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Apr
19
comment The exact definition of a symmetric encryption
@Melab: looking back at it, I'm not particularly happy with my initial answer. I've have a very quick go at improving it, but its still not great.
Apr
19
revised The exact definition of a symmetric encryption
added 623 characters in body
Apr
19
revised The exact definition of a symmetric encryption
deleted 17 characters in body
Apr
19
revised The exact definition of a symmetric encryption
Remove misleading section prompted to revisit by MeLab's comment.
Mar
3
comment Fixed-prefix IV in CBC mode
I think you mean ChosenPlaintextAttacks...
Feb
23
comment Difference between HMAC-SHA1(key, msg) vs SHA1(key + msg)
While concatenation is the common usage, in crypto || is often used to mean "mapped injectively into a single string". Now, this might be the same thing, but only if the first parameter does not 'overflow' some designated datatype. A lot of (admittedly rather petty) attacks exist based on the fact when concatenating $A||0B = A0||B$
Feb
23
comment Does encrypting twice using the same block cipher produce a security weakness?
Although your later comments clarify the situation, your initial claim is false (stream ciphers are catastrophically broken if you double-encrypt under the same key!)
Feb
23
reviewed Approve Obtaining Diffie-Hellman generator
Feb
23
comment Is XSalsa20-Poly1305-SIV a reasonable choice for nonce-misuse-resistant authenticated encryption?
Assuming I've understood what you're asking, your question is equivalent to: "Is SIV a secure mode of operation", which it is [web.cs.ucdavis.edu/~rogaway/papers/keywrap.html]
Feb
23
comment Is it okay to bail out early when length mismatch is detected?
Moreover: how do you know that you have the wrong length? It sounds like you have some external functionality (packet sizes?) around the outside of your AEAD system that tells you what size the content should be. As Richie says, most AEAD schemes do not require an explicit length parameter -- they work it out from the size of their inputs -- so as far as AEAD is concerned there isn't really a concept of 'length mismatch'
Jan
18
revised Simply put, what does “perfect secrecy” mean?
minor typos
Jan
18
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Oct
15
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Aug
22
awarded  Popular Question
Jul
30
awarded  Enlightened
Jul
30
awarded  Nice Answer
Jun
23
awarded  Constituent
Jan
14
comment What is a tweakable block cipher?
@StephenTouset: Changing a key can be very expensive if there is a key expansion routine to calculate (eg AES), whereas tweaking the message need not be. A common method is to take a blockcipher $E_k(\cdot)$ and define the tweakable blockciphers $E^T_k(M):=H(T)\oplus E_k(M \oplus H(T))$ where H is some finite field multiplication with its own secret key (ie $H(T):=T\cdot K_2$). Thus the whole calculation consists of two xors and one finite field multiplication, whereas to tweak the key would require recomputing the whole AES key schedule [as well as opening you up to related key attacks].
Nov
5
revised Why is OCB-AES mode not becoming a standard for authenticated encryption?
shameless hack to change my vote
Nov
5
comment Why is OCB-AES mode not becoming a standard for authenticated encryption?
Agreed, I disagree with my past self. When back at desk will try and hack that vote off