126 lines
3.7 KiB
C#
126 lines
3.7 KiB
C#
/*
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* Copyright (c) 2023 ETH Zürich, Educational Development and Technology (LET)
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*
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/.
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*/
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using System;
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using System.Security.Cryptography;
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using System.Text;
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using SafeExamBrowser.Configuration.Contracts;
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using SafeExamBrowser.Configuration.Contracts.Cryptography;
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using SafeExamBrowser.Configuration.Contracts.Integrity;
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using SafeExamBrowser.Logging.Contracts;
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namespace SafeExamBrowser.Configuration.Cryptography
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{
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public class KeyGenerator : IKeyGenerator
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{
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private readonly object @lock = new object();
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private readonly SHA256Managed algorithm;
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private readonly AppConfig appConfig;
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private readonly IIntegrityModule integrityModule;
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private readonly ILogger logger;
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private string browserExamKey;
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public KeyGenerator(AppConfig appConfig, IIntegrityModule integrityModule, ILogger logger)
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{
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this.algorithm = new SHA256Managed();
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this.appConfig = appConfig;
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this.integrityModule = integrityModule;
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this.logger = logger;
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}
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public string CalculateAppSignatureKey(string connectionToken, string salt)
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{
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if (integrityModule.TryCalculateAppSignatureKey(connectionToken, salt, out var appSignatureKey))
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{
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logger.Debug("Successfully calculated app signature key using integrity module.");
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}
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else
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{
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logger.Error("Failed to calculate app signature key using integrity module!");
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}
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return appSignatureKey;
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}
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public string CalculateBrowserExamKeyHash(string configurationKey, byte[] salt, string url)
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{
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var urlWithoutFragment = url.Split('#')[0];
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var hash = algorithm.ComputeHash(Encoding.UTF8.GetBytes(urlWithoutFragment + (browserExamKey ?? ComputeBrowserExamKey(configurationKey, salt))));
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var key = ToString(hash);
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return key;
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}
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public string CalculateConfigurationKeyHash(string configurationKey, string url)
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{
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var urlWithoutFragment = url.Split('#')[0];
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var hash = algorithm.ComputeHash(Encoding.UTF8.GetBytes(urlWithoutFragment + configurationKey));
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var key = ToString(hash);
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return key;
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}
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public void UseCustomBrowserExamKey(string browserExamKey)
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{
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if (browserExamKey != default)
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{
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this.browserExamKey = browserExamKey;
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logger.Debug("Initialized custom browser exam key.");
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}
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}
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private string ComputeBrowserExamKey(string configurationKey, byte[] salt)
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{
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lock (@lock)
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{
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if (browserExamKey == default)
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{
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logger.Debug("Initializing browser exam key...");
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if (configurationKey == default)
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{
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configurationKey = "";
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logger.Warn("The current configuration does not contain a value for the configuration key!");
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}
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if (salt == default || salt.Length == 0)
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{
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salt = new byte[0];
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logger.Warn("The current configuration does not contain a salt value for the browser exam key!");
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}
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if (integrityModule.TryCalculateBrowserExamKey(configurationKey, ToString(salt), out browserExamKey))
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{
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logger.Debug("Successfully calculated browser exam key using integrity module.");
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}
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else
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{
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logger.Warn("Failed to calculate browser exam key using integrity module! Falling back to simplified calculation...");
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using (var algorithm = new HMACSHA256(salt))
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{
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var hash = algorithm.ComputeHash(Encoding.UTF8.GetBytes(appConfig.CodeSignatureHash + appConfig.ProgramBuildVersion + configurationKey));
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var key = ToString(hash);
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browserExamKey = key;
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}
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}
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}
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}
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return browserExamKey;
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}
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private string ToString(byte[] bytes)
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{
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return BitConverter.ToString(bytes).ToLower().Replace("-", string.Empty);
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}
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}
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}
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