52
votes
Accepted
Largest integer factored by Shor's algorithm?
wondering what the largest integer is which they were able to factor with a small quantum computer
This answer starts with stunt records. See final paragraph for the the largest meaningful record. ...
42
votes
What size should the HMAC key be with SHA-256?
Short answer: 32 bytes of full-entropy key is enough.
Assuming full-entropy key (that is, each bit of key is chosen independently of the others by an equivalent of fair coin toss), the security of ...
- 138k
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:
...
- 91.7k
27
votes
Accepted
Argon2 output length and parameter storage?
You need to consider the weakest link property: a security system is never stronger than its weakest link. Since Argon2 is a password-based function, the weak link here is going to be the strength of ...
- 14.4k
23
votes
What size should the HMAC key be with SHA-256?
The only rule for the key is that it should at least contain 256 bits of randomness. If the key is smaller you may not get the full security of HMAC-SHA-256. The full security of HMAC is basically ...
- 91.7k
21
votes
What is the minimum and maximum value of an RSA private exponent?
In summary:
Modern practice is to fix a small public exponent $e$ such as $e=2^{(2^4)}+1=65537$, then choose a public modulus $N$ of $\text{nlen}$ bits (the key size) with large random prime factors ...
- 138k
17
votes
Accepted
What is the maximum key size for a 128 bit AES?
Despite similarities, it is really important to understand that passwords and cryptographic keys should not be carelessly conflated. Some important contrasts:
Passwords are normally selected by ...
- 14.4k
14
votes
Accepted
How do I get the equivalent strength of an ECC key?
The security level of an elliptic curve group is approximately $\log_2{0.886\sqrt{2^n}}$. You can use this to approximate the security level of a $n$-bit key, eg:
$\log_2{0.886\sqrt{2^{571}}} = 285....
- 330
14
votes
Accepted
Password entropy much lower than entropy of encryption keys. Why is this acceptable?
I know that humans would find it impossible to maintain a 128 bit password -- however, I wonder if there is some technical reason why a 52 bit password would not be as weak as a 52-bit encryption key ...
- 32k
14
votes
Is there any benefit from using SHA-512 over SHA-256 when AES just truncates it anyway?
SHA-512 has both a larger internal state and a higher number of rounds than SHA-256 - which means that it provides a higher bit strength. Somewhat surprisingly it may also outperform SHA-256, as it ...
- 91.7k
14
votes
Brute-forcing Symmetric Keys
No, it doesn't help. It doesn't hurt either; as long as you don't repeat keys, the probability of success is always the same. That is, if there are $2^n$ possible keys, and you test $\lambda$ of ...
- 145k
13
votes
What is the purely mathematical definition of encryption strength?
There isn't just one, because there are many different scenarios where you'd use such a function, where the attacker has lesser or greater powers, or variably stringent success goals—different attack ...
- 14.4k
12
votes
Accepted
Why can't we give block ciphers larger keys?
I saw many people complainig about AES , twofish and serpent that these ciphers all could be crackable in the near future and even today with big datacenters .
This is a good example of why we should ...
- 19.5k
11
votes
What is the maximum key size for a 128 bit AES?
AES-128 requires keys 128 bits in length, period.
As Thomas points out in his comment, the size of characters is encoding-dependent, so there is no well-defined "number of characters" in an AES key. ...
- 11k
11
votes
"128-bit security" or "128 bits of security"?
The term used in the Security level article in Wikipedia is n-bit
security or for example: 128-bit security level
In cryptography, security level is a measure of the strength that a cryptographic ...
- 428
11
votes
Accepted
80-bit collision resistance because of 80-bit x87 registers?
Coprocessors are designed for improved performance in a certain case, and in the case of fixed-width mathematics, I do not believe you would see a performance increase.
I am quite sure that 80-bits ...
- 4,625
10
votes
Why can't we give block ciphers larger keys?
I saw many people complainig about AES , twofish and serpent that these ciphers all could be crackable in the near future and even today with big datacenters
Let me go through some of the lesser ...
- 145k
10
votes
Accepted
What is the recommended minimum key length for ECDSA signature
You are already well on your way if you've found keylength.com. It references the documents that the recommendations are based on, so if you need a more clear reason to go for a specific key size then ...
- 91.7k
10
votes
Nexor, an encryption algorithm that promises unlimited key sizes: Is it safe?
Looking very briefly at the source, it's just using the output of /dev/urandom to generate keys, then using that as a (poor) One-Time Pad. /dev/urandom is just a ChaCha20-based RNG with some decent ...
- 5,786
10
votes
Does this RSA decryption scheme make sense?
You're right, normal RSA does not (and cannot) work like that. Most likely, your client either:
is using RSA with an absurdly low (and insecure) key size,
is not actually using RSA at all, or
has ...
- 45.9k
9
votes
How big an RSA key is considered secure today?
An adversary with a moderately large quantum computer to run Shor's algorithm will cut through a 1024-bit RSA modulus like a hot knife through butter, and maybe through a 2048-bit RSA modulus like a ...
- 47.9k
9
votes
Accepted
AES-256 password cracking time
It will happen immediately, because you just posted your password to the internet, where the adversary is watching.
On the other hand, if you can describe the procedure you used to generate it, we ...
- 47.9k
9
votes
How Secure is gpg Command Using Symmetric AES-256 From Being Cracked
GPG's AES-256 symmetric encryption is believed to be as secure as it is difficult to
guess the passphrase
or compromise the machine used to perform encryption and decryption.
Guessing the passphrase ...
- 138k
9
votes
80-bit collision resistance because of 80-bit x87 registers?
The original SHA-1 and Skipjack specifications did not provide a justification for using a 80-bit key size, so we can only speculate about the reasons. However, it is important to understand that ...
- 36.2k
9
votes
Recommended key size for DKIM
Nobody has publicly announced that they've broken 1024-bit RSA (other than via implementation defects that can happen with any key size), but it's not out of the question. I can't find a recent ...
8
votes
How do I get the equivalent strength of an ECC key?
The previous answer has the correct formula for estimating the security level of prime field elliptic curves. However, the table seems to just list the closest Koblitz curve sizes used, as Richie ...
- 32k
8
votes
Accepted
For RSA keys, is the safety of a given key-length different for signing vs encryption?
No, the security is about identical, as the underlying RSA problem is the same.
Besides that, non-repudiation is usually managed differently with regards to legal requirements. Providing ...
- 91.7k
8
votes
Accepted
How secure is AES-256, but with an effective key length of 56-bits?
It would appear that this is as a consequence of the Wassenaar Arrangement (more detailed explanation). Basically, because cryptography falls under certain laws regarding munitions and arms ...
- 4,012
8
votes
What is the purely mathematical definition of encryption strength?
One of the definition is stated in the other answer (indistinguishably). Another way to assert the strength of an algorithm is to compute the complexity of an attack.
Let us assume $$E : \mathcal{K} \...
- 9,969
8
votes
Why can't we give block ciphers larger keys?
Cryptographers can and we do create block ciphers with bigger keys, take for instance the Threefish cipher which has key/block sizes of 256, 512 and 1024 bits.
However, since strong block ciphers ...
- 91.7k
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