# Is it possible to split a large AES encrypted string and decrypt the parts one by one?

Due to some platform restrictions our decryption algorithm can only handle up to 1 million bytes. The string we receive is larger, having been generated by AES in cipher block chaining (CBC) mode, with PKCS5 padding.

Is it possible to somehow split the encrypted data and decrypt the parts?

• What mode of operation is being used? AES only encrypts blocks of 128bits. – figlesquidge Dec 6 '13 at 14:03
• We use cipher block chaining (CBC) and PKCS5 padding. – h9nry Dec 6 '13 at 14:06
• With no authentication? This tends to be a risky scenario to be in, since unauthenticated encryption may be malleable. – figlesquidge Dec 6 '13 at 14:16
• No, authenticated encryption is about ensuring two things: privacy (no-one can read our encrypted messages) and non-malleability (no-one can create a fake message). Hopefully this blogpost will explain better than I can here – figlesquidge Dec 6 '13 at 14:24
• In the hopes of deriving a clearer explanation that "because you should!", I've started this question - hopefully it'll be of use. – figlesquidge Dec 10 '13 at 16:04

I assume from what you've said that you have a function that will "do" AES-CBC decryption on large amounts of data, and you wish to use this. So, you simply run: $$D_k^{IV}(c_1\ ||c_2\ ||\dots||c_n\ ) = m_1\ ||m_2\ ||\dots||m_n\ \\ D_k^{c_n}(c_{n+1}||c_{n+2}||\dots||c_{2n}) = m_{n+1}||m_{n+2}||\dots||m_{2n}$$