1,276 reputation
722
bio website ethanheilman.tumblr.com
location Cambridge, MA
age 31
visits member for 3 years, 4 months
seen Nov 9 at 20:46

Nov
23
comment Security of simple xor and s-box cipher?
The above scheme does not use multiple rounds but rather uses only one round with a different round key per message block. A multiple round scheme would have the same the same message block be enciphered by more than one round key. You may have conflated CBC with rounds. I think you should probably abandon the CBC stuff until you have a secure block cipher and then figure out how to use that secure block cipher in a CBC mode.
Nov
22
comment Could one construct a cipher that is secure for friendly parties to use but insecure for hostile parties?
@user11342 Where did your answer get moved? I can't seem to find it.
Nov
2
comment How to deduce enigma settings given a partial plaintext?
Yeah, my bad, I redid my calculations and ended up with the same answer as you.
Oct
28
comment How to deduce enigma settings given a partial plaintext?
Nice try, time traveling Alan Turing.
Oct
12
comment Why has the RSA factoring challenge been withdrawn?
Thanks Thomas! Someone should turn this into a question.
Oct
12
comment Why has the RSA factoring challenge been withdrawn?
According the wikipedia page: "[TWIRL'S designers] estimate that if TWIRL were built, it would be able to factor 1024-bit numbers in one year at the cost of 'a few dozen million US dollars'". Is this statement incorrect? Has something new been learned?
Oct
12
comment Why has the RSA factoring challenge been withdrawn?
What about research on custom cryptanalysis hardware such as TWIRL en.wikipedia.org/wiki/TWIRL
Sep
30
comment What is the “Random Oracle Model” and why is it controversial?
+1 for "There is a black box. In the box lives a gnome, with a big book and some dice."
Sep
28
comment Is there a secure cryptosystem that can be performed mentally?
OTP's can be done mentally, would you consider OTPs a fair answer?
Sep
28
comment Time Capsule cryptography?
@IlmariKaronen - Thanks, updated the link. I agree completely, an attacker could run a sequencing side-channel attack against the message. I said the same thing here: the-scientist.com/2011/09/26/encrypting-e-coli . Most security systems are built around models of what an attacker can do. For example AES can be broken by a timing side-channel, but that doesn't mean AES is no longer secure. Furthermore there are some pretty powerful countermeasures that could be used prevent to a sequencing side-channel attack.
Sep
27
comment Would the ability to efficiently find Discrete Logs have any impact on the security of RSA?
Can you find a citation that states that it is an open question if a break in DH would have implications for RSA?
Sep
27
comment How can I prevent a message replay with RSA?
@ChrisSmith Good point! We have three options: (1) as you pointed out storing the previous nonces works, (2). the first time a connection happens the nonce is generated and sent, each subsequent connection the nonce is incremented by 1 (since the attacker doesn't know the value of the nonce the attacker can't predict the new value of the nonce). (3). nonces passed between both parties and combined to generate new nonces (similar to the way syn cookies work en.wikipedia.org/wiki/…).
Sep
27
comment How can I prevent a message replay with RSA?
Why would including the expiration time in the message be insecure?
Sep
27
comment Is Diffie-Hellman mathematically the same as RSA?
I think I might be wrong, but I thought I've heard the claim that breaking discrete log breaks/threatens RSA as well. Maybe this should be a question.
Sep
26
comment What is the best method to determine the language used in a monoalphabetic substitution cipher?
@128 - absence of whitespace is typical, most classical ciphers assume you remove whitespace. Solving classical ciphers can get a bit tricky, if you are really interested I'd recommend: 'Cryptanalysis: a study of ciphers and their solution' ( books.google.com/books/about/Cryptanalysis.html?id=fKNB-7y_Hs4C ). Also 'The American Black Chamber' (amazon.com/American-Black-Chamber-Cryptographic/dp/0894121545) has a bit of advice, examples on solving classical ciphers.
Sep
26
comment What is the best method to determine the language used in a monoalphabetic substitution cipher?
Certainly you can use a monogram frequency chart to guess the language. Another idea would be to use a common word such as in german 'ein' and look for three letters that have the same distance from each other (assuming it is a shift cipher). Where did you get a ciphertext in which you know so little about it, yet you know that it is monoalphabetic?
Sep
24
comment Is there a simple hash function that one can compute without a computer?
@FUZxxl - I think it might be safe enough for class. Not sure how safe it is generally. It would make an interesting crypto question.
Sep
24
comment Is there a simple hash function that one can compute without a computer?
Yes, RO schemes typically require a trusted third party to keep the table of inputs and outputs and to generate new outputs.
Sep
24
comment Is there a simple hash function that one can compute without a computer?
What about a function, $f$, that given a number $n$, indexes e with $n \times 4$ and returns the next 4 digits of e. $$f(n) = e[n*4],e[n*4+1],e[n*4+2],e[n*4+3]$$ So $f(0)$ would return $2718$, f(1) would return $2818$ and so on. Or use a normal number instead of e ( en.wikipedia.org/wiki/Normal_number) is you want to be more formal.
Sep
23
comment New PRG from old
What about using bar $|$ for concatenation? Is $.$ commonly used?