SecureRandom

public class SecureRandom
extends Random

java.lang.Object
   ↳ java.util.Random
     ↳ java.security.SecureRandom


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() 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) 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[])). The first call to nextBytes 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 the reseed 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 all SecureRandom implementations.

Some SecureRandom implementations may accept a SecureRandomParameters parameter in its nextBytes(byte[], java.security.SecureRandomParameters) and reseed(java.security.SecureRandomParameters) 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()

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

SecureRandom(byte[] seed)

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

Protected constructors

SecureRandom(SecureRandomSpi secureRandomSpi, Provider provider)

Creates a SecureRandom object.

Public methods

byte[] generateSeed(int numBytes)

Returns the given number of seed bytes, computed using the seed generation algorithm that this class uses to seed itself.

String getAlgorithm()

Returns the name of the algorithm implemented by this SecureRandom object.

static SecureRandom getInstance(String algorithm)

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

static SecureRandom getInstance(String algorithm, String provider)

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

static SecureRandom getInstance(String algorithm, SecureRandomParameters params)

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

static SecureRandom getInstance(String algorithm, Provider provider)

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

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 specified SecureRandomParameters request.

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 specified SecureRandomParameters request.

static SecureRandom getInstanceStrong()

Returns a SecureRandom object.

SecureRandomParameters getParameters()

Returns the effective SecureRandomParameters for this SecureRandom instance.

final Provider getProvider()

Returns the provider of this SecureRandom object.

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.

void nextBytes(byte[] bytes, SecureRandomParameters params)

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

void nextBytes(byte[] bytes)

Generates a user-specified number of random bytes.

void reseed(SecureRandomParameters params)

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

void reseed()

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

void setSeed(long seed)

Reseeds this random object, using the eight bytes contained in the given long seed.

void setSeed(byte[] seed)

Reseeds this random object with the given seed.

String toString()

Returns a Human-readable string representation of this SecureRandom.

Protected methods

final int next(int numBits)

Generates an integer containing the user-specified number of pseudo-random bits (right justified, with leading zeros).

Inherited methods

Public constructors

SecureRandom

Added in API level 1
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 the SecureRandomSpi 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() method.

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

SecureRandom

Added in API level 1
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 the SecureRandomSpi 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() method.

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

Parameters
seed byte: the seed.

Protected constructors

SecureRandom

Added in API level 1
protected SecureRandom (SecureRandomSpi secureRandomSpi, 
                Provider provider)

Creates a SecureRandom object.

Parameters
secureRandomSpi SecureRandomSpi: the SecureRandom implementation.

provider Provider: the provider.

Public methods

generateSeed

Added in API level 1
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.

Parameters
numBytes int: the number of seed bytes to generate.

Returns
byte[] the seed bytes.

Throws
IllegalArgumentException if numBytes is negative

getAlgorithm

Added in API level 1
public String getAlgorithm ()

Returns the name of the algorithm implemented by this SecureRandom object.

Returns
String the name of the algorithm or unknown if the algorithm name cannot be determined.

getInstance

Added in API level 1
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 the SecureRandomSpi 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() method.

Implementation Note:
  • The JDK Reference Implementation additionally uses the jdk.security.provider.preferred Security property to determine the preferred provider order for the specified algorithm. This may be different than the order of providers returned by Security.getProviders().
Parameters
algorithm String: the name of the RNG algorithm. See the SecureRandom section in the Java Security Standard Algorithm Names Specification for information about standard RNG algorithm names.

Returns
SecureRandom the new SecureRandom object

Throws
NoSuchAlgorithmException if no Provider supports a SecureRandomSpi implementation for the specified algorithm
NullPointerException if algorithm is null

See also:

getInstance

Added in API level 1
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 the SecureRandomSpi 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() method.

Parameters
algorithm String: the name of the RNG algorithm. See the SecureRandom section in the Java Security Standard Algorithm Names Specification for information about standard RNG algorithm names.

provider String: the name of the provider.

Returns
SecureRandom the new SecureRandom object

Throws
IllegalArgumentException if the provider name is null or empty
NoSuchAlgorithmException if a SecureRandomSpi implementation for the specified algorithm is not available from the specified provider
NoSuchProviderException if the specified provider is not registered in the security provider list
NullPointerException if algorithm is null

See also:

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 specified SecureRandomParameters request.

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

Note that the list of registered providers may be retrieved via the Security.getProviders() method.

Implementation Note:
  • The JDK Reference Implementation additionally uses the jdk.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().
Parameters
algorithm String: the name of the RNG algorithm. See the SecureRandom section in the Java Security Standard Algorithm Names Specification for information about standard RNG algorithm names.

params SecureRandomParameters: the SecureRandomParameters the newly created SecureRandom object must support.

Returns
SecureRandom the new SecureRandom object

Throws
IllegalArgumentException if the specified params is null
NoSuchAlgorithmException if no Provider supports a SecureRandomSpi implementation for the specified algorithm and parameters
NullPointerException if algorithm is null

See also:

getInstance

Added in API level 1
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 the SecureRandomSpi implementation from the specified Provider object is returned. Note that the specified Provider object does not have to be registered in the provider list.

Parameters
algorithm String: the name of the RNG algorithm. See the SecureRandom section in the Java Security Standard Algorithm Names Specification for information about standard RNG algorithm names.

provider Provider: the provider.

Returns
SecureRandom the new SecureRandom object

Throws
IllegalArgumentException if the specified provider is null
NoSuchAlgorithmException if a SecureRandomSpi implementation for the specified algorithm is not available from the specified Provider object
NullPointerException if algorithm is null

See also:

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 specified SecureRandomParameters request.

A new SecureRandom object encapsulating the SecureRandomSpi 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() method.

Parameters
algorithm String: the name of the RNG algorithm. See the SecureRandom section in the Java Security Standard Algorithm Names Specification for information about standard RNG algorithm names.

params SecureRandomParameters: the SecureRandomParameters the newly created SecureRandom object must support.

provider String: the name of the provider.

Returns
SecureRandom the new SecureRandom object

Throws
IllegalArgumentException if the provider name is null or empty, or params is null
NoSuchAlgorithmException if the specified provider does not support a SecureRandomSpi implementation for the specified algorithm and parameters
NoSuchProviderException if the specified provider is not registered in the security provider list
NullPointerException if algorithm is null

See also:

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 specified SecureRandomParameters request.

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

Parameters
algorithm String: the name of the RNG algorithm. See the SecureRandom section in the Java Security Standard Algorithm Names Specification for information about standard RNG algorithm names.

params SecureRandomParameters: the SecureRandomParameters the newly created SecureRandom object must support.

provider Provider: the provider.

Returns
SecureRandom the new SecureRandom object

Throws
IllegalArgumentException if the specified provider or params is null
NoSuchAlgorithmException if the specified provider does not support a SecureRandomSpi implementation for the specified algorithm and parameters
NullPointerException if algorithm is null

See also:

getInstanceStrong

Added in API level 26
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 strong SecureRandom 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.

Returns
SecureRandom a strong SecureRandom implementation

Throws
NoSuchAlgorithmException if no algorithm is available

getParameters

public SecureRandomParameters getParameters ()

Returns the effective SecureRandomParameters for this SecureRandom instance.

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

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

Returns
SecureRandomParameters the effective SecureRandomParameters parameters, or null if no parameters were used.

See also:

getProvider

Added in API level 1
public final Provider getProvider ()

Returns the provider of this SecureRandom object.

Returns
Provider the provider of this SecureRandom object.

getSeed

Added in API level 1
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 alternative getInstance methods to obtain a SecureRandom object, and then call the generateSeed method to obtain seed bytes from that object.

Parameters
numBytes int: the number of seed bytes to generate.

Returns
byte[] the seed bytes.

Throws
IllegalArgumentException if numBytes is negative

See also:

nextBytes

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

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

Parameters
bytes byte: the array to be filled in with random bytes

params SecureRandomParameters: additional parameters

Throws
NullPointerException if bytes is null
UnsupportedOperationException if the underlying provider implementation has not overridden this method
IllegalArgumentException if params is null, illegal or unsupported by this SecureRandom

nextBytes

Added in API level 1
public void nextBytes (byte[] bytes)

Generates a user-specified number of random bytes.

Parameters
bytes byte: the array to be filled in with 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.

Parameters
params SecureRandomParameters: extra parameters

Throws
UnsupportedOperationException if the underlying provider implementation has not overridden this method.
IllegalArgumentException if params is null, illegal or unsupported by this SecureRandom

reseed

public void reseed ()

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

Throws
UnsupportedOperationException if the underlying provider implementation has not overridden this method.

setSeed

Added in API level 1
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 with java.util.Random.

Parameters
seed long: the seed.

See also:

setSeed

Added in API level 1
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 if setSeed 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.

Parameters
seed byte: the seed.

See also:

toString

Added in API level 1
public String toString ()

Returns a Human-readable string representation of this SecureRandom.

Returns
String the string representation

Protected methods

next

Added in API level 1
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).

Parameters
numBits int: number of pseudo-random bits to be generated, where 0 <= numBits <= 32.

Returns
int an int containing the user-specified number of pseudo-random bits (right justified, with leading zeros).