PlugRailsCookieSessionStore.MessageEncryptor.verify_and_decrypt

You're seeing just the function verify_and_decrypt, go back to PlugRailsCookieSessionStore.MessageEncryptor module for more information.
Link to this function

verify_and_decrypt(encrypted, secret, sign_secret, cipher \\ :aes_256_cbc)

Decrypts and verifies a message. We need to verify the message in order to avoid padding attacks. Reference: http://www.limited-entropy.com/padding-oracle-attacks