Questions tagged [ctr]

Counter Mode (CTR) is an encryption mode, that builds a random-access stream-cipher from a block-cipher.

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Is there a form of AES encryption where the cipher text matches the length of the clear text and does not show patterns?

I have clear text, usually less than 10 bytes, which contains a few infrequently changing bytes. Reading: http://lollyrock.com/articles/nodejs-encryption/ I tried ...
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Can CTR mode be used with public key cryptography?

Can counter mode be used in some way in public key cryptography? Can we decrypt when counter mode is used if 1. we have the key and 2. we have the cipher text? Is there any other mode in which this is ...
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1answer
373 views

CTR vs CBC cycles per byte with OpenSSL EVP

I did some benchmarking using the EVP calls of OpenSSL in C, for CBC and CTR encryption/decryption. (1) : I got 3.2 Cycles/Byte for CBC encryption. (2) : I got 0.7 Cycles/Byte for CBC decryption, CTR ...
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IND-CPA security of CTR mode

Let's suppose CTR mode is instantiated such that the input to the block-cipher is $\langle \mathrm{IV}+\mathrm{ctr}\rangle$ instead of $\mathrm{IV}\mathbin\|\langle \mathrm{ctr}\rangle$, where $\...
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1answer
241 views

Deterministic encryption for a limited space: using HMAC as IV [duplicate]

I need a deterministic encryption scheme. Objectives: Same plaintext is always encrypted to the same ciphertext. Related plaintexts are encrypted to unrelated ciphertexts. (For example, attacker that ...
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3answers
591 views

Can I create a random access CSPRNG from a hash?

Can I use a keyed hash of a counter as a secure CSPRNG? E.g. 4th random block is Hash(4, seed) If not, what are the possible attacks?
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1answer
580 views

AES CTR with a non random and predictable nonce?

after reading a lot of stuff about AES CTR and its IV i'm still not sure if it is ok to use a predictable IV or not. I searched the web and found some programs / devices that use encryption in this ...
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Encrypting small packets with low data rate

I'm trying to build my own layer of encryption on top of our bluetooth communication. We can't use bonding because of some specific features we need to support. I'm a beginner when it comes to ...
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1answer
214 views

Relevance of Input and Ouput size of PRF on CTR mode

I'm working on exercise 3.25 on Katz' book (2nd ed): Let $F$ be a pseudorandom function such that for $k \in \{0, 1\}^n$ the function $F_k$ maps $\ell_{\text{in}}(n)$-bit inputs to $\ell_{\text{out}...
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1answer
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CPA-security of CTR mode

I'm self-studying Katz' book (2nd ed), and I have a small question about Theorem 3.32, which says: If $F$ is a pseudorandom permutation, then $CTR$ mode is $CPA-$secure. They present a proof there,...
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Using checksum instead of hash for authenticating ciphers

Lets assume for a specific situation we cannot use authenticated encryption like GCM ... If we were to use CTR, what would you think of using a checksum on the plaintext then encrypt the whole. The ...
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1answer
510 views

Can I use MAC as CTR mode nonce, if my messages are non-repeating?

I design encryption for a network protocol with restrictively low MTU. I'm aiming at 128 bit strength; AES-128-CCM would add 128 nonce bits and 128 MAC bits to each packet. Such MTU reduction by 32 ...
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1answer
182 views

Why does the authenticated cipher CCM encrypt the MAC too?

Suppose I have Encryption in CTR-Mode and a CBC-MAC. Why should I also encrypt the MAC? In the documentation of NIST stands: "By encrypting T we avoid CBC-MAC collision attacks". But I can not ...
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Pseudo code for CTR mode encryption

I am following a document here about CTR mode of encryption. I have some point uncleared. At the part 1 Review of Counter-Mode Encryption. Operation. To encrypt using CTR-mode encryption, one ...
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1answer
185 views

Break AES-CFB mode having access to a AES-CTR black box

I need help for a "simple" problem Let's suppose an intercepted ciphertext C encrypted with an AES-CFB mode of operation, using key K and some IV (IV is probably at the begining of C) The system let ...
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1answer
2k views

Is openssl library safe threaded? [closed]

I'm encrypting in C++ with openssl library. Basically I'm using these functions: RAND_bytes(key, 32); AES_set_encrypt_key(...) AES_ctr128_encrypt(...) My purpose is to encrypt parallelly with ...
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Dangers of using AES CTR without authentication - can you recover the plain text?

I understand that if a system uses AES in CTR mode (or other modes) without authentication that an attacker can flip bits to try to get the system to do something it wasn't intended to do (or wasn't ...
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2answers
446 views

Is it feasable to combine ECB and CTR block modes of operation?

Both ECB and CTR block modes have advantages over other block modes. They both support parallel encryption and decryption and random reads/writes. Yet both have major disadvantages: (I'm assuming AES ...
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1answer
1k views

Counter Mode Known plain text attacks

Counter mode uses a counter that is incremented for each block. Doesnt predictability of this counter expose the scheme to attack? I mean if you are incrementing some nonce, then I have a good chance ...
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1answer
293 views

encryption of digest of unkeyed hash function is susceptible to forgery?

I have a question in mind. Suppose we have a collision-resistant unkeyed-hash function $H(·)$ which outputs a 128-bit digest, and a semantic secure symmetric encryption $E(·,·)$ in the following way, ...
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1answer
2k views

Repeated NONCE in CTR mode

My understanding is that, in CTR mode, the NONCE must be unique because, if a NONCE is reused, multiple plaintexts will be XORed with the same value and this gives an attacker leverage to decrypt the ...
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Questions regarding parallel AES CTR with merkle trees

I'm planning to implement a parallelizable variant of AES to end-to-end-encrypt files in a web application. As a mode of operation I'm most likely gonna use CTR mode with a 64 bit IV and 64 bit ...
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860 views

Can we use a sort of “hash” function in CTR mode instead of a block cipher? [duplicate]

CTR mode does not use the decryption function at all, only the encryption function. This may suggest that a different primitive that has this "one-way" property could do the trick. For instance, I ...
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Authenticating a range of a ciphertext

I have some data that I CTR-AES-256 encrypt already. I am aware of ways to validate the integrity of the entire file (ie: a separate MAC of the plaintext, or encrypt then MAC, etc). But none of ...
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What is the LENGTH parameter in EEA2?

In the EEA2 option for encryption in LTE the AES-128 algorithm is mechanized. There, the nonce used in the CTR Mode is a 128-bit value. The first 64-bits are covered by a COUNTER, a BEARER, a ...
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1answer
206 views

Why is a one-time MAC secure for MAC-then-encrypt with randomized-CTR but not randomized-CBC?

The Coursera cryptography 1 course says: If you want to use MAC-then-encrypt mode then you should either use randomized-CTR or randomized-CBC. And if you use randomized-CTR then one-time-MAC ...
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1answer
922 views

How to increment the counter value for CTR mode?

I am testing CTR mode on STM32 board. I want to know how the auto increment of counter happens and what is the method to follow if I have to increment the counter manually.
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1answer
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Security of AES CTR with multiple messages containing the same known plain text

I have this scenario: The same plain text (that is part of an unkown bigger message) is being ecrypted multiple times using AES 128 CTR with a random key, the key is being transfered encrypted using ...
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1answer
13k views

How is decryption done in AES CTR mode?

Since AES CTR mode uses a unique IV and counter to produce the key to XOR with the plain text to get the ciphertext, the question is so as to how decryption is done. Since AES CTR produces a ...
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435 views

mode of operation in cryptography

We need to choose a mode of operation for a Telnet like application where the average message size is between 7 and 1024 bytes. What is the best mode of operation in this case? a. CBC b. CFB c. ...
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1answer
938 views

Security of a simple AEAD construction

Introduction I want securely communicate with a small 8-bit device which has a very limited program program memory (Arduino Uno - 32K). My goal is to minimize code size and RAM usage. There are ...
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1answer
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I homebrewed an algorithm. Is it secure?

This is a bit of a cross-posted question which I first asked at StackOverflow, however I was not satisfied with stackoverflow, so I came here for an in depth answer. I, as they say, "rolled my own ...
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1answer
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Is regular CTR mode vulnerable to any attacks?

Regular plain CTR mode, is it vulnerable to any attack? I plan to write a program to encrypt a drive with ctr mode. Would this be wise? Would AES be better? When I look at this diagram, it looks ...
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1answer
665 views

Dangers of using CTR mode for encryption

I know that it is never suggested to use the same key or key/IV pair twice for a stream cipher or for a block cipher operating in CTR mode. Filesystem-level encryption reuses the same key for a file ...
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1answer
858 views

Multi-target attacks on AES-CTR with a random nonce

128-bit block ciphers are vulnerable to multi-target attacks where the attacker seeks to attack a collection of keys instead of a single key. A simple example: Generate keys $k_1, k_2, ...,k_{2^{40}}...
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Using HMAC as a nonce with AES-CTR encrypt-and-MAC

Suppose I use the following encrypt-and-MAC construction: $E(k_1, k_2, n, m) = E_\text{AES256-CTR}(k_1, n, m) \| \text{HMAC-SHA256}(k_2, m)$, where: $k_1$ and $k_2$ are 256-bit keys $n$ is a nonce $m$...
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1answer
1k views

CTR DRBG dependent on request size?

I am trying to understand the CTR DRBG specification in NIST SP 800-90A. It seems slightly different to a pure stream cipher in that the key and counter are reset after each generate call using the ...
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1answer
485 views

If PRF is non invertible, why does Ctr-mode work?

I am currently taking Dan Boneh's Cryptography I course online. I have just finished the segment discussing the Cipher Block Chaining and Counter-Mode. I am having difficulty understanding why Ctr-...
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1answer
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Best practises to generate IV for AES-CTR in this scenario

I'm building a cloud storage application that will store files on my servers. I want these files to be encrypted with AES in CTR mode and the key will be always the same. I'm wondering what would be ...
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1answer
620 views

Implementing 5 modes of operation with a hash function

Is it possible to implement any of the 5 modes of operation (ECB, CBC, OFB, CFB, CTR) with a hash function?
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Picking a nonce in the context of CCM (CTR with CBC-MAC) mode

After having read the RFC 3610: Counter with CBC-MAC (CCM) and the Wikipedia article on CTR mode, I'm not sure how a nonce should be picked. The RFC says: A nonce N of 15-L octets. Within the ...
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1answer
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Explain same encryption time for two files of different sizes

I am encrypting files in a two layered fashion as follows: Inner layer is 128 bit AES-CTR encryption Outer/Second layer is a simple one time padding with a particular key. While testing the running ...
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AES decryption vs encryption speed

Let's consider the CTR mode. For a faster encryption/decryption, is it preferable to use the decryption operation of AES, or its encryption ?
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549 views

Does CTR mode yield a PRF?

Can the keystream generated by CTR mode be considered a pseudo-random function? I give more information. Assume $f$ is a PRF. We define a function which takes 2 inputs, the desired length, and an ...
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1answer
790 views

File encryption with AES CTR mode

I understand that encrypting files in CTR mode is not perfectly safe, because it potentially leaves the files open for cryptanalysis, if the attacker has access to previous versions of the same files. ...
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Why is it good to split a CTR-mode counter into nonce and counter?

When discussing the CTR mode of block ciphers, Wikipedia says the following: Simply adding or XORing the nonce and counter into a single value would completely break the security under a chosen-...
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Is this commutative encryption protocol secure?

My assumptions are: encryption must be commutative man-in-the-middle is not an issue I would like to implement the following scheme between 2 parties ($A$ and $B$) to exchange a key: Lets $C = P \...
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2answers
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Are all stream-ciphers IND-CPA?

A stream cipher usually generates a stream of pseudo-random bits which get XORed with the plaintext to form the ciphertext. The stream is generated using a given IV / nonce and a secret key. CTR-mode ...
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Turning a 64 bit block cipher into a 128 bit block cipher

There are quite a few block cipher modes of operation that require 128 bits. There are also modes of operation where a higher block size than 128, e.g. a block size of 256 bit would even be practical. ...
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Secure Channel - Encryption and Decryption Keys For A Message

I'm currently reading Cryptography Engineering and in Chapter 7, the topic of secure channel is discussed. The channel described is using CTR for encryption. Below I include images with pseudo-code ...