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transmission/libtransmission/handshake.cc

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// This file Copyright © 2017-2022 Mnemosyne LLC.
// It may be used under GPLv2 (SPDX: GPL-2.0-only), GPLv3 (SPDX: GPL-3.0-only),
// or any future license endorsed by Mnemosyne LLC.
// License text can be found in the licenses/ folder.
#include <algorithm>
#include <array>
#include <cerrno>
#include <chrono>
#include <string_view>
#include <utility>
#include <fmt/format.h>
#include "transmission.h"
#include "clients.h"
#include "crypto-utils.h"
#include "handshake.h"
#include "log.h"
#include "peer-io.h"
#include "timer.h"
#include "tr-assert.h"
#include "tr-buffer.h"
#include "utils.h"
using namespace std::literals;
/* enable LibTransmission extension protocol */
#define ENABLE_LTEP
/* fast extensions */
#define ENABLE_FAST
/* DHT */
#define ENABLE_DHT
/***
****
***/
static auto constexpr HandshakeName = std::array<std::byte, 20>{
std::byte{ 19 }, std::byte{ 'B' }, std::byte{ 'i' }, std::byte{ 't' }, std::byte{ 'T' },
std::byte{ 'o' }, std::byte{ 'r' }, std::byte{ 'r' }, std::byte{ 'e' }, std::byte{ 'n' },
std::byte{ 't' }, std::byte{ ' ' }, std::byte{ 'p' }, std::byte{ 'r' }, std::byte{ 'o' },
std::byte{ 't' }, std::byte{ 'o' }, std::byte{ 'c' }, std::byte{ 'o' }, std::byte{ 'l' }
};
// bittorrent handshake constants
static auto constexpr HandshakeFlagsLen = int{ 8 };
static auto constexpr HandshakeSize = int{ 68 };
static auto constexpr IncomingHandshakeLen = int{ 48 };
// encryption constants
static auto constexpr PadaMaxlen = int{ 512 };
static auto constexpr PadbMaxlen = int{ 512 };
static auto constexpr PadcMaxlen = int{ 512 };
static auto constexpr CryptoProvidePlaintext = int{ 1 };
static auto constexpr CryptoProvideCrypto = int{ 2 };
// "VC is a verification constant that is used to verify whether the
// other side knows S and SKEY and thus defeats replay attacks of the
// SKEY hash. As of this version VC is a String of 8 bytes set to 0x00."
// https://wiki.vuze.com/w/Message_Stream_Encryption
using vc_t = std::array<std::byte, 8>;
static auto constexpr VC = vc_t{};
// how long to wait before giving up on a handshake
static auto constexpr HandshakeTimeoutSec = 30s;
#ifdef ENABLE_LTEP
#define HANDSHAKE_HAS_LTEP(bits) (((bits)[5] & 0x10) != 0)
#define HANDSHAKE_SET_LTEP(bits) ((bits)[5] |= 0x10)
#else
#define HANDSHAKE_HAS_LTEP(bits) (false)
#define HANDSHAKE_SET_LTEP(bits) ((void)0)
#endif
#ifdef ENABLE_FAST
#define HANDSHAKE_HAS_FASTEXT(bits) (((bits)[7] & 0x04) != 0)
#define HANDSHAKE_SET_FASTEXT(bits) ((bits)[7] |= 0x04)
#else
#define HANDSHAKE_HAS_FASTEXT(bits) (false)
#define HANDSHAKE_SET_FASTEXT(bits) ((void)0)
#endif
#ifdef ENABLE_DHT
#define HANDSHAKE_HAS_DHT(bits) (((bits)[7] & 0x01) != 0)
#define HANDSHAKE_SET_DHT(bits) ((bits)[7] |= 0x01)
#else
#define HANDSHAKE_HAS_DHT(bits) (false)
#define HANDSHAKE_SET_DHT(bits) ((void)0)
#endif
/**
***
**/
using DH = tr_message_stream_encryption::DH;
enum handshake_state_t
{
/* incoming */
AWAITING_HANDSHAKE,
AWAITING_PEER_ID,
AWAITING_YA,
AWAITING_PAD_A,
AWAITING_CRYPTO_PROVIDE,
AWAITING_PAD_C,
AWAITING_IA,
AWAITING_PAYLOAD_STREAM,
/* outgoing */
AWAITING_YB,
AWAITING_VC,
AWAITING_CRYPTO_SELECT,
AWAITING_PAD_D,
/* */
N_STATES
};
struct tr_handshake
{
tr_handshake(
std::unique_ptr<tr_handshake_mediator> mediator_in,
std::shared_ptr<tr_peerIo> io_in,
tr_encryption_mode encryption_mode_in)
: mediator{ std::move(mediator_in) }
, io{ std::move(io_in) }
, dh{ mediator->privateKey() }
, encryption_mode{ encryption_mode_in }
{
}
tr_handshake(tr_handshake&&) = delete;
tr_handshake(tr_handshake const&) = delete;
tr_handshake& operator=(tr_handshake&&) = delete;
tr_handshake& operator=(tr_handshake const&) = delete;
~tr_handshake() = default;
[[nodiscard]] auto isIncoming() const noexcept
{
return io->isIncoming();
}
[[nodiscard]] constexpr uint32_t cryptoProvide() const
{
uint32_t provide = 0;
switch (encryption_mode)
{
case TR_ENCRYPTION_REQUIRED:
case TR_ENCRYPTION_PREFERRED:
provide |= CryptoProvideCrypto;
break;
case TR_CLEAR_PREFERRED:
provide |= CryptoProvideCrypto | CryptoProvidePlaintext;
break;
}
return provide;
}
std::unique_ptr<tr_handshake_mediator> const mediator;
bool haveReadAnythingFromPeer = false;
bool haveSentBitTorrentHandshake = false;
std::shared_ptr<tr_peerIo> const io;
DH dh = {};
handshake_state_t state = AWAITING_HANDSHAKE;
tr_encryption_mode encryption_mode;
uint16_t pad_c_len = {};
uint16_t pad_d_len = {};
uint16_t ia_len = {};
uint32_t crypto_select = {};
uint32_t crypto_provide = {};
std::unique_ptr<libtransmission::Timer> timeout_timer;
std::optional<tr_peer_id_t> peer_id;
tr_handshake_done_func done_func = nullptr;
void* done_func_user_data = nullptr;
};
/**
***
**/
#define tr_logAddTraceHand(handshake, msg) tr_logAddTrace(msg, (handshake)->io->addrStr())
static constexpr std::string_view getStateName(handshake_state_t const state)
{
auto state_strings = std::array<std::string_view, N_STATES>{
"awaiting handshake"sv, /* AWAITING_HANDSHAKE */
"awaiting peer id"sv, /* AWAITING_PEER_ID */
"awaiting ya"sv, /* AWAITING_YA */
"awaiting pad a"sv, /* AWAITING_PAD_A */
"awaiting crypto_provide"sv, /* AWAITING_CRYPTO_PROVIDE */
"awaiting pad c"sv, /* AWAITING_PAD_C */
"awaiting ia"sv, /* AWAITING_IA */
"awaiting payload stream"sv, /* AWAITING_PAYLOAD_STREAM */
"awaiting yb"sv, /* AWAITING_YB */
"awaiting vc"sv, /* AWAITING_VC */
"awaiting crypto select"sv, /* AWAITING_CRYPTO_SELECT */
"awaiting pad d"sv /* AWAITING_PAD_D */
};
return state < N_STATES ? state_strings[state] : "unknown state"sv;
}
static void setState(tr_handshake* handshake, handshake_state_t state)
{
tr_logAddTraceHand(handshake, fmt::format("setting to state [{}]", getStateName(state)));
handshake->state = state;
}
static void setReadState(tr_handshake* handshake, handshake_state_t state)
{
setState(handshake, state);
}
static bool buildHandshakeMessage(tr_handshake const* const handshake, uint8_t* buf)
{
auto const& info_hash = handshake->io->torrentHash();
TR_ASSERT_MSG(info_hash != tr_sha1_digest_t{}, "buildHandshakeMessage requires an info_hash");
auto const info = handshake->mediator->torrentInfo(info_hash);
if (!info)
{
return false;
}
uint8_t* walk = buf;
walk = std::copy_n(reinterpret_cast<uint8_t const*>(std::data(HandshakeName)), std::size(HandshakeName), walk);
std::fill_n(walk, HandshakeFlagsLen, 0);
HANDSHAKE_SET_LTEP(walk);
HANDSHAKE_SET_FASTEXT(walk);
/* Note that this doesn't depend on whether the torrent is private.
* We don't accept DHT peers for a private torrent,
* but we participate in the DHT regardless. */
if (handshake->mediator->allowsDHT())
{
HANDSHAKE_SET_DHT(walk);
}
walk += HandshakeFlagsLen;
walk = std::copy_n(reinterpret_cast<char const*>(std::data(info_hash)), std::size(info_hash), walk);
[[maybe_unused]] auto const* const walk_end = std::copy(
std::begin(info->client_peer_id),
std::end(info->client_peer_id),
walk);
TR_ASSERT(walk_end - buf == HandshakeSize);
return true;
}
static ReadState tr_handshakeDone(tr_handshake* handshake, bool is_connected);
enum handshake_parse_err_t
{
HANDSHAKE_OK,
HANDSHAKE_ENCRYPTION_WRONG,
HANDSHAKE_BAD_TORRENT,
HANDSHAKE_PEER_IS_SELF,
};
static handshake_parse_err_t parseHandshake(tr_handshake* handshake, tr_peerIo* peer_io)
{
tr_logAddTraceHand(handshake, fmt::format("payload: need {}, got {}", HandshakeSize, peer_io->readBufferSize()));
if (peer_io->readBufferSize() < HandshakeSize)
{
return HANDSHAKE_ENCRYPTION_WRONG;
}
/* confirm the protocol */
auto name = decltype(HandshakeName){};
peer_io->readBytes(std::data(name), std::size(name));
if (name != HandshakeName)
{
return HANDSHAKE_ENCRYPTION_WRONG;
}
/* read the reserved bytes */
auto reserved = std::array<uint8_t, HandshakeFlagsLen>{};
peer_io->readBytes(std::data(reserved), std::size(reserved));
// torrent hash
auto info_hash = tr_sha1_digest_t{};
peer_io->readBytes(std::data(info_hash), std::size(info_hash));
if (info_hash == tr_sha1_digest_t{} || info_hash != peer_io->torrentHash())
{
tr_logAddTraceHand(handshake, "peer returned the wrong hash. wtf?");
return HANDSHAKE_BAD_TORRENT;
}
// peer_id
auto peer_id = tr_peer_id_t{};
peer_io->readBytes(std::data(peer_id), std::size(peer_id));
handshake->peer_id = peer_id;
/* peer id */
auto const peer_id_sv = std::string_view{ std::data(peer_id), std::size(peer_id) };
tr_logAddTraceHand(handshake, fmt::format("peer-id is '{}'", peer_id_sv));
if (auto const info = handshake->mediator->torrentInfo(info_hash); info && info->client_peer_id == peer_id)
{
tr_logAddTraceHand(handshake, "streuth! we've connected to ourselves.");
return HANDSHAKE_PEER_IS_SELF;
}
/**
*** Extensions
**/
peer_io->enableDHT(HANDSHAKE_HAS_DHT(reserved));
peer_io->enableLTEP(HANDSHAKE_HAS_LTEP(reserved));
peer_io->enableFEXT(HANDSHAKE_HAS_FASTEXT(reserved));
return HANDSHAKE_OK;
}
/***
****
**** OUTGOING CONNECTIONS
****
***/
template<size_t PadMax>
static void sendPublicKeyAndPad(tr_handshake* handshake)
{
auto const public_key = handshake->dh.publicKey();
auto outbuf = std::array<std::byte, std::size(public_key) + PadMax>{};
auto const data = std::data(outbuf);
auto walk = data;
walk = std::copy(std::begin(public_key), std::end(public_key), walk);
walk += handshake->mediator->pad(walk, PadMax);
handshake->io->writeBytes(data, walk - data, false);
}
// 1 A->B: our public key (Ya) and some padding (PadA)
static void sendYa(tr_handshake* handshake)
{
sendPublicKeyAndPad<PadaMaxlen>(handshake);
setReadState(handshake, AWAITING_YB);
}
static constexpr uint32_t getCryptoSelect(tr_encryption_mode encryption_mode, uint32_t crypto_provide)
{
auto choices = std::array<uint32_t, 2>{};
int n_choices = 0;
switch (encryption_mode)
{
case TR_ENCRYPTION_REQUIRED:
choices[n_choices++] = CryptoProvideCrypto;
break;
case TR_ENCRYPTION_PREFERRED:
choices[n_choices++] = CryptoProvideCrypto;
choices[n_choices++] = CryptoProvidePlaintext;
break;
case TR_CLEAR_PREFERRED:
choices[n_choices++] = CryptoProvidePlaintext;
choices[n_choices++] = CryptoProvideCrypto;
break;
}
for (auto const& choice : choices)
{
if ((crypto_provide & choice) != 0)
{
return choice;
}
}
return 0;
}
static ReadState readYb(tr_handshake* handshake, tr_peerIo* peer_io)
{
if (peer_io->readBufferSize() < std::size(HandshakeName))
{
return READ_LATER;
}
bool const is_encrypted = !peer_io->readBufferStartsWith(HandshakeName);
auto peer_public_key = DH::key_bigend_t{};
if (is_encrypted && (peer_io->readBufferSize() < std::size(peer_public_key)))
{
return READ_LATER;
}
tr_logAddTraceHand(handshake, is_encrypted ? "got an encrypted handshake" : "got a plain handshake");
if (!is_encrypted)
{
setState(handshake, AWAITING_HANDSHAKE);
return READ_NOW;
}
handshake->haveReadAnythingFromPeer = true;
// get the peer's public key
peer_io->readBytes(std::data(peer_public_key), std::size(peer_public_key));
handshake->dh.setPeerPublicKey(peer_public_key);
/* now send these: HASH('req1', S), HASH('req2', SKEY) xor HASH('req3', S),
* ENCRYPT(VC, crypto_provide, len(PadC), PadC, len(IA)), ENCRYPT(IA) */
auto outbuf = libtransmission::Buffer{};
/* HASH('req1', S) */
outbuf.add(tr_sha1::digest("req1"sv, handshake->dh.secret()));
auto const& info_hash = peer_io->torrentHash();
TR_ASSERT_MSG(info_hash != tr_sha1_digest_t{}, "readYb requires an info_hash");
/* HASH('req2', SKEY) xor HASH('req3', S) */
{
auto const req2 = tr_sha1::digest("req2"sv, info_hash);
auto const req3 = tr_sha1::digest("req3"sv, handshake->dh.secret());
auto x_or = tr_sha1_digest_t{};
for (size_t i = 0, n = std::size(x_or); i < n; ++i)
{
x_or[i] = req2[i] ^ req3[i];
}
outbuf.add(x_or);
}
/* ENCRYPT(VC, crypto_provide, len(PadC), PadC
* PadC is reserved for future extensions to the handshake...
* standard practice at this time is for it to be zero-length */
peer_io->write(outbuf, false);
peer_io->encryptInit(peer_io->isIncoming(), handshake->dh, info_hash);
outbuf.add(VC);
outbuf.addUint32(handshake->cryptoProvide());
outbuf.addUint16(0);
/* ENCRYPT len(IA)), ENCRYPT(IA) */
if (auto msg = std::array<uint8_t, HandshakeSize>{}; buildHandshakeMessage(handshake, std::data(msg)))
{
outbuf.addUint16(std::size(msg));
outbuf.add(msg);
handshake->haveSentBitTorrentHandshake = true;
}
else
{
return tr_handshakeDone(handshake, false);
}
/* send it */
setReadState(handshake, AWAITING_VC);
peer_io->write(outbuf, false);
return READ_NOW;
}
// MSE spec: "Since the length of [PadB is] unknown,
// A will be able to resynchronize on ENCRYPT(VC)"
static ReadState readVC(tr_handshake* handshake, tr_peerIo* peer_io)
{
auto const info_hash = peer_io->torrentHash();
TR_ASSERT_MSG(info_hash != tr_sha1_digest_t{}, "readVC requires an info_hash");
// find the end of PadB by looking for `ENCRYPT(VC)`
auto needle = VC;
auto filter = tr_message_stream_encryption::Filter{};
filter.encryptInit(true, handshake->dh, info_hash);
filter.encrypt(std::size(needle), std::data(needle));
for (size_t i = 0; i < PadbMaxlen; ++i)
{
if (peer_io->readBufferSize() < std::size(needle))
{
tr_logAddTraceHand(handshake, "not enough bytes... returning read_more");
return READ_LATER;
}
if (peer_io->readBufferStartsWith(needle))
{
tr_logAddTraceHand(handshake, "got it!");
// We already know it's a match; now we just need to
// consume it from the read buffer.
peer_io->decryptInit(peer_io->isIncoming(), handshake->dh, info_hash);
peer_io->readBytes(std::data(needle), std::size(needle));
setState(handshake, AWAITING_CRYPTO_SELECT);
return READ_NOW;
}
peer_io->readBufferDrain(1);
}
tr_logAddTraceHand(handshake, "couldn't find ENCRYPT(VC)");
return tr_handshakeDone(handshake, false);
}
static ReadState readCryptoSelect(tr_handshake* handshake, tr_peerIo* peer_io)
{
static size_t const needlen = sizeof(uint32_t) + sizeof(uint16_t);
if (peer_io->readBufferSize() < needlen)
{
return READ_LATER;
}
uint32_t crypto_select = 0;
peer_io->readUint32(&crypto_select);
handshake->crypto_select = crypto_select;
tr_logAddTraceHand(handshake, fmt::format("crypto select is {}", crypto_select));
if ((crypto_select & handshake->cryptoProvide()) == 0)
{
tr_logAddTraceHand(handshake, "peer selected an encryption option we didn't offer");
return tr_handshakeDone(handshake, false);
}
uint16_t pad_d_len = 0;
peer_io->readUint16(&pad_d_len);
tr_logAddTraceHand(handshake, fmt::format("pad_d_len is {}", pad_d_len));
if (pad_d_len > 512)
{
tr_logAddTraceHand(handshake, "encryption handshake: pad_d_len is too long");
return tr_handshakeDone(handshake, false);
}
handshake->pad_d_len = pad_d_len;
setState(handshake, AWAITING_PAD_D);
return READ_NOW;
}
static ReadState readPadD(tr_handshake* handshake, tr_peerIo* peer_io)
{
size_t const needlen = handshake->pad_d_len;
tr_logAddTraceHand(handshake, fmt::format("pad d: need {}, got {}", needlen, peer_io->readBufferSize()));
if (peer_io->readBufferSize() < needlen)
{
return READ_LATER;
}
peer_io->readBufferDrain(needlen);
setState(handshake, AWAITING_HANDSHAKE);
return READ_NOW;
}
/***
****
**** INCOMING CONNECTIONS
****
***/
static ReadState readHandshake(tr_handshake* handshake, tr_peerIo* peer_io)
{
tr_logAddTraceHand(handshake, fmt::format("payload: need {}, got {}", IncomingHandshakeLen, peer_io->readBufferSize()));
if (peer_io->readBufferSize() < IncomingHandshakeLen)
{
return READ_LATER;
}
handshake->haveReadAnythingFromPeer = true;
if (peer_io->readBufferStartsWith(HandshakeName)) // unencrypted
{
if (handshake->encryption_mode == TR_ENCRYPTION_REQUIRED)
{
tr_logAddTraceHand(handshake, "peer is unencrypted, and we're disallowing that");
return tr_handshakeDone(handshake, false);
}
}
else // either encrypted or corrupt
{
if (handshake->isIncoming())
{
tr_logAddTraceHand(handshake, "I think peer is sending us an encrypted handshake...");
setState(handshake, AWAITING_YA);
return READ_NOW;
}
}
auto name = decltype(HandshakeName){};
peer_io->readBytes(std::data(name), std::size(name));
if (name != HandshakeName)
{
return tr_handshakeDone(handshake, false);
}
/* reserved bytes */
auto reserved = std::array<uint8_t, HandshakeFlagsLen>{};
peer_io->readBytes(std::data(reserved), std::size(reserved));
/**
*** Extensions
**/
peer_io->enableDHT(HANDSHAKE_HAS_DHT(reserved));
peer_io->enableLTEP(HANDSHAKE_HAS_LTEP(reserved));
peer_io->enableFEXT(HANDSHAKE_HAS_FASTEXT(reserved));
/* torrent hash */
auto hash = tr_sha1_digest_t{};
peer_io->readBytes(std::data(hash), std::size(hash));
if (handshake->isIncoming())
{
if (!handshake->mediator->torrentInfo(hash))
{
tr_logAddTraceHand(handshake, "peer is trying to connect to us for a torrent we don't have.");
return tr_handshakeDone(handshake, false);
}
peer_io->setTorrentHash(hash);
}
else // outgoing
{
if (peer_io->torrentHash() != hash)
{
tr_logAddTraceHand(handshake, "peer returned the wrong hash. wtf?");
return tr_handshakeDone(handshake, false);
}
}
/**
*** If it's an incoming message, we need to send a response handshake
**/
if (!handshake->haveSentBitTorrentHandshake)
{
auto msg = std::array<uint8_t, HandshakeSize>{};
if (!buildHandshakeMessage(handshake, std::data(msg)))
{
return tr_handshakeDone(handshake, false);
}
peer_io->writeBytes(std::data(msg), std::size(msg), false);
handshake->haveSentBitTorrentHandshake = true;
}
setReadState(handshake, AWAITING_PEER_ID);
return READ_NOW;
}
static ReadState readPeerId(tr_handshake* handshake, tr_peerIo* peer_io)
{
// read the peer_id
auto peer_id = tr_peer_id_t{};
if (peer_io->readBufferSize() < std::size(peer_id))
{
return READ_LATER;
}
peer_io->readBytes(std::data(peer_id), std::size(peer_id));
handshake->peer_id = peer_id;
auto client = std::array<char, 128>{};
tr_clientForId(std::data(client), std::size(client), peer_id);
tr_logAddTraceHand(
handshake,
fmt::format("peer-id is '{}' ... isIncoming is {}", std::data(client), handshake->isIncoming()));
// if we've somehow connected to ourselves, don't keep the connection
auto const info_hash = peer_io->torrentHash();
auto const info = handshake->mediator->torrentInfo(info_hash);
auto const connected_to_self = info && info->client_peer_id == peer_id;
return tr_handshakeDone(handshake, !connected_to_self);
}
static ReadState readYa(tr_handshake* handshake, tr_peerIo* peer_io)
{
auto peer_public_key = DH::key_bigend_t{};
tr_logAddTraceHand(
handshake,
fmt::format("in readYa... need {}, have {}", std::size(peer_public_key), peer_io->readBufferSize()));
if (peer_io->readBufferSize() < std::size(peer_public_key))
{
return READ_LATER;
}
/* read the incoming peer's public key */
peer_io->readBytes(std::data(peer_public_key), std::size(peer_public_key));
handshake->dh.setPeerPublicKey(peer_public_key);
// send our public key to the peer
tr_logAddTraceHand(handshake, "sending B->A: Diffie Hellman Yb, PadB");
sendPublicKeyAndPad<PadbMaxlen>(handshake);
setReadState(handshake, AWAITING_PAD_A);
return READ_NOW;
}
static ReadState readPadA(tr_handshake* handshake, tr_peerIo* peer_io)
{
// find the end of PadA by looking for HASH('req1', S)
auto const needle = tr_sha1::digest("req1"sv, handshake->dh.secret());
for (size_t i = 0; i < PadaMaxlen; ++i)
{
if (peer_io->readBufferSize() < std::size(needle))
{
tr_logAddTraceHand(handshake, "not enough bytes... returning read_more");
return READ_LATER;
}
if (peer_io->readBufferStartsWith(needle))
{
tr_logAddTraceHand(handshake, "found it... looking setting to awaiting_crypto_provide");
peer_io->readBufferDrain(std::size(needle));
setState(handshake, AWAITING_CRYPTO_PROVIDE);
return READ_NOW;
}
peer_io->readBufferDrain(1U);
}
tr_logAddTraceHand(handshake, "couldn't find HASH('req', S)");
return tr_handshakeDone(handshake, false);
}
static ReadState readCryptoProvide(tr_handshake* handshake, tr_peerIo* peer_io)
{
/* HASH('req2', SKEY) xor HASH('req3', S), ENCRYPT(VC, crypto_provide, len(PadC)) */
uint16_t padc_len = 0;
uint32_t crypto_provide = 0;
auto obfuscated_hash = tr_sha1_digest_t{};
size_t const needlen = sizeof(obfuscated_hash) + /* HASH('req2', SKEY) xor HASH('req3', S) */
std::size(VC) + sizeof(crypto_provide) + sizeof(padc_len);
if (peer_io->readBufferSize() < needlen)
{
return READ_LATER;
}
/* This next piece is HASH('req2', SKEY) xor HASH('req3', S) ...
* we can get the first half of that (the obfuscatedTorrentHash)
* by building the latter and xor'ing it with what the peer sent us */
tr_logAddTraceHand(handshake, "reading obfuscated torrent hash...");
auto req2 = tr_sha1_digest_t{};
peer_io->readBytes(std::data(req2), std::size(req2));
auto const req3 = tr_sha1::digest("req3"sv, handshake->dh.secret());
for (size_t i = 0; i < std::size(obfuscated_hash); ++i)
{
obfuscated_hash[i] = req2[i] ^ req3[i];
}
if (auto const info = handshake->mediator->torrentInfoFromObfuscated(obfuscated_hash); info)
{
bool const client_is_seed = info->is_done;
bool const peer_is_seed = handshake->mediator->isPeerKnownSeed(info->id, peer_io->address());
tr_logAddTraceHand(handshake, fmt::format("got INCOMING connection's encrypted handshake for torrent [{}]", info->id));
peer_io->setTorrentHash(info->info_hash);
if (client_is_seed && peer_is_seed)
{
tr_logAddTraceHand(handshake, "another seed tried to reconnect to us!");
return tr_handshakeDone(handshake, false);
}
}
else
{
tr_logAddTraceHand(handshake, "can't find that torrent...");
return tr_handshakeDone(handshake, false);
}
/* next part: ENCRYPT(VC, crypto_provide, len(PadC), */
auto const& info_hash = peer_io->torrentHash();
TR_ASSERT_MSG(info_hash != tr_sha1_digest_t{}, "readCryptoProvide requires an info_hash");
peer_io->decryptInit(peer_io->isIncoming(), handshake->dh, info_hash);
auto vc_in = vc_t{};
peer_io->readBytes(std::data(vc_in), std::size(vc_in));
peer_io->readUint32(&crypto_provide);
handshake->crypto_provide = crypto_provide;
tr_logAddTraceHand(handshake, fmt::format("crypto_provide is {}", crypto_provide));
peer_io->readUint16(&padc_len);
tr_logAddTraceHand(handshake, fmt::format("padc is {}", padc_len));
if (padc_len > PadcMaxlen)
{
tr_logAddTraceHand(handshake, "peer's PadC is too big");
return tr_handshakeDone(handshake, false);
}
handshake->pad_c_len = padc_len;
setState(handshake, AWAITING_PAD_C);
return READ_NOW;
}
static ReadState readPadC(tr_handshake* handshake, tr_peerIo* peer_io)
{
if (auto const needlen = handshake->pad_c_len + sizeof(uint16_t); peer_io->readBufferSize() < needlen)
{
return READ_LATER;
}
// read the throwaway padc
auto pad_c = std::array<char, PadcMaxlen>{};
peer_io->readBytes(std::data(pad_c), handshake->pad_c_len);
/* read ia_len */
uint16_t ia_len = 0;
peer_io->readUint16(&ia_len);
tr_logAddTraceHand(handshake, fmt::format("ia_len is {}", ia_len));
handshake->ia_len = ia_len;
setState(handshake, AWAITING_IA);
return READ_NOW;
}
static ReadState readIA(tr_handshake* handshake, tr_peerIo* peer_io)
{
size_t const needlen = handshake->ia_len;
tr_logAddTraceHand(handshake, fmt::format("reading IA... have {}, need {}", peer_io->readBufferSize(), needlen));
if (peer_io->readBufferSize() < needlen)
{
return READ_LATER;
}
/**
*** B->A: ENCRYPT(VC, crypto_select, len(padD), padD), ENCRYPT2(Payload Stream)
**/
auto const& info_hash = peer_io->torrentHash();
TR_ASSERT_MSG(info_hash != tr_sha1_digest_t{}, "readIA requires an info_hash");
peer_io->encryptInit(peer_io->isIncoming(), handshake->dh, info_hash);
auto outbuf = libtransmission::Buffer{};
// send VC
tr_logAddTraceHand(handshake, "sending vc");
outbuf.add(VC);
/* send crypto_select */
uint32_t const crypto_select = getCryptoSelect(handshake->encryption_mode, handshake->crypto_provide);
if (crypto_select != 0)
{
tr_logAddTraceHand(handshake, fmt::format("selecting crypto mode '{}'", crypto_select));
outbuf.addUint32(crypto_select);
}
else
{
tr_logAddTraceHand(handshake, "peer didn't offer an encryption mode we like.");
return tr_handshakeDone(handshake, false);
}
tr_logAddTraceHand(handshake, "sending pad d");
/* ENCRYPT(VC, crypto_provide, len(PadD), PadD
* PadD is reserved for future extensions to the handshake...
* standard practice at this time is for it to be zero-length */
outbuf.addUint16(0);
/* maybe de-encrypt our connection */
if (crypto_select == CryptoProvidePlaintext)
{
peer_io->write(outbuf, false);
TR_ASSERT(std::empty(outbuf));
}
tr_logAddTraceHand(handshake, "sending handshake");
/* send our handshake */
if (auto msg = std::array<uint8_t, HandshakeSize>{}; buildHandshakeMessage(handshake, std::data(msg)))
{
outbuf.add(msg);
handshake->haveSentBitTorrentHandshake = true;
}
else
{
return tr_handshakeDone(handshake, false);
}
/* send it out */
peer_io->write(outbuf, false);
/* now await the handshake */
setState(handshake, AWAITING_PAYLOAD_STREAM);
return READ_NOW;
}
static ReadState readPayloadStream(tr_handshake* handshake, tr_peerIo* peer_io)
{
size_t const needlen = HandshakeSize;
tr_logAddTraceHand(
handshake,
fmt::format("reading payload stream... have {}, need {}", peer_io->readBufferSize(), needlen));
if (peer_io->readBufferSize() < needlen)
{
return READ_LATER;
}
/* parse the handshake ... */
handshake_parse_err_t const i = parseHandshake(handshake, peer_io);
tr_logAddTraceHand(handshake, fmt::format("parseHandshake returned {}", i));
if (i != HANDSHAKE_OK)
{
return tr_handshakeDone(handshake, false);
}
/* we've completed the BT handshake... pass the work on to peer-msgs */
return tr_handshakeDone(handshake, true);
}
/***
****
****
****
***/
static ReadState canRead(tr_peerIo* peer_io, void* vhandshake, size_t* piece)
{
TR_ASSERT(tr_isPeerIo(peer_io));
auto* handshake = static_cast<tr_handshake*>(vhandshake);
bool ready_for_more = true;
/* no piece data in handshake */
*piece = 0;
tr_logAddTraceHand(handshake, fmt::format("handling canRead; state is [{}]", getStateName(handshake->state)));
ReadState ret = READ_NOW;
while (ready_for_more)
{
switch (handshake->state)
{
case AWAITING_HANDSHAKE:
ret = readHandshake(handshake, peer_io);
break;
case AWAITING_PEER_ID:
ret = readPeerId(handshake, peer_io);
break;
case AWAITING_YA:
ret = readYa(handshake, peer_io);
break;
case AWAITING_PAD_A:
ret = readPadA(handshake, peer_io);
break;
case AWAITING_CRYPTO_PROVIDE:
ret = readCryptoProvide(handshake, peer_io);
break;
case AWAITING_PAD_C:
ret = readPadC(handshake, peer_io);
break;
case AWAITING_IA:
ret = readIA(handshake, peer_io);
break;
case AWAITING_PAYLOAD_STREAM:
ret = readPayloadStream(handshake, peer_io);
break;
case AWAITING_YB:
ret = readYb(handshake, peer_io);
break;
case AWAITING_VC:
ret = readVC(handshake, peer_io);
break;
case AWAITING_CRYPTO_SELECT:
ret = readCryptoSelect(handshake, peer_io);
break;
case AWAITING_PAD_D:
ret = readPadD(handshake, peer_io);
break;
default:
#ifdef TR_ENABLE_ASSERTS
TR_ASSERT_MSG(false, fmt::format(FMT_STRING("unhandled handshake state {:d}"), handshake->state));
#else
ret = READ_ERR;
break;
#endif
}
if (ret != READ_NOW)
{
ready_for_more = false;
}
else if (handshake->state == AWAITING_PAD_C)
{
ready_for_more = peer_io->readBufferSize() >= handshake->pad_c_len;
}
else if (handshake->state == AWAITING_PAD_D)
{
ready_for_more = peer_io->readBufferSize() >= handshake->pad_d_len;
}
else if (handshake->state == AWAITING_IA)
{
ready_for_more = peer_io->readBufferSize() >= handshake->ia_len;
}
}
return ret;
}
static bool fireDoneFunc(tr_handshake* handshake, bool is_connected)
{
auto result = tr_handshake_result{};
result.handshake = handshake;
result.io = handshake->io;
result.readAnythingFromPeer = handshake->haveReadAnythingFromPeer;
result.isConnected = is_connected;
result.userData = handshake->done_func_user_data;
result.peer_id = handshake->peer_id;
bool const success = (*handshake->done_func)(result);
return success;
}
static ReadState tr_handshakeDone(tr_handshake* handshake, bool is_connected)
{
tr_logAddTraceHand(handshake, is_connected ? "handshakeDone: connected" : "handshakeDone: aborting");
handshake->io->clearCallbacks();
bool const success = fireDoneFunc(handshake, is_connected);
delete handshake;
return success ? READ_LATER : READ_ERR;
}
void tr_handshakeAbort(tr_handshake* handshake)
{
if (handshake != nullptr)
{
tr_handshakeDone(handshake, false);
}
}
static void gotError(tr_peerIo* io, short what, void* vhandshake)
{
int const errcode = errno;
auto* handshake = static_cast<tr_handshake*>(vhandshake);
if (io->socket.type == TR_PEER_SOCKET_TYPE_UTP && !io->isIncoming() && handshake->state == AWAITING_YB)
{
// the peer probably doesn't speak µTP.
auto const info_hash = io->torrentHash();
auto const info = handshake->mediator->torrentInfo(info_hash);
/* Don't mark a peer as non-µTP unless it's really a connect failure. */
if ((errcode == ETIMEDOUT || errcode == ECONNREFUSED) && info)
{
handshake->mediator->setUTPFailed(info_hash, io->address());
}
if (handshake->mediator->allowsTCP() && handshake->io->reconnect() == 0)
{
auto msg = std::array<uint8_t, HandshakeSize>{};
buildHandshakeMessage(handshake, std::data(msg));
handshake->haveSentBitTorrentHandshake = true;
setReadState(handshake, AWAITING_HANDSHAKE);
handshake->io->writeBytes(std::data(msg), std::size(msg), false);
}
}
/* if the error happened while we were sending a public key, we might
* have encountered a peer that doesn't do encryption... reconnect and
* try a plaintext handshake */
if ((handshake->state == AWAITING_YB || handshake->state == AWAITING_VC) &&
handshake->encryption_mode != TR_ENCRYPTION_REQUIRED && handshake->mediator->allowsTCP() &&
handshake->io->reconnect() == 0)
{
auto msg = std::array<uint8_t, HandshakeSize>{};
tr_logAddTraceHand(handshake, "handshake failed, trying plaintext...");
buildHandshakeMessage(handshake, std::data(msg));
handshake->haveSentBitTorrentHandshake = true;
setReadState(handshake, AWAITING_HANDSHAKE);
handshake->io->writeBytes(std::data(msg), std::size(msg), false);
}
else
{
tr_logAddTraceHand(
handshake,
fmt::format("libevent got an error what=={}, errno={} ({})", what, errcode, tr_strerror(errcode)));
tr_handshakeDone(handshake, false);
}
}
/**
***
**/
tr_handshake* tr_handshakeNew(
std::unique_ptr<tr_handshake_mediator> mediator,
std::shared_ptr<tr_peerIo> io,
tr_encryption_mode encryption_mode,
tr_handshake_done_func done_func,
void* done_func_user_data)
{
auto* const handshake = new tr_handshake{ std::move(mediator), std::move(io), encryption_mode };
handshake->done_func = done_func;
handshake->done_func_user_data = done_func_user_data;
handshake->timeout_timer = handshake->mediator->timerMaker().create([handshake]() { tr_handshakeAbort(handshake); });
handshake->timeout_timer->startSingleShot(HandshakeTimeoutSec);
handshake->io->setCallbacks(canRead, nullptr, gotError, handshake);
if (handshake->isIncoming())
{
setReadState(handshake, AWAITING_HANDSHAKE);
}
else if (encryption_mode != TR_CLEAR_PREFERRED)
{
sendYa(handshake);
}
else
{
auto msg = std::array<uint8_t, HandshakeSize>{};
buildHandshakeMessage(handshake, std::data(msg));
handshake->haveSentBitTorrentHandshake = true;
setReadState(handshake, AWAITING_HANDSHAKE);
handshake->io->writeBytes(std::data(msg), std::size(msg), false);
}
return handshake;
}
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