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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AES CBC encoding of decomposable OAuth2 tokens

We're building a stateless RESTful API that relies on the content of OAuth2 tokens to identify users and what they have access to. Users authenticate with other means and are given a token, which they ...
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91 views

Do both client and server need to implement OpenSSL protections to protect from CBC attacks

In regards to the protections implemented in OpenSSL 0.9.6d (described in section 2. http://www.openssl.org/~bodo/tls-cbc.txt). My understanding is that the mechanism prepends an empty record when ...
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283 views

Importance of block size in CBC mode

Why is it necessary to use a sufficiently long block size when implementing a CBC block cipher with a truly random initialization vector? In ECB mode it's easy to get information about the message if ...
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158 views

Why is CBC based on AES malleable in blocks other than the first block?

Perhaps I don't understand the answers to Malleability attacks against encryption without authentication, so trying to be more concrete... please be patient. If I want to change the first block and I ...
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410 views

AES mode for file encryption

I'm working on software that should eventually result in a crypto file system similar to encfs. So far, i have a user password that goes through PBKDF2 using ...
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2answers
273 views

AES-CBC and IV — Encrypting multiple blocks of a file with the same IV?

I have large files which are broken into blocks of 512 KB and uploaded to a server. In my DB I store the IV, and the user knows the security key themselves. When downloading, I use the same IV and ...
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764 views

What are the advantages of CBC over ECB?

From this question I understand that, for a block cipher, using CBC is better than ECB. It seems that if one only has part of the cipher text, then decryption is difficult because the decryption ...
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233 views

Brute Force on 3DES with Reduced Keyspace and Unknown IV

I'm trying to brute force a 3DES problem given a reduced keyspace (ie I know the first half of the key) but with an unknown IV. The code decrypts to plaintext. My first thought was that I could set ...
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258 views

Is it possible to split a large AES encrypted string and decrypt the parts one by one?

Due to some platform restrictions our decryption algorithm can only handle up to 1 million bytes. The string we receive is larger, having been generated by AES in cipher block chaining (CBC) mode, ...
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Increasing the diffusion of the AES-CBC encryption algorithm in pycrypto for python

When encryption is done using the AES-CBC algorithm, the encryption can be thought of as chaining the cipher texts with the previous ones and an IV. However, if its on CBC mode, we give our cipher ...
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How to break AES/CBC/PKCS5 when key and IV are reused?

I'm doing a code review for a crypto solution that reuses the same key with a constant IV. I want to demonstrate that this is not the right way to do things by figuring out the key and decrypting all ...
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1answer
447 views

Is CBC mode encryption vulnerable to a reordering attack?

I know that, if CBC-MAC is used in such a way that the tag is the concatenation of all the blocks' outputs (and not just the last one), it's insecure under CPA for the simplest case of 2 blocks ...
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202 views

Bellovin 96' attack on IPsec ESP protocol on encryption only option

Can you explain the actual attack? Why does the attacker need to firstly send some arbitrary UDP packet? How can the attacker break the privacy between A and B? Link: The article of Bellovin I'm ...
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440 views

Security proofs for CBC mode

I'm looking for different approaches to proofs for the security of CBC mode encryption. What are the best sources of information about this subject?
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374 views

Forge CBC-MAC given the MAC of two messages and of their concatenation

I found this question about CBC-MAC forging and want to make sure I understand it: Let a and be be two strings of block length one. Suppose the sender sends $(a, \text{CBC-MAC}(a)), (b, ...
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1answer
227 views

Generalizing a CBC-MAC collision with a common final message block

I found this question from a textbook but I don't really understand what it is asking: Suppose $c$ is $1$ block long, $a$ and $b$ are strings that are a multiple of the block length, and ...
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393 views

What block cipher is used for CBC-MAC?

What block cipher is used for CBC-MAC? DES, AES, 3DES? Or it doesn't matter?
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2answers
265 views

Does encrypting a hash of the message yield a secure MAC?

Is $mac = enc(hash(x))$ a secure MAC for hash = SHA256 and enc AES in CBC mode or ECB mode. Normally AES-CBC is malleable. But in this case I use a hash function for the one-way property. For ...
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1answer
401 views

Key-size of encryption method

Is there a definied keysize to be used with the "AES_256-CBC" method in PHP, and what is the size? Some background: I want to store encrypted text into a PostgreSQL database. The user who posts the ...
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940 views

Reusing keys with AES-CBC

I heard that key/IV pairs must not be reused in AES-CTR, or when using any stream cipher for that matter. Yet the attacks described do not seem to apply to AES-CBC. Is reusing the same key several ...
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1answer
319 views

Is it safe to use GZIP to avoid padding related attacks

I am designing a library that supports encrypting some in-memory data using AES in CBC mode. Now I need a padding but it seemed to me that Apple's CommonCrypto library and OpenSSL have different ...
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1answer
121 views

Is it dangerous to use plaintext as IV in CBC?

Is it dangerous to use plaintext as IV in CBC? For example: I want to encrypt $p1$, $p2$ with CBC. However I use $p2$ as the IV for CBC purposes. What would be the risk of not using a IV based on ...
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How does a cracker know if they've broken AES-CBC encryption? [duplicate]

How does a cracker know if they've broken CBC or stream encryption? With hashe cracking one would know because you have the password that you started with to test. But for CBC decryption, you have a ...
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445 views

Generating an IV for ESP 3DES-CBC

I have an application written in C, running in Linux. It uses IPsec (ESP) (manually putting the IPSec packet together) to send certain packets. The algorithm used ...
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225 views

Does CBC encryption of a hash provide authenticity?

Given a message $M$ and a cryptographic hash function $H$, let $f(M) = E_K(M || H(M))$ where $E_K$ is AES-128-CBC encryption with PKCS#5 padding. Take $H = \textrm{SHA-256}$ if it matters. In other ...
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How resilient to attackers with extreme resources available is this encryption method?

How could the following scheme be made more resilient to attacks capable of utilizing massive amounts of resources? Assume all random values are cryptographically secure. Key derivation: scrypt ...
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Strength of CBC with Ciphertext Stealing

Is CBC with Ciphertext Stealing (CTS) considerably weaker than CBC with padding such as PKCS7? I would imagine the most common situation where CTS is necessary would be due to some size constraint of ...
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How to use salt when there is only one user

I am encrypting files for storage in an untrusted location, using a custom Java program to do that. There is only one user, but there are many files. I am using AES in CBC mode with PKCS5 padding, and ...
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205 views

Is this method of deterministically using CBC secure?

I'm trying to design a deterministic encryption scheme to enable searching in untrusted databases with the smallest possible IV overhead on ciphertexts. I know it's very bad practice but unfortunately ...
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Can CBC-encrypted files be modified in-place?

When changing a (small) part of the plaintext of a CBC-encrypted file, how much of the file will have to be rewritten? My suspicion is that everything after the modified block will have to be ...
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2answers
558 views

Encrypted counter as IV in CBC mode encryption using same key

I'm currently working with a secure transport protocol that defines the IV to be a counter (incremental nonce) to be encrypted with the same key. This is a followup to a protocol that did not provide ...
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0answers
56 views

CBC with random prefix but no IV - secure? [duplicate]

I'm looking at a datagram-based protocol that encrypts a payload. The payload is encrypted with AES or blowfish in CBC mode, and the result is then authenticated with a HMAC. To save space, the ...
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224 views

Using AES-CBC and providing the IV in file header -> security hole? [duplicate]

currently I audit some code I've written in the past to use KeePass 1.x databases. I stumbled over the following question: The database itself is decrypted with AES-CBC and inside the file header the ...
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760 views

Is the 64-bit blocksize a fatal issue when encrypting TBs of data with Blowfish CBC?

Crashplan uses 448-bit Blowfish to encrypt the data you send them. The mode used is CBC, keys are multiple use, and IVs are generated with SecureRandom from JCE. Keys are not derived from the ...
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Why should I use an Initialization Vector (IV) when I have unique keys?

I took a look at this question. My question is not the same. I've unique keys encrypting (in CBC mode, AES-256) each plaintext, i.e. I do not use a key to encrypt more than one plaintext. Is it ...
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271 views

IV Security Clarification

After doing lots of reading on SO and other websites relating to AES cryptography, I am trying to understand the security issues surrounding IV's. There seems to be a lot of confusion and ...
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455 views

How much data can I encrypt with AES before I need to change the key in CBC mode?

In my cryptography class, the instructor suggested that in order to give the attacker a minimal advantage of $1/2^{32}$, we have to change the key after $2^{48}$ blocks are encrypted. It seems that ...
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1answer
356 views

Alternatives to HMAC + CBC?

I'm looking at using HMAC + CBC. The combination looks like this: ciphertext = AES256(text, k1) data = HMAC-SHA256(iv | ciphertext, k2) | iv | ciphertext Where: ...
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Is it worth applying a MAC on data in a HSM?

I'm building a basic HSM out of an Arduino, and am using the following scheme to store data: Master symmetric key $k_m$ stored in firmware (secure bit set to prevent trivial extraction). Secondary ...
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Is encrypting a single 128 bit block with AES ECB “safe”

I want to encrypt a small piece of data that is less that 16 bytes in size (think SSN), and I'll be using a 256bit encryption key. The typical suggestion is to never use ECB, but if there is just a ...
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1answer
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Can prepending “junk” be equivalent to an IV when encrypting using CBC?

We are encrypting a small positive integer (1-1000) with a constant key using AES256 encryption. We are considering two approaches to make this secure; use an initial vector (which we then need to ...
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3answers
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Ciphers in CBC mode reveal place of change in plaintext

Theoretically, when using a symmetric block cipher in CBC mode, the current block is dependent on the previous block. Suppose one plaintext is encrypted using CBC, and then one bit of it is changed, ...
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1answer
732 views

What are the constraints for an IV using AES in CBC mode?

I'm designing a protocol for use into a VPN software. The VPN frames are encapsulated into AES-256 CBC encrypted frames. I understand that IVs must be uniquely used for each message encrypted with ...
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1answer
467 views

What is the usual block cipher mode that goes with CTS

I am wondering: what is the most common, or most used, block cipher mode used that goes with CTS (CypherText Stealing)? Is it ebc or cbc?
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2answers
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Validating successful decryption in AES

I have a program which uses AES-256 in CBC mode to encrypt and decrypt files. As I have quickly realized, AES will even use an incorrect passphrase to decrypt data, which leaves me with no way to ...
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2answers
609 views

Is storing original file size in an encrypted file header a bad idea?

I'm encrypting a file using AES-256 in CBC mode. I'm padding to the 16-byte input multiple by using the PKCS#7 limit. The problem I'm currently having is that if I'm transferring or reading the ...
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690 views

Implementing PKCS#7 Padding on a Stream of Unknown Length?

I have a fairly simple Python program using PyCrypto to use AES+CBC to encrypt a stream of input. In order to adhere to the 16-byte input size multiple, I've implemented PKCS#7 by hand. (While I know ...
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1answer
688 views

How to break AES CBC with PKCS#5 padding?

If I have a string encoded with AES CBC with PKCS#5 padding, how can one possibly decrypt/crack it? What are the approaches that one can take to break its security?
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Use of CBC-AES-256 to encrypt usernames

I came across some code in our project that encrypts usernames (limited to 50 chars, most are less than 10) using the following parameters: IV size is 16 bytes Key size is 32 bytes Algorithm is ...
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877 views

AES CBC with fixed or predictable IV, what are the risks?

I am making a call with parameters from application "A" to a web application "B". Parameters are in the query string of the URL. They don't need to be hidden. But I want to be sure that nobody can ...