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May
7
comment Key derivation design to avoid key leaks
When the adversary exercises the extra capability, a chosen input is chosen_seed, a known input is seed, an unknown input is key, but what is the output? If there's none, the extra capability is useless! If that is derived_key_2 (my best idea to make sense of the question), how come " second goal is to get derived_key_2 " ? Or perhaps the adversary observes something through a side channel like timing, power analysis?
May
7
comment Information-theoretic bound on leakage by timing measurement
Ah, did not knew fountain codes, that seems a fine (optimal ?) tool for the hypothetical attacker. Yes I'm hoping that the answer to Q3 (assuming Q2) can come from coding theory (which is off my area of practiced expertise, even though I was in modems in an earlier life).
May
6
comment Information-theoretic bound on leakage by timing measurement
The methods you describe work, with the first matching Q1, and "Another approach.." matching Q2. I'm uncertain this optimal, though: getting all the key fragments (or coupons) is long, and perhaps it is better to build a redundant form of the key, and leak that; sort of forward error correction, where error is missing coupons.
May
6
comment Brute-force attack given small search-space and hash prefix
I'm reading " the attacker only has the first 8 characters of the SHA1 " as meaning the first $8\cdot4=32$ bits of the hash. Thus it is expected about $2^{24}$ candidates.
May
5
comment Special case of format-preserving encryption
This is a form of steganography. It is feasible if we accept that ciphertext is significantly larger than plaintext, for some definition of natural language (especially if that includes modern poetry, or something looking like the output of automatic translation, or/and if we feed the encryption program with natural text that it slightly transforms).
May
5
comment RSA : Is finding Public key given Private key as hard as finding Private key given Public key?
@Myth17: Notice that the other question is not quite the same as your's. $\;$ The answer to your question depends on the form in which the private key is known [ $(n,d)$ versus $(p,q,dp,dp,qInv)$ ]; on the key generation method [ how $e$ was chosen ], and what is meant by "finding Public key" [ if we are content with knowing $e$ within addition of a multiple of $\operatorname{lcm}(p-1,q-1)$ ]. Thus there is no yes-or-no answer.
May
4
comment RFC 5297 implementation
Was re-posted (in slightly better form) in this (off-topic) question.
May
4
comment trying to get calculator and RSA calculatin does not make sense
First part (the only one close to being on-topic) is addressed in Calculating RSA private exponent
May
4
comment where can i learn the basics of AES?
I guess your primary reference should be AES Proposal: Rijndael. At a very high level, remember that the key schedule is here to build, from the key, one more (blockwide) subkey than there are rounds, because the block is XORed with a subkey on input, between rounds, and on output; and the subkeys should not be so closely related that it creates a weakness.
May
3
comment Energy necessary for brute-forcing or decryption
@Ricky Demer: According to the private key format, the public modulus is available in clear in the private key. A critical parameter is the value of the s2kcount parameter, which controls the work factor used for stretching of the passphrase. Depending on that, the point of least hardest attack is the passphrase, or the factorization of the public modulus. By default stretching used to hash about $2^{16}$ bytes, but I have read about plans to increase that.
May
2
comment How can AES be considered secure when encrypting large files?
@kasperd: my $r$ is residual risk as a base-2 log, not comparable to the security of a key in bits (the adversary can use more brute force against the later, not former). When making a 100 miles trip by car in the US, one accepts a residual risk of death about $2^{-20}$, compare to $2^{-40}$ of being hacked; see also my consideration about oblivion by asteroid.
May
2
comment SipHash - 64 bit (second) preimage security?
Are you considering SipHash with the supposedly secret key gone public, making SipHash as hash, when normally SipHash is a MAC (aka Pseudo Random Function Family) rather than a hash (aka random public menber of a Pseudo Random Function Family) ?
May
2
comment How can AES be considered secure when encrypting large files?
@kasperd: your rule of thumb is way overly conservative; CBC/CFB modes are good for $b\cdot2^{(b+1-r)/2}$ bits where residual odds of duplication of one block are $2^{-r}$. With AES and $r=40$ (residual odds of one in a million millions, entirely negligible compared to oblivion by asteroid on any given day) that's 3 petabit (nearly 400 terabyte). CTR/OFB modes are good for even more.
May
1
comment Is it safe to derive two different keys with the same password and key derivation function using two different salts?
@rossum: your appendix is to belt what salt+KDF are to suspenders.
May
1
comment If a DES key correctly decrypts one message, what's the probability it's the real key?
Hint: if DES was an ideal cipher, $X\to DES_X(M)$ would behave like a random function, Using this, estimate odds that there exists no $T$ less than $K$, as a function of the value of $K$; then that expected odd for random $K$; then what's asked.
May
1
comment Key space vs Cardinality of 1024-bit RSA
@dave_thompson_085; good catch, thanks
May
1
comment What crypto scheme should I use on embedded m2m system with assymetric uplink/downlink bandwidth?
You should define your objectives better. You mention asymmetric encryption, what about authentication of the origin of messages? Can there be two-way communication with the intended final receiver of the encrypted message? Do you want forward secrecy? Do you need some level of resistance to traffic analysis? What can adversaries attempt (in particular, on what are they in a position to hook a jtag probe or equivalent? On other devices, can they measure the device's power consumption? Exploit EM emission beyond the radio link?) What's the legal context?
May
1
comment Information-theoretic bound on leakage by timing measurement
@kodlu: you are right, the space requirement I gave was way too high, I had left a $\log_2$ factor out. Thanks for pointing that error!
Apr
30
comment Discrete logarithm over prime modulo: small input, large exponent, larger prime
That would not be a hash as usually defined, since part of its definition ($e$) is secret. Rather, that's a keyed one-way function; it is the encryption part of a cipher if $\gcd(e,p-1)=1$
Apr
30
comment Zero knowledge / proof of knowledge sudoku solution
Nice trick.Describing in more details how it's done from a distance would be awesome.