In the nutshell , cryptography zipo za ain tatu,
1. private key/secret key/symmetric cryptography
2. Public key/asymmetric cryptography
3. Hash function cryptography
Kam wadau wengine walivyosem ,most commonly used encryption zinatumia encryption algorithm ya mojawapo kat ya hizo tatu...!
Back to your QN. Ili uweze Ku decrypt/decipher encrypted string , kwanza inabid kufaham ni ain gani ya encryption iliotumika..! Baad ya hapo ndo unawez kuanza mchakato wa kufanya decryption.
NB. Kun encryption nyingine zinahitaj heavy computing resources during decryption.
Hope nime shed some more light on the subject matter.