Pseudorandom Generator (PRG)

A pseudorandom generator (PRG) is a deterministic algorithm that takes a “short” uniformly distributed string, known as the seed, and outputs a longer string that cannot be efficiently distinguished from a uniformly distributed string of that length.

The PseudorandomGenerator abstract class

The main function of this class is getPrgBytes(). It streams the prg bytes and return the reauired amount of pseudo random bytes:

void PseudorandomGenerator::getPRGBytes(vector<byte>& outBytes, int outOffset, int outlen)

Streams the prg bytes.

Parameters:

• outBytes – output bytes. The result of streaming the bytes.
• outOffset – output offset
• outlen – the required output length

Setting the Secret Key

SecretKey PseudorandomGenerator::generateKey(AlgorithmParameterSpec& keyParams)

Generates a secret key to initialize this prg object.

Parameters keyParams:  algorithmParameterSpec contains the required parameters for the key generation Returns:the generated secret key

SecretKey PseudorandomGenerator::generateKey(int keySize)

Generates a secret key to initialize this prg object.

Parameters keySize:  is the required secret key size in bits Returns:the generated secret key

bool PseudorandomGenerator::isKeySet()

An object trying to use an instance of prg needs to check if it has already been initialized.

Returns:true if the object was initialized by calling the function setKey.

void PseudorandomGenerator::setKey(SecretKey& secretKey)

Sets the secret key for this prg. The key can be changed at any time.

Parameters secretKey:  secret key

Basic Usage

//Create secret key and out byte vector
...

//Create a prg
PseudorandomGenerator* prg = new PrgFromOpenSSLAES();
SecretKey secretKey = prg->generateKey(256); //256 is the key size in bits.

//set the key
prg->setKey(secretKey);

//get PRG bytes. The caller is responsible for allocating the out array.
//The result will be put in the out array.
prg->getPRGBytes(out.length, out);

Supported Prg Types

In this section we present the prg functions provided by libscapi.

Class Name	Class Location
ScPrgFromPrf	libscapi/include/primitives/Prg.hpp
PrgFromOpenSSLAES	libscapi/include/primitives/Prf.hpp

The OpenSSL implementation:

Class Name	Class Location
OpenSSLRC4	libscapi/include/primitives/Prg.hpp