How does cross platform AES encryption work?

Question

I have been able to successfully encrypt and decrypt AES-256 in both php and objective-c code. I won't post any code here since I have tried many varieties and none work. I have no idea how these encryption functions work... AES is a standardized algorithm, so why it doesn't work in my thinking boils down to

a) the iv
b) some encoding error
or
c) differences in padding (should be irrelevant for decryption).

If someone has AES functions that work in both php and objective-c that would be wonderful, but if not, any help in understanding what is causing these varied results would be appreciated.

If you want a more narrow question, it is about encodings, iv, and block size of this AES cipher.

1) Does it matter what encoding is used in terms of the key and the plaintext/ciphertext? Basically I'm guessing it is not a problem with the plain text since all the characters that I would use (at least during testing) are standard ASCII symbols. But lets say php strings are ASCII and I am using UTF8 in objective-c... I don't know enough to say if php uses ASCII or if the bytes ie. the key would be different between the two.

2) To my knowledge the ECB mode uses no iv (correct if wrong). CBC mode uses an iv. In this case, the iv must be recorded along with the cipher text. Now this key is 16 or 32 chars long in php (depending on 128 vs 256 block size). This means 16 or 32 bytes? And will the string 1234567890123456789012 be the same in ASCII and UTF8 when converted to bytes?

3) What is the difference between block size and key size in terms of the alogrithm? (again correct if wrong) Basically they are all the same algorithm just different parameters? And using a 256 bit key vs a 128 bit key is just a matter of which key is passed

(Also, note that I have been using base64 encoding to transfer strings between the applications for testing)

Thanks, Elijah

Answer 1

For decryption to work correctly, everything must be exactly the same. Same key, same IV, same mode. In particular the key must be the same. Byte for byte the same. Bit for bit the same. AES is designed to fail to decrypt correctly if even one bit of the key is incorrect.

Reading your question, I suspect that your problem lies with the key. Your real key is not characters, it is bytes. There are a number of different ways to translate between characters and bytes, which can cause decryption to fail. You need to be certain that the two keys match byte for byte, not character for character. At the very least you need to be explicit about what mapping is used. Don't rely on system defaults as they can differ across systems.

Looking at your three questions:

1) For plaintext encoding you will get back exactly what you put in: UTF-8 in, UTF-8 out. If you want to convert to a different encoding then you will have to do it after decryption.

2) You are right that ECB doesn't need an IV, but ECB mode leaks information and should be avoided. Use CBC or CTR mode instead, the same mode at both ends. The IV is tied to the block size, so for AES the IV is always 16 bytes or 128 bits. You cannot guarantee that ASCII and UTF-8 will be the same. UTF might have a BOM at the start. ASCII might have a C-style zero byte at the end. Don't think in terms of characters, think in terms of bytes. Everything has to match at the byte level. In CBC mode a faulty IV will munge up the first block but decrypt subsequent blocks OK.

3) Block size is fixed at 128 bits for AES and cannot be changed. Key sizes are less constrained, and can be 128, 192, or 256 bits. In practice most people seem to use 128 or 256 bits. A block is a conveniently sized processing unit that is built into the cypher at a very low level. The key determines what is done to the block in the course of the processing. That allows more flexibility for the key. The key you enter is used to build some internal structures, the "round keys". This process is called "key expansion". It is the round keys which interact with the block being processed. Because the key is used indirectly there is more flexibility about how large it can be.