Questions tagged [cbc]

Cipher block chaining (CBC) is a method for encrypting large amounts of data with a block cipher that can only encrypt fixed length plaintexts. When used with an unpredictable initialization vector (IV), it is secure against chosen plaintext attacks (CPA-secure).

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Formal security arguments for 3 round feistal network using PRF

There is a proof sketch in Introduction to modern cryptography that a three-round feistel network using pseudorandom round functions is a secure pseudorandom permutation PRP Πk against probabilistic ...
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Online AES Calculator

I am currently looking into how AES works and found a calculator online: http://testprotect.com/appendix/AEScalc I understand that the AES process has 10 rounds of calculations. Does this mean that ...
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Does XORing a random block into a layer of CBC encrypted message add some bit strenght?

Let's suppose I encrypt a message with a block cipher in CBC mode, keep the key and IV secret and XOR a random block of data in each encrypted CBC block, being the XORed block the same for each CBC ...
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Random data CBC padding scheme?

When using AES in CBC mode, padding is required when the last plaintext block has not the length required by the cipher. Let's say $k$ mode bytes would be required to fill the block. My idea was to ...
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Finding IV for forgery of AES CBC-MAC with non-fixed IV

As CBC mode might do several rounds, how it could be possible to find another IV for forgery attack? This is an example from my course: Assume you receive a pair with the plaintext $P_1$ and its MAC ...
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AES-128 CBC encryption : ciphertext size seems too large

I am trying to encrypt messages with the AES symmetric algorithm using C language. I am using a key, an Initialization Vector (IV) and a plaintext, all with a length of 16 bytes. I expected as output ...
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Comparing the security when using fixed IV for CBC and CTR

Assuming that I'm using fixed IV's for CBC mode and CTR mode. I know that in CBC, the blocks depend on the previous ones and on CTR they are all independent. Yet with the same key and fixed IV, the ...
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Proving Security of a Tweakable Block Cipher in CBC Mode (Threefish)

I want to prove the security of a Tweakable Block Cipher in CBC Mode How does one prove the security of a Tweakable block cipher (ie Threefish) in CBC mode with a random IV and a constant (public) ...
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AES/GCM/PKCS5Padding giving issues while AES/CBC/PKCS5Padding works fine for my use case

I have a requirement to read-encypt-save plus its reverse i.e. read-decypt-save files through chunk/block mechanism (single operation not feasible because of huge(GBs) file sizes). So essentially the ...
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How to create the ciphertext that generates a desired plaintext in CBC (previous cipher- and plaintext are known)

I'm struggling with understanding this part of the literature: It can be assumed that the entire ciphertext AND plaintext is known. Considering the encryption method is $C_i = enc_k ( P_i \oplus C_{...
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Exploitability of AES in CBC Mode For Recovery of Plain Text

I'm aware of existing literature that shows in a comprehensive manner just how simple it is to tweak the ciphertext of AES-CBC such that the decrypted message ends up being entirely different than ...
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Substitution ciphers amended with cipher block chaining: susceptible to frequency analysis?

I have been studying ways to amend a simple substitution cipher, and one of the toy suggestions was to use CBC in the following way: identify each letter with a number from $0\ldots 25$ start with a ...
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Finding the IV of AES CBC (CTF)

We have a python program that encrypts/decrypts plaintext given in hex to a cipher using AES CBC. We know that the IV is the same and is not going to change (it is stored in a file). How can we ...
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Is it safe to reuse an IV 100 times to a single key with AES CBC? [duplicate]

I have a unique 6 digit number that I am generating a IV from. I am trying to make it as unique as possible. From my programming, I am able to get it down to only have 100 duplicate IVs per unique 6 ...
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Which encryption technique should be adopted to keep the search functionalities?

I am in exactly the same situation as described in this post. I must be able to encrypt data from a client workstation then send it to an untrusted server (encrypted at rest), then decrypt it only on ...
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AES-CBC decrypts to wrong output?

Someone I know showed this to me, and I can't put a finger to why it's happening. Am I missing something? They encrypt the text, and the output is really guessable. $u\to t$, $i\to h$ (it's one letter ...
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An exercise to show that CBC Chain is insecure

The CBC-Chain mode of operation is a CBC variant in which the IV that is used for the very first message to be encrypted is randomly selected, whereas the IV used for each subsequent encrypted message ...
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Is it more secure to use Diffie-Hellman to generate a symmetric AES key for use with CBC, rather than use CBC directly atop RSA encryption?

Short question: If we assume a one-session use public RSA keypair on both sides, and if we assume that the input stream will be split into blocks -- with CBC encryption -- is there any security ...
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Is ECB mode more suitable than CBC mode while encrypting a long bit string?

I know that ECB is less secure than CBC, but will it be suitable for encrypting a long bit string?
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Is it necessary to use an IV in CBC mode when just encrypting one file? [duplicate]

As far as I know, CBC mode, even with a null IV, will encrypt identical plaintext blocks to unique ciphertext blocks within a single file. The purpose of an IV is to obscure the similarities between ...
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Help me understand how to do a crypto assignment about AES/CBC

I'm doing the Coursera cryptography I course from Dan Boneh. I am attempting to do week 2's programming assignment, but I think I am having some sort of misunderstanding of either the theory, the ...
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Does it increase security if the CBC chunks of a AES-256 encrypted bytestream are uploaded to different clouds?

I want to backup my data according to the 3-2-1 rule. I am already back up to 2 different drives (2 different systems) at home. In addition, I would like to use cloud storage (Google, Amazon, etc.). ...
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Does it increase security if the CBC chunks of a AES-256 encrypted bytestream are uploaded to different clouds? [duplicate]

I want to backup my data according to the 3-2-1 rule. I already backup to 2 different drives (2 different systems) at home. In addition i would like to use cloud storage (Google, Amazon etc.). ...
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CBC mode works well for input greater than “b” bits. What is that “b” bits?

This post on geeksforgeeks.org about block cipher modes of operation, says that one of the advantages of the CBC is that it works well for input greater than b bits. I also saw that in the book "...
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Known plaintext attack on AES with CBC

Assume I have a plaintext and a ciphertext, encrypted using some key and AES with CBC mode. I don't have a service, that I can send something to, so I can't use the Padding Oracle attack. Are there ...
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Padding Oracle Attack CBC Mode: If the set of characters of plaintext is known, then can you speed up the process

Consider AES-256, 128-bit blocks, and PKCS#5 Padding. During a Padding Oracle Attack, a manipulated byte of cipher text is sent to the oracle and we eventually hope to find out what that respective ...
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Block Cipher Mode of Operations

I am studying for a test and I don't seem to understand the Block Cipher and its different modes of operations especially when it comes to encrypting and decryption. Could anyone please help solve ...
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Decrypting first byte of plain-text for AES CBC Padding Oracle Attack

I'm working on padding oracle attack and I have a 16 byte cipher on which I need to perform the attack. I've been able to successfully decrypt all the bytes of the plain-text except first byte and I'm ...
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Is it safe to use the CBC-MAC as a Initial Vector and Key Check Value in deterministic encryption?

I want to deterministically encrypt a short message <50 bytes with AES-128 CBC. I also want to be able to detect if the correct key is being used. Ciphertext and IV integrity will be guaranteed by ...
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CBC-MAC variable-length weakness with MAC size greater than the encryption block size

The title mostly explains it; CBC-MAC variable-length weakness with MAC size greater than the encryption block size I've seen that by having the pairs $(M,T_m)$ and $(N,T_n)$, we can generate a ...
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How to alter text that was encrypted with AES CBC using PKCSPadding7

Assuming, I had a ciphertext along with its decrypted plain text result but not the IV or the Secret Key. (keep in mind that the text was encrypted with AES/CBC/PKCSPadding7). How would I go about ...
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AES Padding (Nullbytes vs PKCS7)

I currently implement the AES(128,192,256)(ECB,CBC). Does the type of padding affect security? I just want to fill the blocks with null bytes.
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Can I and How Decrypt this Online

Although I had a security course in school a long time ago, I am effectively new to cryptography. I have an encrypted text, the encryption key of 44 characters, the encryption method, (Fernet, which ...
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Decrypting an AES string

I'm dealing with a website that loads a javascript file which eventually sets an AES hash/value (whatever it's called) as a cookie. The file is heavily obfuscated making it hard to understand certain ...
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In CBC mode of encryption, what happen if the IV is changed when it arrives at the receiver end? [duplicate]

What will happen if the Initialiazation Vector (IV), i.e, the initial randomness fed in to the first block arrives mangled at receiver end? How will this affect the decryption? Will the receiver be ...
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Find first two blocks of plaintext given a final plaintext block, the IV and ciphertext for CBC?

I have the following: complete ciphertext; iv; last block of plaintext including the padding which is 04040404. The encryption was made using AES-CBC-256. I can ...
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How is it possible that these two encryption functions yield the same result?

I opened this post in stackoverflow some time ago where I explained a problem I had about not finding a way that would allow to successfully get the result of a string encrypted with a "aes-256-...
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Quadruple DES vs AES-128 security

I came across this question regarding the security of Quadruple DES (hypothetically DES done four times) in CBC encryption mode vs security of AES-128 in CBC encryption mode. What parameters help to ...
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Multiple IV CBC

CBC's encryption is not paralelizable but I could divide a 64 block message in 4 16 block parts and encrypt those 4 parts separatly in parallel using a diferent IV for each one. Is this unsecure ? Is ...
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AES-256, CBC, plaintext length is multiple of block size: Does PKCS #7 padding weaken the encryption?

Of course, padding is superfluous when AES with CBC is used if you know that the plaintext length is a multiple of the block size. But does the padding precipitate a weakness in the encryption? ...
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Looking for a way to enlarge a message so that any modification to the “enlarged” message makes recovering the original message impossible

Let's say we have a message m of small size. I am looking for a system $S$ so that $S(m)$ is arbitrarily large, we can easily compute the inverse $I(S(m)) = m$, and any modification to $S(m)$ makes it ...
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Is it okay to have no IV when using AES-CBC with unique keys, that are however derived from one shared key?

Given $\operatorname{AES-CBC}$ (128-bit), with $IV=0$ and unique encryption keys $$K_{Enc} = \operatorname{CMAC}(MK, \text{known_text})$$ where $MK$ is a shared symmetric 128-bit key and $\text{...
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Does AES-CBC always use PKCS#7?

I need to add AES decryption with a 256 bit key to our software, and of course I won't do it myself; rather, I chose a library that works fine for our needs. It supports various modes of operation, ...
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Encryption & decryption in AES CBC mode - Which is faster?

I wrote a small code to compute the time consumption by the AES in CBC mode, I noticed that the encryption is faster than the decryption, my question: Is my result correct? If yes, why is the ...
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CBC encryption + CBC MAC reusing key in MAC-then-Encrypt

I'm reading the Handbook of Applied Cryptography by ‎Alfred J. Menezes et al. Especially, I'm stuck with the case that reusing key for CBC encryption and CBC-MAC in MAC-then-encrypt structure. My ...
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DES-CBC with random IV - how big is the keyspace? Can COTS tools crack it?

I've spent the night reversing the implementation of the 'Basic Text Encryptor' from Jasypt [1]. The algorithm is defined in documentation as 'PBEWithMD5AndDES'. The implementation is this: A ...
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Crypto system design : How to design Encryption for Zero width characters

Zero width character's can be used to hide text in plain sight.I am building a system where user's can provide their secret message , cover message and a password (key) which I would encrypt and give ...
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How are initialization vectors used in TLS 1.1?

From my understanding, TLS 1.0 can use CBC to encrypt data. To do so, it creates one initialization vector (IV), and then uses the previous ciphertext block as the IV for the next record. I made this ...
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Weakness in a CBC-like XOR cipher

A simple symmetric encryption algorithm can be written as follows: Input message M and 64 bit key $K$ Divide M into 64 bit size blocks $B_1...B_n$ Get first block $B_1$ and perform bit-wise $\oplus$ ...
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Is there any way to find out two numbers such that the XOR between them is a given number?

I have the following operation I took from a cryptanalysis I'm performing for a specific CBC encryption where the challenger has provided the key: ...

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