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


Nov
24
revised Inverse of a function $f_k(x) := f (x ⊕ k) ⊕ k$
TeXify
Nov
24
asked Efficient proof of knowledge using Wegman-Carter hash
Nov
24
comment Large file validation on an embedded system through hash and encryption
Can you elaborate on what you trust, and do not trust, on the embedded system? In particular is there some part of the code/data that the adversary is assumed unable to modify? To read? $\;$ Independently: can you program the system at the CPU level, or are you bound to using built-in primitives (as you would be if you could only define bytecode for a virtual machine)? Is MD5 fast enough for your purpose, or do you want something even faster (that seems possible with a Wegman-Carter hash)? And if not, is the bottleneck MD5, or reading the data?
Nov
24
comment Large file validation on an embedded system through hash and encryption
@McMurrich: what you described in above comment is a simple hash tree. As pointed by Richie Frame, it will not detect alteration of a block that was not supposed to have changed, but did (with no change of the stored hash, remaining at its former, original value, no longer matching the file content). I can't tell if that's a problem in your threat model: on one hand you trust the embedded system (when it accesses the data and compute hashes), and on the other you do not (when it comes to not changing this data).
Nov
23
comment Compact digital signature for noisy data
Yes. But unless I miss something, if we use a conventional hash, it seems the verifier will need the exact same message as the prover in order to produce (or check) the hash; thus the scheme won't be more efficient than the "generic but inefficient construction" given, I'm afraid.
Nov
23
comment Compact digital signature for noisy data
Doesn't the proposed protocols require communication from verifier to prover, contrary to the static signature scheme asked?
Nov
23
comment Mifare Classic and crypto1 algorithm
There is a detailed description in: Wee Hon Tan, Practical Attacks on the MIFARE Classic (Department of Computing Research of the Imperial College London; report, September 2009; see in particular section 4). It would be a good idea that you write an answer on your question based on that material.
Nov
23
comment Why we can't implement AES 512 key size?
@owlstead: I am considering doing something about the 2011 paper; the IEEE lists itself as the publisher, the first author had an affiliation, and some papers in that IAS 2011 conference have a summary that looks fair. For the 2010 and 2014 papers, sad truth is that we can not trust publishers at face value, even those not on this list
Nov
22
comment Addition / Multiplication modulo 13
The first table is still not that of a group, but of the commutative monoid $({\mathbb Z_{13}},*)$ (hint: find the neutral element, then if every element has an inverse). Again, find the group you are working with, and check why $g=2$ is a generator by using the definition of that.
Nov
22
comment Addition / Multiplication modulo 13
The (partial) tables are for the commutative monoid $({\mathbb Z_{13_\text h}},*)$ and the Abelian group $(\mathbb Z_{13_\text h},+)$, where $13_\text h=19$, rather than for what's stated. $\;$ By definition $g$ is a generator of a finite group $(S,\circ)$ iff $g\in S$, and any $x\in S$ can be written as $x=g\circ g\circ\dots g\circ g$, for some number of terms. $\;$ Ascertain what group you are working with (including, most importantly, what the group operation is), and apply that definition.
Nov
22
revised Verbatim of early work on public-key cryptography?
Conspiracy theory is weakened by history of CESG; link to that
Nov
21
revised Why we can't implement AES 512 key size?
typo
Nov
21
revised Why we can't implement AES 512 key size?
More precise summary of the criticized article
Nov
21
comment Python. RSA common modulus attack problem
Use Extended Euclidian to compute modular inverse. That maybe what gmpy.invert does.
Nov
21
revised Verbatim of early work on public-key cryptography?
Expand description of Simon Singh's writing
Nov
21
revised Verbatim of early work on public-key cryptography?
Add quote of [6]
Nov
21
revised Verbatim of early work on public-key cryptography?
Add new reference [6], and better link to the FoIA case
Nov
21
revised Is there a way to systematically calculate the public exponent $e$ in RSA?
fix another OB1, polish
Nov
21
revised Which attacks are possible against raw/textbook RSA?
More interesting example
Nov
21
revised Which attacks are possible against raw/textbook RSA?
Add Leak of Jacobi