The Telegram which is a very popular messenger uses a special mode called IGE(Infinite Garble Extension). IGE mode is not included in standard mode of operation recommended by National Institute of Standards and Technology(NIST) in 2001. Block cipher encrypts fixed length of plaintext into the corresponding fixed-length of ciphertext using a secret key shared by two parties and utilizes lots of mode of operation for various length of plaintext.
Even though Telegram uses non-standard IGE mode, Telegram is claimed to be secure and demonstrate their security is stronger than other IM's. Thus, we need to verify the security of IGE mode depending on underlying block ciphers. In this paper, we show that IGE mode block cipher used in Telegram assuming sPRF is not IND-qCPA, but assuming qPRF is IND-qCPA.