SecureRandom

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 1750: Randomness Recommendations for Security.

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

      SecureRandom random = new SecureRandom();
 

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

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 method to generate a given number of seed bytes (to seed other random number generators, for example):

      byte seed[] = random.generateSeed(20);
 
Note: Depending on the implementation, the generateSeed and nextBytes methods may block as entropy is being gathered, for example, if they need to read from /dev/random on various unix-like operating systems. The SHA1PRNG algorithm from the Crypto provider has been deprecated as it was insecure, and also incorrectly used by some apps as a key derivation function. See Security "Crypto" provider deprecated in Android N for details.

Public Constructor Summary

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 Constructor Summary

SecureRandom(SecureRandomSpi secureRandomSpi, Provider provider)
Creates a SecureRandom object.

Public Method Summary

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.
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.
synchronized void
nextBytes(byte[] bytes)
Generates a user-specified number of random bytes.
void
setSeed(long seed)
Reseeds this random object, using the eight bytes contained in the given long seed.
synchronized void
setSeed(byte[] seed)
Reseeds this random object.

Protected Method Summary

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

Inherited Method Summary

Public Constructors

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 Cryptography Architecture Standard Algorithm Name Documentation for information about standard RNG algorithm names.

The returned SecureRandom object has not been seeded. To seed the returned object, call the setSeed method. If setSeed is not called, the first call to nextBytes will force the SecureRandom object to seed itself. This self-seeding will not occur if setSeed was previously called.

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 Cryptography Architecture Standard Algorithm Name Documentation for information about standard RNG algorithm names.

Parameters
seed the seed.

Protected Constructors

protected SecureRandom (SecureRandomSpi secureRandomSpi, Provider provider)

Creates a SecureRandom object.

Parameters
secureRandomSpi the SecureRandom implementation.
provider the provider.

Public Methods

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.

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.

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.

The returned SecureRandom object has not been seeded. To seed the returned object, call the setSeed method. If setSeed is not called, the first call to nextBytes will force the SecureRandom object to seed itself. This self-seeding will not occur if setSeed was previously called.

Parameters
algorithm the name of the RNG algorithm. See the SecureRandom section in the Java Cryptography Architecture Standard Algorithm Name Documentation 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.
See Also

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.

The returned SecureRandom object has not been seeded. To seed the returned object, call the setSeed method. If setSeed is not called, the first call to nextBytes will force the SecureRandom object to seed itself. This self-seeding will not occur if setSeed was previously called.

Parameters
algorithm the name of the RNG algorithm. See the SecureRandom section in the Java Cryptography Architecture Standard Algorithm Name Documentation for information about standard RNG algorithm names.
provider the name of the provider.
Returns
  • the new SecureRandom object.
Throws
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.
IllegalArgumentException if the provider name is null or empty.
See Also

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.

The returned SecureRandom object has not been seeded. To seed the returned object, call the setSeed method. If setSeed is not called, the first call to nextBytes will force the SecureRandom object to seed itself. This self-seeding will not occur if setSeed was previously called.

Parameters
algorithm the name of the RNG algorithm. See the SecureRandom section in the Java Cryptography Architecture Standard Algorithm Name Documentation for information about standard RNG algorithm names.
provider the provider.
Returns
  • the new SecureRandom object.
Throws
NoSuchAlgorithmException if a SecureRandomSpi implementation for the specified algorithm is not available from the specified Provider object.
IllegalArgumentException if the specified provider is null.
See Also

public final Provider getProvider ()

Returns the provider of this SecureRandom object.

Returns
  • the provider of this SecureRandom object.

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.
See Also

public synchronized void nextBytes (byte[] bytes)

Generates a user-specified number of random bytes.

If a call to setSeed had not occurred previously, the first call to this method forces this SecureRandom object to seed itself. This self-seeding will not occur if setSeed was previously called.

Parameters
bytes the array to be filled in with random bytes.

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 the seed.
See Also

public synchronized void setSeed (byte[] seed)

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

Parameters
seed the seed.
See Also

Protected Methods

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 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).