de- (expressing reversal) + cipher

source: [フルメタル・パニックの相良宗介がもらったラブレターを全く見当違いに解読するGIF画像｜無料GIF画像検索 GIFMAGAZINE 66034]

verb with object

convert (a text written in code, or a coded signal) into normal language:

〈暗号など〉を解読する(↔ cipher)

e.g. enable the government to decipher coded computer transmissions.

succeed in understanding, interpreting, or identifying (something):

〈難読文字など〉を判読する, 読みとる

e.g. an expression she could not decipher came and went upon his face.

decipherable |dəˈsīf(ə)rəb(ə)l| adjective

decipherment |dəˈsīfərmənt| noun

early 16th century: from de- (expressing reversal) + cipher1, on the pattern of French déchiffrer.