# Class SecureRandom

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

All Implemented Interfaces:
  `Serializable, RandomGenerator`

public class SecureRandom extends 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()` 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[], SecureRandomParameters)` and `reseed(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.

Implementation Requirements:
  A `SecureRandom` service provider can advertise that it is thread-safe by setting the service provider attribute "ThreadSafe" to "true" when registering the provider. Otherwise, this class will instead synchronize access to the following methods of the `SecureRandomSpi` implementation:
  - `SecureRandomSpi.engineSetSeed(byte[])`
  - `SecureRandomSpi.engineNextBytes(byte[])`
  - `SecureRandomSpi.engineNextBytes(byte[], SecureRandomParameters)`
  - `SecureRandomSpi.engineGenerateSeed(int)`
  - `SecureRandomSpi.engineReseed(SecureRandomParameters)`
Since:
  1.1
External Specifications
  - RFC 4086: Randomness Requirements for Security
  - Security Requirements for Cryptographic Modules
  - Java Security Standard Algorithm Names
See Also:
  - `SecureRandomSpi`
  - `Random`
  - Serialized Form

- ## Nested Class Summary

  ### Nested classes/interfaces declared in interface RandomGenerator

  `RandomGenerator.ArbitrarilyJumpableGenerator, RandomGenerator.JumpableGenerator, RandomGenerator.LeapableGenerator, RandomGenerator.SplittableGenerator, RandomGenerator.StreamableGenerator`

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  ``
  `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`
  `SecureRandom(SecureRandomSpi secureRandomSpi, Provider provider)`
  Creates a `SecureRandom` object.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `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, Provider 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, 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 that was selected by using the algorithms/providers specified in the `securerandom.strongAlgorithms` `Security` property.
  `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.
  `protected final int`
  `next(int numBits)`
  Generates an integer containing the user-specified number of pseudo-random bits (right justified, with leading zeros).
  `void`
  `nextBytes(byte[] bytes)`
  Generates a user-specified number of random bytes.
  `void`
  `nextBytes(byte[] bytes, SecureRandomParameters params)`
  Generates a user-specified number of random bytes with additional parameters.
  `void`
  `reseed()`
  Reseeds this `SecureRandom` with entropy input read from its entropy source.
  `void`
  `reseed(SecureRandomParameters params)`
  Reseeds this `SecureRandom` with entropy input read from its entropy source with additional parameters.
  `void`
  `setSeed(byte[] seed)`
  Reseeds this random object with the given seed.
  `void`
  `setSeed(long seed)`
  Reseeds this random object, using the eight bytes contained in the given `long seed`.
  `String`
  `toString()`
  Returns a Human-readable string representation of this `SecureRandom`.

  ### Methods declared in class Random

  `doubles, doubles, doubles, doubles, from, ints, ints, ints, ints, longs, longs, longs, longs, nextBoolean, nextDouble, nextFloat, nextGaussian, nextInt, nextInt, nextLong`

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

  ### Methods declared in interface RandomGenerator

  `equiDoubles, isDeprecated, nextDouble, nextDouble, nextExponential, nextFloat, nextFloat, nextGaussian, nextInt, nextLong, nextLong`

- ## Constructor Details

  - ### 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 the `SecureRandomSpi` implementation from the first provider that supports a `SecureRandom` (RNG) algorithm is returned. If none of the providers support an 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.

    External Specifications
      - Java Security Standard 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 the `SecureRandomSpi` implementation from the first provider that supports a `SecureRandom` (RNG) algorithm is returned. If none of the providers support an 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` - the seed.
    Throws:
      `NullPointerException` - if `seed` is `null`
    External Specifications
      - Java Security Standard Algorithm Names

  - ### SecureRandom

    protected SecureRandom(SecureRandomSpi secureRandomSpi, Provider provider)
    Creates a `SecureRandom` object.
    Parameters:
      `secureRandomSpi` - the `SecureRandom` implementation.
      `provider` - the provider.

- ## Method Details

  - ### getInstance

    public static SecureRandom getInstance(String algorithm) throws NoSuchAlgorithmException
    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 from the order of providers returned by `Security.getProviders()`.
    Parameters:
      `algorithm` - 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:
      the new `SecureRandom` object
    Throws:
      `NoSuchAlgorithmException` - if no `Provider` supports a `SecureRandomSpi` implementation for the specified algorithm
      `NullPointerException` - if `algorithm` is `null`
    Since:
      1.2
    External Specifications
      - Java Security Standard Algorithm Names
    See Also:
      - `Provider`

  - ### getInstance

    public static SecureRandom getInstance(String algorithm, String provider) throws NoSuchAlgorithmException, NoSuchProviderException
    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` - 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` - the name of the provider.
    Returns:
      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`
    Since:
      1.2
    External Specifications
      - Java Security Standard Algorithm Names
    See Also:
      - `Provider`

  - ### getInstance

    public static SecureRandom getInstance(String algorithm, Provider provider) throws NoSuchAlgorithmException
    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. Note that the specified provider does not have to be registered in the provider list.

    Parameters:
      `algorithm` - 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` - the provider.
    Returns:
      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`
    Since:
      1.4
    External Specifications
      - Java Security Standard Algorithm Names
    See Also:
      - `Provider`

  - ### getInstance

    public static SecureRandom getInstance(String algorithm, SecureRandomParameters params) throws NoSuchAlgorithmException
    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 from the order of providers returned by `Security.getProviders()`.
    Parameters:
      `algorithm` - 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` - the `SecureRandomParameters` the newly created `SecureRandom` object must support.
    Returns:
      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`
    Since:
      9
    External Specifications
      - Java Security Standard Algorithm Names
    See Also:
      - `Provider`

  - ### getInstance

    public static SecureRandom getInstance(String algorithm, SecureRandomParameters params, String provider) throws NoSuchAlgorithmException, NoSuchProviderException
    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` - 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` - the `SecureRandomParameters` the newly created `SecureRandom` object must support.
      `provider` - the name of the provider.
    Returns:
      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`
    Since:
      9
    External Specifications
      - Java Security Standard Algorithm Names
    See Also:
      - `Provider`

  - ### getInstance

    public static SecureRandom getInstance(String algorithm, SecureRandomParameters params, Provider provider) throws NoSuchAlgorithmException
    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. Note that the specified provider does not have to be registered in the provider list.

    Parameters:
      `algorithm` - 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` - the `SecureRandomParameters` the newly created `SecureRandom` object must support.
      `provider` - the provider.
    Returns:
      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`
    Since:
      9
    External Specifications
      - Java Security Standard Algorithm Names
    See Also:
      - `Provider`

  - ### getProvider

    public final Provider getProvider()
    Returns the provider of this `SecureRandom` object.
    Returns:
      the provider of this `SecureRandom` object.

  - ### getAlgorithm

    public String getAlgorithm()
    Returns the name of the algorithm implemented by this `SecureRandom` object.
    Returns:
      the name of the algorithm or `unknown` if the algorithm name cannot be determined.
    Since:
      1.5

  - ### toString

    public String toString()
    Returns a Human-readable string representation of this `SecureRandom`.
    Overrides:
      `toString` in class `Object`
    Returns:
      the string representation

  - ### 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:
      the effective `SecureRandomParameters` parameters, or `null` if no parameters were used.
    Since:
      9
    See Also:
      - `SecureRandomSpi`

  - ### 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 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` - the seed.
    Throws:
      `NullPointerException` - if `seed` is `null`
    See Also:
      - `getSeed(int)`

  - ### 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.

    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`.

    This method is defined for compatibility with `java.util.Random`.

    Overrides:
      `setSeed` in class `Random`
    Parameters:
      `seed` - the seed.
    See Also:
      - `getSeed(int)`

  - ### nextBytes

    public void nextBytes(byte[] bytes)
    Generates a user-specified number of random bytes.
    Specified by:
      `nextBytes` in interface `RandomGenerator`
    Overrides:
      `nextBytes` in class `Random`
    Parameters:
      `bytes` - the array to be filled in with random bytes.
    Throws:
      `NullPointerException` - if `bytes` is `null`

  - ### nextBytes

    public void nextBytes(byte[] bytes, SecureRandomParameters params)
    Generates a user-specified number of random bytes with additional parameters.
    Parameters:
      `bytes` - the array to be filled in with random bytes
      `params` - 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`
    Since:
      9

  - ### 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 the methods inherited from that class (for example, `nextInt`, `nextLong`, and `nextFloat`).
    Overrides:
      `next` in class `Random`
    Parameters:
      `numBits` - number of pseudo-random bits to be generated, where `0 <= numBits <= 32`.
    Returns:
      an `int` containing the user-specified number of pseudo-random bits (right justified, with leading zeros).

  - ### 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 alternative `getInstance` methods to obtain a `SecureRandom` object, and then call the `generateSeed` method to obtain seed bytes from that object.

    Parameters:
      `numBytes` - the number of seed bytes to generate.
    Returns:
      the seed bytes.
    Throws:
      `IllegalArgumentException` - if `numBytes` is negative
    See Also:
      - `setSeed(byte[])`

  - ### 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.
    Parameters:
      `numBytes` - the number of seed bytes to generate.
    Returns:
      the seed bytes.
    Throws:
      `IllegalArgumentException` - if `numBytes` is negative

  - ### getInstanceStrong

    public static SecureRandom getInstanceStrong() throws NoSuchAlgorithmException
    Returns a `SecureRandom` object that was selected by using the algorithms/providers specified in the `securerandom.strongAlgorithms` `Security` property.

    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:
      a strong `SecureRandom` implementation as indicated by the `securerandom.strongAlgorithms` Security property
    Throws:
      `NoSuchAlgorithmException` - if no algorithm is available
    Since:
      1.8
    See Also:
      - `Security.getProperty(String)`

  - ### 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.
    Since:
      9

  - ### 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` - 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`
    Since:
      9