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May
16
comment CCA secure or not?
I do not understand how decryption could be done, or what $\bigoplus0^n$ is supposed to achieve. Is there a typo somewhere?
May
16
comment Authenticate a short message with redundant encryption instead of using a MAC?
@user1636512: your feeling is right (the adversary gets twice as much plaintext with the same key); yet modern ciphers like AES are perfectly fit for this.
May
15
comment Authenticate a short message with redundant encryption instead of using a MAC?
If you can spare a single bit out of the plaintext (say, the leftmost of the 128 bits is always 0 in a valid plaintext), you can have authenticated encryption with a single key: make the cryptogram $E_K(plaintext)\|E_K(plaintext+2^{127})$ and make the obvious checks of the 129 constrained bits on the receiver side. If size is an issue, the second block can be truncated.
May
15
comment Meet-in-the-Middle attack on 3DES
(re-revised) In the section on attacking 2-key 3DES: odds of having made a valid guess at step 1, stated at step 4, are $p/{2^m}$. The storage requirement is OK (I had to get at the article to confirm that). $\;$ Minor TeXpo in the section on attacking 3-key 3DES: $2^56$ should be $2^{56}$
May
14
comment Meet-in-the-Middle attack on 3DES
I'm not seeing that this question is a true duplicate; various other answers do not really describe the basic MitM applied to 3DES. The closest I found is in this question but the table is built my enumerating 2 keys rather than 1, which makes it require a ludicrous amount of memory; hence my answer.
May
10
comment Key Check Value standard pratice for asymetric cryptography?
That indeed matches the question as asked. However there's a significant issue: it's big!
May
7
comment Algorithm for n-of-m keys with partial results
If " nobody is trusted to hold the complete key " in the question rules out that a trusted party (not among those holding a key share) holds the complete key, it's going to be critical to specify what the complete key is supposed to allow.
May
7
comment Information-theoretic bound on leakage by timing measurement
Your answer and comment above seems very close to my goal as set in the second section of the question: we bound the channel capacity in the situation of Q3, disregarding the considerations of Q1 and Q2 (also I realize that a practical adversary can often choose the input to the black box or Smart Card, which is even more handy than having an internal counter in the black box). $\;$ So the only thing left to do is dig the formula for computing the Shannon capacity of the channel in the presence of noise $U$.
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 Defining format-preserving encryption for natural language
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
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.