メモメモ CBC
https://golang.org/pkg/crypto/cipher/#NewCBCDecrypter
https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Cipher_Block_Chaining_.28CBC.29
Advanced Encryption Standard - Wikipedia
https://en.wikipedia.org/wiki/Feistel_cipher#Construction_details
Initialization vector - Wikipedia, the free encyclopedia
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