Wire Cryptography: AES-CTR Cipher and 0xA101 Handshake

// Dispatcher for both login and game cipher modes
void PacketPayload_Decrypt(uint8_t *buf, size_t len) {
    if (DAT_023037e8 /* login_gate */ != 0) {
        if (DAT_023037e4 /* login_submode */ == 2) {
            Login_XORStream(buf, len);      // 0x00401000
        }
    } else if (DAT_023037ea /* game_cipher */ != 0) {
        if (DAT_023037e0 /* ctx_sel */ == 0) {
            StreamCrypt_XOR(0x23037F0, buf, len);   // 0x00404DF0 — main recv ctx
        } else {
            StreamCrypt_AltCtx(0x2303908, buf, len); // 0x00401640 — alt send ctx
        }
    }
}

// PacketStream_XOR — symmetric encrypt/decrypt via AES-CTR keystream
// Server: 0x004E4180 | Client: 0x00404DF0
void PacketStream_XOR(CryptoCtx *ctx, uint8_t *buf, size_t len) {
    uint32_t partial = ctx->partial;  // ctx+0x104: bytes used from current block

    // 1. Drain any leftover keystream from a previous partial block
    if (partial > 0) {
        while (partial < 16 && len > 0) {
            *buf++ ^= ctx->keystream[partial++];  // ctx+0x108
            len--;
        }
        if (partial == 16) partial = 0;
        ctx->partial = partial;
        if (len == 0) return;
    }

    // 2. Full 16-byte blocks
    while (len >= 16) {
        AES_BlockKeystream(ctx->keystream, ctx);  // ECB-encrypt counter → keystream
        // Increment 128-bit LE counter at ctx+0xF4
        if (++ctx->counter[0] == 0)
            if (++ctx->counter[1] == 0)
                if (++ctx->counter[2] == 0)
                    ++ctx->counter[3];
        for (int i = 0; i < 16; i++) buf[i] ^= ctx->keystream[i];
        buf += 16;
        len -= 16;
    }

    // 3. Partial trailing block
    if (len > 0) {
        AES_BlockKeystream(ctx->keystream, ctx);
        if (++ctx->counter[0] == 0)
            if (++ctx->counter[1] == 0)
                if (++ctx->counter[2] == 0)
                    ++ctx->counter[3];
        for (size_t i = 0; i < len; i++) buf[i] ^= ctx->keystream[i];
        ctx->partial = (uint32_t)len;
    }
}

NetworkRecv_SocketPump @ 0x005F438E
  └─ PacketPayload_Decrypt @ 0x00401080
  └─ PacketDispatcher @ 0x005F1E10      (cmp opcode == 0xA101)
       └─ Handler_Packet_A101_KeyMaterial @ 0x005E3D60
            reads body: 3× u8 scalar + u8[64] block_a + u8[128] block_b
            └─ [conn vtable + 0x254] → VA 0x00747798
                 └─ Connection_OnKeyMaterial @ 0x005A4D50
                      ├─ Crypto_ProcessKeyPacket @ 0x00401100   (HMAC/expand pipeline)
                      ├─ NetworkSendKeyFollowUp @ 0x005EC5A0   (send 131-byte ack)
                      └─ Crypto_EnableGameCipher @ 0x00401310  (flip AES-CTR globals)

// Crypto_EnableGameCipher @ 0x00401310
void Crypto_EnableGameCipher(void) {
    DAT_023037e9 = 1;  // enable outbound encrypt
    DAT_023037ea = 1;  // enable inbound decrypt (game cipher on)
}

// Crypto_DeriveSessionKeys @ 0x00401320 (abbreviated from decompiled source)
void Crypto_DeriveSessionKeys(void) {
    uint32_t digest[4];
    SHA256(&DAT_023027C0, digest);  // 0x00404390 over 0x23027C0

    if (DAT_023037e0 == 0) {
        // recv path: write directly to counter slots
        DAT_023038E4 = digest[0];  DAT_023038E8 = digest[1];
        DAT_023038EC = digest[2];  DAT_023038F0 = digest[3];
        DAT_023038F4 = 0;          // partial = 0
        DAT_02303A34 = digest[0];  DAT_02303A38 = digest[1];
        DAT_02303A3C = digest[2];  DAT_02303A40 = digest[3];
        DAT_02303A44 = 0;
    } else {
        // send path: distribute to alt ctx + advance counter
        memcpy((void*)0x2303908, digest, 32);
        Crypto_CounterAdvance();   // 0x00401500 — LCG mutate
        memcpy((void*)0x2303A58, digest, 32);
        DAT_02303938 = 0;
        Crypto_CounterAdvance();
        DAT_02303A88 = 0;
    }

    // Always clear key seed buffer after derive
    DAT_023027C0 = DAT_023027C4 = DAT_023027C8 = DAT_023027CC = 0;
    DAT_023027D0 = DAT_023027D4 = DAT_023027D8 = DAT_023027DC = 0;
}