* A optimised AEAD construct is now proposed (ChaCha20Poly1305@Bitcoin), reducing the required ChaCha20 rounds (compared to the openSSH version).
* introduction of short-command-ID (ex.: uint8_t 13 == INV, etc.) which result in
<pre>
BIP: ???
Layer: Peer Services
Title: Version 2 Peer-to-Peer Message Transport Protocol
Author: Jonas Schnelli <dev@jonasschnelli.ch>
Status: Draft
Type: Standards Track
Created: 2019-03-08
License: PD
</pre>
== Abstract ==
This BIP describes a new Bitcoin peer to peer transport protocol with
opportunistic encryption.
== Motivation ==
The current peer-to-peer protocol is partially inefficient and in plaintext.
With the current unencrypted message transport, BGP hijack, block delay attacks
and message tempering are inexpensive and can be executed in a covert way
(undetectable MITM)<ref>[https://btc-hijack.ethz.ch/files/btc_hijack.pdf
Hijacking Bitcoin: Routing Attacks on Cryptocurrencies - M. Apostolaki, A.
Zohar, L.Vanbever]</ref>.
Adding opportunistic encryption introduces a high risk for attackers of being
detected. Peer operators can compare encryption session IDs or use other form
of authentication schemes <ref
name="bip150">[https://github.com/bitcoin/bips/blob/master/bip-0150.mediawiki
BIP150]</ref> to identify an attack.
Each current version 1 Bitcoin peer-to-peer message uses a double-SHA256
checksum truncated to 4 bytes. Roughly the same amount of computation power
would be required for encrypting and authenticating a peer-to-peer message with
ChaCha20 & Poly1305.
Additionally, this BIP describes a way how data manipulation (blocking or
tempering commands by an intercepting TCP/IP node) would be identifiable by the
communicating peers.
Encrypting traffic between peers is already possible with VPN, tor, stunnel,
curveCP or any other encryption mechanism on a deeper OSI level, however, most
of those solutions require significant knowhow in how to setup such a secure
channel and are therefore not widely deployed.
== Specification ==
<blockquote>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in RFC 2119<ref>[https://tools.ietf.org/html/rfc2119
RFC 2119]</ref>.
</blockquote>
A peer that supports the message transport protocol as defined in this proposal
MUST accept encryption requests from all peers.
Both communication direction share the same shared-secret but have different
symmetric cipher keys.
The encryption handshake MUST happen before sending any other messages to the
responding peer.
If the responding peer closes the connection after sending the handshake
request, the initiating peer MAY try to connect again with the v1 peer-to-peer
transport protocol. Such reconnects allow an attacker to "downgrade" the
encryption to plaintext communication and thus, accepting v1 connections MUST
not be done when the Bitcoin peer-to-peer network uses almost only v2
communication.
=== NODE_P2P_V2 ===
Peers supporting the transport protocol after this proposal MUST signal
<code>NODE_P2P_V2</code>
<pre>
NODE_P2P_V2 = (1 << 11)
</pre>
A peer usually learns an address along with the expected service flags which
MAY be used to filter possible outbound peers.
A peer signaling <code>NODE_P2P_V2</code> MUST accept encrypted communication
specified in this proposal.
Peers MAY only make outbound connections to peers supporting
<code>NODE_P2P_V2</code>.
=== Handshake ===
<pre>
----------------------------------------------------------------------------------------
| Initiator Responder |
| |
| x, X := SECP256k1_KEYGEN() |
| CLIENT_HDATA := X |
| |
| --- CLIENT_HDATA ---> |
| |
| y, Y := SECP256k1_KEYGEN() |
| ECDH_KEY := SECP256k1_ECDH(X,y) |
| SERVER_HDATA := Y |
| |
| <-- SERVER_HDATA ---- |
| |
| ECDH_KEY := SECP256k1_ECDH(x,Y) |
----------------------------------------------------------------------------------------
</pre>
To request encrypted communication (only possible if yet no other messages have
been sent or received), the initiating peer generates an EC secp256k1 ephemeral
key and sends the corresponding 32-byte public key to the responding peer and
waits for the remote 32-byte public key from the counterparty.
ODD secp256k1 public keys MUST be used (public keys starting with 0x02). If the
public key from the generated ephemeral key is an EVEN public key (starting
with 0x03), negating the key and recalculating its public key SHOULD be done.
Only using ODD public makes it more complex to identify the handshake based on
analyzing the traffic.
The handshake request and response message are raw 32byte payloads containing
no header, length or checksum (the pure 32byte payload) and MUST be sent before
anything else.
Public keys starting with the 4-byte network magic are forbidden and MUST lead
to locally re-generate an ephemeral-key.
Pseudocode for the ephemeral-key generation
<pre>
do {
ecdh_key.MakeNewKey();
if (ecdh_key.GetPubKey()[0] == 3) {
ecdh_key.Negate();
}
} while (m_ecdh_key.GetPubKey()[0..3] == NETWORK_MAGIC);
</pre>
Once a peer has received the public key from its counterparty, the shared
secret MUST be calculated by using secp256k1 ECDH.
Private keys will never be transmitted. The shared secret can only be
calculated if an attacker knows at least one private key and the counterparties
public key. This key-exchange is based on the discrete log problem and thus not
sufficiently strong against known forms of possible quantum computer
algorithms. Adding an additional quantum resistant key exchange like NewHope is
possible but out of scope for this proposal.
After a successful handshake, the messages format MUST use the "v2 messages
structure". Non-encrypted v1 messages from the initiating peer MUST lead to an
immediate connection termination.
After a successful handshake, both peers MUST cleanse the ephemeral-session-key
from memory and/or persistence storage.
A peer not supporting this proposal will not perform the described handshake
and thus send a v1 version message.
Peers supporting this BIP MAY optionally allow unencrypted v1 communication by
detecting a v1 version message by the initial 11-byte sequence of <code>4byte
net magic || "version"</code>.
=== Symmetric Encryption Cipher Keys ===
Once the ECDH secret (<code>ECDH_KEY</code>) is calculated on each side, the
symmetric encryption cipher keys MUST be derived with HKDF
<ref>[https://tools.ietf.org/html/rfc5869 HKDF (RFC 5869)]</ref> after the
following specification:
1. HKDF extraction
<code>PRK = HKDF_EXTRACT(hash=SHA256, salt="BitcoinSharedSecret||INITIATOR_32BYTES_PUBKEY||RESPONDER_32BYTES_PUBKEY", ikm=ECDH_KEY)</code>.
2. Derive Key_1_A (K_1 communication direction A)
<code>K1A = HKDF_EXPAND(prk=PRK, hash=SHA256, info="BitcoinK_1_A", L=32)</code>
2. Derive Key_2_A (K_2 communication direction A)
<code>K1B = HKDF_EXPAND(prk=PRK, hash=SHA256, info="BitcoinK_2_A", L=32)</code>
3. Derive Key_1_B (K_1 communication direction B)
<code>K2 = HKDF_EXPAND(prk=PRK, hash=SHA256, info="BitcoinK_1_B", L=32)</code>
3. Derive Key_2_B (K_2 communication direction B)
<code>K2 = HKDF_EXPAND(prk=PRK, hash=SHA256, info="BitcoinK_2_B", L=32)</code>
=== Session ID ===
Both parties MUST also calculate the 256bit session-id using <code>SID =
HKDF_EXPAND(prk=PRK, hash=SHA256, info="BitcoinSessionID", L=32)</code>. The
session-id can be used for authenticating the encryption-session (identity
check).
The session-id MUST be presented to the user on request.
=== ChaCha20-Poly1305@Bitcoin Cipher Suite ===
==== Background ====
ChaCha20 is a stream cipher designed by Daniel Bernstein and described in
<ref>[http://cr.yp.to/chacha/chacha-20080128.pdf ChaCha20]</ref>. It operates
by permuting 128 fixed bits, 128 or 256 bits of key, a 64 bit nonce and a 64
bit counter into 64 bytes of output. This output is used as a keystream, with
any unused bytes simply discarded.
Poly1305 <ref>[http://cr.yp.to/mac/poly1305-20050329.pdf Poly1305]</ref>, also
by Daniel Bernstein, is a one-time Carter-Wegman MAC that computes a 128 bit
integrity tag given a message and a single-use 256 bit secret key.
The chacha20-poly1305@bitcoin combines these two primitives into an
authenticated encryption mode. The construction used is based on that proposed
for TLS by Adam Langley in
<ref>[http://tools.ietf.org/html/draft-agl-tls-chacha20poly1305-03 "ChaCha20
and Poly1305 based Cipher Suites for TLS", Adam Langley]</ref>, but differs in
the layout of data passed to the MAC and in the addition of encryption of the
packet lengths.
==== Detailed Construction ====
The chacha20-poly1305@bitcoin cipher requires two 256 bits of key material as
output from the key exchange. Each key (K_1 and K_2) are used by two separate
instances of chacha20.
The instance keyed by K_1 is a stream cipher that is used only to encrypt the 3
byte packet length field and has its own sequence number. The second instance,
keyed by K_2, is used in conjunction with poly1305 to build an AEAD
(Authenticated Encryption with Associated Data) that is used to encrypt and
authenticate the entire packet.
Two separate cipher instances are used here so as to keep the packet lengths
confidential but not create an oracle for the packet payload cipher by
decrypting and using the packet length prior to checking the MAC. By using an
independently-keyed cipher instance to encrypt the length, an active attacker
seeking to exploit the packet input handling as a decryption oracle can learn
nothing about the payload contents or its MAC (assuming key derivation,
ChaCha20 and Poly1305 are secure).
The AEAD is constructed as follows: for each packet, generate a Poly1305 key by
taking the first 256 bits of ChaCha20 stream output generated using K_2, an IV
consisting of the packet sequence number encoded as an LE uint64 and a ChaCha20
block counter of zero. The K_2 ChaCha20 block counter is then set to the
little-endian encoding of 1 (i.e. {1, 0, 0, 0, 0, 0, 0, 0}) and this instance
is used for encryption of the packet payload.
==== Packet Handling ====
When receiving a packet, the length must be decrypted first. When 3 bytes of
ciphertext length have been received, they may be decrypted.
A ChaCha20 round always calculates 64bytes which is sufficient to crypt 21
times a 3 bytes length field (21*3 = 63). The length field sequence number can
thus be used 21 times (keystream caching).
The length field must be enc-/decrypted with the ChaCha20 keystream keyed with
K_1 defined by block counter 0, the length field sequence number in little
endian and a keystream position from 0 to 60.
Pseudo code example:
<pre>
// init
sequence_nr_payload = 0; //payload sequence number
sequence_nr_length_field = 0; //length field sequence number (will be reused)
aad_length_field_pos = 0; //position in the length field cipher instance keystream chunk
...
// actual encryption
if cache_length_field_sequence_number != sequence_nr_length_field {
cache_keystream_64_bytes = ChaCha20(key=K_1, iv=little_endian(sequence_nr_length_field), counter=0);
cache_length_field_sequence_number = sequence_nr_length_field
}
packet_length = XOR_TO_LE(cache_length_field_sequence_number[aad_length_field_pos - aad_length_field_pos+3], ciphertext[0-3])
sequence_nr_payload++;
aad_length_field_pos += 3; //skip 3 bytes in keystream
if (aad_length_field_pos + 3 > 64) { //if we are outside of the 64byte keystream...
aad_length_field_pos = 0; // reset at position 0
sequence_nr_length_field++; // increase length field sequence number
}
</pre>
Once the entire packet has been received, the MAC MUST be checked before
decryption. A per-packet Poly1305 key is generated as described above and the
MAC tag calculated using Poly1305 with this key over the ciphertext of the
packet length and the payload together. The calculated MAC is then compared in
constant time with the one appended to the packet and the packet decrypted
using ChaCha20 as described above (with K_2, the packet sequence number as
nonce and a starting block counter of 1).
Detection of an invalid MAC MUST lead to immediate connection termination.
To send a packet, first encode the 3 byte length and encrypt it using K_1 as
described above. Encrypt the packet payload (using K_2) and append it to the
encrypted length. Finally, calculate a MAC tag and append it.
The initiating peer MUST use <code>K_1_A, K_2_A</code> to encrypt messages on
the send channel, <code>K_1_B, K_2_B</code> MUST be used to decrypt messages on
the receive channel.
The responding peer MUST use <code>K_1_A, K_2_A</code> to decrypt messages on
the receive channel, <code>K_1_B, K_2_B</code> MUST be used to encrypt messages
on the send channel.
Optimized implementations of ChaCha20-Poly1305@bitcoin are relatively fast in
general, therefore it is very likely that encrypted messages require not more
CPU cycles per bytes then the current unencrypted p2p message format
(ChaCha20/Poly1305 versus double SHA256).
The initial packet sequence numbers are 0.
K_2 ChaCha20 cipher instance (payload) must never reuse a {key, nonce} for
encryption nor may it be used to encrypt more than 2^70 bytes under the same
{key, nonce}.
K_1 ChaCha20 cipher instance (length field/AAD) must never reuse a {key, nonce,
position-in-keystream} for encryption nor may it be used to encrypt more than
2^70 bytes under the same {key, nonce}.
We use message sequence numbers for both communication directions.
<pre>
------------------------------------------------------------------------------------------
| Initiator Responder |
| |
| AEAD() = ChaCha20Poly1305Bitcoin() |
| MSG_A_CIPH = AEAD(k=K_1_A, K_2_A, payload_nonce=0, aad_nonce=0, aad_pos=0, msg) |
| |
| --- MSG_CIPH ---> |
| |
| msg := AEAD(k=K_1_A,K_2_A, n=0, ..., MSG_A_CIPH) |
| |
------------------------------------------------------------------------------------------
</pre>
==== Test Vectors ====
<pre>
message 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
k1 (DATA) 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
k2 (AAD) 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
AAD keystream
76 b8 e0 ad a0 f1 3d 90 40 5d 6a e5 53 86 bd 28 bd d2 19 b8 a0 8d ed 1a a8 36 ef cc 8b 77 0d c7 da 41 59 7c 51 57 48 8d 77 24 e0 3f b8 d8 4a 37 6a 43 b8 f4 15 18 a1 1c c3 87 b6 69 b2 ee 65 86
ciphertext
76 b8 e0 9f 07 e7 be 55 51 38 7a 98 ba 97 7c 73 2d 08 0d cb 0f 29 a0 48 e3 65 69 12 c6 53 3e 32
MAC
d2 fc 11 82 9c 1b 6c 1d f1 f5 51 cd 61 31 ff 08
</pre>
<pre>
message 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
k1 (DATA) 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
k2 (AAD) 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
AAD keystream
76 b8 e0 ad a0 f1 3d 90 40 5d 6a e5 53 86 bd 28 bd d2 19 b8 a0 8d ed 1a a8 36 ef cc 8b 77 0d c7 da 41 59 7c 51 57 48 8d 77 24 e0 3f b8 d8 4a 37 6a 43 b8 f4 15 18 a1 1c c3 87 b6 69 b2 ee 65 86
ciphertext
77 b8 e0 9f 07 e7 be 55 51 38 7a 98 ba 97 7c 73 2d 08 0d cb 0f 29 a0 48 e3 65 69 12 c6 53 3e 32
MAC
ba f0 c8 5b 6d ff 86 02 b0 6c f5 2a 6a ef c6 2e
</pre>
<pre>
message
ff 00 00 f1 95 e6 69 82 10 5f fb 64 0b b7 75 7f 57 9d a3 16 02 fc 93 ec 01 ac 56 f8 5a c3 c1 34 a4 54 7b 73 3b 46 41 30 42 c9 44 00 49 17 69 05 d3 be 59 ea 1c 53 f1 59 16 15 5c 2b e8 24 1a 38 00 8b 9a 26 bc 35 94 1e 24 44 17 7c 8a de 66 89 de 95 26 49 86 d9 58 89 fb 60 e8 46 29 c9 bd 9a 5a cb 1c c1 18 be 56 3e b9 b3 a4 a4 72 f8 2e 09 a7 e7 78 49 2b 56 2e f7 13 0e 88 df e0 31 c7 9d b9 d4 f7 c7 a8 99 15 1b 9a 47 50 32 b6 3f c3 85 24 5f e0 54 e3 dd 5a 97 a5 f5 76 fe 06 40 25 d3 ce 04 2c 56 6a b2 c5 07 b1 38 db 85 3e 3d 69 59 66 09 96 54 6c c9 c4 a6 ea fd c7 77 c0 40 d7 0e af 46 f7 6d ad 39 79 e5 c5 36 0c 33 17 16 6a 1c 89 4c 94 a3 71 87 6a 94 df 76 28 fe 4e aa f2 cc b2 7d 5a aa e0 ad 7a d0 f9 d4 b6 ad 3b 54 09 87 46 d4 52 4d 38 40 7a 6d eb 3a b7 8f ab 78 c9
k1 (DATA) 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
k2 (AAD) ff 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
AAD keystream
c6 40 c1 71 1e 3e e9 04 ac 35 c5 7a b9 79 1c 8a 1c 40 86 03 a9 0b 77 a8 3b 54 f6 c8 44 cb 4b 06 d9 4e 7f c6 c8 00 e1 65 ac d6 61 47 e8 0e c4 5a 56 7f 6c e6 6d 05 ec 0c ae 67 9d ce eb 89 00 17
ciphertext
39 40 c1 e9 2d a4 58 2f f6 f9 2a 77 6a eb 14 d0 14 d3 84 ee b3 0f 66 0d ac f7 0a 14 a2 3f d3 1e 91 21 27 01 33 4e 2c e1 ac f5 19 9d c8 4f 4d 61 dd be 65 71 bc a5 af 87 4b 4c 92 26 c2 6e 65 09 95 d1 57 64 4e 18 48 b9 6e d6 c2 10 2d 54 89 a0 50 e7 1d 29 a5 a6 6e ce 11 de 5f b5 c9 55 8d 54 da 28 fe 45 b0 bc 4d b4 e5 b8 80 30 bf c4 a3 52 b4 b7 06 8e cc f6 56 ba e7 ad 6a 35 61 53 15 fc 7c 49 d4 20 03 88 d5 ec a6 7c 2e 82 2e 06 93 36 c6 9b 40 db 67 e0 f3 c8 12 09 c5 0f 32 16 a4 b8 9f b3 ae 1b 98 4b 78 51 a2 ec 6f 68 ab 12 b1 01 ab 12 0e 1e a7 31 3b b9 3b 5a 0f 71 18 5c 7f ea 01 7d db 92 76 98 61 c2 9d ba 4f bc 43 22 80 d5 df f2 1b 36 d1 c4 c7 90 12 8b 22 69 99 50 bb 18 bf 74 c4 48 cd fe 54 7d 8e d4 f6 57 d8 00 5f dc 0c d7 a0 50 c2 d4 60 50 a4 4c 43 76 35 58 58
MAC
98 1f be 8b 18 42 88 27 6e 7a 93 ea bc 89 9c 4a
</pre>
=== v2 Messages Structure ===
{|class="wikitable"
! Field Size !! Description !! Data type !! Comments
|-
| 3 || length & flag || 23 + 1 bits || Encrypted length of ciphertext payload (not counting the MAC tag) in number of bytes (only 2^23 is usable, most significant bit is the rekey-flag)
|-
| 1-13 || encrypted command || variable || ASCII command (or one byte short command ID)
|-
| ? || encrypted payload || ? || The actual data
|-
| 16 || MAC tag || ? || 128bit MAC-tag
|}
Encrypted messages do not have the 4byte network magic.
The maximum message size is 2^23 (8’388’608) bytes. Future communication MAY
exceed this limit and thus MUST be split into different messages.
Decrypting and processing the message before the authentication succeeds (MAC
verified) MUST not be done.
The 4byte sha256 checksum is no longer required because the AEAD (MAC).
Both peers MUST keep track of the message sequence number (uint32) of sent and
received messages for building a 64-bit symmetric cipher IV.
The command field MUST start with a byte that defines the length of the ASCII
command string up to 12 chars (1 to 12) or a short command ID (see below).
==== Short Command ID ====
To save valuable bandwidth, the v2 message format supports message command
short IDs for message types with high frequency. The ID/string mapping is a
peer to peer arrangement and MAY be negotiated between the initiating and
responding peer. A peer conforming to this proposal MUST support short IDs
based on the table below and SHOULD use short command IDs for outgoing messages.
{|class="wikitable"
! Number !! Command
|-
| 13 || INV
|-
| 14 || HEADERS
|-
| 15 || PING
|-
| 16 || PONG
|-
|}
==== Length comparisons between v1 and v2 messages ====
<pre>
v1 in: 4(Magic)+12(Command)+4(MessageSize)+4(Checksum)+36(Payload) == 60
v2 inv: 3(MessageSize&Flag)+1(Command)+36(Payload)+16(MAC) == 56
(93.33%)
</pre>
<pre>
v1 ping: 4(Magic)+12(Command)+4(MessageSize)+4(Checksum)+8(Payload) == 32
v2 pong: 3(MessageSize&Flag)+1(Command)+8(Payload)+16(MAC) == 28
(87.5%)
</pre>
<pre>
v1 block: 4(Magic)+12(Command)+4(MessageSize)+4(Checksum)+1’048’576(Payload) = 1’048’600
v2 block: 3(MessageSize&Flag)+6(CommandStr)+8(Payload)+16(MAC) == 28 = 1’048’601
(100.000095%)
</pre>
=== Re-Keying ===
Re-keying can be signaled by setting the most significant bit in the length
field before encryption. A peer signaling a rekey MUST use the next key for
encryption messages AFTER the message where the signaling has been done.
A peer identifying a rekey by checking the most significant bit in the envelope
length must use the next key for decrypt messages AFTER the message where the
signaling has been detected.
The next symmetric cipher key MUST be calculated by <code>SHA256(SHA256(session
ID || old_symmetric_cipher_key))</code> and the packet sequence number of the
according encryption direction must be set to 0.
Re-Keying interval is a peer policy with a minimum timespan of 10 seconds.
The Re-Keying must be done after every 1GB of data sent (recommended by RFC4253
SSH Transport) or if the last rekey was more than an hour ago.
Peers calculate the counterparty limits and MUST disconnect immediately if a
violation of the limits has been detected.
=== Risks ===
The encryption does not include an authentication scheme. This BIP does not
cover a proposal to avoid MITM attacks during the encryption initialization.
However, peers MUST show the session-id to the user on request which allows to
identify a MITM by a manual verification on a secure channel.
Optional authentication schemes may be covered by other proposals <ref
name="bip150">[https://github.com/bitcoin/bips/blob/master/bip-0150.mediawiki
BIP150]</ref>.
An attacker could delay or halt v2 protocol enforcement by providing a
reasonable amount of peers not supporting the v2 protocol.
== Compatibility ==
This proposal is backward compatible (as long as not enforced). Non-supporting
peers can still use unencrypted communications.
== Reference implementation ==
* Complete Bitcoin Core implementation: https://github.com/bitcoin/bitcoin/pull/14032
* Reference implementation of the AEAD in C: https://github.com/jonasschnelli/chacha20poly1305
== References ==
<references/>
== Acknowledgements ==
* Pieter Wuille and Gregory Maxwell for most of the ideas in this BIP.
* Tim Ruffing for the review and the hint for the enhancement of the symmetric
key derivation
== Copyright ==
This work is placed in the public domain.