Re: Any of a set of keys for encryption

From: Mike Amling (nospam_at_nospam.com)
Date: 11/09/05 

Date: Wed, 09 Nov 2005 21:40:39 GMT

Sebastian Gottschalk wrote:
> glenlow@pixelglow.com wrote:
>
>
>>Hi All
>>
>>Is there any cipher or cryptosystem that allows any of a limited set of
>>keys to decrypt a message?
>>
>>E.g. if D (K, C) is the decryption function, where K is the key and C
>>is the ciphertext,
>>
>>then given an arbitrary set of keys K1, K2, K3 ... Kn,
>>
>>D (K1, C) == D (K2, C) == D (K3, C) .... == D (Kn, C) == P
>>
>>Would be interested especially if you can build this on top of a stream
>>cipher.
>
>
> That's trivial. Choose a random key K and let (K XOR K1, K XOR K2, ..., K
> XOR Kn, E(K,C)) be the ciphertext.

   Well, that allows anyone who knows one Ki to find all the other Ki's,
which may or may not meet the OP's unspoken requirements. Using bcE(Ki,
K) instead of K XOR Ki, where bcE() is a block cipher, would avoid that.
Also, "E(K,C)" should be "E(K,P)".

--Mike Amling

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