Differential D7494 Diff 23619 src/crypto/chacha_poly_aead.h

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src/crypto/chacha_poly_aead.h

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				// Copyright (c) 2019 The Bitcoin Core developers
				// Distributed under the MIT software license, see the accompanying
				// file COPYING or http://www.opensource.org/licenses/mit-license.php.

				#ifndef BITCOIN_CRYPTO_CHACHA_POLY_AEAD_H
				#define BITCOIN_CRYPTO_CHACHA_POLY_AEAD_H

				#include <crypto/chacha20.h>

				#include <cmath>

				static constexpr int CHACHA20_POLY1305_AEAD_KEY_LEN = 32;
				static constexpr int CHACHA20_POLY1305_AEAD_AAD_LEN = 3; /* 3 bytes length */
				static constexpr int CHACHA20_ROUND_OUTPUT = 64; /* 64 bytes per round */
				static constexpr int AAD_PACKAGES_PER_ROUND = 21; /* 64 / 3 round down*/

				/* A AEAD class for ChaCha20-Poly1305@bitcoin.
				*
				* 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.
				*
				* 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.
				*/

				class ChaCha20Poly1305AEAD {
				private:
				// payload and poly1305 key-derivation cipher instance
				ChaCha20 m_chacha_main;
				// AAD cipher instance (encrypted length)
				ChaCha20 m_chacha_header;
				// aad keystream cache
				uint8_t m_aad_keystream_buffer[CHACHA20_ROUND_OUTPUT];
				// aad keystream cache hint
				uint64_t m_cached_aad_seqnr;

				public:
				ChaCha20Poly1305AEAD(const uint8_t K_1, size_t K_1_len, const uint8_t K_2,
				size_t K_2_len);

				explicit ChaCha20Poly1305AEAD(const ChaCha20Poly1305AEAD &) = delete;

				/**
				* Encrypts/decrypts a packet
				* - seqnr_payload, the message sequence number.
				* - seqnr_aad, the messages AAD sequence number which allows reuse of the
				* AAD keystream.
				* - aad_pos, position to use in the AAD keystream to encrypt the AAD.
				* - dest, output buffer, must be of a size equal or larger than
				* CHACHA20_POLY1305_AEAD_AAD_LEN + payload (+ POLY1305_TAG_LEN in
				* encryption) bytes.
				* - destlen, length of the destination buffer src, the AAD+payload to
				* encrypt or the AAD+payload+MAC to decrypt.
				* - src_len, the length of the source buffer.
				* - is_encrypt, set to true if we encrypt (creates and appends the MAC
				* instead of verifying it).
				*/
				bool Crypt(uint64_t seqnr_payload, uint64_t seqnr_aad, int aad_pos,
				uint8_t dest, size_t dest_len, const uint8_t src,
				size_t src_len, bool is_encrypt);

				/** decrypts the 3 bytes AAD data and decodes it into a uint32_t field */
				bool GetLength(uint32_t *len24_out, uint64_t seqnr_aad, int aad_pos,
				const uint8_t *ciphertext);
				};

				#endif // BITCOIN_CRYPTO_CHACHA_POLY_AEAD_H