Cryptography Stack Exchange
Cryptography Stack Exchange

Questions tagged [key-reuse]

Using the same key for more than one purpose, e.g. to encrypt two messages or to encrypt and authenticate a message. Some cryptosystems may be vulnerable to attacks if keys are inappropriately reused.

Key reuse refers to using the same key for more than one purpose, e.g. to encrypt two messages or to encrypt and authenticate a message. Some cryptosystems (such as the one-time pad or many stream ciphers) may be vulnerable to attacks if keys are inappropriately reused. In some cases, using different but similar ("related") keys may also be problematic.

Common ways to avoid problems arising from key reuse include deriving multiple pseudo-independent keys from a single master key using a key derivation function (KDF) or using unique initialization vectors or "tweaks" to differentiate uses of the same key.

See also: , , ,

114 questions
124
votes
7 answers

Taking advantage of one-time pad key reuse?

Suppose Alice wants to send encryptions (under a one-time pad) of $m_1$ and $m_2$ to Bob over a public channel. Alice and Bob have a shared key $k$; however, both messages are the same length as the key $k$. Since Alice is extraordinary lazy (and…
Elliott
  • 1,711
  • 3
  • 15
  • 9
79
votes
3 answers

How does one attack a two-time pad (i.e. one time pad with key reuse)?

My question might appear the same as the question Taking advantage of one-time pad key reuse?, but actually I did read all the answers and none of them helped me with the details I need. I am new to cryptography and my problem is with two time pad…
Samer Makary
  • 993
  • 1
  • 8
  • 8
25
votes
4 answers

How many RSA keys before a collision?

I was wondering how many possible private/public keys exist? If a million people – for whatever reason – would try to generate 5 keys each in the same minute (on the same date and time) is there a high chance of collision? I believe GUID would…
Nick wheatley
  • 363
  • 1
  • 3
  • 5
22
votes
2 answers

Deriving Keys for Symmetric Encryption and Authentication

So here's the concept. Rather than storing 2 keys and using a random IV, which presents its own problems (key rotation, ensuring no key is used in more than 2^32 cycles, sharing the keys, etc), is it OK to derive the cipher key, authentication key…
ircmaxell
  • 425
  • 3
  • 8
21
votes
1 answer

How to attack a "many-time pad" based on what happens when an ASCII space is XORed with a letter?

I've already sent my correct solution to a homework exercise from Dan Boneh's Introduction to Cryptography class on Coursera: "Let us see what goes wrong when a stream cipher key is used more than once. Below are eleven hex-encoded ciphertexts that…
sunrise
  • 321
  • 1
  • 2
  • 5
20
votes
1 answer

Why must IV/key-pairs not be reused in CTR mode?

Many sources mention that IVs must not be reused with the same key in CTR mode, for encrypting 2 different pieces of data, because that totally destroys security - but I haven't found an explanation so far as to why this is the case. The issue is…
Dexter
  • 647
  • 1
  • 6
  • 10
20
votes
3 answers

Can I use a one time pad key twice with random plaintext?

I understand the basics of OTP: $|\text{key space}| = |\text{plaintext space}|$ implies perfect security, key reuse destroys this. Cryptanalysis on the $N$-Time Pad for $N > 1$ involves finding patterns in the ciphertext; this, however, all seems…
mfsiega
  • 363
  • 2
  • 7
13
votes
2 answers

Is it safe to reuse ECDH asymmetric keys for authentication?

Alice, Bob, and Carol each generate ECDH keypairs. Alice and Bob establish a communication channel and negotiate an AliceBob secret. The question is: Is it safe for Alice and/or Bob to reuse their keypairs to negotiate a new AliceCarol and/or…
Edward Ned Harvey
  • 833
  • 1
  • 7
  • 14
11
votes
3 answers

AES key reuse and guessing the key

Bit of a noob question and a fair bit of Googling didn't help. I understand that the current encryption standard is AES256. A few questions Is the point of this (and other encryption techniques) to reuse the same private key for multiple messages?…
user1936752
  • 778
  • 1
  • 4
  • 17
11
votes
1 answer

Is there any reason not to use Single-Key EM with AES and a constant key?

I've read recently the paper "Minimalism in Cryptography: The Even-Mansour Scheme Revisited" by Dunkelman, Keller and Shamir where it is claimed that the following construction (Single-Key EM) is secure: $$C=K\oplus E(P\oplus K)$$ with $E$ being a…
SEJPM
  • 46,697
  • 9
  • 103
  • 214
10
votes
3 answers

Is RSA in a ECB-like-mode safe for bulk encryption?

Let's say I would like to communicate with my friend using asymmetric/public-key encryption, e.g. RSA. (Note: I do realize that in practice this is done through an intermediate symmetric key, but this question assumes we only use asymmetric…
user541686
  • 1,409
  • 1
  • 11
  • 24
10
votes
4 answers

Problems with using AES Key as IV in CBC-Mode

I'm a pentester and currently analysing a web application which are using some strange encryption scheme. The point is: They encrypt using AES-128, generate a (not cryptographic secure) key and use this as encryption key AND initialization…
Chris
  • 103
  • 1
  • 4
9
votes
2 answers

Is there a downside to encrypting too much data with the same key?

Is there a downside to encrypting too much data? If so, what? I'm guessing things like too much data encrypted with the same key, possibly leaking information due to a collision encrypting non-essential data, wasting resources could be a problem,…
Drathier
  • 1,063
  • 1
  • 8
  • 17
6
votes
1 answer

Public Key generation for Ed25519 vs X25519

It is my understanding that EdDSA uses a slight variant of Curve25519 (typically used for ECDH), called Ed25519. Given the same private key, are the differences between the two algorithms enough to make the resulting public keys different between…
Robert Quattlebaum
  • 534
  • 3
  • 9
6
votes
2 answers

Why are ephemeral/session/temporary keys useful?

One reason that communication protocols use ephemeral keys is to help with implementing Perfect Forward Security. They're also used in SSL to go from using RSA to using a faster symmetric encryption. Why else are ephemeral keys used? In particular,…
theicfire
  • 161
  • 1
  • 1
  • 4
1
2 3 4 5 6 7 8