Encryption Initialization Vector

Question

Using the aes_cfb_encrypt and aes_cfb_decrypt functions, I have the following questions.

1. What is unsigned char *iv (Initialization Vector) in an encryption.
2. Is it required to preserve the *iv for decryption.
3. Each time i encrypt a block of data the *iv is modified, What i have to do with this modified *iv.

4. I am encrypting a large file around 100mb, and passing a random *iv for the very first time, do i have to use the same *iv for the rest of the loop, or i have to use the updated *iv from the last call of encrypt block.

5. Lastly, I am dealing with a structured file, so do i had to use Sizeof(struct) as the length of the buffer or have to use sizeof(struct)*8 as length of the buffer for encryption or decryption. Please guide..

AES_RETURN aes_cfb_encrypt(const unsigned char *ibuf, unsigned char *obuf, int len, unsigned char *iv, aes_encrypt_ctx cx[1]);

AES_RETURN aes_cfb_decrypt(const unsigned char *ibuf, unsigned char *obuf, int len, unsigned char *iv, aes_encrypt_ctx cx[1]);

Answer 1

In answering your questions, please note the following:

PT(x) = Plain Text representation of 'x'
CT(x) = Cipher Text representation of 'x'
Bn    = Logical Data Block 'n' in a sequence of multiple blocks.

1. What is an IV?

IV is short notation for Initialization Vector. It is used in symmetric block-encryption algorithms that perform their encryption in what are called chained or feedback modes. In either, the previous block of encrypted data is used as a piece of functional data "goo" to alter the next block of data to be encrypted. Each successive block of data that is encrypted is fed the prior already-encrypted data block as their blob of goo to use. But what about the first block of plaintext? What does it use for its special sauce? Answer: the IV provided to the function. Pictorially, it looks like the following:

CT(B1) = Encrypt(IV + PT(B1))
CT(B2) = Encrypt(CT(B1) + PT(B2))
CT(B3) = Encrypt(CT(B2) + PT(B3))
...
CT(Bn) = Encrypt(CT(Bn-1) + PT(Bn))

Note: '+' in the above denotes the application of the prior cipher block to the next plaintext block. It is not to be thought of as mathematical-addition. Think of it as "combined with".

The size of the IV must be the same as the block size of the symmetric algorithm being used. Both AES-128-CFB and AES-256-CFB use a 128-bit block size (16 bytes). Therefore, your IV should be 16 bytes of random goo for your purposes in this question, and should be generated on the encryption-side using a secure FIPS-compliant random-source algorithm.

2. Is it required to preserve the IV for decryption?

Yes, but not necessarily in the fashion you may first think. The first IV (provided by you) must be retained somehow. Traditionally, it is sent right where you would think it should be; as the first block of encrypted data. This often freaks people out, they think "But if I send the IV with the data, it isn't as secure, is it?" Think about it this way. How many "IV's" are you sending, anyway? Remember, each data block is encrypted using the prior block of encrypted data as its IV. Therefore, you're actually sending an entire stream of IVs, each encrypted block the IV for the next encrypted block, etc. Where the initial IV is in your output ciphertext is a data-representation question, but where it goes is ultimately irrelevant to the question. It must be preserved. It is possible your API does this for you as part of its output stream (it is not uncommon at all, in fact).

3. Each time I encrypt a block of data the *iv is modified, What i have to do with this modified *iv?

I'm not familiar with the API you're using, but it sounds like you're given the IV to use for the next encryption, which makes perfect sense when you consider how feedback or chaining works for block-mode encryption. You should NOT use the same IV repeatedly. Use the one returned last as the next one. Since your API is modifying the IV in place, it appears the only thing you may need to do is preserve the initial IV somewhere else before sending. I would compare the first ciphertext block against your IV. if they are not the same, you probably need to send your IV, then the cipher text chain in your data stream, and have the receiver aware that the first block is the IV of the decryption.

4. I am encrypting a large file around 100mb, and passing a random *iv for the very first time, do i have to use the same *iv for the rest of the loop, or i have to use the updated *iv from the last call of encrypt block.

See (3). Use the updated IV for each successive block.

5. Lastly, I am dealing with a structured file, so do i Sizeof(struct) as the length of the buffer or have to use sizeof(struct)*8 as length of the buffer for encryption or decryption.?

Use the size of your structure in bytes (not bits). The C/C++ sizeof(yourstruct) should compute this for you, but note if you're encrypting each structure as an independent entity (and not the entire file in one mass), each encryption will carry with it a minimum amount of data added to account for (a) the IV used for that structure, and (b) padding the last block out to an even block boundary, assuming you're using PKCS5 padding. The exact size of an encrypted structure, therefore, would be:

IV + ((sizeof(struct) + 15)/16)*16) bytes.

Again, this is if you're independently encrypting, and storing, each structure as a singular encryption, and again, your API may account for some of this for you.

For more information on symmetric AES, see the AES entry on Wiki. For information on the CFB block cipher mode, see the Block Cipher Modes of Operation article on the same site.

I hope this helps. Do some homework and above all, learn exactly how your API works, which is something I cannot, unfortunately, help you with.