33
votes
What is the maximum size of the plaintext message for RSA OAEP?
The input length of OAEP is directly specified in the standard:
...
- 91.8k
16
votes
Accepted
AES - What is the advantage of a 256-bit key with a 128-bit block cipher?
No, this isn't an oversight. AES is a block cipher, which is a keyed permutation. Now if you have a permutation of, say, three elements there are e few permutations possible:
...
- 91.8k
15
votes
Accepted
How can there be AES-256-GCM, when GCM is defined for 128-sized blocks?
AES has a block-size of 128 bits in all its variants. The number in AES-128/192/256 is the key-size. Rijndael, the block-cipher that became AES, also supports 256 bit blocks, but that part was not ...
- 24.8k
13
votes
Accepted
Why is it OK for secretbox to leak the message length?
It is usually assumed that the length of the message is not secret. Even with padding the approximate length is leaked, and necessarily any encryption reveals a maximum length – or at least ...
- 32k
13
votes
Accepted
What are the risks of using CTR mode with 64 bit blocks?
I believe that he was referring to the misconception that the birthday problem that arises in encryption is only when you use the same counter twice. If a random IV is used, then such a counter ...
- 27.6k
11
votes
Accepted
How does using a really small block size reduce the key space?
How does a small block size reduce the key space?
Let $b$ be the block bit size of a block cipher. There are $2^b$ possible block values, for a plaintext, and for a ciphertext. For a fixed key, there ...
- 138k
9
votes
Accepted
SHA-3 block sizes / bitrate calculation?
First of all, in the case of SHA-3 we don't call it block size but bitrate.
SHA-3 has been formally defined in FIPS 202 and in its reference manual.
We define the sponge function denoted by $\...
- 9,969
8
votes
Accepted
What is a wide block cipher and why does it avoid birthday bound problems?
I don't think the term "wide block cipher" has a hard definition beyond
has a larger block size than the current standard algorithm(s)
which right now in most cases would equal to
has a block ...
- 45.7k
8
votes
AES - What is the advantage of a 256-bit key with a 128-bit block cipher?
There is no NIST oversight here. The key size and the block size are two completely different parameters and issues. The only reason that you need a large block size is because bad things start to ...
- 27.6k
8
votes
What are the risks of using CTR mode with 64 bit blocks?
This answer summarizes what's achieved against CTR mode in the paper pointed by Yehuda Lindell's answer: Gaëtan Leurent and Ferdinand Sibleyras's The Missing Difference Problem, and its Applications ...
- 138k
7
votes
Accepted
How does blocksize affect security?
Block ciphers are usually used in modes of operation. The security of a mode of operation depends on two things: the security of the underlying block cipher, and the security of the mode itself when ...
- 411
7
votes
Accepted
Why are block ciphers mostly used as stream ciphers?
Why are block ciphers mostly used as stream ciphers?
CTR mode doesn't need padding like the CBC mode that caused many attacks over the years knowns as the padding oracle attack [1] [2]. Finally, CBC ...
- 47.6k
7
votes
Accepted
What would be the maximum acceptable block size for disk enryption?
AES-XTS is a mode of operation of AES that encrypts blocks, but is not a blockcipher. It has the security drawback that when information in a large block changes locally (e.g. a single byte), it's ...
- 138k
6
votes
Accepted
What's the proper choice for the block size of a sponge-based hash?
The "block size" matters for Merkle-Damgård functions because the HMAC security proof relies on that block size. For other functions, and in particular sponge functions, the block size for HMAC is ...
- 86.6k
6
votes
What is the typical block size in RSA?
It is seldom a good idea to encipher more than one block with RSA alone, thus the question is moot. One should use hybrid encryption, where the bulk of the data is symmetrically enciphered with a ...
- 138k
6
votes
Does the block size of a symmetric cipher impact the security of the cipher itself?
Does the block size of a symmetric cipher impact the security of the cipher itself?
Yes, absolutely. A small block size limits the amount of data that can be encrypted with a given key, and some ...
- 15.1k
6
votes
Accepted
How many required known plaintexts for an attack are considered insecure?
Firstly, is this a correct threshold for considering a cipher secure?
Not exactly. Security is a spectrum, so what is secure for some applications may not be secure for others. Is a $2^{-64}$ ...
- 15.1k
6
votes
Accepted
How to securely encrypt/decrypt data with a maximum chunk size?
I have a solution that may satisfy you
Splitting the file into parts (chunk) and chaining them is a solution for you. To prevent the truncation we will use the associated data, which is the same for ...
- 47.6k
5
votes
How does blocksize affect security?
Block size does not directly affect the security of the cipher. However, if block size is too small, it can prevent you from using the cipher securely.
The main effect of block size is due to the ...
- 32k
5
votes
Accepted
What is the typical block size in RSA?
Well this depends on your key size.
But, in general:
A 1024-bit RSA key using OAEP padding can encrypt up to (1024/8) – 42 = 128 – 42 = 86 bytes.
A 2048-bit key can encrypt up to (2048/8) – 42 = 256 ...
- 129
5
votes
Can data over a very low bandwidth channel still be encrypted?
If the sender and recipient can agree on the nonce of every message from existing context, so that the nonce either doesn't need to be transmitted or can be derived from metadata that must be ...
- 14.4k
5
votes
what is the block in crypto
A "block" is the amount of bits your block cipher operates on (There are other kinds of ciphers, see below.)
Think of a Caesar cipher, it replaces letters in words. For example with key 13 it ...
- 4,883
4
votes
Accepted
Block cipher block and key sizes
GOST symmetric cipher uses 256-bit keys to encrypt 64-bit blocks. That means there are many keys which give the same result
This is actually incorrect. A block cipher, when given a key, maps $n$-bit ...
- 2,814
4
votes
Accepted
How does Feistel cipher create diffusion between blocks?
If a message is longer than the block length, how would changing one
part of the message affect the encryption of other parts of the
message?
That really doesn't depend on the block cipher in use,...
- 45.7k
4
votes
What's the proper choice for the block size of a sponge-based hash?
Iterated hashes generally accumulate a fixed amount of data and then call a compression function (or a permutation in the case of sponges) on this data together with a chaining-value. The block size ...
- 24.8k
4
votes
Accepted
Can data over a very low bandwidth channel still be encrypted?
Yes, encryption can be without increase of the message size, even if the ciphertext is constrained to the same character set/semantic as the plaintext (e.g. to fit the semantics imposed in the body of ...
- 138k
4
votes
Accepted
Why blake2b is the fastest despite it being 512 bits hashing algorithm?
I am struggling to understand why blake2b is faster than blake2s, despite that blake2b is 512 bits hashing.
Comparing is not easy. They designed
BLAKE2b for 64-bit platforms including NEON enabled ...
- 47.6k
3
votes
Accepted
Weird size of data transmitted in SSLv3
You won't find the answer in the RFC; instead, here's what your SSL implementation is doing: when you send a single byte of data, it actually generates two records.
The first record is empty; so it ...
- 145k
3
votes
Is the risk of a collision for OCB mode mitigated by upping the block size of the cipher?
Using a larger blocksize in a cipher will always aid in mitigating collisions.
For a practical example, take what OWASP says about key management:
If the amount of data encrypted grows beyond a ...
- 45
3
votes
What is the typical block size in RSA?
Typically, what we do when we encrypt a large piece of text with RSA is:
We select a random symmetric key (perhaps, an AES key)
We encrypt that symmetric key with RSA
We then use that symmetric key ...
- 145k
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