Quote from: ozgur on November 10, 2013, 06:49:14 AM

Quote from: DeathAndTaxes on November 10, 2013, 06:32:57 AM

If you cut a key in half then the key strength of brute forcing the other half is reduced by half.

EDSA 256 bit keys has 128 bit strength security.

So if you cut it into two 128 bit "half keys" and the attacker has one it is only 64 bit key strength to resist an attack. While it may take some time and effort that is computationally feasible.

A much better option would be to construct the partial keys using XOR making each key have full strength.

I know what you mean, but to make it clear;

it is not halved, it is ~~2^128~~ 2^64 times easier

I updated the post, your right it is unclear. A clarification though, 256 bit ECDSA only has 128 bit key strength so half the key would be 64 bit key strength (too weak for me to sleep well at night).

So it is 2^64 (or a whole hell of a lot) easier to break then the full key.

Still you are dead on with the larger point. It SIGNIFICANTLY degrades the strength of the key and 64 bit is too close for comfort in my book. Keys with 64 bit key strength have been broken in a distributed computing project as a demonstration. It is almost certain that major nation states have the ability to break keys with 64 bit strength in a reasonable amount of time.