If you read about asymmetric encryption and think about it, you will understand, that if such method existed it would defeat the purpose if ciphering or digital signature.

All this is based on the idea of

*one-way function*, see http://en.wikipedia.org/wiki/One-way_function[^]. The algorithm representing a one-way function is believed or proven to have not

*backdoor*(http://en.wikipedia.org/wiki/Backdoor_(computing)[^]).

Perhaps you need better understanding of

*public-key cryptography*. Please read http://en.wikipedia.org/wiki/Public-key_cryptography[^].

To understand digital signature, read http://en.wikipedia.org/wiki/Digital_signature[^].