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Questions tagged [key-size]

Key size is the size measured in bits of the key used in a cryptographic algorithm.

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Creating new cryptographic algorithm instead of using known algorithm

Does it invent a new cryptographic algorithm in case of military encrypted communications makes the communication safer? Keeping the structure and behavior of the protocol unknown is an advantage?
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1answer
113 views

cryptographic accumulators: accumulator size vs. max. number of set members?

Here's a basic question about cryptographic accumulators, or at least ones based on fixed-size accumulator values (such as RSA ones based on modulo arithmetic), that is strangely not even mentioned in ...
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1answer
37 views

Key, plaintext block and ciphertext block permutations question

Question: If a cipher’s key size is $k$ bits and its block size is $b$ bits, how many possible keys are there, how many possible plaintext blocks are there, and how many possible ciphertext blocks are ...
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109 views

Why is Diffie-Hellman Insecure If Order of the Generator Has Only Small Prime Factors?

In this post from security SE, Tom Leek mentioned that, for Diffie-Hellman to be secure order of the group $g$ should have a prime factor at least $2k$ bits long, where $k$ is the security parameter. ...
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How many bits should I request from PBKDF2 to create a strong password?

I'm building a tool which generates passwords by hashing a service name with a master key. PBKDF2 was primarily chosen due to availability in the Web Crypto API. What would be a secure value for the ...
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1answer
82 views

What is the recommended minimum key length for ECDSA signature

I want to identify the proportion of certificates that use unrecommend ECDSA key length for TLS certificates based on some data I collected. By looking at a standard like NIST for example, I find ...
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3answers
179 views

What is the largest key size that could be carried by a shuffled deck of playing cards?

If you are carrying a standard 52 card pack of cards (no jokers) what is the largest key size, or the amount of data the cards can hold? Without the cards being marked or distinguishable from an ...
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3answers
8k views

Is 80 bits of key size considered safe against brute force attacks?

I came across the KATAN family of ciphers for small domain input blocks. They cipher arbitrary block lengths: 32, 48 and 64, but their key size is 80 bits only. Is 80 bits of key size considered ...
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55 views

Key-space analysis of an algorithm

I have an algorithm that requires a set of keys to encrypt a text (image of size N x N). It is an image encryption algorithm for a project. The key can be thought of as a tuple $x_0, \lambda, d, i, ...
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2answers
52 views

Can one trade off the number of iterations for key length with the PBKDF2?

When using the PBKDF2, is one taking on significant risk if one chooses to lower the number of iterations and choose to have very long keys? Can I XOR my message with a a very long key (plain-text ...
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7answers
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How big an RSA key is considered secure today?

I think 1024 bit RSA keys were considered secure ~5 years ago, but I assume that's not true anymore. Can 2048 or 4096 keys still be relied upon, or have we gained too much computing power in the ...
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2answers
40 views

Maximum number of possible keys for a fixed-length password?

I'm reviewing some information and possibilities regarding key space size following a unique password requirement, as it were. This particular requirement is a fixed-length password of 12 characters, ...
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2answers
106 views

Why would Diffie-Hellman Group 21 be 521 bits rather than 512?

Diffie-Hellman groups tend to increment in key sizes as you'd expect up until group 21. Any idea why the key size is 521 rather than 512? I haven't been able to find an answer at all.
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1answer
255 views

X25519 - why openssl shows 253 bits?

When using ECDHE with Curve25519 with openssl: Server Temp Key: X25519, 253 bits I thought when we use X25519, it use 256 bit key. Why openssl shows the server temp key as 253 bits?
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0answers
52 views

Is it possible to successfully encrypt/decrypt in DUKPT using different sized BDKs?

We are having our P2PE Solution validated against the PCI P2PE standard. We currently use a TDEA 2-key based BDK, and IPEK in both the encryption and decryption environments. ANSI X9.24-1 (2009) ...
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2answers
2k views

What is the minimum and maximum value of an RSA private exponent?

Is it possible to calculate the minimum and maximum value of the private exponent for an RSA key, for example for a 1024 key size? In other words, what is the range of values in which the private ...
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2answers
961 views

Mathematical formula for switching the key for OTP?

Instead of generating the random key for the one time pad cipher over and over again, is there a mathematical formula that allows you to switch the key to a new key? The new key must be as random and ...
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3answers
1k views

1 Billion Bit Encryption?

So, browsing through YouTube, I stumbled on this video interview of John Draper (Captain Crunch), one of the first "hackers". He talks for about 3 minutes (until 27:48) about his home rolled ...
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find keylength in xor-based encryption

I have found interesting answer, written by GeneQ and now want to understand and implement it. I think, that first point of this answer can be rewritten in follwing way: suppose we have n bytes of ...
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1answer
293 views
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1answer
67 views

Cutting down the size of a long blowfish key

I am reverse engineering a software that uses blowfish for encryption. The target uses an implementation of blowfish that allows arbitrary positive key length with a key size larger than 56 bytes. I ...
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2answers
143 views

Arbitrary key size in RC4

Is it possible using RC4 that for e.g. a 17-bit long plaintext i can get a 17-bit long ciphertext? From my understanding, for this example i need a 17-bit long key to do the XOR operation for ...
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1answer
364 views

384-bit ChaCha20 / Salsa20

The standard Salsa20 core is a $\{0,1\}^{384} \to \{0,1\}^{512}$ PRF with a 16 byte constant ($\sigma$ for a 32-byte key), an 8 byte nonce, an 8 byte counter, and a 32 byte key. The hash function does ...
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2answers
395 views

How do I calculate a password space if the password only has eight alphabetic characters (not case sensitive)

I read this wiki article and saw this formula for random password's information entropy. Also the wiki article mention that the symbol count for non case sensitive is 36. Is there a way I can ...
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2answers
169 views

How does the key size in supersingular isogeny schemes relate to their security level?

I'm looking at the De Feo, Jao, and Plût 2014 paper: Towards quantum-resistant cryptosystems from supersingular elliptic curve isogenies. My understanding of section 3.2 Key Exchange, is that Alice'...
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2answers
103 views

Why aren't 4096-bit RSA PGP key blocks $\lceil\frac{4096}{6}\rceil=683$ base64 characters long?

If a 4096-bit RSA PGP key is considered sufficiently long for most purposes, why are so many key blocks around 3000 base64 characters in length? Why aren't $\lceil\frac{4096}{6}\rceil=683$ base64 ...
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1answer
609 views

Will using a 32 character key for Blowfish make it a 32 bit encryption?

I am sorry if it sounds naive but I have been searching and searching regarding what kind of encryption we end up with if we use a key of length 32 characters? This article https://www.embedded.com/...
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2answers
641 views

Should text be hashed before being used as an encryption key?

Let's say I have an API that provides an encryption wrapper around another API. I give the consuming programmer the option of either passing a string or byte array as an encryption key. The byte ...
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2answers
207 views

How to calculate the key size of my cryptographic system

I have a 'for fun' project, which is a pure software based redesign of the Enigma I. I'd like to compute the key length of my design, but I need some help with that. I have a write-up here (see ...
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2answers
92 views

Why is keysize almost always expressed in bits?

I've never actually seen a time when somebody has expressed RSA keysize in bytes, and I haven't seen keysize expressed in bytes in other encryption algorithms either (can't be certain that that's ...
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0answers
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Safely using ciphers that take small key sizes

Is it feasible to use triple encryption for block ciphers with the maximum keysize being the only significant security issue? I mean $C = E_{k_1}(E_{k_2}(E_{k_3}(P)))$ and $P = D_{k_1}(D_{k_2}(D_{k_3}(...
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1answer
62 views

Blockcipher: key space define permutations of blocks

An $n$-bit block cipher is a keyed function $f_k:\{0,1\}^n\rightarrow\{0,1\}^n$. There are $2^n!$ such functions. Most block ciphers seem to have only $2^n$ keys, although there is an AES mode that ...
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1answer
281 views

Key size and finite fields in ECC (References)

So somehow I know that the key size in ECC is defined over the number of elements in a finite field or that it is almost equivalent to that (Correct me if I am wrong). However, other than on Wikipedia ...
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1answer
2k views

Why exactly Blowfish max key length is 448

Could anyone, please, tell me history of birth of Blowfish algorithm? Specifically, what is substantiation of Blowfish key length to be in range from 32 till 448 bits? Why, for example, it wasn't 64-...
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4answers
6k views

Is 512-bit RSA still safe for signature generation?

The standard CSP on Windows XP only supports RSA up to 512-bit, which means that it's the maximum key size I can use for authenticity verification of updates. The public key is embedded in the updater,...
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4answers
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Why can't we give block ciphers larger keys?

I have limited math's knowledge so sorry if this question sounds stupid. I saw many people complaining about AES, Twofish and Serpent that these ciphers all could be crackable in the near future and ...
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165 views

Post-quantum candidates for asymmetric cryptography with small key sizes

One of the things that I have come to like about elliptic curve cryptography is that the key only needs to be twice as long as the desired security level. That's a very small expansion (I also like it ...
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0answers
60 views

Are 80-bit keys considered secure as of late-2017? [duplicate]

I've seen a lot of ciphers with variants that use 80-bit keys, like Grain, Trivium, KLEIN, KATAN & KTANTAN, Piccolo, PRESENT, RC4, Skipjack, and TWINE. Some of these designs are younger than six ...
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4answers
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What are the practical differences between 256-bit, 192-bit, and 128-bit AES encryption?

AES has several different variants: AES-128 AES-192 AES-256 But why would someone prefer use one over another?
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4answers
6k views

Finding length of a key for a given vigenere cipher using Index of Coincidence

I'm developing an algorithm that takes in a vigenere cipher and produces the key length automatically. However, I'm having trouble understanding how to detect the key length when there are multiple ...
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1answer
909 views

AES with small and large string keys

byte *key; String skey = "abcde" If I convert skey to bytes and uses it to encrypt AES 128 bit, 192 and 256 bits. How AES would work with small key sizes or ...
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1answer
88 views

Why should the factors $p,q$ of $n$ be half the bit length of $n$ in RSA?

Why do we want the factors $p,q$ of $n$ to be of size half the bit length of $n$ in RSA? I know $p,q$ need to be large so that it is harder to factor $n$ and if one of $p,q$ is small, then it is more ...
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2answers
524 views

Determining key size in bits by knowing plaintext and ciphertext

I need to know how can I determine the key size in bits knowing the ciphertext and the encryption algorithm used? For example suppose the plaintext was "ABC" and after the encryption it became "DEF" ...
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2answers
133 views

Existence of 99% successful unbounded adversary against randomized secret key encryption scheme with short keys

It can be shown that if $E$ is a deterministic encryption with key of length $n$ and messages of length $l\geq n + 10$ then there exist two messages $m_0$, $m_1$ and a strategy for Eve so that given ...
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1answer
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Does limiting the sales market to U.S. only, releive my application of Export Admin Regs? [closed]

I have a steganography application that uses one-time-pad encryption. Obviously, key lengths can be very long. If I were to limit the sales market on the Microsoft Store to U.S. only, would that be ...
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1answer
451 views

how to calculate the average key search for AES-128bit?

Assume that an attacker has a special purpose application specific integrated circuit (ASIC) which checks $5\times 10^{8}$ keys per second, and she has a budget of $\$1$ million. One ASIC costs $\$...
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1answer
6k views

Is there any benefit from using SHA-512 over SHA-256 when AES just truncates it anyway?

I am still not very clear on how AES-256-CBC can use SHA-512 bit keys, but I assume that it just truncates the 512bit hash down to 256bit. In software like GnuPG and OpenPGP, is there any real benefit ...
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1answer
262 views

Keyspace in truncated MD5 hash?

I have identified a password hashing algorithm that uses a truncated MD5 hash to compare user passwords. This means that any password whose hash matches the same first 23 hex chars (11.5 bytes or 92 ...
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1answer
64 views

Does FHE's longer key bring more security than traditional encryption algorithm?

I check some key size which FHE libraries generated, for example, HElib's key size will be more than 300M, while tfhe's is also ...
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3answers
688 views

How confident can we be that nobody will crack a 128-bit key?

In a context involving a block cipher like AES-128, excluding quantum computers, cryptanalytic breakthrough on AES and implementation attacks (poor TRNG, DPA..), and wrench, how confident can we be ...