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bio website github.com/CodesInChaos
location Frankfurt, Germany
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visits member for 3 years
seen 2 hours ago

Nov
22
comment Can cryptocurrency mining devices be used for cryptanalysis?
Most cryptography is far out of brute-force range, even with ASIC. Password hashing is one of the few exceptions. Among the commonly used password hashes, PBKDF2-HMAC-SHA-256 is closest to bitcoin mining. But I suspect mining ASICs are too specialized to attack it.
Nov
22
comment Creating a secure key
HKDF (with SHA-2 as underlying hash) is the cleanest solution IMO. But a plain SHA-2 hash to extract the entropy would be perfectly fine. // In theory there are some provably secure constructions, in practice SHA-2 I'd prefer a proper hash like SHA-2 over all of them.
Nov
22
comment Block Cipher vs Stream Cipher in Web Application
Web applications typically use HTTPS. Which encrypts the TCP stream using SSL/TLS. SSL can use both stream and block ciphers. In principle they're both fine for this, but SSL made some dumb design choices that weaken block ciphers. On the other hand it only supports a decent stream cipher (AES in GCM mode) in TLS 1.2 which isn't widely supported yet.
Nov
22
comment How to put the hash of a PDF file in the PDF file?
With CRC yes, with MD5 no. Standard practice for similar applications is to ignore the hash or signature part while hashing.
Nov
21
revised Applying differential cryptanalysis to ciphers with addition mod $2^{32}$
edited title
Nov
21
comment Authentication of machine before installation (to stop piracy)
This isn't about crypto. At best it's about security, but even there it's security-through-obscurity.
Nov
21
comment Authentication of machine before installation (to stop piracy)
An attacker can always rewrite the client and remove the the check.
Nov
21
comment When using Curve25519, why does the private key always have a fixed bit at 2^254?
Sorry but I still don't get it. 1) The actual code starts with $P$ and $\infty = (x=1,z=0)$ 2) Why does the code not work if it always starts the ladder at bit 254, but that bit happens to be 0? 3) Your Montgomery ladder link is dead
Nov
21
reviewed Approve suggested edit on hill cipher encryption way 1x3 plaintext matrix
Nov
20
comment When using Curve25519, why does the private key always have a fixed bit at 2^254?
That this answer is tautological. I read the paper including those paragraphs and still Why does having a fixed position for the leading 1 improve performance? I didn't notice an algorithm in the code that relies on this bit being set. Does the code produce incorrect results if this bit is wrong?
Nov
20
reviewed Approve suggested edit on How to prove this LFSR equation?
Nov
20
reviewed Approve suggested edit on hybrid PKE scheme CCA2 insecurity
Nov
20
revised How can I implement the elliptic curve MOV attack myself?
added 20 characters in body
Nov
18
comment Why not just generate random strings for one-time password (OTP)?
Your idea has been used for online banking. My bank used to send me a list with 100 single-use TANs. Then for each transaction ask for one of them.
Nov
18
comment Efficient parameters for group Diffie-Hellman
@poncho Could you describe how to use that oracle to break CDH?
Nov
18
answered Efficient parameters for group Diffie-Hellman
Nov
18
comment Efficient parameters for group Diffie-Hellman
@daniel How would you do that? multiplying public keys translates to addition in the exponent.
Nov
18
comment Efficient parameters for group Diffie-Hellman
@RickyDemer It's large enough to make group DH annoying since you need one round of communication for every member in the group.
Nov
18
comment Efficient parameters for group Diffie-Hellman
You'd need to compute $K^{(a^{-1})}$. Only those who hold the private key $a$ can do this. Multiplying with $(g^a)^{-1} = g^{-a}$ would subtract $a$ from the exponent, not divide the exponent by $a$.
Nov
14
comment Elliptic Curve Encryption Ciphertext Size
Depends on the details of the ECIES implementation. The overhead is one ECC point. For example with an 256 bit curve (offering 128 bits of security) and compressed points the overhead would be 32 bytes. With uncompressed points you need 64 bytes. Often you'll lose a bit more for headers. The symmetric encryption might cost a bit as well for padding or MACs.