; dh_gex.inc - Diffie Hellman Group exchange
;
; Copyright (C) 2015-2021 Jeffrey Amelynck
;
; This program is free software: you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation, either version 3 of the License, or
; (at your option) any later version.
;
; This program is distributed in the hope that it will be useful,
; but WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
; GNU General Public License for more details.
;
; You should have received a copy of the GNU General Public License
; along with this program. If not, see <http://www.gnu.org/licenses/>.
; https://www.ietf.org/rfc/rfc4419.txt
; TODO: dont convert mpints to little endian immediately.
; Or maybe even better, not at all.
proc dh_gex
locals
dh_f_big dd ?
endl
;----------------------------------------------
; >> Send Diffie-Hellman Group Exchange Request
DEBUGF 2, "Sending GEX\n"
stdcall ssh_send_packet, con, ssh_gex_req, ssh_gex_req.length, 0
cmp eax, -1
je .socket_err
;---------------------------------------------
; << Parse Diffie-Hellman Group Exchange Group
stdcall ssh_recv_packet, con, 0
cmp eax, -1
je .socket_err
cmp [con.rx_buffer.message_code], SSH_MSG_KEX_DH_GEX_GROUP
jne proto_err
DEBUGF 2, "Received GEX group\n"
mov esi, con.rx_buffer+sizeof.ssh_packet_header
DEBUGF 1, "DH modulus (p): "
stdcall mpint_to_little_endian, con.dh_p, esi
add esi, 4
add esi, eax
stdcall mpint_print, con.dh_p
DEBUGF 1, "DH base (g): "
stdcall mpint_to_little_endian, con.dh_g, esi
add esi, 4
add esi, eax
stdcall mpint_print, con.dh_g
;-------------------------------------------
; >> Send Diffie-Hellman Group Exchange Init
; generate a random number x, where 1 < x < (p-1)/2
mov edi, con.dh_x+4
mov [con.dh_x], DH_PRIVATE_KEY_SIZE/8
mov ecx, DH_PRIVATE_KEY_SIZE/8/4
@@:
push ecx
call MBRandom
pop ecx
stosd
dec ecx
jnz @r
; If the highest bit is set, add a zero byte
shl eax, 1
jnc @f
mov byte[edi], 0
inc dword[con.dh_x]
@@:
DEBUGF 1, "DH x: "
stdcall mpint_print, con.dh_x
; Compute e = g^x mod p
stdcall mpint_modexp, con.dh_e, con.dh_g, con.dh_x, con.dh_p
stdcall mpint_shrink, con.dh_e
DEBUGF 1, "DH e: "
stdcall mpint_print, con.dh_e
; Create group exchange init packet
mov edi, con.tx_buffer.message_code
mov al, SSH_MSG_KEX_DH_GEX_INIT
stosb
stdcall mpint_to_big_endian, edi, con.dh_e
DEBUGF 2, "Sending GEX init\n"
mov ecx, dword[con.tx_buffer.message_code+1]
bswap ecx
add ecx, 5
stdcall ssh_send_packet, con, con.tx_buffer.message_code, ecx, 0
cmp eax, -1
je .socket_err
;---------------------------------------------
; << Parse Diffie-Hellman Group Exchange Reply
stdcall ssh_recv_packet, con, 0
cmp eax, -1
je .socket_err
cmp [con.rx_buffer.message_code], SSH_MSG_KEX_DH_GEX_REPLY
jne .proto_err
DEBUGF 2, "Received GEX Reply\n"
;--------------------------------
; HASH: string K_S, the host key
mov esi, con.rx_buffer+sizeof.ssh_packet_header
mov edx, [esi]
bswap edx
add edx, 4
lea ebx, [esi+edx]
mov [dh_f_big], ebx
invoke sha256_update, con.temp_ctx, esi, edx
;--------------------------------------------------------------------------
; HASH: uint32 min, minimal size in bits of an acceptable group
; uint32 n, preferred size in bits of the group the server will send
; uint32 max, maximal size in bits of an acceptable group
invoke sha256_update, con.temp_ctx, ssh_gex_req+sizeof.ssh_packet_header-ssh_packet_header.message_code, 12
;----------------------------
; HASH: mpint p, safe prime
stdcall mpint_shrink, con.dh_p
stdcall mpint_to_big_endian, con.mpint_tmp, con.dh_p
lea edx, [eax+4]
invoke sha256_update, con.temp_ctx, con.mpint_tmp, edx
;----------------------------------------
; HASH: mpint g, generator for subgroup
stdcall mpint_shrink, con.dh_g
stdcall mpint_to_big_endian, con.mpint_tmp, con.dh_g
lea edx, [eax+4]
invoke sha256_update, con.temp_ctx, con.mpint_tmp, edx
;---------------------------------------------------
; HASH: mpint e, exchange value sent by the client
mov esi, con.tx_buffer+sizeof.ssh_packet_header
mov edx, [esi]
bswap edx
add edx, 4
invoke sha256_update, con.temp_ctx, esi, edx
;---------------------------------------------------
; HASH: mpint f, exchange value sent by the server
mov esi, [dh_f_big]
mov edx, [esi]
bswap edx
add edx, 4
invoke sha256_update, con.temp_ctx, esi, edx
stdcall mpint_to_little_endian, con.dh_f, [dh_f_big]
mov esi, [dh_f_big]
add esi, eax
add esi, 4
DEBUGF 1, "DH f: "
stdcall mpint_print, con.dh_f
stdcall mpint_to_little_endian, con.dh_signature, esi
DEBUGF 1, "DH signature: "
stdcall mpint_print, con.dh_signature
;--------------------------------------
; Calculate shared secret K = f^x mod p
stdcall mpint_modexp, con.rx_buffer, con.dh_f, con.dh_x, con.dh_p
stdcall mpint_shrink, con.rx_buffer
DEBUGF 1, "DH K: "
stdcall mpint_print, con.rx_buffer
; We always need it in big endian order, so store it as such.
stdcall mpint_to_big_endian, con.dh_K, con.rx_buffer
mov [con.dh_K_length], eax
;-----------------------------------
; HASH: mpint K, the shared secret
mov edx, [con.dh_K_length]
add edx, 4
invoke sha256_update, con.temp_ctx, con.dh_K, edx
;-------------------------------
; Finalize the exchange hash (H)
invoke sha256_final, con.temp_ctx
mov esi, con.temp_ctx.hash
mov edi, con.dh_H
mov ecx, SHA256_HASH_SIZE/4
rep movsd
DEBUGF 1, "Exchange hash H: "
stdcall dump_hex, con.dh_H, 8
; TODO: skip this block when re-keying
mov esi, con.dh_H
mov edi, con.session_id
mov ecx, SHA256_HASH_SIZE/4
rep movsd
;---------------
; Calculate keys
; First, calculate partial hash of K and H so we can re-use it for every key.
invoke sha256_init, con.k_h_ctx
mov edx, [con.dh_K_length]
add edx, 4
invoke sha256_update, con.k_h_ctx, con.dh_K, edx
invoke sha256_update, con.k_h_ctx, con.dh_H, 32
;---------------------------------------------------------------
; Initial IV client to server: HASH(K || H || "A" || session_id)
mov esi, con.k_h_ctx
mov edi, con.temp_ctx
mov ecx, sizeof.crash_ctx/4
rep movsd
mov [con.session_id_prefix], 'A'
invoke sha256_update, con.temp_ctx, con.session_id_prefix, 32+1
invoke sha256_final, con.temp_ctx.hash
mov edi, con.tx_iv
mov esi, con.temp_ctx
mov ecx, SHA256_HASH_SIZE/4
rep movsd
DEBUGF 1, "Remote IV: "
stdcall dump_hex, con.tx_iv, 8
;---------------------------------------------------------------
; Initial IV server to client: HASH(K || H || "B" || session_id)
mov esi, con.k_h_ctx
mov edi, con.temp_ctx
mov ecx, sizeof.crash_ctx/4
rep movsd
inc [con.session_id_prefix]
invoke sha256_update, con.temp_ctx, con.session_id_prefix, 32+1
invoke sha256_final, con.temp_ctx
mov edi, con.rx_iv
mov esi, con.temp_ctx
mov ecx, SHA256_HASH_SIZE/4
rep movsd
DEBUGF 1, "Local IV: "
stdcall dump_hex, con.rx_iv, 8
;-------------------------------------------------------------------
; Encryption key client to server: HASH(K || H || "C" || session_id)
mov esi, con.k_h_ctx
mov edi, con.temp_ctx
mov ecx, sizeof.crash_ctx/4
rep movsd
inc [con.session_id_prefix]
invoke sha256_update, con.temp_ctx, con.session_id_prefix, 32+1
invoke sha256_final, con.temp_ctx
mov edi, con.tx_enc_key
mov esi, con.temp_ctx
mov ecx, SHA256_HASH_SIZE/4
rep movsd
DEBUGF 1, "Remote key: "
stdcall dump_hex, con.tx_enc_key, 8
;-------------------------------------------------------------------
; Encryption key server to client: HASH(K || H || "D" || session_id)
mov esi, con.k_h_ctx
mov edi, con.temp_ctx
mov ecx, sizeof.crash_ctx/4
rep movsd
inc [con.session_id_prefix]
invoke sha256_update, con.temp_ctx, con.session_id_prefix, 32+1
invoke sha256_final, con.temp_ctx
mov edi, con.rx_enc_key
mov esi, con.temp_ctx
mov ecx, SHA256_HASH_SIZE/4
rep movsd
DEBUGF 1, "Local key: "
stdcall dump_hex, con.rx_enc_key, 8
;------------------------------------------------------------------
; Integrity key client to server: HASH(K || H || "E" || session_id)
mov esi, con.k_h_ctx
mov edi, con.temp_ctx
mov ecx, sizeof.crash_ctx/4
rep movsd
inc [con.session_id_prefix]
invoke sha256_update, con.temp_ctx, con.session_id_prefix, 32+1
invoke sha256_final, con.temp_ctx
mov edi, con.tx_int_key
mov esi, con.temp_ctx
mov ecx, SHA256_HASH_SIZE/4
rep movsd
DEBUGF 1, "Remote Integrity key: "
stdcall dump_hex, con.tx_int_key, 8
;------------------------------------------------------------------
; Integrity key server to client: HASH(K || H || "F" || session_id)
mov esi, con.k_h_ctx
mov edi, con.temp_ctx
mov ecx, sizeof.crash_ctx/4
rep movsd
inc [con.session_id_prefix]
invoke sha256_update, con.temp_ctx, con.session_id_prefix, 32+1
invoke sha256_final, con.temp_ctx
mov edi, con.rx_int_key
mov esi, con.temp_ctx
mov ecx, SHA256_HASH_SIZE/4
rep movsd
DEBUGF 1, "Local Integrity key: "
stdcall dump_hex, con.rx_int_key, 8
;-------------------------------------
; << Parse Diffie-Hellman New Keys MSG
stdcall ssh_recv_packet, con, 0
cmp eax, -1
je .socket_err
cmp [con.rx_buffer.message_code], SSH_MSG_NEWKEYS
jne .proto_err
DEBUGF 2, "Received New Keys\n"
;-------------------------------
; >> Reply with New Keys message
stdcall ssh_send_packet, con, ssh_new_keys, ssh_new_keys.length, 0
xor eax, eax
ret
.socket_err:
DEBUGF 3, "Socket error during key exchange!\n"
mov eax, 1
ret
.proto_err:
DEBUGF 3, "Protocol error during key exchange!\n"
mov eax, 2
ret
endp