Cryptography Stack Exchange Community Digest

Top new questions this week:

Can I encrypt a message by swapping bits in the text?

I have tried out an encryption method, in which I swap bits in the text. The text length is N bit, then I generate several random number pairs in the range 0..N-1, as [n,k] pairs. After that I swap ...

encryption random-number-generator xor  
asked by Konstantin 13 votes
answered by Luis Casillas 31 votes

(updated) Utilizing a non-computable function to create a one-way function

Why can't uncomputable functions be adapted to serve as theoretically perfect one-way functions? This has been bugging me for years, and I've never been able to track down an explanation of why it ...

pseudo-random-function one-way-function computational-complexity-theory  
asked by Trev 7 votes
answered by Yehuda Lindell 19 votes

Does having a hash of a password jeopardize the security of plaintext that was encrypted with that password?

A malicious actor wants to decrypt ciphertext that was encrypted with a password. He also possesses the hash (bcrypt/PBKDF2) of that password. Does he have any significant practical advantage in ...

encryption hash attack password-hashing dictionary-attack  
asked by OwenFlanders 5 votes
answered by fgrieu 11 votes

Itoh Tsuji algorithm

I'd like to use the Itoh-Tsujii algorithm for a dynamic substitution table, but I do not get the following line: $$r\ \gets\ (p^m - 1)\,/\,(p - 1)$$ And why can $r$ be used to calculate the ...

asked by MrXeth 4 votes
answered by Thomas Pornin 6 votes

Possible issues with ECIES using a static (non-ephemeral) sender key

After looking at this bitcoin-related ECIES repo I believe the intention is that, at least in some cases, the recipient would know that the message is intended for them and use an secp256k1 public key ...

authentication authenticated-encryption ecies  
asked by thesquaregroot 3 votes
answered by SEJPM 0 votes

Computational trapdoor where the problem is tractable for both parties but easier for one

Usually the sort of trapdoors which are talked about are designed such as to make the computation intractable for one party and tractable for the other. But what if one party merely has a big ...

rsa prime-numbers factoring trapdoor  
asked by Richard Flarinson 3 votes
answered by Mark 2 votes

Is this a safe way to prove the knowledge of an ECDSA Signature?

I think that I've found a good solution to prove the knowledge of an ECDSA signature without revealing it. In short terms it consists in generating an ECDSA signature using the point $R$ as generator, ...

elliptic-curves zero-knowledge-proofs  
asked by Jan Moritz 3 votes

Greatest hits from previous weeks:

Creating cryptographic algorithms at runtime

Would it be possible to create a program with which to create a cryptographic algorithm (i.e. encryption or hash) using well-known elements of other algorithms in the same way that algorithms "reuse" ...

algorithm-design forward-secrecy  
asked by Arget 17 votes
answered by Cort Ammon 34 votes

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 ...

public-key rsa key-size factoring  
asked by Inaimathi 113 votes
answered by fgrieu 137 votes

How much would it cost in U.S. dollars to brute force a 256 bit key in a year?

I am often told that any key can be broken and that it is only a matter of time and resources for any key to be broken. I know that this is technically true. However, I think that there is probably a ...

asked by John Fischer 193 votes
answered by mikeazo 166 votes

What are the differences between a digital signature, a MAC and a hash?

A message may be accompanied with a digital signature, a MAC or a message hash, as a proof of some kind. Which assurances does each primitive provide to the recipient? What kind of keys are needed?

hash signature authentication mac non-repudiation  
asked by Flimm 180 votes
answered by Flimm 204 votes

How do ciphers change plaintext into numeric digits for computing?

For example, in RSA, we use this for encryption: $ciphertext = (m^e \mod n)$ and for decryption. If our message is "hello world", then what number do we have to put as $m$ in the RSA formula?

asked by Ted 15 votes
answered by marstato 11 votes


I have three questions: Would you use HMAC-SHA1 or HMAC-SHA256 for message authentication? How much HMAC-SHA256 is slower than HMAC-SHA1? Are the security improvements of SHA256 (over SHA1) enough ...

hash hmac sha-256 sha-1 sha-2  
asked by Mario 28 votes
answered by poncho 35 votes

What is the difference between PKCS#5 padding and PKCS#7 padding

One runtime platform provides an API that supplies PKCS#5 padding for block cipher modes such as ECB and CBC. These modes have been defined for the triple DES, AES and Blowfish block ciphers. The ...

encryption aes block-cipher modes-of-operation padding  
asked by Maarten Bodewes 82 votes
answered by Maarten Bodewes 104 votes

Can you answer these questions?

Proving the knowlege of e-th root in an non-interactive way

Just like in this question: Protocol for proof of knowledge of $l$-th root I want to prove that for $u^e = w$ I know $u$ without revealing it. Three other requirements are: e is small (65537) The ...

rsa number-theory  
asked by Jan Moritz 1 vote

Security limits of password-based encryption

A reasonable way to do password-based encryption is: put the passphrase┬╣ and nonce in a well-parametrized memory-hard iterated hash function like Argon2, to derive a key 128-bit or larger. output ...

asked by fgrieu 2 votes

Quantum-secure obfuscation

My question is a follow-up to a recent question regarding quantum-secure time-lock puzzles (TLPs). TLPs can be built (in principle) from indistinguishability obfuscation (iO) [BGJ+,BGL+] as noted in ...

post-quantum-cryptography quantum-cryptography obfuscation  
asked by Occams_Trimmer 2 votes
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