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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1answer
487 views

New AES/CBC Hack [closed]

Can someone view this video and comment? https://cipherloc.net/cipherloc-presents-details-of-encryption-attacks/ The concept is presented between 8 and 13 minutes, and then results are presented at ...
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1answer
821 views

Decrypt simple CBC message with known IV using XOR encryption

I've been given a ciphertext message, encrypted using CBC. I know that the block size is 12 bytes and I know the plaintext value of the first block. I also know that the encryption function is a ...
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2answers
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CBC-MAC insecure with random IV

Prove that the following modification of $\operatorname{CBC-MAC}$ does not yield a secure fixed-length $\operatorname{MAC}$: Modify $\operatorname{CBC-MAC}$ so that a random $IV$ is used each time a ...
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1answer
730 views

Is it possible to obtain AES-128 key from a known ciphertext-plaintext-iv tuple used the encrypt the plaintext? [duplicate]

i have a file, which was encrypted with AES-128 in CBC mode. I know the content of original file and know the iv used to encrypt that file.So, just wondering if it's possible to recover the key used ...
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1answer
209 views

When using AES-CBC-128, what attacks become feasible as you decrease the entropy of the IV?

16 bytes is the recommended entropy for the IV, AFAICT. Is 12 bytes of randomness with 4 fixed (across invocations of this encrypt function) bytes at the end, significantly worse? How so? What about 8 ...
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2answers
93 views

security of design for sending files

let's say I have a protocol to send a fi le $F$. I am intending to employ AES in CBC mode for encrypting $F$ where the $IV$ is randomly chosen. For authenticity, I am employing $HMAC$ to compute a ...
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2answers
328 views

using hash function of (key||plaintext) instead of random value for IV

for a protocol which implements AES under CBC mode, an $IV $(initialization vector) is required for each encryption. Instead of picking the $IV$ randomly, is it advisable for us to set the $IV$ to be $...
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1answer
111 views

Is This method of generating Initialisation Vector for Cipher Block Chaining secure?

I have worries about security defined in BACnet protocol (ASHRAE/ANSI Standard 135, ISO 16484-5). Full text of security is here: http://bacnet.org/Addenda/Add-135-2008g.pdf and here is my summary: In ...
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6answers
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Why not authenticate full-disk encryption?

Common FDE software (TrueCrypt, BitLocker, dm-crypt) doesn't authenticate ciphertext stored on the disk. The commonly cited reason is "it would take too much space", reasoning that you would need an ...
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3answers
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Chosen Plaintext Attack on DES/AES

PERSON A sends messages to PERSON B using AES, CBC, PKCS5Padding with an IV that is incremented by one after each message. If PERSON Z knows that the first block of the plain-text of any message is ...
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2answers
2k views

What does (block cipher) decryption parallelizable mean?

I've tried searching this answer on here and on google, but I cant seem to find a succinct answer (maybe because its obvious). My understanding from wiki/tutorialspoint/ppt is that for CBC/CFB, ...
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1answer
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What is CCM — Counter Mode Cipher Block Chaining Message Authentication Code Protocol

CCM puts together two different modes of operation (at least in the name). Does it mean data is encrypted using counter mode and then encrypted using CBC; kind of a double encryption?
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724 views

CBC mode is not IND-CCA2 secure with no prediction of IV

How can it be explained that CBC mode is not IND-CCA2 secure, in case that the IV can't be predicted by adversary. Edit: In decryption of CBC, for each block it is XORed with previous block's cipher ...
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1answer
3k views

Why do the majority of SSL cipher suite used CBC?

I am looking at a wireshark capture of the cipher suites sent by my browser to the server during an SSL handshake; however, almost 90% of them use CBC, with 2 or 3 having GCM. Why is CBC most used? ...
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AES CBC Initialization vector - Security issues [duplicate]

I have 2 questions regarding security issues related to initialization vectors used in AES CBC Consider that I'm using a sequence number (which increments with every encryption) as an initialization ...
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1answer
160 views

Difference between XORing the IV and just pre-pending it to the plaintext before encryption with AES-CBC [duplicate]

The normal way of using AES-CBC is to XOR the IV with the first plaintext block This way you have to pass the IV along the ciphertext. What if instead of XORing we just put a fixed size randomIV in ...
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2answers
819 views

How does a Padding Oracle Attack work in practice?

I have understood how the padding oracle attack works in theory. Now, I am trying to visualize it in a real scenario. Can someone please give a real scenario in which an attacker can insert himself ...
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Best way to generate a IV for AES-CBC when encrypting files?

The situation is that I want to use symmetric encryption to store some info in files. I'm going to use AES-CBC algorithm. The user is going to remember the password (meaning it will not be stored ...
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1answer
171 views

Can you help me understand some details about the attack using SSL3 POODLE bytes?

I have read some articles or books (e.g., “Bulletproof SSL and TLS”) about POODLE, but they still don't answer my questions – which are: From https://www.openssl.org/~bodo/ssl-poodle.pdf, ...
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2answers
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Clarification on the origin of `01` in this oracle padding attack?

I have found what I believe to be a straightforward explanation of the oracle padding attack here: http://robertheaton.com/2013/07/29/padding-oracle-attack/ I am however getting confused by something ...
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1answer
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HMAC vs CMAC, which is faster?

I want to know which is faster-HMAC or CMAC.I know CMAC is based on block cipher encryption,where output of each block is X-OR with the next plain text block. If security is not a concern,which will ...
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1answer
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bruteforce on aes 256 cbc mode [closed]

i have a series of encrypted text , they encrypted by aes 256 bit cbc mode for every encrypted text there is an iv that i have them also and i know its algorithm is aes 256 bit cbc mode i can ...
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2answers
564 views

CBC mode with same plaintext

I'm working on a file encryption system where multiple versions of the same file might exist. The filenames are encrypted using AES in CBC mode with a random IV and stored in metadata. Since versions ...
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1answer
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Generating an IV for AES-CBC

I am faced with the task of generating a secure 256-bit IV for AES-CBC. I possess the following resources: A 32-bit microprocessor A 32-bit PRNG (can read/reseed) A 64-bit clock counter (can read) ...
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1answer
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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
532 views

Is double AES-CBC encryption with the same key and IV unsafe?

If some plaintext encrypted twice with the same AES function (CBC-mode), same IV and same key, is it possible to retrieve either the key or the plaintext?
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1answer
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CPA on theoretical OTP with CBC mode

Body of question Note: this is THEORETICAL exercise. I understand OTP and CBC does not get along. I have to find a CPA on CBC enc-mode using OTP as the encryption algorithm, s.t a new key is ...
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1answer
3k views

Question about Cipher Block Chaining(CBC)

I'm studying at the moment the CBC Encryption method,and I was asked in an excercise how many bits would be wrong after decryption if during the transmission 2 bits are interchanged. My guess is that ...
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2answers
429 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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Parallel file encryption and authentication for large files

For a cloud storage upload client I want to encrypt files during upload, so that they cannot be read by the cloud service provider or anyone who gains access to the account. I would like to use some ...
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2answers
812 views

How to crack unknown 8-bit encryption algorithm?

Can anyone tell me how to crack the encryption of 8 bit algorithm, when you have some clue that CBC mode is used, but the algorithm is secret. You also have some encrypted data (chosen plain text), ...
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850 views

Why can't we use the first block of AES-CBC as MAC

As per topic, why can't we use the first block as MAC instead of using the last block? what's the rationale?
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Does length prepending fix the key reuse problems with CBC and CBC-MAC?

As I hope everybody is well-aware, re-using the key for CBC-MAC and CBC is a really bad idea. Now, there's also the paradigm to prepend the message length to the CBC-MAC (as "associated data") to ...
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1answer
375 views

Disk encryption using AES-CBC: is reliance on previous block data a downside?

I have seen drives that boast that they use AES with CBC mode, which doesn't really make sense in my opinion, sure it's more secure than ECB (but actually, leaving security aside, ECB is better suited ...
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1answer
548 views

File using same key on XOR cipher. Using CBC [closed]

I have some files that are encrypted using an unknown key and it's using the CBC method to encrypt the files. When I XOR two files together the first two bytes are always the same, but different for ...
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1answer
206 views

Is it safe to use the same IV for multiple encryptions in AES CBC mode when you are always encrypting random bytes?

I am generating random bytes (16 or 32 bytes at a time), and I want to encrypt them using a 128 bit AES key in CBC mode. Am I really weakening this horribly if I am reusing the same IV and key each ...
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1answer
67 views

Proper authenticated encryption

I am using AES-CBC, authenticating using HMAC-SHA512 and I was wondering whether my thought process and implementation are correct. The message send over the network is: HMAC(64Bytes),IV(16Bytes),...
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0answers
637 views

TLS padding format

My question is about how the padding is formated in TLS1.0, 1.1, 1.2, 1.3 I came across the PKCS7 format for the padding: ...
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0answers
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What is the benefit of artificially padding messages? [duplicate]

For ECB, CBC, and CFB, padding is required when the plaintext does not fill out a complete multiple of the block size. However, I have heard that is good practice to pad all messages, even if the ...
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0answers
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Which protocols / file formats use CBC with a random IV and without a MAC?

Disclaimer: I am a noob so this may well be a stupid question or one that's been asked before. I didn't see it come up after searching, though, so I'm going to assume it hasn't been asked here. The ...
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1answer
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Is there any benefit to verifying PKCS#7 padding when using AES CBC and HMAC?

As I'm using HMAC to ensure integrity and authenticity of the ciphertext, is there any additional security benefit to verifying PKCS#7 padding when decrypting using AES CBC? A previous question asked ...
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2answers
1k views

How to show that this modification of CBC-MAC is insecure?

I'm working on some cryptography problems, found this and I'm not sure how to solve it: Modify CBC-MAC so that all blocks $t_1,\dots,t_l$ are output rather than just $t_l$ and prove it is not ...
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1answer
670 views

Fixed-prefix IV in CBC mode

I know the Initialization Vector needs to be unpredictable. Does it apply to the IV as a whole, or to every bit in the IV? I have multiple devices with poor entropy sharing the same key and I want to ...
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1answer
366 views

What about using AES-ECB with random IV?

Let P1P2...Pn be n blocks plaintext, and M0 be a random IV. Then a n+1 blocks message M0M1...Mn can be constructed by setting Mi+1 = Mi xor Pi+1. Finally, we use AES-ECB to encrypt this message. Is it ...
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2answers
286 views

DIY over-encryption with a related key

I know next to nothing about cryptography, which is bad because I'm a programmer. I was exploring some code the other day and the encryption part of it seemed a little strange to me. I'd like the name/...
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Is reusing keys for CBC and CBC-MAC secure when using encrypt-then-MAC?

I know reusing keys is a bad idea, especially for CBC and CBC-MAC, but the example for why on Wikipedia seemed to use encrypt-and-MAC. It seems obvious that the last block of encryption will be equal ...
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2answers
3k views

AES-128 Decryption in Objective-C just slightly off [closed]

I have been asked to integrate a home-grown CRM system with a 3rd-party data service provider (DSP). The DSP has encrypted the information they are providing. I only have their decryption ...
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2answers
851 views

Why IV does not have to be secret yet has to be random [duplicate]

The IV used in schemes such as CBC has to be random and unpredictable. But at the same time it does not have to be kept secret. If the IV does not have to be secret, why does it have to be random ...
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1answer
297 views

Should I be concerned about padding oracle attacks if the lifespan of my key is ~1 minute?

I'm using 4096-bit RSA to encrypt a 256-bit AES key generated client-side. The RSA ciphertext is then sent to the server, and the server decrypts the RSA ciphertext to acquire the AES key. ...
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2answers
2k views

AES key equal to IV (CBC mode)

Application uses AES in CBC mode with 128 bit key. In this implementation key is always same and is equal to IV. Can attacker get the key or decrypt data? If he knows: First and second blocks of ...