YouTube-Manager/Manager.YouTube/Util/Cipher/CipherDecoder.cs

using System.Collections.Frozen;
using System.Text;
using System.Text.RegularExpressions;
using Manager.YouTube.Util.Cipher.Operations;

namespace Manager.YouTube.Util.Cipher;

public partial class CipherDecoder
{
    public required string Version { get; init; }
    public readonly IReadOnlySet<ICipherOperation> Operations;

    private CipherDecoder(IEnumerable<ICipherOperation> operations)
    {
        Operations = operations.ToFrozenSet();
        if (Operations.Count == 0)
        {
            throw new ArgumentNullException(nameof(operations), "No decipher operations given.");
        }
    }

    public static async Task<CipherDecoder> CreateAsync(string relativeUrl, string version, YouTubeClient? client = null)
    {
        var operations = await GetCipherOperations(relativeUrl, client);
        var decoder = new CipherDecoder(operations)
        {
            Version = version
        };
        return decoder;
    }

    public string Decipher(string? signatureCipher)
    {
        if (string.IsNullOrEmpty(signatureCipher))
        {
            return "";
        }

        var urlBuilder = new StringBuilder();

        var indexStart = signatureCipher.IndexOf("s=", StringComparison.Ordinal);
        var indexEnd = signatureCipher.IndexOf("&", StringComparison.Ordinal);
        var signature = signatureCipher.Substring(indexStart, indexEnd);

        indexStart = signatureCipher.IndexOf("&sp", StringComparison.Ordinal);
        indexEnd = signatureCipher.IndexOf("&url", StringComparison.Ordinal);
        var spParam = signatureCipher.Substring(indexStart, indexEnd - indexStart);

        indexStart = signatureCipher.IndexOf("&url", StringComparison.Ordinal);
        var videoUrl = signatureCipher[indexStart..];


        signature = signature[(signature.IndexOf('=') + 1)..];
        spParam = spParam[(spParam.IndexOf('=') + 1)..];
        videoUrl = videoUrl[(videoUrl.IndexOf('=') + 1)..];
        if (string.IsNullOrWhiteSpace(signature))
        {
            throw new InvalidOperationException("Invalid signature.");
        }
        var signatureDeciphered = Operations.Aggregate(signature, (acc, op) => op.Decipher(acc));

        urlBuilder.Append(videoUrl);
        urlBuilder.Append($"&{spParam}=");
        urlBuilder.Append(signatureDeciphered);
        return urlBuilder.ToString();
    }

    private static async Task<IEnumerable<ICipherOperation>> GetCipherOperations(string relativeUrl, YouTubeClient? client = null)
    {
        var downloadRequest = new HttpRequestMessage(HttpMethod.Get, new Uri($"{NetworkService.Origin}/{relativeUrl}"));
        var downloadResponse = await NetworkService.DownloadBytesAsync(downloadRequest, client);
        if (!downloadResponse.IsSuccess)
        {
            return [];
        }

        var playerJs = Encoding.UTF8.GetString(downloadResponse.Value.Data);
        if (string.IsNullOrWhiteSpace(playerJs))
        {
            return [];
        }
        
        var functionBody = FunctionBodyRegex().Match(playerJs).Groups[0].ToString();
        var definitionBody = DefinitionBodyRegex().Match(functionBody).Groups[1].Value;
        var decipherDefinition = Regex.Match(playerJs, $@"var\s+{definitionBody}=\{{(\w+:function\(\w+(,\w+)?\)\{{(.*?)\}}),?\}};", RegexOptions.Singleline).Groups[0].ToString();

        SortedSet<ICipherOperation> operations = [];
        foreach (var statement in functionBody.Split(';'))
        {
            // Get the name of the function called in this statement
            var calledFuncName = StatementFunctionNameRegex().Match(statement).Groups[1].Value;
            if (string.IsNullOrWhiteSpace(calledFuncName))
                continue;

            if (Regex.IsMatch(decipherDefinition, $@"{Regex.Escape(calledFuncName)}:\bfunction\b\([a],b\).(\breturn\b)?.?\w+\."))
            {
                var index = int.Parse(OperationIndexRegex().Match(statement).Groups[1].Value);
                operations.Add(new CipherSlice(index));
            }
            else if (Regex.IsMatch(decipherDefinition, $@"{Regex.Escape(calledFuncName)}:\bfunction\b\(\w+\,\w\).\bvar\b.\bc=a\b"))
            {
                var index = int.Parse(OperationIndexRegex().Match(statement).Groups[1].Value);
                operations.Add(new CipherSwap(index));
            }
            else if (Regex.IsMatch(decipherDefinition, $@"{Regex.Escape(calledFuncName)}:\bfunction\b\(\w+\)"))
            {
                operations.Add(new CipherReverse());
            }
        }

        return operations;
    }

    
    [GeneratedRegex(@"([A-Za-z_$][A-Za-z0-9_$]*)=function\([A-Za-z_$][A-Za-z0-9_$]*\)\{\s*([A-Za-z_$][A-Za-z0-9_$]*)=\2\.split\(\x22\x22\);[\s\S]*?return\s+\2\.join\(\x22\x22\)\s*\}")]
    private static partial Regex FunctionBodyRegex();
    [GeneratedRegex("([\\$_\\w]+).\\w+\\(\\w+,\\d+\\);")]
    private static partial Regex DefinitionBodyRegex();
    
    
    [GeneratedRegex(@"\(\w+,(\d+)\)")]
    private static partial Regex OperationIndexRegex();
    [GeneratedRegex(@"\w+(?:.|\[)(\""?\w+(?:\"")?)\]?\(")]
    private static partial Regex StatementFunctionNameRegex();
}