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Aug
15
accepted What security flaw exists in MAC authentication scenario?
Aug
15
revised What security flaw exists in MAC authentication scenario?
Answer selection rational
Aug
15
comment What security flaw exists in MAC authentication scenario?
I concur a digital signature (asymmetric keys) would be better than a MAC were it not for 3 things: A and B are cooperative, thus B would not forge a message, non-repudiation is not needed & the DS is about 2x as long as a MAC. I'll expand more in the Post.
Aug
14
comment What security flaw exists in MAC authentication scenario?
Selectively blocking - (you keep finding these Swiss holes in my cheese.) The messages are complete unto themselves and a missed one is irrelevant. This also was unstated, but then I didn't consider it either.
Aug
14
comment What security flaw exists in MAC authentication scenario?
An order caveat - The message order, as it turns out is not significantly important. This was unstated, but then I didn't consider it until you provided it (and a solution).
Aug
14
awarded  Commentator
Aug
14
comment What security flaw exists in MAC authentication scenario?
You have a good point. The idea, though not stated clearly enough, is that $A$ & $B(s)$ are cooperative and "$A$ and $B$ keep their keys secret." and in a later comment "B only talks to himself". Given your good idea, if B ever talks to himself via a public message, a different MAC key/clone should be used. But as you suggest, if A or one of the clone B is compromised, the gig is up - which really the same as the "secret" key being comprised. I'll ponder this more.
Aug
14
awarded  Editor
Aug
14
comment What security flaw exists in MAC authentication scenario?
@Perseids Yes, all Bobs are clones and share the same secret keys with A.
Aug
14
revised What security flaw exists in MAC authentication scenario?
confindentiality to authenticity
Aug
14
awarded  Custodian
Aug
14
reviewed Approve suggested edit on What security flaw exists in MAC authentication scenario?
Aug
14
comment What security flaw exists in MAC authentication scenario?
Thanks for asking. (A)lice communicates with many (B)obs - they are all clones. (B)ob listens to the world and wants to authenticate messages to see if they originated from A. 'B' does not talk to 'A'. No need for 'A' to verify anything. 'B' only talks to himself. No public keys involved, just private keys that A & B hold secret for the MAC. Goal: as posted.
Aug
14
asked What security flaw exists in MAC authentication scenario?
Aug
2
comment Digital Signature size requirement
What values for k & L would you recommend?
Jul
31
comment Digital Signature size requirement
I see that when your initial post mentioned "2⋅k bits", I took that to mean 2048 bits. Now I see the revised edit details the meaning of 'k'. Further, my needs do not need a digital signature (an error on my part) as I do not need repudiation, a MAC will suffice. So I estimate a 256 bit is sufficiently secure. Your thoughts on that or shall I post a new question?
Jul
31
comment Digital Signature size requirement
@Ricky Demer Thanks for the added info. As 2 kbit will not fit into the smaller RFID tags, looks like a MAC will have to do. I'm thinking 256 or 512 bit MAC will suffice, but that sounds like another question.
Jul
25
awarded  Scholar
Jul
25
comment Digital Signature size requirement
I see 2048 bits is the standard for today, with little regard to the size of the document. 1k may work, but its becoming dated. Thanks for the MAC idea.
Jul
25
comment Digital Signature size requirement
@e-sushi Thank-you for your BLS suggestion.