Initialization Vector (IV) is a binary sequence used by block ciphers in most modes of operation. This post serves as a quick overview of how IV is used in one of the most popular Cipher Block Chaining mode.
So where does IV come from? To quote Wikipedia:
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Randomization is crucial for encryption schemes to achieve semantic security, a property whereby repeated usage of the scheme under the same key does not allow an attacker to infer relationships between segments of the encrypted message.
Probably the most famous example showing what “semantic security” means is the Linux penguin (on the left) encrypted using ECB mode (in the center) and CBC mode (on the right).
- If you are encrypting data, you need to generate an IV of the proper size. If you are decrypting data, and trying to use an IV that is not 16 bytes, then you have an incorrect assumption about either the cipher algorithm or your IV. (AES-256-CBC always has a block size of 16 bytes.).
- With AES-CBC you usually need a random IV. However, in the case where you use each key only once, like when using password-based encryption with random salts for each file, you can use a fixed, zero IV.
So what is the problem with the penguin above? ECB splits all data in block and encrypts each of the using the same key. Thus, the same plaintext results in the same ciphertext. On the image above, the same plain areas are transformed into the same cipher areas which let attacher to see relationships between areas.
Even bigger problem is that if attacker can make the system encrypt an arbitrary plaintext, she can then compare the result with the original ciphertext and guess the original plaintext. While this sounds like a long shot, it is actually a known TLS 1.0 vulnerability called TLS CBC IV or BEAST attack that was first discovered in 2001 and then was demonstrated in 2011 by Juliano Rizzo and Thai Duong. See Duong’s post, Thierry Zoller’s post, and TOR team’s overview for more details.
.(AES) for the data encryption. A random 256-bit key and an initialization vector (IV) are generated using the built-in system secret random generator. Supported key lengths and IV lengths 1 You can use only hexadecimal characters, newlines, tabulators and new line characters if you decrypt a string. 2 Input text has an autodetect feature at your disposal. The autodetect detects for you if the content of Input text field is in form of a plain text or a hexadecimal string. You can turn off the feature by clicking on 'OFF'.
One of the popular block cipher modes that solves this problem is Cipher Block Chaining (CBC) that uses outputted ciphertext of the previous block to obfuscate plaintext block before encrypting it.
In contrast, each of the other modes describe a process where ciphertext from one block encryption step gets intermixed with the data from the next encryption step. To initiate this process, an additional input value is required to be mixed with the first block, and which is referred to as an initialization vector. For example, the cipher-block chaining (CBC) mode requires a random value of the cipher’s block size as additional input, and adds it to the first plaintext block before subsequent encryption. In turn, the ciphertext produced in the first encryption step is added to the second plaintext block, and so on. The ultimate goal for encryption schemes is to provide semantic security: by this property, it is practically impossible for an attacker to draw any knowledge from observed ciphertext.
In cipher-block chaining mode (CBC mode), the IV must, in addition to being unique, be unpredictable at encryption time. Is there anything similar to crystal disk info that works. In particular, the (previously) common practice of re-using the last ciphertext block of a message as the IV for the next message is insecure (for example, this method was used by SSL 2.0). If an attacker knows the IV (or the previous block of ciphertext) before he specifies the next plaintext, he can check his guess about plaintext of some block that was encrypted with the same key before. This is known as the TLS CBC IV attack, also called the BEAST attack.[8]
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From SO comments:
Aes Key And Iv Generator Reviews
The key protects the encrypted data, whereas the use of a random IV ensures that information is not leaked by the ciphertext itself. IT does this by preventing identical plaintexts from producing identical ciphertext when encrypted using the same key
References:
- Different symmetric encryption modes (StackOverflow)
- Block cipher modes of operation (Wikipedia)
- Initialization Vector (Wikipedia)
- Salt vs IV when using AES-CBC (StackExchange)
- Explanation on why IV is important in AES (StackExchange)
- Issues with static IV in AES (StackExchange)
PROGRAMMING INTERFACE
Internal state is maintained in an opaque structure that is returnedfrom the Init function. In ECB mode the state is not affected bythe input but for CBC mode some input dependent state is maintainedand may be reset by calling the Reset function with a newinitialization vector value.
Construct a new AES key schedule using the specified key data and thegiven initialization vector. The initialization vector is not usedwith ECB mode but is important for CBC mode.See MODES OF OPERATION for details about cipher modes.
Use a prepared key acquired by calling Init to encrypt theprovided data. The data argument should be a binary array that is amultiple of the AES block size of 16 bytes. The result is a binaryarray the same size as the input of encrypted data.
Decipher data using the key. Note that the same key may be used toencrypt and decrypt data provided that the initialization vector isreset appropriately for CBC mode.
Reset the initialization vector. This permits the programmer to re-usea key and avoid the cost of re-generating the key schedule where thesame key data is being used multiple times.
This should be called to clean up resources associated with Key.Once this function has been called the key may not be used again.