All crypto algorithms have the asymetric property. Which means if you know the plain text and the algorithm, you can get the crypted text within expected time and operations, usually several seconds.

However, if you know the algorithm and the crypted text, it is not possible to use available machine power to get the plain text in a practical period. For example, to get the plain text for DES with given crypted text, one needs ALL the PC/Server/Workstations in USA for a week.

So....

Suggest to read some about cryptography.