SecureRandom  |  API reference  |  Android Developers (original) (raw)

public class SecureRandom
` extends [Random](/reference/java/util/Random) `

This class provides a cryptographically strong random number generator (RNG).

A cryptographically strong random number minimally complies with the statistical random number generator tests specified in FIPS 140-2, Security Requirements for Cryptographic Modules, section 4.9.1. Additionally, SecureRandom must produce non-deterministic output. Therefore any seed material passed to a SecureRandom object must be unpredictable, and all SecureRandom output sequences must be cryptographically strong, as described in RFC 4086: Randomness Requirements for Security.

Many SecureRandom implementations are in the form of a pseudo-random number generator (PRNG, also known as deterministic random bits generator or DRBG), which means they use a deterministic algorithm to produce a pseudo-random sequence from a random seed. Other implementations may produce true random numbers, and yet others may use a combination of both techniques.

A caller obtains a SecureRandom instance via the no-argument constructor or one of the getInstance methods. For example:

SecureRandom r1 = new SecureRandom(); SecureRandom r2 = SecureRandom.getInstance("NativePRNG"); SecureRandom r3 = SecureRandom.getInstance("DRBG", DrbgParameters.instantiation(128, RESEED_ONLY, null));

The third statement above returns a SecureRandom object of the specific algorithm supporting the specific instantiate parameters. The implementation's effective instantiated parameters must match this minimum request but is not necessarily the same. For example, even if the request does not require a certain feature, the actual instantiation can provide the feature. An implementation may lazily instantiate a SecureRandom until it's actually used, but the effective instantiate parameters must be determined right after it's created and [getParameters()](/reference/java/security/SecureRandom#getParameters%28%29) should always return the same result unchanged.

Typical callers of SecureRandom invoke the following methods to retrieve random bytes:

SecureRandom random = new SecureRandom(); byte[] bytes = new byte[20]; random.nextBytes(bytes);

Callers may also invoke the [generateSeed(int)](/reference/java/security/SecureRandom#generateSeed%28int%29) method to generate a given number of seed bytes (to seed other random number generators, for example):

byte[] seed = random.generateSeed(20);

A newly created PRNG SecureRandom object is not seeded (except if it is created by [SecureRandom(byte[])](/reference/java/security/SecureRandom#SecureRandom%28byte[]%29)). The first call tonextBytes will force it to seed itself from an implementation- specific entropy source. This self-seeding will not occur if setSeed was previously called.

A SecureRandom can be reseeded at any time by calling thereseed or setSeed method. The reseed method reads entropy input from its entropy source to reseed itself. The setSeed method requires the caller to provide the seed.

Please note that reseed may not be supported by allSecureRandom implementations.

Some SecureRandom implementations may accept a[SecureRandomParameters](/reference/java/security/SecureRandomParameters) parameter in its[nextBytes(byte[], java.security.SecureRandomParameters)](/reference/java/security/SecureRandom#nextBytes%28byte[],%20java.security.SecureRandomParameters%29) and[reseed(java.security.SecureRandomParameters)](/reference/java/security/SecureRandom#reseed%28java.security.SecureRandomParameters%29) methods to further control the behavior of the methods.

Note: Depending on the implementation, the generateSeed,reseed and nextBytes methods may block as entropy is being gathered, for example, if the entropy source is /dev/random on various Unix-like operating systems.

Thread safety

SecureRandom objects are safe for use by multiple concurrent threads.

Summary

Public constructors

SecureRandom

public SecureRandom ()

Constructs a secure random number generator (RNG) implementing the default random number algorithm.

This constructor traverses the list of registered security Providers, starting with the most preferred Provider. A new SecureRandom object encapsulating theSecureRandomSpi implementation from the first Provider that supports a SecureRandom (RNG) algorithm is returned. If none of the Providers support a RNG algorithm, then an implementation-specific default is returned.

Note that the list of registered providers may be retrieved via the [Security.getProviders()](/reference/java/security/Security#getProviders%28%29) method.

See the SecureRandom section in the Java Security Standard Algorithm Names Specification for information about standard RNG algorithm names.

SecureRandom

public SecureRandom (byte[] seed)

Constructs a secure random number generator (RNG) implementing the default random number algorithm. The SecureRandom instance is seeded with the specified seed bytes.

This constructor traverses the list of registered security Providers, starting with the most preferred Provider. A new SecureRandom object encapsulating theSecureRandomSpi implementation from the first Provider that supports a SecureRandom (RNG) algorithm is returned. If none of the Providers support a RNG algorithm, then an implementation-specific default is returned.

Note that the list of registered providers may be retrieved via the [Security.getProviders()](/reference/java/security/Security#getProviders%28%29) method.

See the SecureRandom section in the Java Security Standard Algorithm Names Specification for information about standard RNG algorithm names.

Protected constructors

SecureRandom

protected SecureRandom (SecureRandomSpi secureRandomSpi, Provider provider)

Creates a SecureRandom object.

Public methods

generateSeed

public byte[] generateSeed (int numBytes)

Returns the given number of seed bytes, computed using the seed generation algorithm that this class uses to seed itself. This call may be used to seed other random number generators.

getAlgorithm

public String getAlgorithm ()

Returns the name of the algorithm implemented by thisSecureRandom object.

getInstance

public static SecureRandom getInstance (String algorithm)

Returns a SecureRandom object that implements the specified Random Number Generator (RNG) algorithm.

This method traverses the list of registered security Providers, starting with the most preferred Provider. A new SecureRandom object encapsulating theSecureRandomSpi implementation from the first Provider that supports the specified algorithm is returned.

Note that the list of registered providers may be retrieved via the [Security.getProviders()](/reference/java/security/Security#getProviders%28%29) method.

Implementation Note:

• The JDK Reference Implementation additionally uses thejdk.security.provider.preferred [Security](/reference/java/security/Security#getProperty%28java.lang.String%29) property to determine the preferred provider order for the specified algorithm. This may be different than the order of providers returned by[Security.getProviders()](/reference/java/security/Security#getProviders%28%29).

getInstance

public static SecureRandom getInstance (String algorithm, String provider)

Returns a SecureRandom object that implements the specified Random Number Generator (RNG) algorithm.

A new SecureRandom object encapsulating theSecureRandomSpi implementation from the specified provider is returned. The specified provider must be registered in the security provider list.

Note that the list of registered providers may be retrieved via the [Security.getProviders()](/reference/java/security/Security#getProviders%28%29) method.

getInstance

public static SecureRandom getInstance (String algorithm, SecureRandomParameters params)

Returns a SecureRandom object that implements the specified Random Number Generator (RNG) algorithm and supports the specifiedSecureRandomParameters request.

This method traverses the list of registered security Providers, starting with the most preferred Provider. A new SecureRandom object encapsulating theSecureRandomSpi implementation from the first Provider that supports the specified algorithm and the specifiedSecureRandomParameters is returned.

Note that the list of registered providers may be retrieved via the [Security.getProviders()](/reference/java/security/Security#getProviders%28%29) method.

Implementation Note:

• The JDK Reference Implementation additionally uses thejdk.security.provider.preferred property to determine the preferred provider order for the specified algorithm. This may be different than the order of providers returned by[Security.getProviders()](/reference/java/security/Security#getProviders%28%29).

getInstance

public static SecureRandom getInstance (String algorithm, Provider provider)

Returns a SecureRandom object that implements the specified Random Number Generator (RNG) algorithm.

A new SecureRandom object encapsulating theSecureRandomSpi implementation from the specified Provider object is returned. Note that the specified Provider object does not have to be registered in the provider list.

getInstance

public static SecureRandom getInstance (String algorithm, SecureRandomParameters params, String provider)

Returns a SecureRandom object that implements the specified Random Number Generator (RNG) algorithm and supports the specifiedSecureRandomParameters request.

A new SecureRandom object encapsulating theSecureRandomSpi implementation from the specified provider is returned. The specified provider must be registered in the security provider list.

Note that the list of registered providers may be retrieved via the [Security.getProviders()](/reference/java/security/Security#getProviders%28%29) method.

getInstance

public static SecureRandom getInstance (String algorithm, SecureRandomParameters params, Provider provider)

Returns a SecureRandom object that implements the specified Random Number Generator (RNG) algorithm and supports the specifiedSecureRandomParameters request.

A new SecureRandom object encapsulating theSecureRandomSpi implementation from the specifiedProvider object is returned. Note that the specifiedProvider object does not have to be registered in the provider list.

getInstanceStrong

public static SecureRandom getInstanceStrong ()

Returns a SecureRandom object. In Android this is equivalent to get a SHA1PRNG from AndroidOpenSSL. Some situations require strong random values, such as when creating high-value/long-lived secrets like RSA public/private keys. To help guide applications in selecting a suitable strongSecureRandom implementation, Java distributions include a list of known strong SecureRandom implementations in the securerandom.strongAlgorithms Security property.

Every implementation of the Java platform is required to support at least one strong SecureRandom implementation.

getParameters

public SecureRandomParameters getParameters ()

Returns the effective [SecureRandomParameters](/reference/java/security/SecureRandomParameters) for thisSecureRandom instance.

The returned value can be different from theSecureRandomParameters object passed into a getInstance method, but it cannot change during the lifetime of thisSecureRandom object.

A caller can use the returned value to find out what features thisSecureRandom supports.

getProvider

public final Provider getProvider ()

Returns the provider of this SecureRandom object.

getSeed

public static byte[] getSeed (int numBytes)

Returns the given number of seed bytes, computed using the seed generation algorithm that this class uses to seed itself. This call may be used to seed other random number generators.

This method is only included for backwards compatibility. The caller is encouraged to use one of the alternativegetInstance methods to obtain a SecureRandom object, and then call the generateSeed method to obtain seed bytes from that object.

nextBytes

public void nextBytes (byte[] bytes, SecureRandomParameters params)

Generates a user-specified number of random bytes with additional parameters.

nextBytes

public void nextBytes (byte[] bytes)

Generates a user-specified number of random bytes.

reseed

public void reseed (SecureRandomParameters params)

Reseeds this SecureRandom with entropy input read from its entropy source with additional parameters.

Note that entropy is obtained from an entropy source. While some data in params may contain entropy, its main usage is to provide diversity.

reseed

public void reseed ()

Reseeds this SecureRandom with entropy input read from its entropy source.

setSeed

public void setSeed (long seed)

Reseeds this random object, using the eight bytes contained in the given long seed. The given seed supplements, rather than replaces, the existing seed. Thus, repeated calls are guaranteed never to reduce randomness.

This method is defined for compatibility withjava.util.Random.

setSeed

public void setSeed (byte[] seed)

Reseeds this random object with the given seed. The seed supplements, rather than replaces, the existing seed. Thus, repeated calls are guaranteed never to reduce randomness.

A PRNG SecureRandom will not seed itself automatically ifsetSeed is called before any nextBytes or reseed calls. The caller should make sure that the seed argument contains enough entropy for the security of this SecureRandom.

toString

public String toString ()

Returns a Human-readable string representation of thisSecureRandom.

Protected methods

next

protected final int next (int numBits)

Generates an integer containing the user-specified number of pseudo-random bits (right justified, with leading zeros). This method overrides a java.util.Random method, and serves to provide a source of random bits to all of the methods inherited from that class (for example, nextInt,nextLong, and nextFloat).