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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:
Key generatorI just purchased a pirated cd of diablo 2 lod and i install it on the pc but it keeps on asking for a cd key.here in this country games. Use the -keyfile and -ivfile options to specify as a file or use the -key and -iv options to enter them at the command prompt. There is no limit on the length of the password. AES Key Generator. Home / AES Key Generator. AES Key Generator Devon 2020-04-17T12:05:59-07:00. Please enjoy your free AES Key! New(key, AES.MODECTR, counter=ctr) # Encrypt and return IV and ciphertext. This calls the system's random number # generator (RNG).
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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).
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.
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. 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]
From SO comments:
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
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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)
What is AES CBC
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AES-CBC (cipher block chaining) mode is one of the most used symmetric encryption algorithms. The data size must be nonzero and multiple of 16 bytes, which is the size of a “block”. The data is split into 16-byte blocks before encryption or decryption is started, then the operation is performed on each of the blocks. Each block is connected (“chained”) to the two blocks before and after it, respectively. i.e., a block takes a 16-byte IV (initialization vector) from its immediately previous block as input, and outputs a 16-byte IV to the block immediately following it. Of course, the 16-byte ciphertext is also output. The very first block of data (where its previous block does not exist) takes the IV as input from the user; the last block outputs the IV to the user.
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Very important note: choice of IV is critical to the security of CBC mode. The requirement is that the initial IV must be “unpredictable”, in other words, randomly generated before the encryption starts. Never use a hardcoded IV! Never pick the IV from a well-known database! Once encryption is completed, the value of IV is not a secret, and can be transmitted to the decryptor in plaintext.