A word that describes both "encoding" and "decoding"?
Cryptography
is the study of encryption and decryption.
codec
is software/hardware that does both encoding and decoding.
Encode means : "To translate information into code"
Decode means : "To translate code into information"
Therefore translate would be used to describe them both.
Since translate may seem vague I would suggest some synonyms: transcribe, transpose, or convert.