[cryptography] Homomorphic split-key encryption OR snake oil crypto

James A. Donald jamesd at echeque.com
Sun Feb 19 16:34:15 EST 2012

On 2012-02-20 2:08 AM, Florian Weimer wrote:
 >> Can somebody explain me how this so-called Homomorphic split-key
 >> encryption works?

Homomorphic means you combine the keys without finding out the key that 
you are combining - Everyone gives you an encrypted copy of their key 
fragment, and when you are done, you have an encrypted copy of the 
combined key.

 > Isn't this just a protocal which performs a cryptographic primitive
 > using split key material, without actually recombining the keys?
 > (Traditional Shamir secret sharing needs a trust party for key
 > recombination.)
 > If yes, you might want to look for "RSA Threshold Cryptography" and
 > similar work.

My understanding is that RSA Threshold always requires a "trusted" 
party, which makes it useless.  If you have a party that is actually 
trusted, just let him count the votes or whatever.  The cryptography 
does not do you any good.

The only protocol that I am aware of that performs cryptographic 
operations on a split key with needing a trusted party,  uses Gap Diffie 
Hellman groups.

All known Gap Diffie Hellman Groups consist of an elliptic curve which 
supports a bilinear pairing from the curve to integers modulo some large 

More information about the cryptography mailing list