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

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What are the practical difference 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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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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977 views

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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2answers
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Are there practical upper limits of RSA key lengths?

Suppose one wanted to use RSA encryption for the sole purpose of sending key bits for use in symmetric crypto systems, a dedicated key exchange system so to speak. And say you didn't think that the ...
8
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1answer
3k views

How strong is the ECDSA algorithm?

Some cryptographic algorithms are as strong as the size of their key is, while other have some weaknesses that limit their strength (such as SHA-1). How strong is the ECDSA algorithm, and does that ...
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What should be the size of a Diffie-Hellman private key?

I'm implementing the SRP-6 protocol, which relies on discrete logarithms for it's security (essentially Diffie-Hellman). The RFC documents state: The private values $a$ and $b$ SHOULD be at least ...
7
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4answers
243 views

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 ...
6
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2answers
857 views

How much can we compress RSA public keys?

I am wondering to what degree we can define an RSA variant, with a security argument that it is as safe as regular RSA with a given modulus size $m$ (e.g. $m=2048$), in which the public key has a ...
5
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3answers
630 views

Why is the complexity of RSA-1024 80 bit and not 86 bit?

Why is the complexity of RSA-1024 80 bit and not 86.76611925028119 bit? Here is the complexity for the GNFS (pulled from the linked Wikipedia article): $$\exp\left( ...
5
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2answers
633 views

RC2, RC4, RC5 key length

While reading the Wiki page on RC4 I noticed that the key size must be in the range of 40–2,048 bits. So should I understand like this, that I can have a key that has length of, lets say 333 bits? ...
5
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1answer
153 views

Digital signature for an expiring password

I would like to create a publicly available tool that is used to generate a password that gives a limited time access to a resource. My idea is to digitally sign the current date with a private key ...
5
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2answers
216 views

Relation between key size and PRNG state size

Some (supposedly) cryptographically secure PRNGs have an internal state of only 160 bits or less. When the algorithm is otherwise properly designed, that seems like enough to generate a 128 bit key ...
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485 views

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 ...
4
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2answers
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SHA-256 and AES-128

Does it make an encrypted string more secure if I use SHA256(x) instead of x as the secret key for AES-128 encryption? I do know that SHA-256 produces 64 characters of hashed string regardless of what ...
4
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3answers
422 views

Is there a practical security difference between XXX-bit encryption?

I know I'm treading in dangerous waters asking this - my comprehension of cryptography math is sorely lacking. On the flip side it gives me massive admiration for what many of you are able to do. ...
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2answers
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RC4 Keylength Limits

While reading the Wiki page on RC4 I noticed that the key size must be in the range of 40–2,048 bits. Why is that? Is there a reason it can't have a lower or higher length?
4
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1answer
422 views

How were the AES key and block length subsets of Rijndael selected?

My intuition tells me it's a trade off between speed and security, but how did the standardisation process select these three seemingly arbitrary key lengths (namely, AES-128, AES-192, AES-256).
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3answers
275 views

Comparison: complexity measures vs. security

Assume that you want to compare several cryptographic primitives (say, encryption schemes), and choose one. You need to consider several complexity measures, such as the key length, encryption time, ...
3
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3answers
649 views

Export from US of crypto software with key-size > 56 bits still needs permission?

Elsewhere someone stated that he couldn't publish his crypto software on the Internet, because US Export Regulations require approval, if the key size is greater than 56 bits. I know that several ...
3
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2answers
429 views

Why does the recommended key size between symmetric and assymetric encryption differ greatly?

In various articles it is mentioned that for secure communications, the recommended key sizes are 128-bit key size for symmetric encryption (which makes it $2^{128}$ possible keys?) and 2048-bit key ...
3
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2answers
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How many keys does the Playfair Cipher have?

I was just studying the Playfair cipher and from what I've understood, it is just a slightly better version of a Caesar cipher, in that it isn't actually mono-alphabetic but rather the 'digrams' are ...
3
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1answer
382 views

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 ...
2
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3answers
991 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 ...
2
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1answer
490 views

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. ...
2
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2answers
686 views

AES Key Length vs Block Length

This answer points out that certain key and block lengths were a requirement for the AES submissions: The candidate algorithm shall be capable of supporting key-block combinations with sizes of ...
2
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1answer
284 views

Padding for the TEA

Sorry if this is a noob questions, but I finally figured out how to implement the Tiny Encryption Algorithm in C++. My question is what to do about padding the key and the plaintext? I know that ...
2
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1answer
190 views

Why can ECC key sizes be smaller than RSA keys for similar security?

I understand how ECC is based on the discrete log problem and RSA on integer factorization. I've read several references that show how a solution to either of these problems can typically be adapted ...
2
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1answer
117 views

RSA key length choice for TLS when confidentiality not important?

I've a couple of questions about this, the scenario is embedding a key pair in a hardware device which will speak with other hardware devices over tls. computing resources are an issue so i want to ...
2
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1answer
404 views

Is it possible to change PGP / GPG Key Size on an exisiting key?

Is it possible for myself to change the key size (say from 2048 to 4096) on an EXISTING PGP / GPG key, and just republishing that key? Or do I have to generate a new key all together? I want to say ...
2
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1answer
79 views

Hill cipher cryptanalysis - known plaintext known key size

Hello I want to know how to go about this problem I know the plaintext "abcdef" and the ciphertext. The key size is 2. I really can't figure out how to find the key for decrypting and encrypting.
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66 views

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 ...
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1answer
126 views

How can encryption software accept password lengths which are not one of the AES key lengths?

AES comes with key sizes of 128, 192, and 256 bit. But in Truecrypt or other crypto software we can use passwords of different length, even less than 128 bit or more than 256 bit. How is this ...
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1answer
220 views

What is total key space in transposition algorithms

How we can measure key space in transposition algorithms? Should we specify the method, like rail fence?
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1answer
603 views

AES key/ciphertext space sizes

This is giving me a brain ache now... If I have AES-128, block is 128 bit, then every plaintext (128-bit) can be encrypted to some ciphertext that is also 128-bit. This is the block size. But: 128-bit ...
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1answer
195 views

Key sizes for discrete logarithm based methods

I have a question regarding the key generation process of methods that are based on the discrete logarithm problem. This site gives some good insights, but I don't fully grasp it I think: ...
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1answer
252 views

Can iterated key expansion in Blowfish slow down bruteforce attacks on small key sizes?

Suppose I have to use 64-bit keys for encryption (e.g. to comply with export restrictions). For this question, assume this key is truly random, and the encryption algorithm is Blowfish. Blowfish key ...
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1answer
258 views

Which encodings have |encoding key| >> |decoding key|?

I'm looking for an encoding scheme that requires a very large encoding key E (>10MB) and suffices with a relatively small decoding key ...
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0answers
136 views

Are any Certificate Authorities using 4096 bit keys for their intermediate and root certificates? [closed]

Are any Certificate Authorities using 4096 bit keys for their intermediate and root certificates? I know you can, of course, use a 4096 bit key for your server certificate, but I usually see 2048 bit ...
0
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1answer
123 views

What key length is required to keep simple keyed “hash” secure?

In a previous question, I described a particular keyed "hash" that mapped a 5-digit input code into a 5-digit output code. It used a 8-bit key which is very insecure - more than 99% of the time, you ...
0
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1answer
94 views

Size of Parameters in Polynomial Key-Splitting Algorithm

I've seen the elegant way of splitting a key among different people so that only a certain number need to be present to re-compute the key, yet nobody has enough information to re-compute the key on ...
0
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1answer
180 views

Appropriate AES key length for short term protection

Currently I'm doing some initial research for a fully connected distributed network communication model. The context here is a peer-to-peer multiplayer protocol, using a majority-voting system. More ...
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Adequate DH secret size [duplicate]

What's the adequate secret size for Diffie-Hellman when using a 2048 bits safe prime as modp value? KeyLength.com recommends 224 bit but I don't understand a thing: if the security is "bounded" by the ...