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.
See Also
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
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.
|
static SecureRandom |
getInstanceStrong()
Returns a
SecureRandom object. |
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 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
- a strong
SecureRandom
implementation
Throws
NoSuchAlgorithmException | if no algorithm is available |
---|
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).