de‧ci‧pher / dɪˈsaɪfə $ -ər / verb [ transitive ]
1 to find the meaning of something that is difficult to read or understand → indecipherable :
She studied the envelope, trying to decipher the handwriting.
2 to change a message written in a code into ordinary language so that you can read it SYN decode
— decipherment noun [ uncountable ]