Questions tagged [key-size]

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

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How secure is AES-256, but with an effective key length of 56-bits?

I need to encrypt a few drives using Kaspersky Total Security. I however noted something peculiar, Kaspersky's description of their cipher is: Data will be protected using AES-256 encryption with ...
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272 views

How can I determine key length for message encrypted by many-time-pad?

For message $ m $ and key $ k $ I get encrypted message $ c $ ($ m \oplus k = c $) where $ length(k) < length(m) $. Additionally I know that message $ m $ is English text. Instead of using pure ...
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Does a key derivation function always generate an even distribution of bits in the resulting key?

I am a student of crypto. During our study course, we went into the AES-128 algorithm. All the examples given used an initial key k, 128 bits and we only briefly touched on the key derivation of ...
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Security strength of RSA in relation with the modulus size

NIST SP 800-57 §5.6.1 p.62–64 specifies a correspondence between RSA modulus size $n$ and expected security strength $s$ in bits: ...
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Why are the bit lengths of keys and digests equal in Lamport signatures?

In Lamport's one time signature scheme: One way function to convert a pseudo random number private key to a public key takes $\{0,1\}^n$ and returns $\{0,1\}^n$. Cryptographic hash function to ...
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1answer
129 views

Master keys' strengh

If I am going to use a master key to generate a session key, why the master key must be "stronger" than the session key? Like should not be distributing 128-bit AES keys derived from a 56-bit DES key....
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1answer
453 views

Choosing primes in the Paillier cryptosystem

In the first step of key generation phase in Paillier cryptosystem given here. It's given that $$\operatorname{length}(p) = \operatorname{length}(q) ) \implies \operatorname{gcd}(pq,(p-1)(q-1))=1$$ ...
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Why are there $2^{56}$ possible DES keys when there are 64 key bits?

On forums I read DES has $2^{56}$ possible keys so it takes x times to brute force it with hardware x. But there are 64 total bits of which are 8 parity bits. But those parity bits are a ...
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Block cipher block and key sizes

Is there any point in making block size less than key size in block cipher? For example, Russian GOST symmetric cipher uses 256-bit keys to encrypt 64-bit blocks. That means there are many keys which ...
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1answer
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How many bits of symmetric security does RSA-3072 actually provide? [duplicate]

I know that the sizes are standardized to $1024$, $2048$ etc. but they actually provide different security than $128$, $256$... for example $\texttt{RSA-}2048$ is actually $112$ bits of security and $...
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1answer
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PyCrypto and AES: can i use a longer key than the block size (16byte)?

I see that in many scripts are used keys of 16 characters for a block of 16 byte, but can I use key of 40 (for example)? And then why in some scripts with aes module the block size is set to 32 byte ...
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438 views

Diffie-Hellman key size to symmetric formula?

I've noticed that on every site I checked I kept finding the relationship between D-H key size and symmetric encryption equivalent as a table, like: 384 - 56 768 - 80 1792 - 112 2304 - 128 10753 -...
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1answer
432 views

How to calculate secret key size of a CP-ABE scheme

How can I calculate the real size of key in a CP-ABE scheme. For example, I have this GSWV scheme: Fuchun Guo, Willy Susilo, Duncan Wong, Vijay Varadharajan: CP-ABE with constant-size keys for ...
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Why are recomended RSA key lengths so high? [duplicate]

I was checking for authoritative sources to back up my recommendation of a minimum RSA key length of 1024 and was shocked to find that NIST 800-56Br1 and FIPS 186-4 both recommend at least 2048 bits ...
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1answer
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Is a 4DES or 5DES system possible?

We know that 3DES is created with $E_{K_3}(D_{K_2}(E_{K_1}(m)))$ to extend DES's key length. Is it possible to extend it further by repeating this pattern? Perhaps using $E_{K_5}(D_{K_4}(E_{K_3}(D_{...
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How to calculate key size of a security algorithm?

There are lots of security algorithms. One of the way to measure security of a cryptography algorithm is to find out its key size. There are many key size of a single algorithm. ECC (Elliptic ...
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Should the most significant byte of a cryptograhic key be nonzero?

Is it safer to give a non zero value to the most significant byte (MSB) of a cryptographic key to ensure its length (not 31 instead of 32 for an EC private key on a 256-bit curve for example)? Or is ...
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1answer
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Why is Rijndael key length restricted?

Why is Rijndael restricted to key length in $\lbrace 128, 160, 192, 224, 256\rbrace$ bits (and not larger)? The algorithm looks to me like it would support an arbitrary-sized key (multiple of $32$). ...
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How do I get the equivalent strength of an ECC key?

I know how to calculate the comparable symmetric strength of an RSA modulus: calculate the running time for a field sieve. This is how NIST gives approximate symmetric sizes for asymmetric algos in ...
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4answers
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Does it make sense to have a 4000 bit long key?

My question is about the key length of a asymetric algorithm. How is it possible to memorize or remember a key that is about 4000 bits long?
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1answer
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When NIST disallows the use of 1024-bit keys, what effect will that have on SHA-3 (with max. 512 bits)?

Quoting the article Gone in 60 Months or Less: The National Institute of Standards and Technology (NIST) has disallowed the use of 1024-bit keys after 31 December 2013 because they are insecure. ...
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RSA incorrectly translates for small keys [duplicate]

I've been learning about RSA and wrote my own implementation. I don't pretend to have intuitive understanding of RSA or that I understand why it works, but I believe to have some basic understanding ...
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1answer
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What part of an RSA key is referred to by the number of bits? [duplicate]

When we speak about 4096- or 2048-bit RSA keys, what part of the key is this number of bits? The public key comprises both the modulus and the public exponent, and the strength of the key can also be ...
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1answer
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Are private and public key sizes of Elliptic curve related?

I'm new to elliptic curve cryptography. I just want to know in the case where I take a random number (private key) and find its associated public key, does the size of the public key depends upon ...
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Is there a public key encryption scheme with optimal key size?

Symmetric encryption schemes such as AES have known security levels equal to their key sizes (i.e. breaking an encryption with an $n$ bit key needs about $2^n$ work steps). Elliptic curve encryption ...
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The benefit of PBKDF2 for big keyfile?

The man page of cryptsetup says: Whenever a passphrase is added to a LUKS header (luksAddKey, luksFormat), the user may specify how much the time the passphrase processing should consume. The ...
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1answer
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Calculating the average key search time of DES

I am trying to understand how to calculate the average key search time given a specific scenario: Suppose we have a program that uses standard DES with 56 key bits, and we can test 10^6 keys per ...
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Does the length of the keyword used for keytable for Playfair matter?

So for Playfair, we choose a keyword and put that in the keytable with no duplicates. Then, we fill the rest of the table with alphabets in order that are not in the keyword without 'J'. Does the ...
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2answers
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How to generate backup key for PBKD functions?

I would like to encrypt some files using AES, Ill derive key from password using some standard PBKD function and use that as AES encryption key. My question is, if user forgets the main password, ...
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2answers
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Is there a standard format for compact 56-bit DES keys?

DES keys are traditionally stored as 8-byte blobs, with only the high 7 bits of each byte being meaningful to the algorithm; the low bit of each byte is set so that the population count of the byte is ...
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1answer
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Subkey generation in OMAC1/CMAC with XTEA?

All implementations of CMAC that I have found so far use a block cipher with the same key and block size. However, XTEA enciphers a 64 bit block by using a 128 bit key. If ...
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Key Size for Symmetric Homomorphic Encryption Over the Integers

In the paper Fully Homomorphic Encryption over the Integers, it mentions a symmetric key scheme on page 1 and 2. Key Generation: Pick a random odd number $p \epsilon [2^{N-1},2^N)$ Encrypt A Bit m: $...
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1answer
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Recovering key length from transposition block cipher ciphertext

First of all I would like to say hello to all StackExchange users. So far I've been stalking the forum for some time, now it's time to leave the shadows. In my master thesis I'm writing about ...
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1answer
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ElGamal and Paillier key sizes for short messages

I am using ElGamal and Paillier schemes to encrypt a large number of short messages: typical 4-byte integers. I do this for the homomorphic properties of these schemes. However, the way the ...
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1answer
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Can there be a need for 1024-bit (symmetric) encryption?

I think we are all aware of the CAESAR-competition. Now the aim of this competition is to select a (portfolio of) winner(s) which provide authenticated encryption. I'll now assume that the results ...
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1answer
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Why is the public/private key length used in libsodium so much shorter than needed for RSA [duplicate]

Reading the libsodium source I see that is uses a key length of 32 bytes (256 bit) for private/public key encryption. For RSA private/public key encryption a key length of 2048 (or 4095) bit is ...
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1answer
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Cardinality of the key space $\{0,1\}^\ell$

Imagine we have a scheme whose key is chosen from the set $\{0,1\}^\ell$. My question is: What is the size of key space for this scheme?
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1answer
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Calculate the security of a cascade cipher

I’m reading “The Security of Multiple Encryption in the Ideal Cipher Model” and I’m having a hard time understanding something. How do I calculate the bounds of cascade cipher from the following ...
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1answer
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Is 256 a reasonable length for a Diffie-Hellman safe prime?

There's a program that uses the following OpenSSL function... DH_generate_parameters(256, 5, NULL, NULL) ...to create DH parameters which are later used for ...
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1answer
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Will non-ECC algorithms like RSA eventually become too inefficient?

The strength of symmetric and asymmetric encryption schemes scales with the key length, but there is a difference between symmetric algorithms like AES and asymmetric algorithms like RSA. For example,...
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1answer
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Compacting a substitution cipher key

In a substitution cipher, I recieve 3 bits and than I return another 3 bits. The key of this cipher could be a matrix represented as: ...
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1answer
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Required key size for an ideal cipher with a 32 bit block size

For an ideal block cipher, what size key would be required when the cipher has a 32-bit block size?
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Key sizes supported by 3DES

I read about 3DES encryption but I don't understand exactly what the specific meaning of key size is. For example I found that 3DES supports 64, 128, 192, and 256 bits key size. What are all the ...
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Practical Attack on RSA

Currently I am designing an RSA based application, and I am thinking of how long should the key be in order to be secure against attacks. I know that RSA 4096 bit key can be recovered using Sound ...
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Why are RSA key sizes almost always a power of two?

I know that other bit sizes are possible, e.g. this HTTPS server seems to have a 9000 bit key https://www.ssllabs.com/ssltest/analyze.html?d=qqq.gg, but it's very rare that one sees a key not of size ...
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Key Length vs KeySpace

I am a newbie here, this is my first post. I have searched the forum but I haven't found a clear answer to this. In general. What is the relationship between key length and the keyspace? (if there ...
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1answer
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Can one deduce the symmetric key size used to encrypt a given a piece of ciphertext?

Assume an attacker captures a piece of ciphertext encrypted with a symmetric key. He knows all the details of the algorithm used to encrypt except for the size of the key used in this case. Can he ...
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2answers
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Discrepancy with reported key size for EDE ciphers in Qualys SSL Labs

The following two cipher suites were reported by Qualys SSL Labs as being 112-bit: SSL_CK_DES_192_EDE3_CBC_WITH_MD5 TLS_RSA_WITH_3DES_EDE_CBC_SHA However, all other references I can find say that ...
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2answers
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How much extra information is in an RSA public key?

I'm trying to calculate the size of an RSA public key in Ruby. I've retrieved the key in PEM format, and once I've decoded the base64 part from the PEM format, I get the size in bytes. What I find is ...
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1answer
366 views

Equal length of primes in paillier cryptosystem

In the key generation step of paillier cryptosystem , In order to satisfy $\gcd(pq,(p-1)(q-1))=1$ , we can take equal length primes. Instead of taking(length as parameter to generate $p,q$) equal ...