# Tag Info

Accepted

### 7zip : Why does encrypting the same file with AES-256 not give the same output?

This is expected behavior since 7zip uses Cipher Block Chaining (CBC) mode for encryption. For which you need the Initialization Vector (IV) to be unique and unpredictable. It was using 64-bit IV but ...
• 43.4k
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### Why was AES CBC removed in TLS 1.3?

Short: CBC mode in context of TLS protocol has had security issues, and would have had to be reworked. AES-CBC mode combined with decent HMAC can be as secure as AES-GCM. However, combining the ...
• 5,239
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### Why does "2xAES-256" provide "99.99%" security strength whereas "1xAES-128" provides "40%"?

It's meaningless nonsense. I would be inclined to avoid spending any money with these people. If you scroll down on this page, you'll find a table labelled key size vs. time to crack, according to ...
• 2,013
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### Is there any area where AES-CBC cannot be used ? If so, why?

CBC does not perform authentication This property makes it less suitable for places where authentication is required, basically any transport protocol. TLS uses CBC, but by default performs ...

### Why is CBC with predictable IV considered insecure against chosen-plaintext attack?

If your IV is predictable this is as (in)secure as assuming that you have a zero vector IV. And a zero vector IV allows you to perform a so-called Adaptive Chosen Plaintext Attack (ACPA). Why? ...
• 345

### 7zip : Why does encrypting the same file with AES-256 not give the same output?

Encrypting the same input multiple times, normally, is supposed to produce different outputs each time. This is so that an eavesdropper not only cannot tell that the input was ...
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### Why is padding used in CBC mode?

So, I'll answer the theoretical part of your question, since we need a key to address the practical part. Why is padding used in CBC? Blockcipher such as AES are encrypting blocks of a fixed given ...
• 7,246

### Why not authenticate full-disk encryption?

Authentication and probabilistic encryption are two desirable features which each take up a small amount of extra space. And you are absolutely right that the percentage of space consumed is of no ...
• 1,377

### Why was AES CBC removed in TLS 1.3?

TLS 1.3 is a reboot of the TLS protocol which focused on up to date cryptography rather than backwards compatibility. Now CBC is not as secure as you make it to be, and the way that it was used in TLS ...
• 85.1k
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### Bit Flipping Attack on CBC Mode

The Bit Flipping attack Decryption process in CBC mode is performed as \begin{align} P_1 =& Dec_k(C_1) \oplus IV\\ P_i =& Dec_k(C_i) \oplus C_{i-1},\;\; 1 < i \leq nb, \end{align} where $nb$...
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### Why should I use an Initialization Vector (IV) when I have unique keys?

You should use random IV even when unique keys are used. This prevents key-collision attack where the attacker collects number of cryptograms that have been encrypted with unique keys and brute-...
• 311

### Why not authenticate full-disk encryption?

So, are there reasons for not using authentication that I'm missing? I believe that the real reason is not actually space, but time. As you said, storing the tags would not require that much space. ...
• 132k

### Difference between a nonce and IV

With CBC mode the initialization vector is referred to as IV, because it is not nonce. There are ways to construct nonce so that it does not meet the needs of CBC mode. Random IV is one generation ...
• 5,239
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### Why not authenticate full-disk encryption?

With 4096-byte sectors, space is a complete non-issue, less than 1 % Problem 1: 10GB per TB is not a "complete non-issue" for many people. Problem 2: If the checksums are inside of their data ...
• 1,187
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### Are there any benefits to using ECB over CBC?

ECB benefits: It's a tiny bit easier to implement. It allows for parallel encryption and decryption (CBC only decryption). A single corrupted cipher block corrupts only one block of plain text(in ...
• 10.5k

### Is DES secure under CBC?

No, it will be insecure. There are two reasons; Due to the smaller key size 56-bit; DES was tested for brute-force attack since published. DES_CHALL, 96 days to find the CES challenge key in 1997. ...
• 43.4k
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### Error propagation in CBC mode

In CBC mode the decryption equation is $P_i = D_k(C_i) \oplus C_{i-1}$. If you received a corrupted $C_i$, $P_i$ and $P_{i+1}$ will be decrypted wrong, but $P_{i+2}$ no longer depends on $C_i$ and ...
• 31.3k
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### Why can't we use the first block of AES-CBC as MAC

The MAC value should be calculated over all of the input, not just the first block. The chaining of CBC makes sure that the bits in the last block of ciphertext depends on all the previous blocks.
• 85.1k

### Predictable, but private IV in CBC

If a nonce $N_i$ is even, then the binary numeral for it its increment $N_{i+1} = N_i + 1$ differs from $N_i$ only in its least significant bit; and if $N_i$ is odd, its increment is even. This means ...
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### What is the correct way to implement PBKDF2 + AES CBC + HMAC?

You'll find there's a lot of splitting hairs regarding this topic, especially key derivation. But yes, your pseudocode is fine, although you may want to revise (0, 128) => (0, 127) and (129, 256) => (...
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### Random data CBC padding scheme?

Padding is dangerous. CBC mode with padding is secure against chosen-plaintext attacks, where the adversary can convince the legitimate party to encrypt messages and obtain the ciphertexts. But it is ...
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### Modes of operation that allow padding oracle attacks

In the padding oracle attack you have an oracle that only tells you whether a particular chosen ciphertext decrypts to a correctly padded plaintext. That oracle is used to build a last word oracle, ...
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### Why is CBC with predictable IV considered insecure against chosen-plaintext attack?

The answer by mwhs is very wrong about CBC-MAC and its use of IV!! It is perfectly fine and secure to use the same IV for CBC-MAC! In fact, Jonathan Katz and Yehuda Lindell recommend using zero vector ...
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### Why is Fernet only AES-128-CBC?

This seems to me to be less secure... Do you have a plausible adversary that can break AES-128? AES-128 is believed (to the best of knowledge) to require \$O(2^{...
• 132k
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### CBC-Mode Infinite Garble Extension

The infinite garble extension makes sure that if a ciphertext block is changed that this block and each block after it doesn't decrypt correctly. The way that additional plaintext is affected when the ...
• 85.1k

### Are there any benefits to using ECB over CBC?

From a theoretical point as a mode of operation ECB mode has only one advantage over all of the other modes: it doesn't require an IV. That means that the ciphertext doesn't expand if the message is a ...
• 85.1k

### AES key equal to IV (CBC mode)

In this answer I'm assuming that a key is used to encrypt more than one message. The first weakness is that CBC with fixed IV leaks if messages share a common prefix. The second weakness is that it ...
• 24k

### mode of operation in cryptography

I would pick e) none of the above. None of those modes offers integrity protection, so unless integrity is handled elsewhere, your application is wildly insecure. An attacker could modify bits in ...
• 37.7k