Random Number Generator (RNG) -- guide

• Overview

• Base classes and their derived classes

• Dependencies

The Security crypto component exposes the Random API whose broad purpose is the generation of cryptographically strong random numbers.

The Random number library is supplied by random.dll and the classes implementing the Random algorithms are provided by the Random Server, randsvr.exe (which is for internal use only).

Several cryptographic applications rely upon the randomness, unpredictability and irreproducibility of the random number generator, such as:

• one-time pads

• key generation

• random nonces

• Initialization Vectors (IVs)

• salts to be hashed with passwords

• unique parameters in signing operations.

The RNG uses the RANROT algorithm seeded by random data available on the target hardware (e.g. free running counters available on ARM processors).

In order to be fully compliant with DSS (Digital Signature Standard), applications using the cryptography library must supply a FIPS-186-2 CR 1 compliant random number generator. The library provides a mechanism for using such a random number generator if required.

The diagram below shows the main classes used in the RNG, which are implemented in random.dll. The colour of the boxes indicates the type of Symbian class, i.e., M, C, R or T class. For detailed information on each component see the Cryptography API Reference material.

TRandom is a cryptographically stong random number generator. Its declaration is:

class TRandom
    {
public:
    IMPORT_C static void Random(TDes8& aDestination);
    };

TRandom::Random() generates random bytes by first connecting to the random number generation server (using RRandomSession). It the attempt to connect fails, TRandom::Random() panics with a category of Randsvr connect.

The server then fills aDestination with randomly generated bytes up to its current length (not its maximum length). If this fails, TRandom::Random() panics with a category of Randsvr get. If aDestination is 1024 or more bytes long, multiple calls are made to the server. Finally, TRandom closes the session.

TRandom can be used like this:

HBufC8* rand = HBufC8::NewLC(5);
TPtr8 pRand=rand->Des();
pRand.SetLength(5);
TRandom::Random(pRand);
...
CleanupStack::PopAndDestroy(rand);

The diagram linked to below shows the inter-dependencies between random.dll and its clients:

• Dependency diagram for random.dll