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qaw
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PostPosted: Sun Nov 15, 2009 1:06 pm    Post subject: Can anyone help? Reply with quote

We've been learning about RSA and other encryption in college.
Just been given a question to answer:

Produce a protocol so that for different users, A,B,C, and D can access 4 pieces of secret information, 1,2,3 and 4, however

A can only access 1 and 4.
B can only access 2 and 4.
C can only access 3 and 4.
D can only access 4

Now i understand rsa pretty well but this is baffling me and ive googled it to death but every example of rsa is just alice and bob exchanging one message between themselves. Can someone point me in the right direction?
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capi
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PostPosted: Sun Nov 15, 2009 4:01 pm    Post subject: Reply with quote

How detailed must this protocol be? (must you describe things like initial key distribution, etc? I'm assuming you don't)

I'm thinking you can solve that problem rather simply with public-key cryptography... Remember, it is asymmetric encryption; you use one key to encrypt a message, and another key to decrypt it.

Can't really think of anything else I can tell you without giving away the answer...


Last edited by capi on Wed Dec 09, 2009 11:48 pm; edited 1 time in total
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qaw
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PostPosted: Sun Nov 15, 2009 5:14 pm    Post subject: Reply with quote

Israel,

Thanks for the reply, yes question wants the answer as a public key method. I still don't get it though! I know the rule for encryption is encrypt with public key, decrypt with private.
Here is what I've come up with, does this sound remotely correct?

Piece 1 should be encrypted with A's public key so only A can decrypt with it with its private key
Piece 2 should be encrypted with B's public key so only B can decrypt with its private key
Piece 3 should be encrypted with C's public key so only C can decrypt with its private key
Piece 4 can be accessed by all of them. This is the bit I'm stuck on, it has to be encrypted to begin with, would it work to have it encrypted with A's public key first so they can access it, then they encrypt in with B's public key, send it to them, they access it with their private key, and carry on passing it on until D gets it.
Am I close or 10000 miles from the answer?!! ANNOYED!?!
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capi
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PostPosted: Mon Nov 16, 2009 12:56 am    Post subject: Reply with quote

qaw wrote:
Am I close or 10000 miles from the answer?!! ANNOYED!?!

You're right on the first 3 pieces, but the fourth one is somewhat off...

It is possible to make piece 4 decryptable by A, B, C and D without requiring any kind of cooperation between them. GnuPG and PGP, for example, allow you to do precisely that. I suggest you look into how PGP accomplishes that -- the first parts of the article I linked above should give you a nudge in the right direction.


Last edited by capi on Wed Dec 09, 2009 11:48 pm; edited 1 time in total
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petebote
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PostPosted: Tue Nov 17, 2009 1:07 pm    Post subject: Reply with quote

Hello,
I faced the same exercise... Very Happy
Where is the link about PGP?
Thank you for help
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capi
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PostPosted: Tue Nov 17, 2009 7:33 pm    Post subject: Reply with quote

petebote wrote:
Hello,
I faced the same exercise... Very Happy
Where is the link about PGP?
Thank you for help

The link is in my first reply to this thread. Just scroll up.
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petebote
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PostPosted: Wed Nov 18, 2009 1:02 pm    Post subject: Reply with quote

ah ok thank you!
I read it but still I didn't get your idea?!
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