stream-cinema/CinemaLib/Webshare/LinkGenerator.cs

using System;
using System.Diagnostics.CodeAnalysis;
using System.Security.Cryptography;
using System.Text;
using System.Xml;

namespace Cinema.Webshare;

public class LinkGenerator
{
  private static readonly BaseEncoding UnixMD5 = new BaseEncoding("./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz", false);
  private static readonly Uri WebshareApiUri = new Uri("https://webshare.cz/api/");

  /// <summary>
  /// Generates a download link for the given Webshare download.
  /// </summary>
  /// <param name="fileId">Webshare file identifier.</param>
  /// <param name="fileName">Webshare file name.</param>
  /// <param name="cancel">Asynchronous cancellation.</param>
  /// <returns>Time-limited download link.</returns>
  public async static Task<Uri> GenerateDownloadLinkAsync(string fileId, string fileName, CancellationToken cancel)
  {
    // Obtain the download password salt
    // Example response: <?xml version="1.0" encoding="UTF-8"?><response><status>OK</status><salt>UX8y8Fpa</salt><app_version>30</app_version></response>
    Uri saltUri = new Uri(WebshareApiUri, "file_password_salt/");
    HttpResponseMessage saltRes = await Program._http.PostAsync(saltUri, new FormUrlEncodedContent(new[] { new KeyValuePair<string, string>("ident", fileId) }), cancel);
    if (!saltRes.IsSuccessStatusCode)
      throw new IOException("Failed to get password salt from Webshare API.");

    XmlReader saltR = XmlReader.Create(saltRes.Content.ReadAsStream());
    saltR.ReadStartElement("response");
    Exception? e = GetExceptionIfStatusNotOK(saltR, out string? code);
    string password;
    if (e != null)
    {
      if (code == "FILE_PASSWORD_SALT_FATAL_2")
      {
        // No password is set
        password = "";
      }
      else
        throw e;
    }
    else
    {
      // Calculate the download password
      string salt = saltR.ReadElementContentAsString("salt", "");
      saltR.ReadElementContentAsString("app_version", "");
      saltR.ReadEndElement(); // response

      password = CalculatePassword(fileId, fileName, salt);
    }

    // Obtain the download link
    // Example response: <?xml version="1.0" encoding="UTF-8"?><response><status>OK</status><link>https://free.19.dl.wsfiles.cz/9209/...</link><app_version>30</app_version></response>
    Uri linkUri = new Uri(WebshareApiUri, "file_link/");
    HttpResponseMessage linkRes = await Program._http.PostAsync(linkUri, new FormUrlEncodedContent(new[] {
      new KeyValuePair<string, string>("ident", fileId),
      new KeyValuePair<string, string>("download_type", "video_stream"),
      new KeyValuePair<string, string>("device_uuid", Guid.NewGuid().ToString("X")),
      new KeyValuePair<string, string>("device_res_x", "3840"),
      new KeyValuePair<string, string>("device_res_y", "2160"),
      new KeyValuePair<string, string>("password", password),
      }), cancel
    );
    if (!linkRes.IsSuccessStatusCode)
      throw new IOException("Failed to get download link from Webshare API.");

    XmlReader linkR = XmlReader.Create(linkRes.Content.ReadAsStream());
    linkR.ReadStartElement("response");
    ThrowIfStatusNotOK(linkR);
    string link = linkR.ReadElementContentAsString("link", "");
    linkR.ReadElementContentAsString("app_version", "");
    linkR.ReadEndElement(); // response

    return new Uri(link);
  }

  private static Exception? GetExceptionIfStatusNotOK(XmlReader r, out string? code)
  {
    string status = r.ReadElementContentAsString("status", "");
    if (status == "OK") {
      code = null;
      return null;
    }

    code = r.ReadElementContentAsString("code", "");
    string message = r.ReadElementContentAsString("message", "");
    return new IOException(code + ": " + message);
  }

  private static void ThrowIfStatusNotOK(XmlReader r)
  {
    Exception? e = GetExceptionIfStatusNotOK(r, out _);
    if (e != null)
      throw e;
  }

  /// <summary>
  /// Calculates the Webshare download password from file identifier, name and salt.
  /// </summary>
  /// <param name="fileId">Webshare file identifier.</param>
  /// <param name="fileName">Webshare file name.</param>
  /// <param name="salt">Download salt obtained from https://webshare.cz/api/file_password_salt/.</param>
  /// <returns>Password to use to obtain a download link using https://webshare.cz/api/file_link/.</returns>
  private static string CalculatePassword(string fileId, string fileName, string salt)
  {
    if (fileId == null || fileName == null || salt == null)
      throw new ArgumentNullException();

    // Example
    //  name = "bIaFGdkvKiiTBo2_S3E"
    //  id = "12kRBqAYQS2"
    //  password = sha256(name + id) = "1e98873b57b7660cba7267c191f2d1fb7d198d062eb2fe0686502883042c4105"
    //  crypt = md5crypt(password, salt: "UX8y8Fpa") = "$1$UX8y8Fpa$stY.bUYGBsmAlc6IIxnHU0"
    //  result = sha1(crypt) = "8ceff8e5abf9aa375a8a5b05871b6cd6fb7fd185")
    string toHash = fileName + fileId;
    byte[] password = SHA256.HashData(Encoding.UTF8.GetBytes(toHash));
    string passwordS = Convert.ToHexString(password).ToLowerInvariant();
    string crypt = MD5Crypt(passwordS, salt);
    byte[] result = SHA1.HashData(Encoding.ASCII.GetBytes(crypt));
    return Convert.ToHexString(result).ToLowerInvariant();
  }

  private static string MD5Crypt(string password, string salt)
  {
    string prefixString = "$1$";
    byte[] prefixBytes = Encoding.ASCII.GetBytes(prefixString);
    byte[] saltBytes = Encoding.ASCII.GetBytes(salt);

    byte[] key = Encoding.ASCII.GetBytes(password);
    byte[] truncatedSalt = TruncateAndCopy(saltBytes, 8);
    byte[] crypt = Crypt(key, truncatedSalt, prefixBytes, MD5.Create());

    string result = prefixString
        + salt + '$'
        + UnixMD5.GetString(crypt);

    return result.TrimEnd('=');
  }

  private static byte[] Crypt(byte[] key, byte[] salt, byte[] prefix, HashAlgorithm A)
  {
    byte[] H = null, I = null;

    A.Initialize();
    AddToDigest(A, key);
    AddToDigest(A, salt);
    AddToDigest(A, key);
    FinishDigest(A);

    I = (byte[])A.Hash.Clone();

    A.Initialize();
    AddToDigest(A, key);
    AddToDigest(A, prefix);
    AddToDigest(A, salt);

    AddToDigestRolling(A, I, 0, I.Length, key.Length);

    int length = key.Length;
    for (int i = 0; i < 31 && length != 0; i++)
    {
      AddToDigest(A, new[] { (length & (1 << i)) != 0 ? (byte)0 : key[0] });
      length &= ~(1 << i);
    }
    FinishDigest(A);

    H = (byte[])A.Hash.Clone();

    for (int i = 0; i < 1000; i++)
    {
      A.Initialize();
      if ((i & 1) != 0) { AddToDigest(A, key); }
      if ((i & 1) == 0) { AddToDigest(A, H); }
      if ((i % 3) != 0) { AddToDigest(A, salt); }
      if ((i % 7) != 0) { AddToDigest(A, key); }
      if ((i & 1) != 0) { AddToDigest(A, H); }
      if ((i & 1) == 0) { AddToDigest(A, key); }
      FinishDigest(A);

      Array.Copy(A.Hash, H, H.Length);
    }

    byte[] crypt = new byte[H.Length];
    int[] permutation = new[] { 11, 4, 10, 5, 3, 9, 15, 2, 8, 14, 1, 7, 13, 0, 6, 12 };
    Array.Reverse(permutation);
    for (int i = 0; i < crypt.Length; i++)
    {
      crypt[i] = H[permutation[i]];
    }

    return crypt;
  }

  private static void AddToDigest(HashAlgorithm algorithm, byte[] buffer)
  {
    AddToDigest(algorithm, buffer, 0, buffer.Length);
  }

  private static void AddToDigest(HashAlgorithm algorithm, byte[] buffer, int offset, int count)
  {
    algorithm.TransformBlock(buffer, offset, count, buffer, offset);
  }

  private static void AddToDigestRolling(HashAlgorithm algorithm, byte[] buffer, int offset, int inputCount, int outputCount)
  {
    int count;
    for (count = 0; count < outputCount; count += inputCount)
    {
      AddToDigest(algorithm, buffer, offset, Math.Min(outputCount - count, inputCount));
    }
  }

  private static void FinishDigest(HashAlgorithm algorithm)
  {
    algorithm.TransformFinalBlock(new byte[0], 0, 0);
  }

  private static byte[] TruncateAndCopy(byte[] buffer, int maxLength)
  {
    byte[] truncatedBuffer = new byte[Math.Min(buffer.Length, maxLength)];
    Array.Copy(buffer, truncatedBuffer, truncatedBuffer.Length);
    return truncatedBuffer;
  }
}