable to use a computer in order to communicate securely. However, I can think of many practical situations in which mental ...
John Gietzen's user avatar
  • 1,505
44 votes
4 answers

Cryptography algorithms that take longer to solve on a GPU than a CPU

I know that Graphics cards are faster at solving algorithms like SHA-256 because of the many builtin processors, but are there Algorithms that take actually longer on a Graphics card than on a modern ...
user51749's user avatar
  • 449
44 votes
2 answers

What's the fundamental difference between Diffie-Hellman and RSA?

What is the difference in the purpose of DH and RSA? Aren't they both public-key encryption?
user541686's user avatar
  • 1,349
42 votes
12 answers

Differences between industrial and military cryptography

Industrial and military cryptography should follow the same basic rules, but what does make them different is: Higher key length, protocols unknown to the civilian world, and perhaps unique methods ...
R1w's user avatar
  • 1,960
42 votes
12 answers

Is it possible to create a "digital seal" to tell if a document has been opened?

So, in real life we have a handful of ways to leave a physical mark on a packet to know if it has been opened without authorization (eg. you can use 'opened' security tapes, or you can put a signature ...
Jaime Silva's user avatar
42 votes
6 answers

Is Convergent Encryption really secure?

Recently a company called Bitcasa demonstrated a product of cloud storage. they indicated that they would use "Convergent Encryption" to secure your data and de-duplicate, essentially one copy of the ...
Justin King's user avatar
41 votes
2 answers

Why do we use encrypt-decrypt-encrypt (EDE) in 3DES, rather than encrypting three times?

I'm wondering why we use encrypt-decrypt-encrypt (EDE) sequence in 3DES (also known as DES-EDE, TDES or TDEA) with three keys instead of three times encryption (EEE) with three different keys?
alaamub's user avatar
  • 583
40 votes
9 answers

What's the truth about this "absolutely unbreakable" cipher?

A story appeared on Forbes today claiming that we now have an absolutely unbreakable cipher. It cites a paper published in Nature Communications. However, I'm skeptical: But what if there were a ...
Joseph Sible-Reinstate Monica's user avatar
40 votes
2 answers

How does order-preserving encryption work?

Order-preserving encryption (OPE) is, apparently, a method of encrypting data so that it's possible to make efficient inequality comparisons on the encrypted items without decrypting them. I've been ...
Ilmari Karonen's user avatar
39 votes
3 answers

Why is CBC with predictable IV considered insecure against chosen-plaintext attack?

I just learned that using CBC encryption with an IV which is predictable is not secure. From what I understand, using certain plain texts, and then guessing the IV that it uses, the attacker can ...
asudhak's user avatar
  • 491
38 votes
4 answers

How can one securely generate an asymmetric key pair from a short passphrase?

Background info: I am planning on making a filehost with which one can encrypt and upload files. To protect the data against any form of hacking, I'd like not to know the encryption key ($K$) used for ...
Flumble's user avatar
  • 485
38 votes
2 answers

Why should I use Authenticated Encryption instead of just encryption?

There are various different modes of operation for block cipher use, some of which provide "encryption" and some of which provide authenticated encryption. Why should I use an authenticated ...
Cryptographeur's user avatar
37 votes
2 answers

Why should I use an Initialization Vector (IV) when I have unique keys?

I took a look at “Why, or when, to use an Initialization Vector?” but my question is not the same. I have unique keys encrypting each plaintext (in CBC mode, AES-256) and I do not use a key to ...
Ashwin's user avatar
  • 473
37 votes
1 answer

What is the advantage of XTS over CBC mode (with diffuser)?

I have some problems in understanding the "advantage" of AES-XTS compared to CBC with diffuser. I read something about FileVault, in this paper they mention the two modes of operations XTS and CBC (...
tommynogger's user avatar
35 votes
10 answers

Encryption that purposefully take hours to decrypt

My problem: I want to block sites on my router. I want to generate new password for my router after blocking sites. This new password I want to encrypt. But to decrypt it, I want it to take 2 to 8 ...
Matt Rybin's user avatar
33 votes
3 answers

How can I make sure non-open source programs are really using end-to-end encryption?

Without going deep into math, is there a way to make sure that non-open source programs like WhatsApp, FaceTime, Zoom, etc. are really using end-to-end encryption instead of just 'regular encryption' (...
Fulalas's user avatar
  • 431
33 votes
2 answers

7zip : Why does encrypting the same file with AES-256 not give the same output?

Using 7-zip 19.00, on Windows 10 1909, build 18363.592, I encrypted a text file with the contents "hello there" using AES-256 and the password "123". I did this two times, the exact same procedure, ...
super's user avatar
  • 463
33 votes
5 answers

Any practical uses of machine learning for cryptography?

I am about to go study for my masters in machine learning, data mining and high performance computing, but have recently become very interested in cryptography after taking Dan Boneh's Cryptography ...
Michael Aquilina's user avatar
33 votes
2 answers

Why do we need special key-wrap algorithms?

Wikipedia says: Key Wrap constructions are a class of symmetric encryption algorithms designed to encapsulate (encrypt) cryptographic key material. We are using these algorithms to encrypt (and ...
Paŭlo Ebermann's user avatar
32 votes
2 answers

Using the same RSA keypair to sign and encrypt

The RSA signature operation is basically the same as encrypting with the private key. In particular, both operations use the same kind of keys. Is it safe to use the same RSA keypair both for ...
Lay András's user avatar

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