"Decryption" (暗号解読, Angō Kaidoku) is episode 154 of the Naruto: Shippūden anime.
Synopsis
While looking at Jiraiya's code, Naruto states that the "9" they've been looking at is actually a writing quirk of Jiraiya's penmanship, and that it is actually the katakana letter "ta"(タ), something only Naruto would have known. As Naruto reveals that he only learned of the quirk due to Jiraiya's novels, the Konoha Cryptanalysis Team state that the message must have been meant for Naruto, and that the other numbers must mean page numbers from the Icha Icha series. To Kakashi's embarrassment, he is forced to read excerpts from Jiraiya's adult books out loud until they manage to decipher the code — "The real one's not among them".
After relaying the decryption to Fukasaku, who remembers Jiraiya telling him that neither of the Six Paths of Pain looked like Nagato, Fukasaku asks Tsunade's permission to bring Naruto with him to Mount Myōboku, in order for Naruto to learn Sage Mode. Naruto accepts, due to his desire to avenge Jiraiya's death, and is reverse summoned to Mount Myōboku. As Naruto and Fukasaku leave, Tsunade muses how the Great Toad Sage had predicted Jiraiya writing novels and how those very novels lead to the decryption of his code, and decides to believe in the toad's prognostication that Naruto is the Child of the Prophecy.
At Mount Myōboku, Naruto discovers that he was summoned there by Gamakichi, as toads can summon humans who have signed a blood contract with them. After having one of Shima's meals, Fukasaku explains the basics behind senjutsu, with Gamakichi simplifying it for the genin. After Fukasaku states that Naruto must learn to acquire natural energy, Gamakichi tells Naruto that he would have to die first.
