Ethan Heilman
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# 101 Comments

 Jan 3 comment Why programming languages don't provide simple encryption methods? Can anyone provide examples of security failures that resulted from the lack of ease of use of Encryption libraries? Nov 28 comment Where is the proof of security of Diffie's cipher? Can something be perfectly secure and still vulnerable to a known-plaintext key recovery? Nov 23 comment Security of simple xor and s-box cipher? +1, this is an excellent answer, thanks Thomas. Nov 23 comment Security of simple xor and s-box cipher? Probably a question on notation would be in order but let me try to provide a concise explanation here. $E : K \times M \rightarrow M$ means $E$ is a function that uses $K$ to map values of $M$ to other values of $M$. That is the encryption function $E$ uses a key $K$ to map a message to another message with message-space $M$ where messsage-space is the space of all possible messages. It is a notational definition of encryption. Nov 23 comment Security of simple xor and s-box cipher? Problem: nothing is getting mixed outside of these $16$-bit chunks (a bit in the first $16$-bits never effects the outcome of a bit in any other chunk). That is, the block cipher is really a $16$-bit block cipher. You can brute force and attack each of these $16$-bit chunks. To make it stronger we need diffuse the bits outside of your chunk. You have key substitution network, but what you really want is a key permutation/substitution network ( en.wikipedia.org/wiki/Substitution-permutation_network ). Nov 23 comment Security of simple xor and s-box cipher? If your message block size is $256$-bits and your s-box size is $16$-bit what exactly is $S[M]$ doing (looking up $16$-bit chunks of $M$ in $S$? Nov 23 comment Security of simple xor and s-box cipher? How do you combine these rounds? $C_n = S[K_0 \oplus M_n] \oplus S[K_1 \oplus M_n] \oplus ... S[K_r \oplus M_n]$ ? Nov 23 comment Security of simple xor and s-box cipher? @Polynomial - Consider the case in which you only have one message block, $C_n = S[K_0 \oplus M_0]$. $S$ is invertible and so we are back to the known plaintext attack. You need the second xor of the key to prevent an attacker from inverting S ($C_n = S[K_0 \oplus M_0] \oplus K_0$) . The approach you should take is, to build a secure block cipher that only processes one message block. If it isn't secure with only one block it is unlikely to be secure with more than one block. 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/…).