%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 2004-2014. All Rights Reserved.
%%
%% The contents of this file are subject to the Erlang Public License,
%% Version 1.1, (the "License"); you may not use this file except in
%% compliance with the License. You should have received a copy of the
%% Erlang Public License along with this software. If not, it can be
%% retrieved online at http://www.erlang.org/.
%%
%% Software distributed under the License is distributed on an "AS IS"
%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
%% the License for the specific language governing rights and limitations
%% under the License.
%%
%% %CopyrightEnd%
%%
%%
%%% Description: SSH transport protocol
-module(ssh_transport).
-include_lib("public_key/include/public_key.hrl").
-include_lib("kernel/include/inet.hrl").
-include("ssh_transport.hrl").
-include("ssh.hrl").
-export([versions/2, hello_version_msg/1]).
-export([next_seqnum/1, decrypt_first_block/2, decrypt_blocks/3,
supported_algorithms/0, supported_algorithms/1,
default_algorithms/0, default_algorithms/1,
is_valid_mac/3,
handle_hello_version/1,
key_exchange_init_msg/1,
key_init/3, new_keys_message/1,
handle_kexinit_msg/3, handle_kexdh_init/2,
handle_kex_dh_gex_group/2, handle_kex_dh_gex_reply/2,
handle_new_keys/2, handle_kex_dh_gex_request/2,
handle_kexdh_reply/2,
unpack/3, decompress/2, ssh_packet/2, pack/2, msg_data/1,
sign/3, verify/4]).
%%%----------------------------------------------------------------------------
%%%
%%% There is a difference between supported and default algorithms. The
%%% SUPPORTED algorithms can be handled (maybe untested...). The DEFAULT ones
%%% are announced in ssh_msg_kexinit and in ssh:default_algorithms/0 to the
%%% user.
%%%
%%% A supported algorithm can be requested in the option 'preferred_algorithms',
%%% but may give unexpected results because of being promoted to default.
%%%
%%% This makes it possible to add experimental algorithms (in supported_algorithms)
%%% and test them without letting the default users know about them.
%%%
default_algorithms() -> [{K,default_algorithms(K)} || K <- algo_classes()].
algo_classes() -> [kex, public_key, cipher, mac, compression].
default_algorithms(compression) ->
%% Do not announce '[email protected]' because there seem to be problems
supported_algorithms(compression, same(['[email protected]']));
default_algorithms(Alg) ->
supported_algorithms(Alg).
supported_algorithms() -> [{K,supported_algorithms(K)} || K <- algo_classes()].
supported_algorithms(kex) ->
['diffie-hellman-group1-sha1'];
supported_algorithms(public_key) ->
ssh_auth:default_public_key_algorithms();
supported_algorithms(cipher) ->
Supports = crypto:supports(),
CipherAlgos = [{aes_ctr, 'aes128-ctr'}, {aes_cbc128, 'aes128-cbc'}, {des3_cbc, '3des-cbc'}],
Algs = [SshAlgo ||
{CryptoAlgo, SshAlgo} <- CipherAlgos,
lists:member(CryptoAlgo, proplists:get_value(ciphers, Supports, []))],
same(Algs);
supported_algorithms(mac) ->
Supports = crypto:supports(),
HashAlgos = [{sha256, 'hmac-sha2-256'}, {sha, 'hmac-sha1'}],
Algs = [SshAlgo ||
{CryptoAlgo, SshAlgo} <- HashAlgos,
lists:member(CryptoAlgo, proplists:get_value(hashs, Supports, []))],
same(Algs);
supported_algorithms(compression) ->
same(['none','zlib','[email protected]']).
supported_algorithms(Key, [{client2server,BL1},{server2client,BL2}]) ->
[{client2server,As1},{server2client,As2}] = supported_algorithms(Key),
[{client2server,As1--BL1},{server2client,As2--BL2}];
supported_algorithms(Key, BlackList) ->
supported_algorithms(Key) -- BlackList.
same(Algs) -> [{client2server,Algs}, {server2client,Algs}].
%%%----------------------------------------------------------------------------
versions(client, Options)->
Vsn = proplists:get_value(vsn, Options, ?DEFAULT_CLIENT_VERSION),
{Vsn, format_version(Vsn, software_version(Options))};
versions(server, Options) ->
Vsn = proplists:get_value(vsn, Options, ?DEFAULT_SERVER_VERSION),
{Vsn, format_version(Vsn, software_version(Options))}.
software_version(Options) ->
case proplists:get_value(id_string, Options) of
undefined ->
"Erlang"++ssh_vsn();
{random,Nlo,Nup} ->
random_id(Nlo,Nup);
ID ->
ID
end.
ssh_vsn() ->
try {ok,L} = application:get_all_key(ssh),
proplists:get_value(vsn,L,"")
of
"" -> "";
VSN when is_list(VSN) -> "/" ++ VSN;
_ -> ""
catch
_:_ -> ""
end.
random_id(Nlo, Nup) ->
[crypto:rand_uniform($a,$z+1) || _<- lists:duplicate(crypto:rand_uniform(Nlo,Nup+1),x) ].
hello_version_msg(Data) ->
[Data,"\r\n"].
next_seqnum(SeqNum) ->
(SeqNum + 1) band 16#ffffffff.
decrypt_first_block(Bin, #ssh{decrypt_block_size = BlockSize} = Ssh0) ->
<<EncBlock:BlockSize/binary, EncData/binary>> = Bin,
{Ssh, <<?UINT32(PacketLen), _/binary>> = DecData} =
decrypt(Ssh0, EncBlock),
{Ssh, PacketLen, DecData, EncData}.
decrypt_blocks(Bin, Length, Ssh0) ->
<<EncBlocks:Length/binary, EncData/binary>> = Bin,
{Ssh, DecData} = decrypt(Ssh0, EncBlocks),
{Ssh, DecData, EncData}.
is_valid_mac(_, _ , #ssh{recv_mac_size = 0}) ->
true;
is_valid_mac(Mac, Data, #ssh{recv_mac = Algorithm,
recv_mac_key = Key, recv_sequence = SeqNum}) ->
Mac == mac(Algorithm, Key, SeqNum, Data).
yes_no(Ssh, Prompt) ->
(Ssh#ssh.io_cb):yes_no(Prompt, Ssh).
format_version({Major,Minor}, SoftwareVersion) ->
"SSH-" ++ integer_to_list(Major) ++ "." ++
integer_to_list(Minor) ++ "-" ++ SoftwareVersion.
handle_hello_version(Version) ->
try
StrVersion = trim_tail(Version),
case string:tokens(Version, "-") of
[_, "2.0" | _] ->
{{2,0}, StrVersion};
[_, "1.99" | _] ->
{{2,0}, StrVersion};
[_, "1.3" | _] ->
{{1,3}, StrVersion};
[_, "1.5" | _] ->
{{1,5}, StrVersion}
end
catch
error:_ ->
{undefined, "unknown version"}
end.
key_exchange_init_msg(Ssh0) ->
Msg = kex_init(Ssh0),
{SshPacket, Ssh} = ssh_packet(Msg, Ssh0),
{Msg, SshPacket, Ssh}.
kex_init(#ssh{role = Role, opts = Opts, available_host_keys = HostKeyAlgs}) ->
Random = ssh_bits:random(16),
PrefAlgs =
case proplists:get_value(preferred_algorithms,Opts) of
undefined ->
default_algorithms();
Algs0 ->
Algs0
end,
kexinit_message(Role, Random, PrefAlgs, HostKeyAlgs).
key_init(client, Ssh, Value) ->
Ssh#ssh{c_keyinit = Value};
key_init(server, Ssh, Value) ->
Ssh#ssh{s_keyinit = Value}.
kexinit_message(_Role, Random, Algs, HostKeyAlgs) ->
#ssh_msg_kexinit{
cookie = Random,
kex_algorithms = to_strings( get_algs(kex,Algs) ),
server_host_key_algorithms = HostKeyAlgs,
encryption_algorithms_client_to_server = c2s(cipher,Algs),
encryption_algorithms_server_to_client = s2c(cipher,Algs),
mac_algorithms_client_to_server = c2s(mac,Algs),
mac_algorithms_server_to_client = s2c(mac,Algs),
compression_algorithms_client_to_server = c2s(compression,Algs),
compression_algorithms_server_to_client = s2c(compression,Algs),
languages_client_to_server = [],
languages_server_to_client = []
}.
c2s(Key, Algs) -> x2y(client2server, Key, Algs).
s2c(Key, Algs) -> x2y(server2client, Key, Algs).
x2y(DirectionKey, Key, Algs) -> to_strings(proplists:get_value(DirectionKey, get_algs(Key,Algs))).
get_algs(Key, Algs) -> proplists:get_value(Key, Algs, default_algorithms(Key)).
to_strings(L) -> lists:map(fun erlang:atom_to_list/1, L).
new_keys_message(Ssh0) ->
{SshPacket, Ssh} =
ssh_packet(#ssh_msg_newkeys{}, Ssh0),
{ok, SshPacket, Ssh}.
handle_kexinit_msg(#ssh_msg_kexinit{} = CounterPart, #ssh_msg_kexinit{} = Own,
#ssh{role = client} = Ssh0) ->
{ok, Algoritms} = select_algorithm(client, Own, CounterPart),
case verify_algorithm(Algoritms) of
true ->
key_exchange_first_msg(Algoritms#alg.kex,
Ssh0#ssh{algorithms = Algoritms});
_ ->
%% TODO: Correct code?
throw(#ssh_msg_disconnect{code = ?SSH_DISCONNECT_PROTOCOL_ERROR,
description = "Selection of key exchange"
" algorithm failed",
language = "en"})
end;
handle_kexinit_msg(#ssh_msg_kexinit{} = CounterPart, #ssh_msg_kexinit{} = Own,
#ssh{role = server} = Ssh) ->
{ok, Algoritms} = select_algorithm(server, CounterPart, Own),
{ok, Ssh#ssh{algorithms = Algoritms}}.
%% TODO: diffie-hellman-group14-sha1 should also be supported.
%% Maybe check more things ...
verify_algorithm(#alg{kex = 'diffie-hellman-group1-sha1'}) ->
true;
verify_algorithm(#alg{kex = 'diffie-hellman-group-exchange-sha1'}) ->
true;
verify_algorithm(_) ->
false.
key_exchange_first_msg('diffie-hellman-group1-sha1', Ssh0) ->
{G, P} = dh_group1(),
{Private, Public} = dh_gen_key(G, P, 1024),
{SshPacket, Ssh1} = ssh_packet(#ssh_msg_kexdh_init{e = Public}, Ssh0),
{ok, SshPacket,
Ssh1#ssh{keyex_key = {{Private, Public}, {G, P}}}};
key_exchange_first_msg('diffie-hellman-group-exchange-sha1', Ssh0) ->
Min = ?DEFAULT_DH_GROUP_MIN,
NBits = ?DEFAULT_DH_GROUP_NBITS,
Max = ?DEFAULT_DH_GROUP_MAX,
{SshPacket, Ssh1} =
ssh_packet(#ssh_msg_kex_dh_gex_request{min = Min,
n = NBits, max = Max},
Ssh0),
{ok, SshPacket,
Ssh1#ssh{keyex_info = {Min, Max, NBits}}}.
handle_kexdh_init(#ssh_msg_kexdh_init{e = E}, Ssh0) ->
{G, P} = dh_group1(),
if
1=<E, E=<(P-1) ->
{Private, Public} = dh_gen_key(G, P, 1024),
K = ssh_math:ipow(E, Private, P),
Key = get_host_key(Ssh0),
H = kex_h(Ssh0, Key, E, Public, K),
H_SIG = sign_host_key(Ssh0, Key, H),
{SshPacket, Ssh1} = ssh_packet(#ssh_msg_kexdh_reply{public_host_key = Key,
f = Public,
h_sig = H_SIG
}, Ssh0),
{ok, SshPacket, Ssh1#ssh{keyex_key = {{Private, Public}, {G, P}},
shared_secret = K,
exchanged_hash = H,
session_id = sid(Ssh1, H)}};
true ->
Error = {error,bad_e_from_peer},
Disconnect = #ssh_msg_disconnect{
code = ?SSH_DISCONNECT_KEY_EXCHANGE_FAILED,
description = "Key exchange failed, 'f' out of bounds",
language = "en"},
throw({Error, Disconnect})
end.
handle_kex_dh_gex_group(#ssh_msg_kex_dh_gex_group{p = P, g = G}, Ssh0) ->
{Private, Public} = dh_gen_key(G,P,1024),
{SshPacket, Ssh1} =
ssh_packet(#ssh_msg_kex_dh_gex_init{e = Public}, Ssh0),
{ok, SshPacket,
Ssh1#ssh{keyex_key = {{Private, Public}, {G, P}}}}.
handle_new_keys(#ssh_msg_newkeys{}, Ssh0) ->
try install_alg(Ssh0) of
#ssh{} = Ssh ->
{ok, Ssh}
catch
error:_Error -> %% TODO: Throw earlier ....
throw(#ssh_msg_disconnect{code = ?SSH_DISCONNECT_PROTOCOL_ERROR,
description = "Install alg failed",
language = "en"})
end.
%% %% Select algorithms
handle_kexdh_reply(#ssh_msg_kexdh_reply{public_host_key = HostKey, f = F,
h_sig = H_SIG},
#ssh{keyex_key = {{Private, Public}, {_G, P}}} = Ssh0) when 1=<F, F=<(P-1)->
K = ssh_math:ipow(F, Private, P),
H = kex_h(Ssh0, HostKey, Public, F, K),
case verify_host_key(Ssh0, HostKey, H, H_SIG) of
ok ->
{SshPacket, Ssh} = ssh_packet(#ssh_msg_newkeys{}, Ssh0),
{ok, SshPacket, Ssh#ssh{shared_secret = K,
exchanged_hash = H,
session_id = sid(Ssh, H)}};
Error ->
Disconnect = #ssh_msg_disconnect{
code = ?SSH_DISCONNECT_KEY_EXCHANGE_FAILED,
description = "Key exchange failed",
language = "en"},
throw({Error, Disconnect})
end;
handle_kexdh_reply(#ssh_msg_kexdh_reply{}, _SSH) ->
Error = {error,bad_f_from_peer},
Disconnect = #ssh_msg_disconnect{
code = ?SSH_DISCONNECT_KEY_EXCHANGE_FAILED,
description = "Key exchange failed, 'f' out of bounds",
language = "en"},
throw({Error, Disconnect}).
handle_kex_dh_gex_request(#ssh_msg_kex_dh_gex_request{min = _Min,
n = _NBits,
max = _Max}, Ssh0) ->
{G,P} = dh_group1(), %% TODO real imp this seems to be a hack?!
{Private, Public} = dh_gen_key(G, P, 1024),
{SshPacket, Ssh} =
ssh_packet(#ssh_msg_kex_dh_gex_group{p = P, g = G}, Ssh0),
{ok, SshPacket,
Ssh#ssh{keyex_key = {{Private, Public}, {G, P}}}}.
handle_kex_dh_gex_reply(#ssh_msg_kex_dh_gex_reply{public_host_key = HostKey,
f = F,
h_sig = H_SIG},
#ssh{keyex_key = {{Private, Public}, {G, P}},
keyex_info = {Min, Max, NBits}} =
Ssh0) ->
K = ssh_math:ipow(F, Private, P),
H = kex_h(Ssh0, HostKey, Min, NBits, Max, P, G, Public, F, K),
case verify_host_key(Ssh0, HostKey, H, H_SIG) of
ok ->
{SshPacket, Ssh} = ssh_packet(#ssh_msg_newkeys{}, Ssh0),
{ok, SshPacket, Ssh#ssh{shared_secret = K,
exchanged_hash = H,
session_id = sid(Ssh, H)}};
_Error ->
Disconnect = #ssh_msg_disconnect{
code = ?SSH_DISCONNECT_KEY_EXCHANGE_FAILED,
description = "Key exchange failed",
language = "en"},
throw(Disconnect)
end.
%% select session id
sid(#ssh{session_id = undefined}, H) ->
H;
sid(#ssh{session_id = Id}, _) ->
Id.
%%
%% The host key should be read from storage
%%
get_host_key(SSH) ->
#ssh{key_cb = Mod, opts = Opts, algorithms = ALG} = SSH,
case Mod:host_key(ALG#alg.hkey, Opts) of
{ok, #'RSAPrivateKey'{} = Key} ->
Key;
{ok, #'DSAPrivateKey'{} = Key} ->
Key;
Result ->
exit({error, {Result, unsupported_key_type}})
end.
sign_host_key(_Ssh, #'RSAPrivateKey'{} = Private, H) ->
Hash = sha, %% Option ?!
_Signature = sign(H, Hash, Private);
sign_host_key(_Ssh, #'DSAPrivateKey'{} = Private, H) ->
Hash = sha, %% Option ?!
_RawSignature = sign(H, Hash, Private).
verify_host_key(SSH, PublicKey, Digest, Signature) ->
case verify(Digest, sha, Signature, PublicKey) of
false ->
{error, bad_signature};
true ->
known_host_key(SSH, PublicKey, public_algo(PublicKey))
end.
public_algo(#'RSAPublicKey'{}) ->
'ssh-rsa';
public_algo({_, #'Dss-Parms'{}}) ->
'ssh-dss'.
accepted_host(Ssh, PeerName, Opts) ->
case proplists:get_value(silently_accept_hosts, Opts, false) of
true ->
yes;
false ->
yes_no(Ssh, "New host " ++ PeerName ++ " accept")
end.
known_host_key(#ssh{opts = Opts, key_cb = Mod, peer = Peer} = Ssh,
Public, Alg) ->
PeerName = peer_name(Peer),
case Mod:is_host_key(Public, PeerName, Alg, Opts) of
true ->
ok;
false ->
case accepted_host(Ssh, PeerName, Opts) of
yes ->
Mod:add_host_key(PeerName, Public, Opts);
no ->
{error, rejected}
end
end.
%% Each of the algorithm strings MUST be a comma-separated list of
%% algorithm names (see ''Algorithm Naming'' in [SSH-ARCH]). Each
%% supported (allowed) algorithm MUST be listed in order of preference.
%%
%% The first algorithm in each list MUST be the preferred (guessed)
%% algorithm. Each string MUST contain at least one algorithm name.
select_algorithm(Role, Client, Server) ->
{Encrypt, Decrypt} = select_encrypt_decrypt(Role, Client, Server),
{SendMac, RecvMac} = select_send_recv_mac(Role, Client, Server),
{Compression, Decompression} =
select_compression_decompression(Role, Client, Server),
C_Lng = select(Client#ssh_msg_kexinit.languages_client_to_server,
Server#ssh_msg_kexinit.languages_client_to_server),
S_Lng = select(Client#ssh_msg_kexinit.languages_server_to_client,
Server#ssh_msg_kexinit.languages_server_to_client),
HKey = select_all(Client#ssh_msg_kexinit.server_host_key_algorithms,
Server#ssh_msg_kexinit.server_host_key_algorithms),
HK = case HKey of
[] -> undefined;
[HK0|_] -> HK0
end,
%% Fixme verify Kex against HKey list and algorithms
Kex = select(Client#ssh_msg_kexinit.kex_algorithms,
Server#ssh_msg_kexinit.kex_algorithms),
Alg = #alg{kex = Kex,
hkey = HK,
encrypt = Encrypt,
decrypt = Decrypt,
send_mac = SendMac,
recv_mac = RecvMac,
compress = Compression,
decompress = Decompression,
c_lng = C_Lng,
s_lng = S_Lng},
%%ct:pal("~p~n Client=~p~n Server=~p~n Alg=~p~n",[Role,Client,Server,Alg]),
{ok, Alg}.
select_encrypt_decrypt(client, Client, Server) ->
Encrypt =
select(Client#ssh_msg_kexinit.encryption_algorithms_client_to_server,
Server#ssh_msg_kexinit.encryption_algorithms_client_to_server),
Decrypt =
select(Client#ssh_msg_kexinit.encryption_algorithms_server_to_client,
Server#ssh_msg_kexinit.encryption_algorithms_server_to_client),
{Encrypt, Decrypt};
select_encrypt_decrypt(server, Client, Server) ->
Decrypt =
select(Client#ssh_msg_kexinit.encryption_algorithms_client_to_server,
Server#ssh_msg_kexinit.encryption_algorithms_client_to_server),
Encrypt =
select(Client#ssh_msg_kexinit.encryption_algorithms_server_to_client,
Server#ssh_msg_kexinit.encryption_algorithms_server_to_client),
{Encrypt, Decrypt}.
select_send_recv_mac(client, Client, Server) ->
SendMac = select(Client#ssh_msg_kexinit.mac_algorithms_client_to_server,
Server#ssh_msg_kexinit.mac_algorithms_client_to_server),
RecvMac = select(Client#ssh_msg_kexinit.mac_algorithms_server_to_client,
Server#ssh_msg_kexinit.mac_algorithms_server_to_client),
{SendMac, RecvMac};
select_send_recv_mac(server, Client, Server) ->
RecvMac = select(Client#ssh_msg_kexinit.mac_algorithms_client_to_server,
Server#ssh_msg_kexinit.mac_algorithms_client_to_server),
SendMac = select(Client#ssh_msg_kexinit.mac_algorithms_server_to_client,
Server#ssh_msg_kexinit.mac_algorithms_server_to_client),
{SendMac, RecvMac}.
select_compression_decompression(client, Client, Server) ->
Compression =
select(Client#ssh_msg_kexinit.compression_algorithms_client_to_server,
Server#ssh_msg_kexinit.compression_algorithms_client_to_server),
Decomprssion =
select(Client#ssh_msg_kexinit.compression_algorithms_server_to_client,
Server#ssh_msg_kexinit.compression_algorithms_server_to_client),
{Compression, Decomprssion};
select_compression_decompression(server, Client, Server) ->
Decomprssion =
select(Client#ssh_msg_kexinit.compression_algorithms_client_to_server,
Server#ssh_msg_kexinit.compression_algorithms_client_to_server),
Compression =
select(Client#ssh_msg_kexinit.compression_algorithms_server_to_client,
Server#ssh_msg_kexinit.compression_algorithms_server_to_client),
{Compression, Decomprssion}.
install_alg(SSH) ->
SSH1 = alg_final(SSH),
SSH2 = alg_setup(SSH1),
alg_init(SSH2).
alg_setup(SSH) ->
ALG = SSH#ssh.algorithms,
SSH#ssh{kex = ALG#alg.kex,
hkey = ALG#alg.hkey,
encrypt = ALG#alg.encrypt,
decrypt = ALG#alg.decrypt,
send_mac = ALG#alg.send_mac,
send_mac_size = mac_digest_size(ALG#alg.send_mac),
recv_mac = ALG#alg.recv_mac,
recv_mac_size = mac_digest_size(ALG#alg.recv_mac),
compress = ALG#alg.compress,
decompress = ALG#alg.decompress,
c_lng = ALG#alg.c_lng,
s_lng = ALG#alg.s_lng,
algorithms = undefined
}.
alg_init(SSH0) ->
{ok,SSH1} = send_mac_init(SSH0),
{ok,SSH2} = recv_mac_init(SSH1),
{ok,SSH3} = encrypt_init(SSH2),
{ok,SSH4} = decrypt_init(SSH3),
{ok,SSH5} = compress_init(SSH4),
{ok,SSH6} = decompress_init(SSH5),
SSH6.
alg_final(SSH0) ->
{ok,SSH1} = send_mac_final(SSH0),
{ok,SSH2} = recv_mac_final(SSH1),
{ok,SSH3} = encrypt_final(SSH2),
{ok,SSH4} = decrypt_final(SSH3),
{ok,SSH5} = compress_final(SSH4),
{ok,SSH6} = decompress_final(SSH5),
SSH6.
select_all(CL, SL) when length(CL) + length(SL) < 50 ->
A = CL -- SL, %% algortihms only used by client
%% algorithms used by client and server (client pref)
lists:map(fun(ALG) -> list_to_atom(ALG) end, (CL -- A));
select_all(_CL, _SL) ->
throw(#ssh_msg_disconnect{code = ?SSH_DISCONNECT_PROTOCOL_ERROR,
description = "Too many algorithms",
language = "en"}).
select([], []) ->
none;
select(CL, SL) ->
C = case select_all(CL,SL) of
[] -> undefined;
[ALG|_] -> ALG
end,
C.
ssh_packet(#ssh_msg_kexinit{} = Msg, Ssh0) ->
BinMsg = ssh_message:encode(Msg),
Ssh = key_init(Ssh0#ssh.role, Ssh0, BinMsg),
pack(BinMsg, Ssh);
ssh_packet(Msg, Ssh) ->
BinMsg = ssh_message:encode(Msg),
pack(BinMsg, Ssh).
pack(Data0, #ssh{encrypt_block_size = BlockSize,
send_sequence = SeqNum, send_mac = MacAlg,
send_mac_key = MacKey}
= Ssh0) when is_binary(Data0) ->
{Ssh1, Data} = compress(Ssh0, Data0),
PL = (BlockSize - ((4 + 1 + size(Data)) rem BlockSize)) rem BlockSize,
PaddingLen = if PL < 4 -> PL + BlockSize;
true -> PL
end,
Padding = ssh_bits:random(PaddingLen),
PacketLen = 1 + PaddingLen + size(Data),
PacketData = <<?UINT32(PacketLen),?BYTE(PaddingLen),
Data/binary, Padding/binary>>,
{Ssh2, EncPacket} = encrypt(Ssh1, PacketData),
MAC = mac(MacAlg, MacKey, SeqNum, PacketData),
Packet = [EncPacket, MAC],
Ssh = Ssh2#ssh{send_sequence = (SeqNum+1) band 16#ffffffff},
{Packet, Ssh}.
unpack(EncodedSoFar, ReminingLenght, #ssh{recv_mac_size = MacSize} = Ssh0) ->
SshLength = ReminingLenght - MacSize,
{NoMac, Mac, Rest} = case MacSize of
0 ->
<<NoMac0:SshLength/binary,
Rest0/binary>> = EncodedSoFar,
{NoMac0, <<>>, Rest0};
_ ->
<<NoMac0:SshLength/binary,
Mac0:MacSize/binary,
Rest0/binary>> = EncodedSoFar,
{NoMac0, Mac0, Rest0}
end,
{Ssh1, DecData, <<>>} =
case SshLength of
0 ->
{Ssh0, <<>>, <<>>};
_ ->
decrypt_blocks(NoMac, SshLength, Ssh0)
end,
{Ssh1, DecData, Rest, Mac}.
msg_data(PacketData) ->
<<Len:32, PaddingLen:8, _/binary>> = PacketData,
DataLen = Len - PaddingLen - 1,
<<_:32, _:8, Data:DataLen/binary,
_:PaddingLen/binary>> = PacketData,
Data.
sign(SigData, Hash, #'DSAPrivateKey'{} = Key) ->
DerSignature = public_key:sign(SigData, Hash, Key),
#'Dss-Sig-Value'{r = R, s = S} = public_key:der_decode('Dss-Sig-Value', DerSignature),
<<R:160/big-unsigned-integer, S:160/big-unsigned-integer>>;
sign(SigData, Hash, Key) ->
public_key:sign(SigData, Hash, Key).
verify(PlainText, Hash, Sig, {_, #'Dss-Parms'{}} = Key) ->
<<R:160/big-unsigned-integer, S:160/big-unsigned-integer>> = Sig,
Signature = public_key:der_encode('Dss-Sig-Value', #'Dss-Sig-Value'{r = R, s = S}),
public_key:verify(PlainText, Hash, Signature, Key);
verify(PlainText, Hash, Sig, Key) ->
public_key:verify(PlainText, Hash, Sig, Key).
%% public key algorithms
%%
%% ssh-dss REQUIRED sign Raw DSS Key
%% ssh-rsa RECOMMENDED sign Raw RSA Key
%% x509v3-sign-rsa OPTIONAL sign X.509 certificates (RSA key)
%% x509v3-sign-dss OPTIONAL sign X.509 certificates (DSS key)
%% spki-sign-rsa OPTIONAL sign SPKI certificates (RSA key)
%% spki-sign-dss OPTIONAL sign SPKI certificates (DSS key)
%% pgp-sign-rsa OPTIONAL sign OpenPGP certificates (RSA key)
%% pgp-sign-dss OPTIONAL sign OpenPGP certificates (DSS key)
%%
%% key exchange
%%
%% diffie-hellman-group1-sha1 REQUIRED
%%
%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% Encryption
%%
%% chiphers
%%
%% 3des-cbc REQUIRED
%% three-key 3DES in CBC mode
%% blowfish-cbc OPTIONAL Blowfish in CBC mode
%% twofish256-cbc OPTIONAL Twofish in CBC mode,
%% with 256-bit key
%% twofish-cbc OPTIONAL alias for "twofish256-cbc" (this
%% is being retained for
%% historical reasons)
%% twofish192-cbc OPTIONAL Twofish with 192-bit key
%% twofish128-cbc OPTIONAL Twofish with 128-bit key
%% aes256-cbc OPTIONAL AES in CBC mode,
%% with 256-bit key
%% aes192-cbc OPTIONAL AES with 192-bit key
%% aes128-cbc RECOMMENDED AES with 128-bit key
%% serpent256-cbc OPTIONAL Serpent in CBC mode, with
%% 256-bit key
%% serpent192-cbc OPTIONAL Serpent with 192-bit key
%% serpent128-cbc OPTIONAL Serpent with 128-bit key
%% arcfour OPTIONAL the ARCFOUR stream cipher
%% idea-cbc OPTIONAL IDEA in CBC mode
%% cast128-cbc OPTIONAL CAST-128 in CBC mode
%% none OPTIONAL no encryption; NOT RECOMMENDED
%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
encrypt_init(#ssh{encrypt = none} = Ssh) ->
{ok, Ssh};
encrypt_init(#ssh{encrypt = '3des-cbc', role = client} = Ssh) ->
IV = hash(Ssh, "A", 64),
<<K1:8/binary, K2:8/binary, K3:8/binary>> = hash(Ssh, "C", 192),
{ok, Ssh#ssh{encrypt_keys = {K1,K2,K3},
encrypt_block_size = 8,
encrypt_ctx = IV}};
encrypt_init(#ssh{encrypt = '3des-cbc', role = server} = Ssh) ->
IV = hash(Ssh, "B", 64),
<<K1:8/binary, K2:8/binary, K3:8/binary>> = hash(Ssh, "D", 192),
{ok, Ssh#ssh{encrypt_keys = {K1,K2,K3},
encrypt_block_size = 8,
encrypt_ctx = IV}};
encrypt_init(#ssh{encrypt = 'aes128-cbc', role = client} = Ssh) ->
IV = hash(Ssh, "A", 128),
<<K:16/binary>> = hash(Ssh, "C", 128),
{ok, Ssh#ssh{encrypt_keys = K,
encrypt_block_size = 16,
encrypt_ctx = IV}};
encrypt_init(#ssh{encrypt = 'aes128-cbc', role = server} = Ssh) ->
IV = hash(Ssh, "B", 128),
<<K:16/binary>> = hash(Ssh, "D", 128),
{ok, Ssh#ssh{encrypt_keys = K,
encrypt_block_size = 16,
encrypt_ctx = IV}};
encrypt_init(#ssh{encrypt = 'aes128-ctr', role = client} = Ssh) ->
IV = hash(Ssh, "A", 128),
<<K:16/binary>> = hash(Ssh, "C", 128),
State = crypto:stream_init(aes_ctr, K, IV),
{ok, Ssh#ssh{encrypt_keys = K,
encrypt_block_size = 16,
encrypt_ctx = State}};
encrypt_init(#ssh{encrypt = 'aes128-ctr', role = server} = Ssh) ->
IV = hash(Ssh, "B", 128),
<<K:16/binary>> = hash(Ssh, "D", 128),
State = crypto:stream_init(aes_ctr, K, IV),
{ok, Ssh#ssh{encrypt_keys = K,
encrypt_block_size = 16,
encrypt_ctx = State}}.
encrypt_final(Ssh) ->
{ok, Ssh#ssh{encrypt = none,
encrypt_keys = undefined,
encrypt_block_size = 8,
encrypt_ctx = undefined
}}.
encrypt(#ssh{encrypt = none} = Ssh, Data) ->
{Ssh, Data};
encrypt(#ssh{encrypt = '3des-cbc',
encrypt_keys = {K1,K2,K3},
encrypt_ctx = IV0} = Ssh, Data) ->
Enc = crypto:block_encrypt(des3_cbc, [K1,K2,K3], IV0, Data),
IV = crypto:next_iv(des3_cbc, Enc),
{Ssh#ssh{encrypt_ctx = IV}, Enc};
encrypt(#ssh{encrypt = 'aes128-cbc',
encrypt_keys = K,
encrypt_ctx = IV0} = Ssh, Data) ->
Enc = crypto:block_encrypt(aes_cbc128, K,IV0,Data),
IV = crypto:next_iv(aes_cbc, Enc),
{Ssh#ssh{encrypt_ctx = IV}, Enc};
encrypt(#ssh{encrypt = 'aes128-ctr',
encrypt_ctx = State0} = Ssh, Data) ->
{State, Enc} = crypto:stream_encrypt(State0,Data),
{Ssh#ssh{encrypt_ctx = State}, Enc}.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% Decryption
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
decrypt_init(#ssh{decrypt = none} = Ssh) ->
{ok, Ssh};
decrypt_init(#ssh{decrypt = '3des-cbc', role = client} = Ssh) ->
{IV, KD} = {hash(Ssh, "B", 64),
hash(Ssh, "D", 192)},
<<K1:8/binary, K2:8/binary, K3:8/binary>> = KD,
{ok, Ssh#ssh{decrypt_keys = {K1,K2,K3}, decrypt_ctx = IV,
decrypt_block_size = 8}};
decrypt_init(#ssh{decrypt = '3des-cbc', role = server} = Ssh) ->
{IV, KD} = {hash(Ssh, "A", 64),
hash(Ssh, "C", 192)},
<<K1:8/binary, K2:8/binary, K3:8/binary>> = KD,
{ok, Ssh#ssh{decrypt_keys = {K1, K2, K3}, decrypt_ctx = IV,
decrypt_block_size = 8}};
decrypt_init(#ssh{decrypt = 'aes128-cbc', role = client} = Ssh) ->
{IV, KD} = {hash(Ssh, "B", 128),
hash(Ssh, "D", 128)},
<<K:16/binary>> = KD,
{ok, Ssh#ssh{decrypt_keys = K, decrypt_ctx = IV,
decrypt_block_size = 16}};
decrypt_init(#ssh{decrypt = 'aes128-cbc', role = server} = Ssh) ->
{IV, KD} = {hash(Ssh, "A", 128),
hash(Ssh, "C", 128)},
<<K:16/binary>> = KD,
{ok, Ssh#ssh{decrypt_keys = K, decrypt_ctx = IV,
decrypt_block_size = 16}};
decrypt_init(#ssh{decrypt = 'aes128-ctr', role = client} = Ssh) ->
IV = hash(Ssh, "B", 128),
<<K:16/binary>> = hash(Ssh, "D", 128),
State = crypto:stream_init(aes_ctr, K, IV),
{ok, Ssh#ssh{decrypt_keys = K,
decrypt_block_size = 16,
decrypt_ctx = State}};
decrypt_init(#ssh{decrypt = 'aes128-ctr', role = server} = Ssh) ->
IV = hash(Ssh, "A", 128),
<<K:16/binary>> = hash(Ssh, "C", 128),
State = crypto:stream_init(aes_ctr, K, IV),
{ok, Ssh#ssh{decrypt_keys = K,
decrypt_block_size = 16,
decrypt_ctx = State}}.
decrypt_final(Ssh) ->
{ok, Ssh#ssh {decrypt = none,
decrypt_keys = undefined,
decrypt_ctx = undefined,
decrypt_block_size = 8}}.
decrypt(#ssh{decrypt = none} = Ssh, Data) ->
{Ssh, Data};
decrypt(#ssh{decrypt = '3des-cbc', decrypt_keys = Keys,
decrypt_ctx = IV0} = Ssh, Data) ->
{K1, K2, K3} = Keys,
Dec = crypto:block_decrypt(des3_cbc, [K1,K2,K3], IV0, Data),
IV = crypto:next_iv(des3_cbc, Data),
{Ssh#ssh{decrypt_ctx = IV}, Dec};
decrypt(#ssh{decrypt = 'aes128-cbc', decrypt_keys = Key,
decrypt_ctx = IV0} = Ssh, Data) ->
Dec = crypto:block_decrypt(aes_cbc128, Key,IV0,Data),
IV = crypto:next_iv(aes_cbc, Data),
{Ssh#ssh{decrypt_ctx = IV}, Dec};
decrypt(#ssh{decrypt = 'aes128-ctr',
decrypt_ctx = State0} = Ssh, Data) ->
{State, Enc} = crypto:stream_decrypt(State0,Data),
{Ssh#ssh{decrypt_ctx = State}, Enc}.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% Compression
%%
%% none REQUIRED no compression
%% zlib OPTIONAL ZLIB (LZ77) compression
%% openssh_zlib OPTIONAL ZLIB (LZ77) compression
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
compress_init(SSH) ->
compress_init(SSH, 1).
compress_init(#ssh{compress = none} = Ssh, _) ->
{ok, Ssh};
compress_init(#ssh{compress = zlib} = Ssh, Level) ->
Zlib = zlib:open(),
ok = zlib:deflateInit(Zlib, Level),
{ok, Ssh#ssh{compress_ctx = Zlib}};
compress_init(#ssh{compress = '[email protected]'} = Ssh, Level) ->
Zlib = zlib:open(),
ok = zlib:deflateInit(Zlib, Level),
{ok, Ssh#ssh{compress_ctx = Zlib}}.
compress_final(#ssh{compress = none} = Ssh) ->
{ok, Ssh};
compress_final(#ssh{compress = zlib, compress_ctx = Context} = Ssh) ->
zlib:close(Context),
{ok, Ssh#ssh{compress = none, compress_ctx = undefined}};
compress_final(#ssh{compress = '[email protected]', authenticated = false} = Ssh) ->
{ok, Ssh};
compress_final(#ssh{compress = '[email protected]', compress_ctx = Context, authenticated = true} = Ssh) ->
zlib:close(Context),
{ok, Ssh#ssh{compress = none, compress_ctx = undefined}}.
compress(#ssh{compress = none} = Ssh, Data) ->
{Ssh, Data};
compress(#ssh{compress = zlib, compress_ctx = Context} = Ssh, Data) ->
Compressed = zlib:deflate(Context, Data, sync),
{Ssh, list_to_binary(Compressed)};
compress(#ssh{compress = '[email protected]', authenticated = false} = Ssh, Data) ->
{Ssh, Data};
compress(#ssh{compress = '[email protected]', compress_ctx = Context, authenticated = true} = Ssh, Data) ->
Compressed = zlib:deflate(Context, Data, sync),
{Ssh, list_to_binary(Compressed)}.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% Decompression
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
decompress_init(#ssh{decompress = none} = Ssh) ->
{ok, Ssh};
decompress_init(#ssh{decompress = zlib} = Ssh) ->
Zlib = zlib:open(),
ok = zlib:inflateInit(Zlib),
{ok, Ssh#ssh{decompress_ctx = Zlib}};
decompress_init(#ssh{decompress = '[email protected]'} = Ssh) ->
Zlib = zlib:open(),
ok = zlib:inflateInit(Zlib),
{ok, Ssh#ssh{decompress_ctx = Zlib}}.
decompress_final(#ssh{decompress = none} = Ssh) ->
{ok, Ssh};
decompress_final(#ssh{decompress = zlib, decompress_ctx = Context} = Ssh) ->
zlib:close(Context),
{ok, Ssh#ssh{decompress = none, decompress_ctx = undefined}};
decompress_final(#ssh{decompress = '[email protected]', authenticated = false} = Ssh) ->
{ok, Ssh};
decompress_final(#ssh{decompress = '[email protected]', decompress_ctx = Context, authenticated = true} = Ssh) ->
zlib:close(Context),
{ok, Ssh#ssh{decompress = none, decompress_ctx = undefined}}.
decompress(#ssh{decompress = none} = Ssh, Data) ->
{Ssh, Data};
decompress(#ssh{decompress = zlib, decompress_ctx = Context} = Ssh, Data) ->
Decompressed = zlib:inflate(Context, Data),
{Ssh, list_to_binary(Decompressed)};
decompress(#ssh{decompress = '[email protected]', authenticated = false} = Ssh, Data) ->
{Ssh, Data};
decompress(#ssh{decompress = '[email protected]', decompress_ctx = Context, authenticated = true} = Ssh, Data) ->
Decompressed = zlib:inflate(Context, Data),
{Ssh, list_to_binary(Decompressed)}.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% MAC calculation
%%
%% hmac-sha1 REQUIRED HMAC-SHA1 (digest length = key
%% length = 20)
%% hmac-sha1-96 RECOMMENDED first 96 bits of HMAC-SHA1 (digest
%% length = 12, key length = 20)
%% hmac-md5 OPTIONAL HMAC-MD5 (digest length = key
%% length = 16)
%% hmac-md5-96 OPTIONAL first 96 bits of HMAC-MD5 (digest
%% length = 12, key length = 16)
%% none OPTIONAL no MAC; NOT RECOMMENDED
%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
send_mac_init(SSH) ->
case SSH#ssh.role of
client ->
KeySize =mac_key_size(SSH#ssh.send_mac),
Key = hash(SSH, "E", KeySize),
{ok, SSH#ssh { send_mac_key = Key }};
server ->
KeySize = mac_key_size(SSH#ssh.send_mac),
Key = hash(SSH, "F", KeySize),
{ok, SSH#ssh { send_mac_key = Key }}
end.
send_mac_final(SSH) ->
{ok, SSH#ssh { send_mac = none, send_mac_key = undefined }}.
recv_mac_init(SSH) ->
case SSH#ssh.role of
client ->
Key = hash(SSH, "F", mac_key_size(SSH#ssh.recv_mac)),
{ok, SSH#ssh { recv_mac_key = Key }};
server ->
Key = hash(SSH, "E", mac_key_size(SSH#ssh.recv_mac)),
{ok, SSH#ssh { recv_mac_key = Key }}
end.
recv_mac_final(SSH) ->
{ok, SSH#ssh { recv_mac = none, recv_mac_key = undefined }}.
mac(none, _ , _, _) ->
<<>>;
mac('hmac-sha1', Key, SeqNum, Data) ->
crypto:hmac(sha, Key, [<<?UINT32(SeqNum)>>, Data]);
mac('hmac-sha1-96', Key, SeqNum, Data) ->
crypto:hmac(sha, Key, [<<?UINT32(SeqNum)>>, Data], mac_digest_size('hmac-sha1-96'));
mac('hmac-md5', Key, SeqNum, Data) ->
crypto:hmac(md5, Key, [<<?UINT32(SeqNum)>>, Data]);
mac('hmac-md5-96', Key, SeqNum, Data) ->
crypto:hmac(md5, Key, [<<?UINT32(SeqNum)>>, Data], mac_digest_size('hmac-md5-96'));
mac('hmac-sha2-256', Key, SeqNum, Data) ->
crypto:hmac(sha256, Key, [<<?UINT32(SeqNum)>>, Data]).
%% return N hash bytes (HASH)
hash(SSH, Char, Bits) ->
HASH =
case SSH#ssh.kex of
'diffie-hellman-group1-sha1' ->
fun(Data) -> crypto:hash(sha, Data) end;
'diffie-hellman-group-exchange-sha1' ->
fun(Data) -> crypto:hash(sha, Data) end;
_ ->
exit({bad_algorithm,SSH#ssh.kex})
end,
hash(SSH, Char, Bits, HASH).
hash(_SSH, _Char, 0, _HASH) ->
<<>>;
hash(SSH, Char, N, HASH) ->
K = ssh_bits:mpint(SSH#ssh.shared_secret),
H = SSH#ssh.exchanged_hash,
SessionID = SSH#ssh.session_id,
K1 = HASH([K, H, Char, SessionID]),
Sz = N div 8,
<<Key:Sz/binary, _/binary>> = hash(K, H, K1, N-128, HASH),
Key.
hash(_K, _H, Ki, N, _HASH) when N =< 0 ->
Ki;
hash(K, H, Ki, N, HASH) ->
Kj = HASH([K, H, Ki]),
hash(K, H, <<Ki/binary, Kj/binary>>, N-128, HASH).
kex_h(SSH, Key, E, F, K) ->
L = ssh_bits:encode([SSH#ssh.c_version, SSH#ssh.s_version,
SSH#ssh.c_keyinit, SSH#ssh.s_keyinit,
ssh_message:encode_host_key(Key), E,F,K],
[string,string,binary,binary,binary,
mpint,mpint,mpint]),
crypto:hash(sha,L).
kex_h(SSH, Key, Min, NBits, Max, Prime, Gen, E, F, K) ->
L = if Min==-1; Max==-1 ->
Ts = [string,string,binary,binary,binary,
uint32,
mpint,mpint,mpint,mpint,mpint],
ssh_bits:encode([SSH#ssh.c_version,SSH#ssh.s_version,
SSH#ssh.c_keyinit,SSH#ssh.s_keyinit,
ssh_message:encode_host_key(Key), NBits, Prime, Gen, E,F,K],
Ts);
true ->
Ts = [string,string,binary,binary,binary,
uint32,uint32,uint32,
mpint,mpint,mpint,mpint,mpint],
ssh_bits:encode([SSH#ssh.c_version,SSH#ssh.s_version,
SSH#ssh.c_keyinit,SSH#ssh.s_keyinit,
ssh_message:encode_host_key(Key), Min, NBits, Max,
Prime, Gen, E,F,K], Ts)
end,
crypto:hash(sha,L).
mac_key_size('hmac-sha1') -> 20*8;
mac_key_size('hmac-sha1-96') -> 20*8;
mac_key_size('hmac-md5') -> 16*8;
mac_key_size('hmac-md5-96') -> 16*8;
mac_key_size('hmac-sha2-256')-> 32*8;
mac_key_size(none) -> 0.
mac_digest_size('hmac-sha1') -> 20;
mac_digest_size('hmac-sha1-96') -> 12;
mac_digest_size('hmac-md5') -> 20;
mac_digest_size('hmac-md5-96') -> 12;
mac_digest_size('hmac-sha2-256') -> 32;
mac_digest_size(none) -> 0.
peer_name({Host, _}) ->
Host.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%
%% Diffie-Hellman utils
%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
dh_group1() ->
{2, 16#FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF}.
dh_gen_key(G, P, _) ->
{Public, Private} = crypto:generate_key(dh, [P, G]),
{crypto:bytes_to_integer(Private), crypto:bytes_to_integer(Public)}.
trim_tail(Str) ->
lists:reverse(trim_head(lists:reverse(Str))).
trim_head([$\s|Cs]) -> trim_head(Cs);
trim_head([$\t|Cs]) -> trim_head(Cs);
trim_head([$\n|Cs]) -> trim_head(Cs);
trim_head([$\r|Cs]) -> trim_head(Cs);
trim_head(Cs) -> Cs.
