21,905 reputation
12991
bio website
location Paris, France
age
visits member for 3 years, 4 months
seen 15 hours ago

I'm an engineer with experience in applied cryptography, in particular in Smart Card systems.


Feb
9
comment Is 80 bits of key size considered safe against brute force attacks?
Your butlast comment makes a good argument for using wide session keys when things need to be kept secret for a long time, together with a cryptosystem giving forward secrecy: that insures that future breaks of long term keys (by quantum computer if we trust your intuition, or just by some compromise of the system holding them if you ask me) won't allow to decipher past intercepts.
Feb
9
comment RSA assumption and cryptography
Hint: if $D(c,e,n)$ does not succeed to find $m$ with $c=m^e\bmod{n}$, can we can reformulate that same question in a variant and submit some internal portion of that variant to that same magic D algorithm? Now, if that new strategy consistently failed, would "D succeeds in 1% of cases" hold?
Feb
9
comment How can I simulate and measure brute force hacking using RSA?
Did you mean 160 digits (about 530 bits) rather than 160 bit? That's more like the limit of brute-forcing RSA for an individual.
Feb
9
comment Prove preimage resistance property
@user3283751: The counterexample itself is fine (though I would also decide a value of $n$), but the proof is lacking (in detail), that is: is not detailed enough to be acceptable. You need to prove that finding a premimage by $h$ for almost every element of $Im(h)$ is easy, by explaining how that can be done assuming what you hypothesized. Also, be sure you understand/justify on what grounds you can "suppose $x\mapsto g(x)$ is a preimage resistant function"; it is not immediately evident that because you have homework about preimage resistant functions, such things must exist.
Feb
9
revised RSA assumption and cryptography
Polish
Feb
9
reviewed Approve RSA assumption and cryptography
Feb
9
comment Prove preimage resistance property
@user3283751: With that definition of preimage resistant where hard is absolute rather than determined by the width of the result, I doubt that "both f and g must be preimage resistant if h is to be preimage resistant". Hint: what would a counterexample look like? What properties must it have? Find one, that's easy unless I err deeply!
Feb
8
comment Prove preimage resistance property
Does your definition of preimage resistance for $f:\{0,1\}^*\mapsto\{0,1\}^n$ require the hardness of finding a preimage to be some function of $n$? I can't think of a proof of what you ask without that (much to the contrary if the required difficulty was independent of $n$ it seems possible to exhibit a counterexample).
Feb
8
comment Computationaly hard detokenization algorithm for credit card numbers
As explained in my answer modified per the updated question, the requirement of resisting a hack into the vault demonstrably leads to poor security even with a vault consuming more kilowatts than a standard plug allows, and detokenization credentials assumed safely kept. Why not turn the problem around, use a truly secure vault like a security-evaluated Smart Card, and assume the vault secure?
Feb
8
revised Computationaly hard detokenization algorithm for credit card numbers
List possible improvements
Feb
8
revised Computationaly hard detokenization algorithm for credit card numbers
Change X to M since X is used in the satement for something entirely different, and other polish
Feb
8
revised Computationaly hard detokenization algorithm for credit card numbers
Polish
Feb
8
revised Computationaly hard detokenization algorithm for credit card numbers
Expand after unserstanding the modified question
Feb
8
revised Computationaly hard detokenization algorithm for credit card numbers
Expand after unserstanding the modified question
Feb
8
revised Computationaly hard detokenization algorithm for credit card numbers
Adapt to new statement; not proofread
Feb
7
revised Computationaly hard detokenization algorithm for credit card numbers
added 1 characters in body
Feb
7
revised Computationaly hard detokenization algorithm for credit card numbers
Polish
Feb
7
revised Computationaly hard detokenization algorithm for credit card numbers
Polish
Feb
7
answered Computationaly hard detokenization algorithm for credit card numbers
Feb
7
comment Is 80 bits of key size considered safe against brute force attacks?
By the account of Don Coppersmith, Differential Cryptanalysis (which motivates your "20 years ahead of academia") was known to academia by 1989, and to IBM in 1974. Any reference to that agency knowing it by 1969 or earlier? Whatever the offset, I attribute it to a general advance of military cryptography before 1990. For what we know, academia has largely caught-up on a theoretical standpoint (clearly not on the standpoint of brute force, and likely not for interception and other technological attacks).