diff --git a/src/AuthenticationMiddleware.php b/src/AuthenticationMiddleware.php
index 978a528..2177f21 100644
--- a/src/AuthenticationMiddleware.php
+++ b/src/AuthenticationMiddleware.php
@@ -70,28 +70,6 @@ public function getAuthorizationResult()
      */
     private function buildWwwAuthenticateHeader(AuthorizationResultInterface $result)
     {
-        $scheme = $result->getScheme();
-        $challenge = $this->buildChallengeString($result->getChallenge());
-
-        if (empty($challenge)) {
-            return $scheme;
-        }
-
-        return sprintf('%s %s', $scheme, $challenge);
-    }
-
-    /**
-     * @param array $serviceChallenge
-     *
-     * @return type
-     */
-    private function buildChallengeString(array $serviceChallenge)
-    {
-        $challengePairs = [];
-
-        foreach ($serviceChallenge as $challenge => $value) {
-            $challengePairs[] = sprintf('%s="%s"', $challenge, addslashes($value));
-        }
-        return implode(', ', $challengePairs);
+        return Util::buildHeader($result->getScheme(), $result->getChallenge());
     }
 }
\ No newline at end of file
diff --git a/src/AuthorizationResult.php b/src/AuthorizationResult.php
index 320486f..41c6558 100644
--- a/src/AuthorizationResult.php
+++ b/src/AuthorizationResult.php
@@ -64,6 +64,27 @@ public static function notAuthorized($scheme, array $challenge = [], array $attr
         return $instance;
     }
 
+    /**
+     * @param string $scheme
+     * @param array $challenge
+     * @param array $attributes
+     *
+     * @return self
+     */
+    public static function error($scheme, $error, $errorDescription, array $challenge = [], array $attributes = [])
+    {
+        $challenge['error'] = $error;
+        $challenge['error_description'] = $errorDescription;
+
+        $instance = new self();
+        $instance->isAuthorized = false;
+        $instance->scheme = $scheme;
+        $instance->challenge = $challenge;
+        $instance->attributes = $attributes;
+
+        return $instance;
+    }
+
     /**
      * @return array
      */
diff --git a/src/BasicAuthorizationService.php b/src/BasicAuthorizationService.php
index 9916f39..efbb06e 100644
--- a/src/BasicAuthorizationService.php
+++ b/src/BasicAuthorizationService.php
@@ -51,18 +51,14 @@ public function authorize(ServerRequestInterface $request)
             if ($result === true) {
                 return AuthorizationResult::authorized(self::SCHEME, [], ['user-ID' => $userId]);
             } elseif ($result === false) {
-                return AuthorizationResult::notAuthorized(self::SCHEME, [
+                return AuthorizationResult::error(self::SCHEME, 'Invalid credentials', 'Login and/or password are invalid', [
                     'realm' => $this->realm,
-                    'error' => 'Invalid credentials',
-                    'error_description' => 'Login and/or password are invalid',
                 ]);
             }
             throw new UnexpectedValueException(sprintf('%s\'s result must be a boolean value', UserPasswordInterface::class));
         }
-        return AuthorizationResult::notAuthorized(self::SCHEME, [
+        return AuthorizationResult::error(self::SCHEME, 'Invalid header', 'Cannot read user-ID and password from header', [
             'realm' => $this->realm,
-            'error' => '',
-            'error_description' => '',
         ]);
     }
 
diff --git a/src/CredentialAdapter/ArrayUserPassword.php b/src/CredentialAdapter/ArrayUserPassword.php
index 1cfb753..f64aaba 100644
--- a/src/CredentialAdapter/ArrayUserPassword.php
+++ b/src/CredentialAdapter/ArrayUserPassword.php
@@ -2,7 +2,10 @@
 
 namespace PhpMiddleware\HttpAuthentication\CredentialAdapter;
 
-final class ArrayUserPassword implements UserPasswordInterface
+use PhpMiddleware\HttpAuthentication\CredentialAdapter\Exception\UsernameNotFoundException;
+use PhpMiddleware\HttpAuthentication\Util;
+
+final class ArrayUserPassword implements UserPasswordInterface, HashProviderInterface
 {
     /**
      * @var array
@@ -25,6 +28,24 @@ public function __construct(array $users)
      */
     public function authenticate($username, $password)
     {
-        return isset($this->users[$username]) && $this->users[$username] === $password;
+        return $this->isUserNameExists($username) && $this->users[$username] === $password;
+    }
+
+    private function isUserNameExists($username)
+    {
+        return isset($this->users[$username]);
     }
+
+    public function getHash($username, $realm)
+    {
+        if (!$this->isUserNameExists($username)) {
+            throw new UsernameNotFoundException('Username does not exist');
+        }
+        return Util::md5Implode([
+            $username,
+            $realm,
+            $this->users[$username],
+        ]);
+    }
+
 }
diff --git a/src/CredentialAdapter/Exception/UsernameNotFoundException.php b/src/CredentialAdapter/Exception/UsernameNotFoundException.php
new file mode 100644
index 0000000..1e0b5de
--- /dev/null
+++ b/src/CredentialAdapter/Exception/UsernameNotFoundException.php
@@ -0,0 +1,8 @@
+<?php
+
+namespace PhpMiddleware\HttpAuthentication\CredentialAdapter\Exception;
+
+class UsernameNotFoundException extends \DomainException
+{
+
+}
diff --git a/src/CredentialAdapter/HashProviderInterface.php b/src/CredentialAdapter/HashProviderInterface.php
new file mode 100644
index 0000000..12460b4
--- /dev/null
+++ b/src/CredentialAdapter/HashProviderInterface.php
@@ -0,0 +1,8 @@
+<?php
+
+namespace PhpMiddleware\HttpAuthentication\CredentialAdapter;
+
+interface HashProviderInterface
+{
+    public function getHash($username, $realm);
+}
diff --git a/src/DigestAuthorizationService.php b/src/DigestAuthorizationService.php
index 2fadb6e..48c3ece 100644
--- a/src/DigestAuthorizationService.php
+++ b/src/DigestAuthorizationService.php
@@ -1,14 +1,83 @@
 <?php
 
-
 namespace PhpMiddleware\HttpAuthentication;
 
+use PhpMiddleware\HttpAuthentication\CredentialAdapter\Exception\UsernameNotFoundException;
+use PhpMiddleware\HttpAuthentication\CredentialAdapter\HashProviderInterface;
 use Psr\Http\Message\ServerRequestInterface;
 
 final class DigestAuthorizationService implements AuthorizationServiceInterface
 {
+    private $hashProvider;
+    private $realm;
+
+    public function __construct(HashProviderInterface $hashProvider, $realm)
+    {
+        $this->hashProvider = $hashProvider;
+        $this->realm = $realm;
+    }
+
     public function authorize(ServerRequestInterface $request)
     {
-        throw new \BadMethodCallException('Not implemented');
+        $header = $request->getHeaderLine('Authorization');
+
+        $authorization = $this->parseAuthorizationHeader($header);
+
+        if (!$authorization) {
+            return AuthorizationResult::error('digest', 'Invalid header', 'Cannot read data from Authorization header', [
+                'realm' => $this->realm,
+            ]);
+        }
+
+        $result = $this->checkAuthentication($authorization, $request->getMethod());
+
+        if ($result) {
+            return AuthorizationResult::authorized('digest');
+        }
+        return AuthorizationResult::notAuthorized('digest', [], $authorization);
+    }
+
+    private function checkAuthentication(array $authorization, $method)
+    {
+        if ($authorization['realm'] !== $this->realm) {
+            return false;
+        }
+        try {
+            $A1 = $this->hashProvider->getHash($authorization['username'], $this->realm);
+        } catch (UsernameNotFoundException $exception) {
+            return false;
+        }
+
+        $A2 = Util::md5Implode([$method, $authorization['uri']]);
+
+        $realResponse = Util::md5Implode([$A1, $authorization['nonce'], $A2]);
+
+        return $authorization['response'] === $realResponse;
+    }
+
+    private function parseAuthorizationHeader($header)
+    {
+        if (strpos($header, 'Digest') !== 0) {
+            return false;
+        }
+
+        $neededParts = ['nonce' => 1, 'realm' => 1, 'username' => 1, 'uri' => 1, 'response' => 1];
+        $neededPartsString = implode('|', array_keys($neededParts));
+        $data = [];
+
+        preg_match_all('@('.$neededPartsString.')=(?:([\'"])([^\2]+?)\2|([^\s,]+))@', substr($header, 7), $matches, PREG_SET_ORDER);
+
+        if (is_array($matches)) {
+            foreach ($matches as $match) {
+                $data[$match[1]] = $match[3] ?: $match[4];
+                unset($neededParts[$match[1]]);
+            }
+        }
+
+        if (!empty($neededParts)) {
+            return false;
+        }
+
+        return $data;
     }
 }
diff --git a/src/RequestBuilder/Digest.php b/src/RequestBuilder/Digest.php
new file mode 100644
index 0000000..29cfe10
--- /dev/null
+++ b/src/RequestBuilder/Digest.php
@@ -0,0 +1,50 @@
+<?php
+
+namespace PhpMiddleware\HttpAuthentication\RequestBuilder;
+
+use PhpMiddleware\HttpAuthentication\Util;
+use Psr\Http\Message\RequestInterface;
+
+/**
+ * @link https://tools.ietf.org/html/rfc2069
+ */
+final class Digest implements RequestBuilderInterface
+{
+    private $username;
+    private $password;
+    private $realm;
+    private $nonce;
+
+    public function __construct($username, $password, $realm, $nonce)
+    {
+        $this->username = $username;
+        $this->password = $password;
+        $this->realm = $realm;
+        $this->nonce = $nonce;
+    }
+
+    /**
+     * @param RequestInterface $request
+     *
+     * @return RequestInterface
+     */
+    public function authenticate(RequestInterface $request)
+    {
+        $uri = (string) $request->getUri();
+
+        $a1 = Util::md5Implode([$this->username, $this->realm, $this->password]);
+        $a2 = Util::md5Implode([$request->getMethod(), $uri]);
+
+        $response = Util::md5Implode([$a1, $this->nonce, $a2]);
+
+        $value = Util::buildHeader('Digest', [
+            'username' => $this->username,
+            'realm' => $this->realm,
+            'nonce' => $this->nonce,
+            'uri' => $uri,
+            'response' => $response,
+        ]);
+
+        return $request->withHeader('Authorization', $value);
+    }
+}
diff --git a/src/Util.php b/src/Util.php
new file mode 100644
index 0000000..af545da
--- /dev/null
+++ b/src/Util.php
@@ -0,0 +1,55 @@
+<?php
+
+namespace PhpMiddleware\HttpAuthentication;
+
+final class Util
+{
+    /**
+     * @codeCoverageIgnore
+     */
+    private function __construct()
+    {
+    }
+
+    /**
+     * @param array $params
+     *
+     * @return string md5
+     */
+    public static function md5Implode(array $params)
+    {
+        return md5(implode(':', $params));
+    }
+
+    /**
+     * @param string $scheme
+     * @param array $challenges
+     *
+     * @return string
+     */
+    public static function buildHeader($scheme, array $challenges)
+    {
+        $challenge = self::buildChallengeString($challenges);
+
+        if (empty($challenge)) {
+            return $scheme;
+        }
+
+        return sprintf('%s %s', $scheme, $challenge);
+    }
+
+    /**
+     * @param array $serviceChallenge
+     *
+     * @return string
+     */
+    private static function buildChallengeString(array $serviceChallenge)
+    {
+        $challengePairs = [];
+
+        foreach ($serviceChallenge as $challenge => $value) {
+            $challengePairs[] = sprintf('%s="%s"', $challenge, addslashes($value));
+        }
+        return implode(', ', $challengePairs);
+    }
+}
diff --git a/test/CredentialAdapter/ArrayUserPasswordTest.php b/test/CredentialAdapter/ArrayUserPasswordTest.php
index 6560c1e..de04a27 100644
--- a/test/CredentialAdapter/ArrayUserPasswordTest.php
+++ b/test/CredentialAdapter/ArrayUserPasswordTest.php
@@ -3,6 +3,7 @@
 namespace PhpMiddlewareTest\HttpAuthentication\CredentialAdapter;
 
 use PhpMiddleware\HttpAuthentication\CredentialAdapter\ArrayUserPassword;
+use PhpMiddleware\HttpAuthentication\CredentialAdapter\Exception\UsernameNotFoundException;
 use PHPUnit_Framework_TestCase;
 
 class ArrayUserPasswordTest extends PHPUnit_Framework_TestCase
@@ -38,6 +39,20 @@ public function testNotAuthenticate($username, $password)
         $this->assertFalse($result);
     }
 
+    public function testGetHash()
+    {
+        $result = $this->adapter->getHash('boo', 'any-realm');
+
+        $this->assertSame(32, strlen($result));
+    }
+
+    public function testInvalidUsername()
+    {
+        $this->setExpectedException(UsernameNotFoundException::class);
+
+        $this->adapter->getHash('baz', 'any-realm');
+    }
+
     public function correctDataProvider()
     {
         return [
diff --git a/test/DigestAuthorizationServiceTest.php b/test/DigestAuthorizationServiceTest.php
new file mode 100644
index 0000000..0d4c1ff
--- /dev/null
+++ b/test/DigestAuthorizationServiceTest.php
@@ -0,0 +1,73 @@
+<?php
+
+namespace PhpMiddlewareTest\HttpAuthentication;
+
+use PhpMiddleware\HttpAuthentication\AuthorizationResultInterface;
+use PhpMiddleware\HttpAuthentication\CredentialAdapter\ArrayUserPassword;
+use PhpMiddleware\HttpAuthentication\DigestAuthorizationService;
+use PHPUnit_Framework_TestCase;
+use Zend\Diactoros\ServerRequest;
+
+class DigestAuthorizationServiceTest extends PHPUnit_Framework_TestCase
+{
+    protected $service;
+    protected $hashProvider;
+    protected $serverRequest;
+
+    protected function setUp()
+    {
+        $this->hashProvider = new ArrayUserPassword([
+            'boo' => 'foo',
+        ]);
+        $this->service = new DigestAuthorizationService($this->hashProvider, 'realm');
+        $this->serverRequest = new ServerRequest();
+    }
+
+    public function testNotAuthRequest()
+    {
+        $result = $this->service->authorize($this->serverRequest);
+
+        $this->assertInstanceOf(AuthorizationResultInterface::class, $result);
+        $this->assertFalse($result->isAuthorized());
+    }
+
+    public function testAuthRequest()
+    {
+        $header = 'Digest username="boo", realm="realm", nonce="nonce", uri="/boo/bar", response="13f87e6ef7f79c68f8721d3e6b9e45e5"';
+        $request = $this->serverRequest->withHeader('Authorization', $header);
+
+        $result = $this->service->authorize($request);
+
+        $this->assertTrue($result->isAuthorized());
+    }
+
+    public function testNotAuthRequestWithoutRealm()
+    {
+        $header = 'Digest username="boo", nonce="nonce", uri="/boo/bar", response="13f87e6ef7f79c68f8721d3e6b9e45e5"';
+        $request = $this->serverRequest->withHeader('Authorization', $header);
+
+        $result = $this->service->authorize($request);
+
+        $this->assertFalse($result->isAuthorized());
+    }
+
+    public function testNotAuthRequestRealmIsDifferent()
+    {
+        $header = 'Digest username="boo", realm="realmus", nonce="nonce", uri="/boo/bar", response="13f87e6ef7f79c68f8721d3e6b9e45e5"';
+        $request = $this->serverRequest->withHeader('Authorization', $header);
+
+        $result = $this->service->authorize($request);
+
+        $this->assertFalse($result->isAuthorized());
+    }
+
+    public function testNotAuthRequestUsernameNotExists()
+    {
+        $header = 'Digest username="booz", realm="realm", nonce="nonce", uri="/boo/bar", response="13f87e6ef7f79c68f8721d3e6b9e45e5"';
+        $request = $this->serverRequest->withHeader('Authorization', $header);
+
+        $result = $this->service->authorize($request);
+
+        $this->assertFalse($result->isAuthorized());
+    }
+}
diff --git a/test/RequestBuilder/DigestTest.php b/test/RequestBuilder/DigestTest.php
new file mode 100644
index 0000000..5de3821
--- /dev/null
+++ b/test/RequestBuilder/DigestTest.php
@@ -0,0 +1,33 @@
+<?php
+
+namespace PhpMiddlewareTest\HttpAuthentication\RequestBuilder;
+
+use PhpMiddleware\HttpAuthentication\RequestBuilder\Digest;
+use PHPUnit_Framework_TestCase;
+use Psr\Http\Message\RequestInterface;
+use Zend\Diactoros\Request;
+use Zend\Diactoros\Uri;
+
+class DigestTest extends PHPUnit_Framework_TestCase
+{
+    protected $requestBuilder;
+    protected $request;
+
+    protected function setUp()
+    {
+        $this->requestBuilder = new Digest('boo', 'foo', 'realm', 'nonce');
+        $uri = new Uri('/boo/bar');
+        $this->request = new Request($uri, 'GET');
+    }
+
+    public function testAuthorizeRequest()
+    {
+        $result = $this->requestBuilder->authenticate($this->request);
+
+        $this->assertNotSame($this->request, $result);
+        $this->assertInstanceOf(RequestInterface::class, $result);
+        $expected = 'Digest username="boo", realm="realm", nonce="nonce", uri="/boo/bar", response="13f87e6ef7f79c68f8721d3e6b9e45e5"';
+
+        $this->assertSame($expected, $result->getHeaderLine('Authorization'));
+    }
+}