ssh_packet.h

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// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
/**
 * @file ssh_packet.h
 * @brief SSH binary packet protocol: framing, AES-256-CTR encryption,
 *        HMAC-SHA2-256 integrity, and sequence-number tracking.
 *
 * ═══════════════════════════════════════════════════════════════════════════
 * WIRE FORMAT (RFC 4253 §6)
 * ═══════════════════════════════════════════════════════════════════════════
 *
 *  Each SSH packet on the wire (after KEX completes):
 *
 *    [4 bytes] packet_length    - big-endian uint32 (does NOT include itself
 *                                 or the MAC; includes padding_length, payload,
 *                                 and random_padding fields)
 *    [1 byte]  padding_length   - number of bytes of random padding
 *    [N bytes] payload          - the SSH message
 *    [P bytes] random_padding   - random bytes; total pkt_len % blocksize == 0
 *    [32 bytes] MAC             - HMAC-SHA2-256 over:
 *                                   uint32(seq_no) || packet_length ||
 *                                   padding_length || payload || random_padding
 *
 *  AES-256-CTR encrypts: packet_length || padding_length || payload || padding
 *  The MAC is computed over the PLAINTEXT (before encryption) prepended with
 *  the 4-byte sequence number.  This is the "encrypt-then-MAC" variant used
 *  by openssh with hmac-sha2-256 (ETM) - but RFC 4253 default is MAC-then-
 *  encrypt.  We implement STANDARD RFC 4253 (MAC over plaintext, then encrypt)
 *  to match the base SSH specification.
 *
 * ═══════════════════════════════════════════════════════════════════════════
 * MAC-VERIFY-BEFORE-USE INVARIANT
 * ═══════════════════════════════════════════════════════════════════════════
 *
 *  On receive: decrypt → verify HMAC → then process the payload.
 *  If HMAC verification fails: close the connection immediately, zero the
 *  decrypted buffer.  Do NOT process or reflect any byte of the payload.
 *
 *  This ordering closes the "BEAST" class of attack where an attacker can
 *  influence decryption outputs by injecting invalid packets.
 *
 * ═══════════════════════════════════════════════════════════════════════════
 * SEQUENCE NUMBER OVERFLOW GUARD
 * ═══════════════════════════════════════════════════════════════════════════
 *
 *  SSH sequence numbers are 32-bit values that wrap at 2^32.  RFC 4253 does
 *  not mandate rekeying before wrap; however, CTR-mode keystream repetition
 *  at wrap would be a catastrophic confidentiality failure.
 *
 *  Policy: close the connection if seq_no_send or seq_no_recv reaches
 *  SSH_SEQ_CLOSE_THRESHOLD.  A future rekey implementation would reset the
 *  counters instead.
 *
 * ═══════════════════════════════════════════════════════════════════════════
 * PADDING
 * ═══════════════════════════════════════════════════════════════════════════
 *
 *  AES-256-CTR block size = 16 bytes.  RFC 4253 §6 requires the padded
 *  packet (packet_length + 1 + payload + padding) to be a multiple of
 *  max(8, cipher_block_size) = 16.  Minimum padding is 4 bytes.
 *
 *  padding_len = (16 - ((5 + payload_len) % 16)) % 16
 *  if (padding_len < 4) padding_len += 16;
 *
 * @author  Douglas Quigg (dstroy0)
 * @date    2026
 */
 
#ifndef DETERMINISTICESPASYNCWEBSERVER_SSH_PACKET_H
#define DETERMINISTICESPASYNCWEBSERVER_SSH_PACKET_H
 
#include "ServerConfig.h"
#include "network_drivers/presentation/ssh/crypto/ssh_hmac_sha256.h"
#include "network_drivers/presentation/ssh/transport/ssh_keymat.h"
#include <stddef.h>
#include <stdint.h>
 
// ---------------------------------------------------------------------------
// Sequence number overflow threshold
// ---------------------------------------------------------------------------
 
/**
 * @brief Close the connection when seq_no reaches this value.
 *
 * Set to 0xFFFFFFF0 (16 below the 32-bit wrap) as a conservative margin.
 * This prevents CTR keystream reuse that would occur at wrap.
 * A rekey implementation would reset this counter; until then, we close.
 */
#define SSH_SEQ_CLOSE_THRESHOLD 0xFFFFFFF0u
 
// ---------------------------------------------------------------------------
// SSH message type constants (RFC 4253)
// ---------------------------------------------------------------------------
 
#define SSH_MSG_DISCONNECT 1
#define SSH_MSG_IGNORE 2
#define SSH_MSG_UNIMPLEMENTED 3
#define SSH_MSG_SERVICE_REQUEST 5
#define SSH_MSG_SERVICE_ACCEPT 6
#define SSH_MSG_EXT_INFO 7 // RFC 8308 extension negotiation
#define SSH_MSG_KEXINIT 20
#define SSH_MSG_NEWKEYS 21
#define SSH_MSG_KEXDH_INIT 30
#define SSH_MSG_KEXDH_REPLY 31
#define SSH_MSG_USERAUTH_REQUEST 50
#define SSH_MSG_USERAUTH_FAILURE 51
#define SSH_MSG_USERAUTH_SUCCESS 52
#define SSH_MSG_USERAUTH_PK_OK 60
#define SSH_MSG_GLOBAL_REQUEST 80  // RFC 4254 §4 (e.g. tcpip-forward for ssh -R)
#define SSH_MSG_REQUEST_SUCCESS 81 // RFC 4254 §4 reply to a want_reply global request
#define SSH_MSG_REQUEST_FAILURE 82 // RFC 4254 §4 reply: request refused / unrecognized
#define SSH_MSG_CHANNEL_OPEN 90
#define SSH_MSG_CHANNEL_OPEN_CONFIRM 91
#define SSH_MSG_CHANNEL_OPEN_FAILURE 92
#define SSH_MSG_CHANNEL_WINDOW_ADJUST 93
#define SSH_MSG_CHANNEL_DATA 94
#define SSH_MSG_CHANNEL_EOF 96
#define SSH_MSG_CHANNEL_CLOSE 97
#define SSH_MSG_CHANNEL_REQUEST 98
#define SSH_MSG_CHANNEL_SUCCESS 99
#define SSH_MSG_CHANNEL_FAILURE 100
 
// ---------------------------------------------------------------------------
// Disconnect reason codes (RFC 4253 §11.1)
// ---------------------------------------------------------------------------
 
#define SSH_DISCONNECT_PROTOCOL_ERROR 2
#define SSH_DISCONNECT_MAC_ERROR 5
#define SSH_DISCONNECT_TOO_MANY_CONNECTIONS 11
#define SSH_DISCONNECT_NO_MORE_AUTH_METHODS_AVAILABLE 14 // RFC 4250 §4.2.2
 
// ---------------------------------------------------------------------------
// Packet state per connection
// ---------------------------------------------------------------------------
 
/**
 * @brief Per-connection SSH binary packet state.
 *
 * Allocated in ssh_pool[] (BSS); one entry per SSH connection slot.
 * Key material is in ssh_keys[] - a separate BSS symbol to prevent linear
 * overflow from packet buffers into key material.
 */
struct SshPacketState
{
    uint32_t seq_no_send; ///< Outgoing sequence number (incremented per packet).
    uint32_t seq_no_recv; ///< Incoming sequence number (incremented per packet).
    bool kex_active;      ///< True while KEX is in progress (no user data).
    // Encryption activates per direction (RFC 4253 sec 7.3): our outbound turns on when we send our
    // SSH_MSG_NEWKEYS, our inbound when we receive the peer's. The send path (pack) reads enc_out; the
    // receive path (unpack) reads enc_in. A strict peer may activate its send direction before we
    // activate ours, so the two are tracked independently rather than as one flag.
    bool enc_out; ///< True once we have sent our NEWKEYS (outbound cipher/MAC active).
    bool enc_in;  ///< True once we have received the peer's NEWKEYS (inbound cipher/MAC active).
 
    // Receive reassembly: we may receive partial packets across TCP segments.
    uint8_t rx_buf[SSH_PKT_BUF_SIZE]; ///< Raw receive buffer (from transport).
    size_t rx_len;                    ///< Bytes currently in rx_buf.
};
 
/** @brief Static packet state pool (BSS). One entry per SSH slot. */
extern SshPacketState ssh_pkt[MAX_SSH_CONNS];
 
// ---------------------------------------------------------------------------
// Wire buffer sizing
// ---------------------------------------------------------------------------
 
// Worst-case on-wire bytes for a payload of up to SSH_PKT_BUF_SIZE: the 4-byte packet_length, the
// 1-byte padding_length, the effective payload, worst-case padding, and the largest MAC tag. When
// s2c compression is built in, the "effective payload" is the compressor's worst-case output
// (ssh_deflate_bound of a full payload) since fixed-Huffman can slightly expand incompressible data.
// Callers MUST size the wire buffer with this so a compressed packet never overflows and desyncs the
// stateful cipher / compression stream (a dropped packet mid-stream would corrupt the session).
#if DETWS_ENABLE_SSH_ZLIB
#define SSH_MAX_EFFECTIVE_PAYLOAD (2 + SSH_PKT_BUF_SIZE + (SSH_PKT_BUF_SIZE >> 3) + 32) // = ssh_deflate_bound()
#else
#define SSH_MAX_EFFECTIVE_PAYLOAD (SSH_PKT_BUF_SIZE)
#endif
#define SSH_MAX_PAD 32 // worst-case padding across block-8 / block-16 modes (min-4 rule)
#define SSH_MAX_MAC 64 // largest MAC tag (hmac-sha2-512); chacha's Poly1305 tag is 16
#define SSH_WIRE_CAP ((size_t)(4 + 1 + SSH_MAX_EFFECTIVE_PAYLOAD + SSH_MAX_PAD + SSH_MAX_MAC))
 
// ---------------------------------------------------------------------------
// Public API
// ---------------------------------------------------------------------------
 
/**
 * @brief Initialize the packet state for SSH connection slot @p i.
 *
 * Zeroes seq numbers; sets encrypted=false, kex_active=true.
 *
 * @param i  SSH slot index.
 */
void ssh_pkt_init(uint8_t i);
 
/**
 * @brief Build and send one SSH binary packet.
 *
 * Frames @p payload according to RFC 4253 §6:
 *   - Adds random padding to align to 16-byte boundary.
 *   - If encrypted: encrypts with AES-256-CTR, appends HMAC-SHA2-256 MAC.
 *   - Increments seq_no_send; closes connection if threshold reached.
 *
 * The serialized packet is written into @p out.  *@p out_len is set to the
 * number of bytes written.  @p out must be at least
 * (4 + 1 + payload_len + 16 + 32) bytes.
 *
 * @param i           SSH slot index.
 * @param payload     Plaintext SSH message payload.
 * @param payload_len Length of @p payload.
 * @param out         Output buffer for the wire packet.
 * @param out_len     Set to the number of bytes written into @p out.
 * @param out_cap     Capacity of @p out.
 * @return 0 on success, -1 on overflow or sequence-number exhaustion.
 */
int ssh_pkt_send(uint8_t i, const uint8_t *payload, size_t payload_len, uint8_t *out, size_t *out_len, size_t out_cap);
 
/**
 * @brief Callback invoked once per complete, verified inbound SSH message.
 *
 * @param slot         SSH slot index.
 * @param msg_type     First payload byte (SSH message number).
 * @param payload      Decrypted message payload (includes @p msg_type at [0]).
 * @param payload_len  Length of @p payload.
 */
typedef void (*ssh_msg_handler_t)(uint8_t slot, uint8_t msg_type, const uint8_t *payload, size_t payload_len);
 
/**
 * @brief Receive and process one or more SSH binary packets from @p data.
 *
 * Appends @p len bytes from @p data to the receive buffer for slot @p i,
 * then extracts complete packets.  For each complete packet:
 *   - If encrypted: decrypts with AES-256-CTR, verifies HMAC-SHA2-256.
 *     Closes connection (returns -1) on MAC failure without processing payload.
 *   - Increments seq_no_recv; closes connection if threshold reached.
 *   - Calls @p handler(slot, msg_type, payload, payload_len) for the payload.
 *
 * @param i        SSH slot index.
 * @param data     Received bytes (from TCP).
 * @param len      Number of bytes in @p data.
 * @param handler  Callback invoked once per complete, verified packet.
 * @return 0 on success, -1 on MAC failure or sequence-number exhaustion
 *         (caller must close the TCP connection).
 */
int ssh_pkt_recv(uint8_t i, const uint8_t *data, size_t len, ssh_msg_handler_t handler);
 
/**
 * @brief Send SSH_MSG_DISCONNECT with reason @p reason_code.
 *
 * Sends the packet, then zeroes the packet state and key material for slot @p i.
 *
 * @param i            SSH slot index.
 * @param reason_code  One of SSH_DISCONNECT_* constants.
 * @param out          Output buffer for the wire packet.
 * @param out_len      Set to the number of bytes written.
 * @param out_cap      Capacity of @p out.
 * @return 0 on success, -1 on error.
 */
int ssh_pkt_disconnect(uint8_t i, uint32_t reason_code, uint8_t *out, size_t *out_len, size_t out_cap);
 
#endif // DETERMINISTICESPASYNCWEBSERVER_SSH_PACKET_H