Both SRPServer and SRPClient extend the same base class and share the same group parameters and settings. Client and server must use the same parameters, hash algorithm, and padding setting. Otherwise the proof check fails at login.

Group Parameters

SRP computes all values modulo a large safe prime N, with a generator g and a multiplier k. The package provides a default, secure set of these values:

• SRPServer::DEFAULT_PRIME: The large safe prime N, in decimal form.
• SRPServer::DEFAULT_GENERATOR: The generator g, 2.
• SRPServer::DEFAULT_KEY: The multiplier k, in hexadecimal form.

create() uses these defaults when you pass no arguments. Use it unless you need to interoperate with another implementation that requires different values.

use Windwalker\SRP\SRPClient;
use Windwalker\SRP\SRPServer;

$server = SRPServer::create();
$client = SRPClient::create();

To use custom values, pass BigInteger objects to the constructor. The brick/math package provides the BigInteger class:

use Brick\Math\BigInteger;
use Windwalker\SRP\SRPServer;

$server = new SRPServer(
    BigInteger::of(SRPServer::DEFAULT_PRIME),
    BigInteger::of(SRPServer::DEFAULT_GENERATOR),
    BigInteger::fromBase(SRPServer::DEFAULT_KEY, 16),
);

TIP

Passing the default values explicitly, as above, produces the same result as SRPServer::create(). Use it as a starting point when you change only one of the values.

Configuration Array

Use createFromConfig() to build an instance from an array. Keys are prime, generator, key, hasher, and size. The prime and generator values are decimal strings, and key is a hexadecimal string. Each value can also be a BigInteger object:

use Windwalker\SRP\SRPServer;

$server = SRPServer::createFromConfig([
    'prime' => SRPServer::DEFAULT_PRIME,
    'generator' => SRPServer::DEFAULT_GENERATOR,
    'key' => SRPServer::DEFAULT_KEY,
    'hasher' => 'sha256',
    'size' => 256,
]);

WARNING

The string parsing differs from create(). create() reads all string arguments as hexadecimal, so pass BigInteger objects to create() when you use a decimal prime.

Secret Size

The random secrets a and b are generated with a fixed byte length. The default is 256 bits, which is 32 bytes. Change it with setSize(), which takes bits, or with setLength(), which takes bytes:

$server->setSize(512); // 512 bits, same as setLength(64)

$client->setLength(32); // 32 bytes, the default

Hash Algorithm

All hashes in the protocol use one hasher. The default is sha256. Call setHasher() to choose another algorithm:

• sha1, sha256, sha384, sha512
• blake2s-256
• blake2b-224, blake2b-256, blake2b-384, blake2b-512

BLAKE2 algorithms use the sodium extension (sodium_crypto_generichash), so it must be available.

$server->setHasher('sha512');
$client->setHasher('sha512');

You can also pass a closure. It receives the input string and must return a hexadecimal string, which the package parses as a number:

$hasher = fn(string $str): string => hash('sha3-256', $str);

$server->setHasher($hasher);
$client->setHasher($hasher);

WARNING

Use the same hasher on the server and the client. The package does not send the hasher name to the other side. A mismatch causes login to fail for every user.

Padding

By default, values are padded to the byte length of the prime before they are hashed. This matches RFC 5054. Some implementations do not pad before hashing. To interoperate with them, disable padding with enablePad():

$server->enablePad(false);
$client->enablePad(false);

$server->isPadEnabled(); // false

WARNING

Padding must match on both sides. If only one side disables it, the proof check fails. The failure reports an invalid proof, not a configuration error.

Chaining

All setters return the same instance, so you can chain them:

$server = SRPServer::create()
    ->setHasher('sha512')
    ->setSize(512)
    ->enablePad(true);