the files
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niko
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using System.Security.Cryptography;
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using Newtonsoft.Json.Linq;
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namespace Campus_live_rewrite.api.server
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{
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/// <summary>
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/// Offline verification of a Steam login's platform_auth ticket - the SteamID64 a login may be
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/// trusted as. Port of RecFlare auth src/steam-ticket.ts (reference server).
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///
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/// A Steam login posts platform_auth = {"Ticket":"<hex>","AppId":"471710"}. The ticket's
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/// ownership section is signed by Steam's "System" RSA key (RSA-SHA1), so it verifies OFFLINE -
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/// no publisher Web API key (which only Rec Room has for app 471710), no network call - and the
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/// SteamID64 inside is one Steam itself vouched for. The posted platform_id field is NOT: it is
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/// whatever the client typed, and a SteamID64 is public (it's on every Steam profile). Before
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/// this, a cached_login with someone else's SteamID and account id and no device id minted a
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/// token for their account. Byte layout per DoctorMcKay's steam-appticket.
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/// </summary>
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public static class SteamTicket
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{
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// Steam's "System" public key (SPKI DER), from @doctormckay/steam-crypto system.pem.
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private const string SystemPublicKeySpki =
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"MIGdMA0GCSqGSIb3DQEBAQUAA4GLADCBhwKBgQDf7BrWLBBmLBc1OhSwfFkRf53T" +
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"2Ct64+AVzRkeRuh7h3SiGEYxqQMUeYKO6UWiSRKpI2hzic9pobFhRr3Bvr/WARvY" +
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"gdTckPv+T1JzZsuVcNfFjrocejN1oWI0Rrtgt4Bo+hOneoo3S57G9F1fOpn5nsQ6" +
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"6WOiu4gZKODnFMBCiQIBEQ==";
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public readonly record struct Result(bool Ok, ulong SteamId, uint AppId, string Reason);
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public readonly record struct Parsed(ulong SteamId, uint AppId, long ExpiresAtMs, int SignedStart, int SignedEnd, int SignatureAt);
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/// <summary>
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/// The SteamID64 a platform_auth payload proves, or Ok=false (with why) when it is missing,
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/// malformed, expired, or not signed by Steam. Only ever returns an id Steam signed.
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/// </summary>
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public static Result Verify(string? platformAuth, DateTimeOffset? now = null)
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{
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if (string.IsNullOrWhiteSpace(platformAuth)) return Fail("no Steam ticket");
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string? hex;
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try { hex = (string?)JObject.Parse(platformAuth)["Ticket"]; }
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catch { return Fail("platform_auth is not JSON"); }
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if (string.IsNullOrEmpty(hex)) return Fail("no Ticket in platform_auth");
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byte[] buf;
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try { buf = Convert.FromHexString(hex); }
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catch { return Fail("Ticket is not hex"); }
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var t = Parse(buf);
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if (t == null) return Fail("malformed Steam ticket");
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long nowMs = (now ?? DateTimeOffset.UtcNow).ToUnixTimeMilliseconds();
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if (t.Value.ExpiresAtMs != 0 && t.Value.ExpiresAtMs < nowMs) return Fail("expired Steam ticket");
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if (!VerifySignature(buf, t.Value)) return Fail("Steam ticket signature does not verify");
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return new Result(true, t.Value.SteamId, t.Value.AppId, "");
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}
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private static Result Fail(string why) => new(false, 0, 0, why);
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/// <summary>The ticket's fields and signed range WITHOUT checking the signature (null = malformed).</summary>
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public static Parsed? Parse(byte[] buf)
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{
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int pos = 0, limit = buf.Length;
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uint U32() { uint v = BitConverter.ToUInt32(buf, pos); pos += 4; return v; }
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ushort U16() { ushort v = BitConverter.ToUInt16(buf, pos); pos += 2; return v; }
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ulong U64() { ulong v = BitConverter.ToUInt64(buf, pos); pos += 8; return v; }
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try
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{
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if (U32() == 20)
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{
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// full ticket: GC token + session header come before the ownership ticket
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pos += 8; // gcToken
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pos += 8; // steamID (read from the ownership section instead)
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U32(); // tokenGenerated
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if (U32() != 24) return null; // session header length
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pos += 8; // unknown1, unknown2
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U32(); // session external IP
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pos += 4; // filler
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U32(); // client connection time
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U32(); // client connection count
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if (U32() + pos != limit) return null; // ownership-section length check
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}
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else pos -= 4; // a bare ownership ticket
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int ownStart = pos;
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int ownLen = (int)U32(); // includes itself
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if (ownStart + ownLen != limit && ownStart + ownLen + 128 != limit) return null;
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U32(); // version
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ulong steamId = U64();
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uint appId = U32();
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U32(); U32(); U32(); U32(); // ownership external / internal IP, flags, generated
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long expiresAtMs = (long)U32() * 1000;
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int licenses = U16();
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for (int i = 0; i < licenses; i++) U32();
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int dlcs = U16();
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for (int i = 0; i < dlcs; i++)
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{
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U32(); // dlc appId
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int dl = U16();
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for (int j = 0; j < dl; j++) U32();
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}
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U16(); // reserved
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if (pos + 128 != limit) return null; // a signature is required
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return new Parsed(steamId, appId, expiresAtMs, ownStart, ownStart + ownLen, pos);
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}
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catch (ArgumentException) { return null; } // ran off the end
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catch (IndexOutOfRangeException) { return null; }
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}
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public static bool VerifySignature(byte[] buf, Parsed t)
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{
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try
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{
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using var rsa = RSA.Create();
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rsa.ImportSubjectPublicKeyInfo(Convert.FromBase64String(SystemPublicKeySpki), out _);
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return rsa.VerifyData(buf.AsSpan(t.SignedStart, t.SignedEnd - t.SignedStart),
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buf.AsSpan(t.SignatureAt, 128), HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1);
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}
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catch { return false; }
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}
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}
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}
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