Add authproc-filters to generate Subject-id & Pairwise-id (#1435)

* Add SubjectID authproc

* Add PairwiseID authproc

* Add tests

modules/core/docs/authproc_pairwiseid.md

+`core:PairwiseID`
+===================
+
+Filter to insert a pairwise-id that complies with the following specification;
+http://docs.oasis-open.org/security/saml-subject-id-attr/v1.0/saml-subject-id-attr-v1.0.pdf
+
+This filter will take an attribute and a scope as input and transforms this into a anonymized and scoped
+identifier that is globally unique for a given user & service provider combination.
+
+Examples
+--------
+
+    'authproc' => [
+        50 => [
+            'class' => 'core:PairwiseID',
+            'identifyingAttribute' => 'uid',
+            'scope' => 'example.org',
+        ],
+    ],

modules/core/docs/authproc_subjectid.md

+`core:SubjectID`
+===================
+
+Filter to insert a subject-id that complies with the following specification;
+http://docs.oasis-open.org/security/saml-subject-id-attr/v1.0/saml-subject-id-attr-v1.0.pdf
+
+This filter will take an attribute and a scope as input and transforms this into a scoped identifier that is globally unique for a given user.
+
+Note:
+-----
+If privacy is of your concern, you may want to use the PairwiseID-filter instead.
+
+Examples
+--------
+
+    'authproc' => [
+        50 => [
+            'class' => 'core:SubjectID',
+            'identifyingAttribute' => 'uid',
+            'scope' => 'example.org',
+        ],
+    ],

modules/core/lib/Auth/Process/PairwiseID.php

+<?php
+
+declare(strict_types=1);
+
+namespace SimpleSAML\Module\core\Auth\Process;
+
+use Exception;
+use SAML2\Constants;
+use SAML2\XML\saml\NameID;
+use SimpleSAML\Assert\Assert;
+use SimpleSAML\Auth;
+use SimpleSAML\Utils;
+
+/**
+ * Filter to generate the Pairwise ID attribute.
+ *
+ * See: http://docs.oasis-open.org/security/saml-subject-id-attr/v1.0/csprd01/saml-subject-id-attr-v1.0-csprd01.html
+ *
+ * By default, this filter will generate the ID based on the UserID of the current user.
+ * This is generated from the attribute configured in 'identifyingAttribute' in the
+ * authproc-configuration.
+ *
+ * Example - generate from attribute:
+ * <code>
+ * 'authproc' => [
+ *   50 => [
+ *       'core:PairwiseID',
+ *       'identifyingAttribute' => 'uid',
+ *       'scope' => 'example.org',
+ *   ]
+ * ]
+ * </code>
+ *
+ * @package SimpleSAMLphp
+ */
+class PairwiseID extends SubjectID
+{
+    /**
+     * The name for this class
+     */
+    public const NAME = 'PairwiseID';
+
+    /**
+     * @var \SimpleSAML\Utils\Config
+     */
+    protected $configUtils;
+
+
+    /**
+     * Initialize this filter.
+     *
+     * @param array &$config  Configuration information about this filter.
+     * @param mixed $reserved  For future use.
+     */
+    public function __construct(array &$config, $reserved)
+    {
+        parent::__construct($config, $reserved);
+
+        $this->configUtils = new Utils\Config();
+    }
+
+
+    /**
+     * Apply filter to add the Pairwise ID.
+     *
+     * @param array &$state  The current state.
+     */
+    public function process(array &$state): void
+    {
+        $userID = $this->getIdentifyingAttribute($state);
+
+        if (!empty($state['saml:RequesterID'])) {
+            // Proxied request - use actual SP entity ID
+            $sp_entityid = $state['saml:RequesterID'][0];
+        } else {
+            $sp_entityid = $state['core:SP'];
+        }
+
+        // Calculate hash
+        $salt = $this->configUtils->getSecretSalt();
+        $hash = hash('sha256', $salt . '|' . $userID . '|' . $sp_entityid, false);
+
+        $value = strtolower($hash . '@' . $this->scope);
+        $this->validateGeneratedIdentifier($value);
+
+        $state['Attributes'][Constants::ATTR_PAIRWISE_ID] = [$value];
+    }
+
+
+    /**
+     * Inject the \SimpleSAML\Utils\Config dependency.
+     *
+     * @param \SimpleSAML\Utils\Config $configUtils
+     */
+    public function setConfigUtils(Utils\Config $configUtils): void
+    {
+        $this->configUtils = $configUtils;
+    }
+}

modules/core/lib/Auth/Process/SubjectID.php

+<?php
+
+declare(strict_types=1);
+
+namespace SimpleSAML\Module\core\Auth\Process;
+
+use Exception;
+use SAML2\Constants;
+//use SAML2\Exception\ProtocolViolationException
+use SAML2\XML\saml\NameID;
+use SimpleSAML\Assert\Assert;
+use SimpleSAML\Auth;
+use SimpleSAML\Logger;
+
+/**
+ * Filter to generate the subject ID attribute.
+ *
+ * See: http://docs.oasis-open.org/security/saml-subject-id-attr/v1.0/csprd01/saml-subject-id-attr-v1.0-csprd01.html
+ *
+ * By default, this filter will generate the ID based on the UserID of the current user.
+ * This is generated from the attribute configured in 'identifyingAttribute' in the
+ * authproc-configuration.
+ *
+ * Example - generate from attribute:
+ * <code>
+ * 'authproc' => [
+ *   50 => [
+ *       'core:SubjectID',
+ *       'identifyingAttribute' => 'uid',
+ *       'scope' => 'example.org',
+ *   ]
+ * ]
+ * </code>
+ *
+ * @package SimpleSAMLphp
+ */
+class SubjectID extends Auth\ProcessingFilter
+{
+    /**
+     * The name for this class
+     */
+    public const NAME = 'SubjectID';
+
+    /**
+     * The regular expression to match the scope
+     *
+     * @var string
+     */
+    public const SCOPE_PATTERN = '/^[a-z0-9][a-z0-9.-]{0,126}$/i';
+
+    /**
+     * The regular expression to match the specifications
+     *
+     * @var string
+     */
+    public const SPEC_PATTERN = '/^[a-z0-9][a-z0-9=-]{0,126}@[a-z0-9][a-z0-9.-]{0,126}$/i';
+
+    /**
+     * The regular expression to match worrisome identifiers that need to raise a warning
+     *
+     * @var string
+     */
+    public const WARN_PATTERN = '/^[a-z0-9][a-z0-9=-]{3,126}@[a-z0-9][a-z0-9.-]{3,126}$/i';
+
+    /**
+     * The attribute we should generate the subject id from.
+     *
+     * @var string
+     */
+    protected string $identifyingAttribute;
+
+    /**
+     * The scope to use for this attribute.
+     *
+     * @var string
+     */
+    protected string $scope;
+
+    /**
+     * @var \SimpleSAML\Logger|string
+     * @psalm-var \SimpleSAML\Logger|class-string
+     */
+    protected $logger = Logger::class;
+
+
+    /**
+     * Initialize this filter.
+     *
+     * @param array &$config  Configuration information about this filter.
+     * @param mixed $reserved  For future use.
+     */
+    public function __construct(array &$config, $reserved)
+    {
+        parent::__construct($config, $reserved);
+
+        Assert::keyExists($config, 'identifyingAttribute', "Missing mandatory 'identifyingAttribute' config setting.");
+        Assert::keyExists($config, 'scope', "Missing mandatory 'scope' config setting.");
+        Assert::stringNotEmpty($config['identifyingAttribute']);
+        Assert::regex(
+            $config['scope'],
+            self::SCOPE_PATTERN,
+            'core:' . static::NAME . ': \'scope\' contains illegal characters.'
+//            ProtocolViolationException::class
+        );
+
+        $this->identifyingAttribute = $config['identifyingAttribute'];
+        $this->scope = $config['scope'];
+    }
+
+
+    /**
+     * Apply filter to add the subject ID.
+     *
+     * @param array &$state  The current state.
+     */
+    public function process(array &$state): void
+    {
+        $userID = $this->getIdentifyingAttribute($state);
+
+        $value = strtolower($userID . '@' . $this->scope);
+        $this->validateGeneratedIdentifier($value);
+
+        $state['Attributes'][Constants::ATTR_SUBJECT_ID] = [$value];
+    }
+
+
+    /**
+     * Retrieve the identifying attribute from the state and test it for erroneous conditions
+     *
+     * @param array $state
+     * @return string
+     * @throws \SimpleSAML\Assert\AssertionFailedException if the pre-conditions are not met
+     */
+    protected function getIdentifyingAttribute(array $state): string
+    {
+        Assert::keyExists($state, 'Attributes');
+        Assert::keyExists(
+            $state['Attributes'],
+            $this->identifyingAttribute,
+            sprintf(
+                "core:" . static::NAME . ": Missing attribute '%s', which is needed to generate the ID.",
+                $this->identifyingAttribute
+            )
+        );
+
+        $userID = $state['Attributes'][$this->identifyingAttribute][0];
+        Assert::stringNotEmpty($userID, 'core' . static::NAME . ': \'identifyingAttribute\' cannot be an empty string.');
+
+        return $userID;
+    }
+
+
+    /**
+     * Test the generated identifier to ensure compliancy with the specifications.
+     * Log a warning when the generated value is considered to be weak
+     *
+     * @param string $value
+     * @return void
+     * @throws \SimpleSAML\Assert\AssertionFailedException if the post-conditions are not met
+     */
+    protected function validateGeneratedIdentifier(string $value): void
+    {
+        Assert::regex(
+            $value,
+            self::SPEC_PATTERN,
+            'core:' . static::NAME . ': Generated ID \'' . $value . '\' contains illegal characters.'
+//            ProtocolViolationException::class
+        );
+
+        if (preg_match(self::WARN_PATTERN, $value) === 0) {
+            $this->logger::warning('core:' . static::NAME . ': Generated ID \'' . $value . '\' can hardly be considered globally unique.');
+        }
+    }
+
+
+    /**
+     * Inject the \SimpleSAML\Logger dependency.
+     *
+     * @param \SimpleSAML\Logger $logger
+     */
+    public function setLogger(Logger $logger): void
+    {
+        $this->logger = $logger;
+    }
+}

tests/modules/core/lib/Auth/Process/PairwiseIDTest.php

+<?php
+
+declare(strict_types=1);
+
+namespace SimpleSAML\Test\Module\core\Auth\Process;
+
+use PHPUnit\Framework\TestCase;
+use RuntimeException;
+use SAML2\Constants;
+use SAML2\XML\saml\NameID;
+use SimpleSAML\Assert\AssertionFailedException;
+use SimpleSAML\Configuration;
+use SimpleSAML\Logger;
+use SimpleSAML\Module\core\Auth\Process\PairwiseID;
+use SimpleSAML\Utils;
+
+/**
+ * Test for the core:PairwiseID filter.
+ *
+ * @covers \SimpleSAML\Module\core\Auth\Process\PairwiseID
+ */
+class PairwiseIDTest extends TestCase
+{
+    /** @var \SimpleSAML\Configuration */
+    protected Configuration $config;
+
+    /** @var \SimpleSAML\Utils\Config */
+    protected static Utils\Config $configUtils;
+
+    /** @var \SimpleSAML\Logger */
+    protected static Logger $logger;
+
+
+    /**
+     * Set up for each test.
+     */
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        self::$configUtils = new class () extends Utils\Config {
+            public function getSecretSalt(): string
+            {
+                // stub
+                return 'secretsalt';
+            }
+        };
+
+        self::$logger = new class () extends Logger {
+            public static function warning(string $string): void
+            {
+                // stub
+                throw new RuntimeException($string);
+            }
+        };
+    }
+
+
+    /**
+     * Helper function to run the filter with a given configuration.
+     *
+     * @param array $config  The filter configuration.
+     * @param array $request  The request state.
+     * @return array  The state array after processing.
+     */
+    private static function processFilter(array $config, array $request): array
+    {
+        $filter = new PairwiseID($config, null);
+        $filter->setConfigUtils(self::$configUtils);
+        $filter->setLogger(self::$logger);
+        $filter->process($request);
+        return $request;
+    }
+
+
+    /**
+     * Test the most basic functionality
+     */
+    public function testBasic(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'ex-ample.org'];
+        $request = [
+            'Attributes' => ['uid' => ['u=se-r2']],
+            'core:SP' => 'urn:sp',
+        ];
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_PAIRWISE_ID, $attributes);
+        $this->assertMatchesRegularExpression(
+            PairwiseID::SPEC_PATTERN,
+            $attributes[Constants::ATTR_PAIRWISE_ID][0]
+        );
+        $this->assertEquals(
+            '53d4f7fe57fb597ada481e81e0f15048bc610774cbb5614ea38f08ea918ba199@ex-ample.org',
+            $attributes[Constants::ATTR_PAIRWISE_ID][0]
+        );
+    }
+
+
+    /**
+     * Test the most basic functionality on proxied request
+     */
+    public function testBasicProxiedRequest(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'ex-ample.org'];
+        $request = [
+            'Attributes' => ['uid' => ['u=se-r2']],
+            'saml:RequesterID' => [0 => 'urn:sp'],
+        ];
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_PAIRWISE_ID, $attributes);
+        $this->assertMatchesRegularExpression(
+            PairwiseID::SPEC_PATTERN,
+            $attributes[Constants::ATTR_PAIRWISE_ID][0]
+        );
+        $this->assertEquals(
+            '53d4f7fe57fb597ada481e81e0f15048bc610774cbb5614ea38f08ea918ba199@ex-ample.org',
+            $attributes[Constants::ATTR_PAIRWISE_ID][0]
+        );
+    }
+
+
+    /**
+     * Test the proxied request with multiple hops
+     */
+    public function testProxiedRequestMultipleHops(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'ex-ample.org'];
+        $request = [
+            'Attributes' => ['uid' => ['u=se-r2']],
+            'saml:RequesterID' => [0 => 'urn:sp', 1 => 'urn:some:sp', 2 => 'urn:some:other:sp'],
+        ];
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_PAIRWISE_ID, $attributes);
+        $this->assertMatchesRegularExpression(
+            PairwiseID::SPEC_PATTERN,
+            $attributes[Constants::ATTR_PAIRWISE_ID][0]
+        );
+        $this->assertEquals(
+            '53d4f7fe57fb597ada481e81e0f15048bc610774cbb5614ea38f08ea918ba199@ex-ample.org',
+            $attributes[Constants::ATTR_PAIRWISE_ID][0]
+        );
+    }
+
+
+    /**
+     * Test that illegal characters in scope throws an exception.
+     */
+    public function testScopeIllegalCharacterThrowsException(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'ex%ample.org'];
+        $request = [
+            'Attributes' => ['uid' => ['user2']],
+            'core:SP' => 'urn:sp',
+        ];
+
+        $this->expectException(AssertionFailedException::class);
+        self::processFilter($config, $request);
+    }
+
+
+    /**
+     * Test that generated ID's for the same user, but different SP's are NOT equal
+     */
+    public function testUniqueIdentifierPerSPSameUser(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'example.org'];
+        $request = [
+            'Attributes' => ['uid' => ['user1']],
+            'core:SP' => 'urn:sp',
+        ];
+
+        // Generate first ID
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_PAIRWISE_ID, $attributes);
+        $value1 = $attributes[Constants::ATTR_PAIRWISE_ID][0];
+
+        // Switch SP
+        $request['core:SP'] = 'urn:some:other:sp';
+
+        // Generate second ID
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_PAIRWISE_ID, $attributes);
+        $value2 = $attributes[Constants::ATTR_PAIRWISE_ID][0];
+
+        $this->assertNotSame($value1, $value2);
+    }
+
+
+    /**
+     * Test that generated ID's for different users, but the same SP's are NOT equal
+     */
+    public function testUniqueIdentifierPerUserSameSP(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'example.org'];
+        $request = [
+            'Attributes' => ['uid' => ['user1']],
+            'core:SP' => 'urn:sp',
+        ];
+
+        // Generate first ID
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_PAIRWISE_ID, $attributes);
+        $value1 = $attributes[Constants::ATTR_PAIRWISE_ID][0];
+
+        // Switch user
+        $request['Attributes'] = ['uid' => ['user2']];
+
+        // Generate second ID
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_PAIRWISE_ID, $attributes);
+        $value2 = $attributes[Constants::ATTR_PAIRWISE_ID][0];
+
+        $this->assertNotSame($value1, $value2);
+    }
+
+
+    /**
+     * Test that generated ID's for the same user and same SP, but with a different salt are NOT equal
+     */
+    public function testUniqueIdentifierDifferentSalts(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'example.org'];
+        $request = [
+            'Attributes' => ['uid' => ['user1']],
+            'core:SP' => 'urn:sp',
+        ];
+
+        // Generate first ID
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_PAIRWISE_ID, $attributes);
+        $value1 = $attributes[Constants::ATTR_PAIRWISE_ID][0];
+
+        // Change the salt
+        self::$configUtils = new class () extends Utils\Config {
+            public function getSecretSalt(): string
+            {
+                // stub
+                return 'pepper';
+            }
+        };
+
+        // Generate second ID
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_PAIRWISE_ID, $attributes);
+        $value2 = $attributes[Constants::ATTR_PAIRWISE_ID][0];
+
+        $this->assertNotSame($value1, $value2);
+    }
+
+
+    /**
+     * Test that generated ID's for the same user and same SP, but with a different scope are NOT equal
+     */
+    public function testUniqueIdentifierDifferentScopes(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'example.org'];
+        $request = [
+            'Attributes' => ['uid' => ['user1']],
+            'core:SP' => 'urn:sp',
+        ];
+
+        // Generate first ID
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_PAIRWISE_ID, $attributes);
+        $value1 = $attributes[Constants::ATTR_PAIRWISE_ID][0];
+
+        // Change the scope
+        $config['scope'] = 'example.edu';
+
+        // Generate second ID
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_PAIRWISE_ID, $attributes);
+        $value2 = $attributes[Constants::ATTR_PAIRWISE_ID][0];
+
+        $this->assertNotSame($value1, $value2);
+
+        $this->assertMatchesRegularExpression(
+            '/@example.org$/i',
+            $value1
+        );
+        $this->assertMatchesRegularExpression(
+            '/@example.edu$/i',
+            $value2
+        );
+    }
+
+
+    /**
+     * Test that weak identifiers log a warning
+     */
+    public function testWeakIdentifierLogsWarning(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'b'];
+        $request = [
+            'Attributes' => ['uid' => ['a']],
+            'core:SP' => 'urn:sp',
+        ];
+
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage(
+            'core:PairwiseID: Generated ID \'c5b54935db5e291a6b94688921fa77ced8ce425ce8c61a448bd4997f494dbebe@b\' can hardly be considered globally unique.'
+        );
+
+        self::processFilter($config, $request);
+    }
+}

tests/modules/core/lib/Auth/Process/SubjectIDTest.php

+<?php
+
+declare(strict_types=1);
+
+namespace SimpleSAML\Test\Module\core\Auth\Process;
+
+use PHPUnit\Framework\TestCase;
+use RuntimeException;
+use SAML2\Constants;
+use SAML2\XML\saml\NameID;
+use SimpleSAML\Assert\AssertionFailedException;
+use SimpleSAML\Configuration;
+use SimpleSAML\Logger;
+use SimpleSAML\Module\core\Auth\Process\SubjectID;
+use SimpleSAML\Utils;
+
+/**
+ * Test for the core:SubjectID filter.
+ *
+ * @covers \SimpleSAML\Module\core\Auth\Process\SubjectID
+ */
+class SubjectIDTest extends TestCase
+{
+    /** @var \SimpleSAML\Configuration */
+    protected Configuration $config;
+
+    /** @var \SimpleSAML\Logger */
+    protected static Logger $logger;
+
+
+    /**
+     * Set up for each test.
+     */
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        self::$logger = new class () extends Logger {
+            public static function warning(string $string): void
+            {
+                // stub
+                throw new RuntimeException($string);
+            }
+        };
+    }
+
+
+    /**
+     * Helper function to run the filter with a given configuration.
+     *
+     * @param array $config  The filter configuration.
+     * @param array $request  The request state.
+     * @return array  The state array after processing.
+     */
+    private static function processFilter(array $config, array $request): array
+    {
+        $filter = new SubjectID($config, null);
+        $filter->setLogger(self::$logger);
+        $filter->process($request);
+        return $request;
+    }
+
+
+    /**
+     * Test the most basic functionality
+     */
+    public function testBasic(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'ex-ample.org'];
+        $request = [
+            'Attributes' => ['uid' => ['u=se-r2']],
+        ];
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_SUBJECT_ID, $attributes);
+        $this->assertMatchesRegularExpression(
+            SubjectID::SPEC_PATTERN,
+            $attributes[Constants::ATTR_SUBJECT_ID][0]
+        );
+        $this->assertEquals('u=se-r2@ex-ample.org', $attributes[Constants::ATTR_SUBJECT_ID][0]);
+    }
+
+
+    /**
+     * Test that illegal characters in userID throws an exception.
+     */
+    public function testUserIDIllegalCharacterThrowsException(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'example.org'];
+        $request = [
+            'Attributes' => ['uid' => ['u=se+r2']],
+        ];
+
+        $this->expectException(AssertionFailedException::class);
+        self::processFilter($config, $request);
+    }
+
+
+    /**
+     * Test that illegal characters in scope throws an exception.
+     */
+    public function testScopeIllegalCharacterThrowsException(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'ex%ample.org'];
+        $request = [
+            'Attributes' => ['uid' => ['user2']],
+        ];
+
+        $this->expectException(AssertionFailedException::class);
+        self::processFilter($config, $request);
+    }
+
+
+    /**
+     * Test that generated ID's for different users, but the same SP's are NOT equal
+     */
+    public function testUniqueIdentifierPerUserSameSP(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'example.org'];
+        $request = [
+            'Attributes' => ['uid' => ['user1']],
+        ];
+
+        // Generate first ID
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_SUBJECT_ID, $attributes);
+        $value1 = $attributes[Constants::ATTR_SUBJECT_ID][0];
+
+        // Switch user
+        $request['Attributes'] = ['uid' => ['user2']];
+
+        // Generate second ID
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_SUBJECT_ID, $attributes);
+        $value2 = $attributes[Constants::ATTR_SUBJECT_ID][0];
+
+        $this->assertNotSame($value1, $value2);
+    }
+
+
+    /**
+     * Test that generated ID's for the same user and same SP, but with a different scope are NOT equal
+     */
+    public function testUniqueIdentifierDifferentScopes(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'example.org'];
+        $request = [
+            'Attributes' => ['uid' => ['user1']],
+        ];
+
+        // Generate first ID
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_SUBJECT_ID, $attributes);
+        $value1 = $attributes[Constants::ATTR_SUBJECT_ID][0];
+
+        // Change the scope
+        $config['scope'] = 'example.edu';
+
+        // Generate second ID
+        $result = self::processFilter($config, $request);
+        $attributes = $result['Attributes'];
+        $this->assertArrayHasKey(Constants::ATTR_SUBJECT_ID, $attributes);
+        $value2 = $attributes[Constants::ATTR_SUBJECT_ID][0];
+
+        $this->assertNotSame($value1, $value2);
+
+        $this->assertMatchesRegularExpression(
+            '/@example.org$/i',
+            $value1
+        );
+        $this->assertMatchesRegularExpression(
+            '/@example.edu$/i',
+            $value2
+        );
+    }
+
+
+    /**
+     * Test that weak identifiers log a warning
+     */
+    public function testWeakIdentifierLogsWarning(): void
+    {
+        $config = ['identifyingAttribute' => 'uid', 'scope' => 'b'];
+        $request = [
+            'Attributes' => ['uid' => ['a']],
+        ];
+
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('core:SubjectID: Generated ID \'a@b\' can hardly be considered globally unique.');
+
+        self::processFilter($config, $request);
+    }
+}