[Cryptography] Mathematically, how do proxy signatures work?

[Ben Laurie]

On 20 March 2014 18:32, Bear <bear at sonic.net> wrote:
> I would like to understand the mathematics that allow these derived
> keys to be created and the mathematics that allow Alice to identify
> and decrypt messages encrypted using these derived keys.  I would
> like to know what Hard mathematical problem prevents Carol, Dave,
> etc, from being able to identify Bob's derived key as being part
> of the set derived from Alice's persistent key.  And if there are
> any special properties that the persistent key Alice publishes must
> have in order to work with this scheme, I would like to understand
> those too.

Doesn't ElGamal encryption have this property?