340 reputation
18
bio website
location
age
visits member for 2 years, 1 month
seen yesterday

Dec
10
comment Deriving Keys for Symmetric Encryption and Authentication
@PaŭloEbermann: For "definition" you are certainly right. But the security aspect is IMHO the same, thus establishing the usefulness of the idea I mentioned. Note that if a sequence obtained from counter mode (via increasing by 1) can be securely used, then an arbitray sub-sequence, say consisting of the 5-th, 24-th, 88-th, 109-th etc. of the sequence, should also be secure.
Dec
10
comment Deriving Keys for Symmetric Encryption and Authentication
I remember to have mentioned the method of deriving keys from a master key via a block cipher in counter mode many years ago in another forum, thinking though even at that time that the idea very probably was old and well known. Recently however in discussions elsewhere I got the impression that it may not have been widely known, probably due to lack of mention in the literature. The idea of having a hierarchy of master keys to derive session keys dynamically was mentioned in my code JADE in s13.zetaboards.com/Crypto/index
Dec
10
comment Deriving Keys for Symmetric Encryption and Authentication
@PaŭloEbermann: Right. For counter mode the key is secret as usual. The plaintext part is an arbitrary numerical value, normally successsively counted up by 1 (or by any amount), but that input could just as well form any arbitrary sequence of values (e.g. containing time and message serial numbers, etc. etc., arbeit not necessarily to be very strongly guarded for security resaons).
Dec
9
answered Deriving Keys for Symmetric Encryption and Authentication
Oct
9
awarded  Teacher
Oct
9
answered One time pad key exchange
Oct
3
awarded  Commentator
Oct
3
comment How could block encryption in counter mode be secure from viewpoint of entropy?
@PaŭloEbermann: But estimates based on computing power could be difficult/problematic due to uncertainty in predictions of future developments either in software (methods of computation) or in hardware, if the secrecy has to be maintained for quite a long time. I vaguely remember to have seen a paper where Rivest made such a claim for a RSA key size that is way smaller than what is currently considered to be appropriate.
Oct
1
awarded  Student
Oct
1
comment How could block encryption in counter mode be secure from viewpoint of entropy?
Ok, but then how much security measures does one need in order to protect against a "practical" adversary? That's a rather fuzzy and hard to determine matter, isn't it?
Oct
1
comment How could block encryption in counter mode be secure from viewpoint of entropy?
I surmise that it seems anyway difficult to characterize a nice goal of design for stream encryptions, noting on the other hand that the perfect security of OTP is based on the concept of entropy.
Oct
1
comment How could block encryption in counter mode be secure from viewpoint of entropy?
But do you consider my figure of entropy correct? If so, what is the magnitude of entropy that a key stream bit would need to have in practice for a more or less secure stream encryption?
Oct
1
asked How could block encryption in counter mode be secure from viewpoint of entropy?
Sep
22
comment Difference between stream cipher and block cipher
@CodesInChaos. Sorry I missed to answer your main point. I was referring to Ebermann's sentence "A stream cipher usually translates the bits/bytes/... of the stream differently, depending ...". Doesn't that "differently" correspond to the notion of variablity?
Sep
22
comment Difference between stream cipher and block cipher
@CodesInChaos. Different applications have different performance requirements. To encrypt e.g. an email, one doesn't need the performance that would be desirable for encryption of, say, a video-file.
Sep
22
comment Difference between stream cipher and block cipher
If I don't err, "chaining" in block encryption is normally employed in the context of "block chaining", i.e. rendering the successive blocks dependent on one another so as to make the analysis more difficult. So IMHO ECB would have by definition no chaining effect as such.
Sep
22
comment Difference between stream cipher and block cipher
@PauloEbermann. IMHO you answered for me a question of CodesinChaos conscerning "dynamics and variability".
Sep
22
comment Difference between stream cipher and block cipher
I questioned whether it is necessary to have two different terms. According to what you explained, a stream cipher is simply a special case of a block cipher, i.e. one for the limiting case where the n in the set {0,1}^n is 1. So I would argue for not maintaining the current distinction of terminologies.
Sep
22
asked Why is there a strong distinction between stream and block ciphers?