Initialization Vector
Documentation 
#include <cryptopp/cryptlib.h>

An Initialization Vector or IV is an input to a cryptographic algorithm used to provide the initial state. The IV is part of an algorithm's security parameters, including key and other state. The IV is typically random or pseudorandom, but sometimes an IV only needs to be unpredictable or unique.
Initialization Vectors are usually considered public information, but they can be kept secret. The IV used to create the ciphertext should be included in a mac of the ciphertext. Also see Authenticated Encryption.
A Nonce is an IV that is guaranteed to be unique. It is usually implemented as a persistent counter. Also see CTR mode.
Crypto++ provides the SimpleKeyingInterface class and several class methods to determine the IV requirements. The functions include IVSize
, DefaultIVLength
, MinIVLength
, MaxIVLength
, IVRequirement
, CanUseRandomIVs
, anUsePredictableIVs
, and CanUseStructuredIVs
.
To set a new IV on a keyed cipher use Resynchronize
. To generate a new IV call GetNextIV
. When using GetNextIV
, each class can implement a different way to generate an IV based on IVRequirement
.
IVRequirement
IVRequirement
is part of SimpleKeyingInterface
. It is used to determine what kind of IV is needed to key an algorithm. The values returned by IVRequirement
is an enumeration and listed below.
Value  Meaning 

UNIQUE_IV  The IV must be unique. 
RANDOM_IV  The IV must be random and possibly predictable. 
UNPREDICTABLE_RANDOM_IV  The IV must be random and unpredictable. 
INTERNALLY_GENERATED_IV  The IV is set by the object. 
NOT_RESYNCHRONIZABLE  The object does not use an IV. 
IV Generation
You can use GetNextIV
to generate an IV using code similar to below. The buffer size must be at least IVSize
in length. The cipher class must implement SimpleKeyingInterface
. The SecByteBlock is used for secure storage. The AutoSeededRandomPool is a random number generator.
AES::Encryption enc; SecByteBlock key(enc.DefaultKeyLength()), iv(enc.IVSize()); AutoSeededRandomPool prng; prng.GenerateBlock(key, key.size()); enc.GetNextIV(prng, iv); enc.SetKeyWithIV(key, key.size(), iv);
If you want an UNPREDICTABLE_RANDOM_IV
then you can use the AutoSeededRandomPool
directly as shown below.
AutoSeededRandomPool prng; prng.GenerateBlock(iv, iv.size());
Authenticated Encryption
Authenticated Encryption provides authenticity assurances over messages. If your application uses an IV, then the IV should be included in the MAC calculation over the ciphertext. That is, the MAC should be calculated as [math]\displaystyle{ mac = Mac_k(iv  ciphertext) }[/math], where [math]\displaystyle{ ciphertext }[/math] is the result of an encryption operation. Then send the 3tuple [math]\displaystyle{ (iv, ciphertext, mac) }[/math] to the other party.