%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 2013-2017. All Rights Reserved.
%%
%% Licensed under the Apache License, Version 2.0 (the "License");
%% you may not use this file except in compliance with the License.
%% You may obtain a copy of the License at
%%
%% http://www.apache.org/licenses/LICENSE-2.0
%%
%% Unless required by applicable law or agreed to in writing, software
%% distributed under the License is distributed on an "AS IS" BASIS,
%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
%% See the License for the specific language governing permissions and
%% limitations under the License.
%%
%% %CopyrightEnd%
%%
%%----------------------------------------------------------------------
%% Purpose: Handle DTLS record protocol. (Parts that are not shared with SSL/TLS)
%%----------------------------------------------------------------------
-module(dtls_record).
-include("dtls_record.hrl").
-include("ssl_internal.hrl").
-include("ssl_alert.hrl").
-include("dtls_handshake.hrl").
-include("ssl_cipher.hrl").
%% Handling of incoming data
-export([get_dtls_records/2, init_connection_states/2, empty_connection_state/1]).
-export([save_current_connection_state/2, next_epoch/2, get_connection_state_by_epoch/3, replay_detect/2,
init_connection_state_seq/2, current_connection_state_epoch/2]).
%% Encoding
-export([encode_handshake/4, encode_alert_record/3,
encode_change_cipher_spec/3, encode_data/3, encode_plain_text/5]).
%% Decoding
-export([decode_cipher_text/2]).
%% Protocol version handling
-export([protocol_version/1, lowest_protocol_version/1, lowest_protocol_version/2,
highest_protocol_version/1, highest_protocol_version/2,
is_higher/2, supported_protocol_versions/0,
is_acceptable_version/2, hello_version/2]).
-export_type([dtls_version/0, dtls_atom_version/0]).
-type dtls_version() :: ssl_record:ssl_version().
-type dtls_atom_version() :: dtlsv1 | 'dtlsv1.2'.
-define(REPLAY_WINDOW_SIZE, 64).
-compile(inline).
%%====================================================================
%% Handling of incoming data
%%====================================================================
%%--------------------------------------------------------------------
-spec init_connection_states(client | server, one_n_minus_one | zero_n | disabled) ->
ssl_record:connection_states().
%% %
%
%% Description: Creates a connection_states record with appropriate
%% values for the initial SSL connection setup.
%%--------------------------------------------------------------------
init_connection_states(Role, BeastMitigation) ->
ConnectionEnd = ssl_record:record_protocol_role(Role),
Initial = initial_connection_state(ConnectionEnd, BeastMitigation),
Current = Initial#{epoch := 0},
InitialPending = ssl_record:empty_connection_state(ConnectionEnd, BeastMitigation),
Pending = empty_connection_state(InitialPending),
#{saved_read => Current,
current_read => Current,
pending_read => Pending,
saved_write => Current,
current_write => Current,
pending_write => Pending}.
empty_connection_state(Empty) ->
Empty#{epoch => undefined, replay_window => init_replay_window(?REPLAY_WINDOW_SIZE)}.
%%--------------------------------------------------------------------
-spec save_current_connection_state(ssl_record:connection_states(), read | write) ->
ssl_record:connection_states().
%%
%% Description: Returns the instance of the connection_state map
%% where the current read|write state has been copied to the save state.
%%--------------------------------------------------------------------
save_current_connection_state(#{current_read := Current} = States, read) ->
States#{saved_read := Current};
save_current_connection_state(#{current_write := Current} = States, write) ->
States#{saved_write := Current}.
next_epoch(#{pending_read := Pending,
current_read := #{epoch := Epoch}} = States, read) ->
States#{pending_read := Pending#{epoch := Epoch + 1,
replay_window := init_replay_window(?REPLAY_WINDOW_SIZE)}};
next_epoch(#{pending_write := Pending,
current_write := #{epoch := Epoch}} = States, write) ->
States#{pending_write := Pending#{epoch := Epoch + 1,
replay_window := init_replay_window(?REPLAY_WINDOW_SIZE)}}.
get_connection_state_by_epoch(Epoch, #{current_write := #{epoch := Epoch} = Current},
write) ->
Current;
get_connection_state_by_epoch(Epoch, #{saved_write := #{epoch := Epoch} = Saved},
write) ->
Saved;
get_connection_state_by_epoch(Epoch, #{current_read := #{epoch := Epoch} = Current},
read) ->
Current;
get_connection_state_by_epoch(Epoch, #{saved_read := #{epoch := Epoch} = Saved},
read) ->
Saved.
set_connection_state_by_epoch(WriteState, Epoch, #{current_write := #{epoch := Epoch}} = States,
write) ->
States#{current_write := WriteState};
set_connection_state_by_epoch(WriteState, Epoch, #{saved_write := #{epoch := Epoch}} = States,
write) ->
States#{saved_write := WriteState};
set_connection_state_by_epoch(ReadState, Epoch, #{current_read := #{epoch := Epoch}} = States,
read) ->
States#{current_read := ReadState};
set_connection_state_by_epoch(ReadState, Epoch, #{saved_read := #{epoch := Epoch}} = States,
read) ->
States#{saved_read := ReadState}.
%%--------------------------------------------------------------------
-spec init_connection_state_seq(dtls_version(), ssl_record:connection_states()) ->
ssl_record:connection_state().
%%
%% Description: Copy the read sequence number to the write sequence number
%% This is only valid for DTLS in the first client_hello
%%--------------------------------------------------------------------
init_connection_state_seq({254, _},
#{current_read := #{epoch := 0, sequence_number := Seq},
current_write := #{epoch := 0} = Write} = ConnnectionStates0) ->
ConnnectionStates0#{current_write => Write#{sequence_number => Seq}};
init_connection_state_seq(_, ConnnectionStates) ->
ConnnectionStates.
%%--------------------------------------------------------
-spec current_connection_state_epoch(ssl_record:connection_states(), read | write) ->
integer().
%%
%% Description: Returns the epoch the connection_state record
%% that is currently defined as the current connection state.
%%--------------------------------------------------------------------
current_connection_state_epoch(#{current_read := #{epoch := Epoch}},
read) ->
Epoch;
current_connection_state_epoch(#{current_write := #{epoch := Epoch}},
write) ->
Epoch.
%%--------------------------------------------------------------------
-spec get_dtls_records(binary(), binary()) -> {[binary()], binary()} | #alert{}.
%%
%% Description: Given old buffer and new data from UDP/SCTP, packs up a records
%% and returns it as a list of tls_compressed binaries also returns leftover
%% data
%%--------------------------------------------------------------------
get_dtls_records(Data, <<>>) ->
get_dtls_records_aux(Data, []);
get_dtls_records(Data, Buffer) ->
get_dtls_records_aux(list_to_binary([Buffer, Data]), []).
%%====================================================================
%% Encoding DTLS records
%%====================================================================
%%--------------------------------------------------------------------
-spec encode_handshake(iolist(), dtls_version(), integer(), ssl_record:connection_states()) ->
{iolist(), ssl_record:connection_states()}.
%
%% Description: Encodes a handshake message to send on the ssl-socket.
%%--------------------------------------------------------------------
encode_handshake(Frag, Version, Epoch, ConnectionStates) ->
encode_plain_text(?HANDSHAKE, Version, Epoch, Frag, ConnectionStates).
%%--------------------------------------------------------------------
-spec encode_alert_record(#alert{}, dtls_version(), ssl_record:connection_states()) ->
{iolist(), ssl_record:connection_states()}.
%%
%% Description: Encodes an alert message to send on the ssl-socket.
%%--------------------------------------------------------------------
encode_alert_record(#alert{level = Level, description = Description},
Version, ConnectionStates) ->
#{epoch := Epoch} = ssl_record:current_connection_state(ConnectionStates, write),
encode_plain_text(?ALERT, Version, Epoch, <<?BYTE(Level), ?BYTE(Description)>>,
ConnectionStates).
%%--------------------------------------------------------------------
-spec encode_change_cipher_spec(dtls_version(), integer(), ssl_record:connection_states()) ->
{iolist(), ssl_record:connection_states()}.
%%
%% Description: Encodes a change_cipher_spec-message to send on the ssl socket.
%%--------------------------------------------------------------------
encode_change_cipher_spec(Version, Epoch, ConnectionStates) ->
encode_plain_text(?CHANGE_CIPHER_SPEC, Version, Epoch, ?byte(?CHANGE_CIPHER_SPEC_PROTO), ConnectionStates).
%%--------------------------------------------------------------------
-spec encode_data(binary(), dtls_version(), ssl_record:connection_states()) ->
{iolist(),ssl_record:connection_states()}.
%%
%% Description: Encodes data to send on the ssl-socket.
%%--------------------------------------------------------------------
encode_data(Data, Version, ConnectionStates) ->
#{epoch := Epoch} = ssl_record:current_connection_state(ConnectionStates, write),
encode_plain_text(?APPLICATION_DATA, Version, Epoch, Data, ConnectionStates).
encode_plain_text(Type, Version, Epoch, Data, ConnectionStates) ->
Write0 = get_connection_state_by_epoch(Epoch, ConnectionStates, write),
{CipherFragment, Write1} = encode_plain_text(Type, Version, Data, Write0),
{CipherText, Write} = encode_dtls_cipher_text(Type, Version, CipherFragment, Write1),
{CipherText, set_connection_state_by_epoch(Write, Epoch, ConnectionStates, write)}.
%%====================================================================
%% Decoding
%%====================================================================
decode_cipher_text(#ssl_tls{epoch = Epoch} = CipherText, ConnnectionStates0) ->
ReadState = get_connection_state_by_epoch(Epoch, ConnnectionStates0, read),
decode_cipher_text(CipherText, ReadState, ConnnectionStates0).
%%====================================================================
%% Protocol version handling
%%====================================================================
%%--------------------------------------------------------------------
-spec protocol_version(dtls_atom_version() | dtls_version()) ->
dtls_version() | dtls_atom_version().
%%
%% Description: Creates a protocol version record from a version atom
%% or vice versa.
%%--------------------------------------------------------------------
protocol_version('dtlsv1.2') ->
{254, 253};
protocol_version(dtlsv1) ->
{254, 255};
protocol_version({254, 253}) ->
'dtlsv1.2';
protocol_version({254, 255}) ->
dtlsv1.
%%--------------------------------------------------------------------
-spec lowest_protocol_version(dtls_version(), dtls_version()) -> dtls_version().
%%
%% Description: Lowes protocol version of two given versions
%%--------------------------------------------------------------------
lowest_protocol_version(Version = {M, N}, {M, O}) when N > O ->
Version;
lowest_protocol_version({M, _}, Version = {M, _}) ->
Version;
lowest_protocol_version(Version = {M,_}, {N, _}) when M > N ->
Version;
lowest_protocol_version(_,Version) ->
Version.
%%--------------------------------------------------------------------
-spec lowest_protocol_version([dtls_version()]) -> dtls_version().
%%
%% Description: Lowest protocol version present in a list
%%--------------------------------------------------------------------
lowest_protocol_version([]) ->
lowest_protocol_version();
lowest_protocol_version(Versions) ->
[Ver | Vers] = Versions,
lowest_list_protocol_version(Ver, Vers).
%%--------------------------------------------------------------------
-spec highest_protocol_version([dtls_version()]) -> dtls_version().
%%
%% Description: Highest protocol version present in a list
%%--------------------------------------------------------------------
highest_protocol_version([]) ->
highest_protocol_version();
highest_protocol_version(Versions) ->
[Ver | Vers] = Versions,
highest_list_protocol_version(Ver, Vers).
%%--------------------------------------------------------------------
-spec highest_protocol_version(dtls_version(), dtls_version()) -> dtls_version().
%%
%% Description: Highest protocol version of two given versions
%%--------------------------------------------------------------------
highest_protocol_version(Version = {M, N}, {M, O}) when N < O ->
Version;
highest_protocol_version({M, _},
Version = {M, _}) ->
Version;
highest_protocol_version(Version = {M,_},
{N, _}) when M < N ->
Version;
highest_protocol_version(_,Version) ->
Version.
%%--------------------------------------------------------------------
-spec is_higher(V1 :: dtls_version(), V2::dtls_version()) -> boolean().
%%
%% Description: Is V1 > V2
%%--------------------------------------------------------------------
is_higher({M, N}, {M, O}) when N < O ->
true;
is_higher({M, _}, {N, _}) when M < N ->
true;
is_higher(_, _) ->
false.
%%--------------------------------------------------------------------
-spec supported_protocol_versions() -> [dtls_version()].
%%
%% Description: Protocol versions supported
%%--------------------------------------------------------------------
supported_protocol_versions() ->
Fun = fun(Version) ->
protocol_version(Version)
end,
case application:get_env(ssl, dtls_protocol_version) of
undefined ->
lists:map(Fun, supported_protocol_versions([]));
{ok, []} ->
lists:map(Fun, supported_protocol_versions([]));
{ok, Vsns} when is_list(Vsns) ->
supported_protocol_versions(lists:map(Fun, Vsns));
{ok, Vsn} ->
supported_protocol_versions([Fun(Vsn)])
end.
supported_protocol_versions([]) ->
Vsns = case sufficient_dtlsv1_2_crypto_support() of
true ->
?ALL_DATAGRAM_SUPPORTED_VERSIONS;
false ->
?MIN_DATAGRAM_SUPPORTED_VERSIONS
end,
application:set_env(ssl, dtls_protocol_version, Vsns),
Vsns;
supported_protocol_versions([_|_] = Vsns) ->
case sufficient_dtlsv1_2_crypto_support() of
true ->
Vsns;
false ->
case Vsns -- ['dtlsv1.2'] of
[] ->
?MIN_SUPPORTED_VERSIONS;
NewVsns ->
NewVsns
end
end.
%%--------------------------------------------------------------------
-spec is_acceptable_version(dtls_version(), Supported :: [dtls_version()]) -> boolean().
%%
%% Description: ssl version 2 is not acceptable security risks are too big.
%%
%%--------------------------------------------------------------------
is_acceptable_version(Version, Versions) ->
lists:member(Version, Versions).
-spec hello_version(dtls_version(), [dtls_version()]) -> dtls_version().
hello_version(Version, Versions) ->
case dtls_v1:corresponding_tls_version(Version) of
TLSVersion when TLSVersion >= {3, 3} ->
Version;
_ ->
lowest_protocol_version(Versions)
end.
%%--------------------------------------------------------------------
%%% Internal functions
%%--------------------------------------------------------------------
initial_connection_state(ConnectionEnd, BeastMitigation) ->
#{security_parameters =>
ssl_record:initial_security_params(ConnectionEnd),
epoch => undefined,
sequence_number => 0,
replay_window => init_replay_window(?REPLAY_WINDOW_SIZE),
beast_mitigation => BeastMitigation,
compression_state => undefined,
cipher_state => undefined,
mac_secret => undefined,
secure_renegotiation => undefined,
client_verify_data => undefined,
server_verify_data => undefined
}.
get_dtls_records_aux(<<?BYTE(?APPLICATION_DATA),?BYTE(MajVer),?BYTE(MinVer),
?UINT16(Epoch), ?UINT48(SequenceNumber),
?UINT16(Length), Data:Length/binary, Rest/binary>>,
Acc) ->
get_dtls_records_aux(Rest, [#ssl_tls{type = ?APPLICATION_DATA,
version = {MajVer, MinVer},
epoch = Epoch, sequence_number = SequenceNumber,
fragment = Data} | Acc]);
get_dtls_records_aux(<<?BYTE(?HANDSHAKE),?BYTE(MajVer),?BYTE(MinVer),
?UINT16(Epoch), ?UINT48(SequenceNumber),
?UINT16(Length),
Data:Length/binary, Rest/binary>>, Acc) when MajVer >= 128 ->
get_dtls_records_aux(Rest, [#ssl_tls{type = ?HANDSHAKE,
version = {MajVer, MinVer},
epoch = Epoch, sequence_number = SequenceNumber,
fragment = Data} | Acc]);
get_dtls_records_aux(<<?BYTE(?ALERT),?BYTE(MajVer),?BYTE(MinVer),
?UINT16(Epoch), ?UINT48(SequenceNumber),
?UINT16(Length), Data:Length/binary,
Rest/binary>>, Acc) ->
get_dtls_records_aux(Rest, [#ssl_tls{type = ?ALERT,
version = {MajVer, MinVer},
epoch = Epoch, sequence_number = SequenceNumber,
fragment = Data} | Acc]);
get_dtls_records_aux(<<?BYTE(?CHANGE_CIPHER_SPEC),?BYTE(MajVer),?BYTE(MinVer),
?UINT16(Epoch), ?UINT48(SequenceNumber),
?UINT16(Length), Data:Length/binary, Rest/binary>>,
Acc) ->
get_dtls_records_aux(Rest, [#ssl_tls{type = ?CHANGE_CIPHER_SPEC,
version = {MajVer, MinVer},
epoch = Epoch, sequence_number = SequenceNumber,
fragment = Data} | Acc]);
get_dtls_records_aux(<<0:1, _CT:7, ?BYTE(_MajVer), ?BYTE(_MinVer),
?UINT16(Length), _/binary>>,
_Acc) when Length > ?MAX_CIPHER_TEXT_LENGTH ->
?ALERT_REC(?FATAL, ?RECORD_OVERFLOW);
get_dtls_records_aux(<<1:1, Length0:15, _/binary>>,_Acc)
when Length0 > ?MAX_CIPHER_TEXT_LENGTH ->
?ALERT_REC(?FATAL, ?RECORD_OVERFLOW);
get_dtls_records_aux(Data, Acc) ->
case size(Data) =< ?MAX_CIPHER_TEXT_LENGTH + ?INITIAL_BYTES of
true ->
{lists:reverse(Acc), Data};
false ->
?ALERT_REC(?FATAL, ?UNEXPECTED_MESSAGE)
end.
%%--------------------------------------------------------------------
init_replay_window(Size) ->
#{size => Size,
top => Size,
bottom => 0,
mask => 0 bsl 64
}.
replay_detect(#ssl_tls{sequence_number = SequenceNumber}, #{replay_window := Window}) ->
is_replay(SequenceNumber, Window).
is_replay(SequenceNumber, #{bottom := Bottom}) when SequenceNumber < Bottom ->
true;
is_replay(SequenceNumber, #{size := Size,
top := Top,
bottom := Bottom,
mask := Mask}) when (SequenceNumber >= Bottom) andalso (SequenceNumber =< Top) ->
Index = (SequenceNumber rem Size),
(Index band Mask) == 1;
is_replay(_, _) ->
false.
update_replay_window(SequenceNumber, #{replay_window := #{size := Size,
top := Top,
bottom := Bottom,
mask := Mask0} = Window0} = ConnectionStates) ->
NoNewBits = SequenceNumber - Top,
Index = SequenceNumber rem Size,
Mask = (Mask0 bsl NoNewBits) bor Index,
Window = Window0#{top => SequenceNumber,
bottom => Bottom + NoNewBits,
mask => Mask},
ConnectionStates#{replay_window := Window}.
%%--------------------------------------------------------------------
encode_dtls_cipher_text(Type, {MajVer, MinVer}, Fragment,
#{epoch := Epoch, sequence_number := Seq} = WriteState) ->
Length = erlang:iolist_size(Fragment),
{[<<?BYTE(Type), ?BYTE(MajVer), ?BYTE(MinVer), ?UINT16(Epoch),
?UINT48(Seq), ?UINT16(Length)>>, Fragment],
WriteState#{sequence_number => Seq + 1}}.
encode_plain_text(Type, Version, Data, #{compression_state := CompS0,
epoch := Epoch,
sequence_number := Seq,
cipher_state := CipherS0,
security_parameters :=
#security_parameters{
cipher_type = ?AEAD,
bulk_cipher_algorithm =
BulkCipherAlgo,
compression_algorithm = CompAlg}
} = WriteState0) ->
{Comp, CompS1} = ssl_record:compress(CompAlg, Data, CompS0),
AAD = calc_aad(Type, Version, Epoch, Seq),
TLSVersion = dtls_v1:corresponding_tls_version(Version),
{CipherFragment, CipherS1} =
ssl_cipher:cipher_aead(BulkCipherAlgo, CipherS0, Seq, AAD, Comp, TLSVersion),
{CipherFragment, WriteState0#{compression_state => CompS1,
cipher_state => CipherS1}};
encode_plain_text(Type, Version, Fragment, #{compression_state := CompS0,
epoch := Epoch,
sequence_number := Seq,
cipher_state := CipherS0,
security_parameters :=
#security_parameters{compression_algorithm = CompAlg,
bulk_cipher_algorithm =
BulkCipherAlgo}
}= WriteState0) ->
{Comp, CompS1} = ssl_record:compress(CompAlg, Fragment, CompS0),
WriteState1 = WriteState0#{compression_state => CompS1},
MAC = calc_mac_hash(Type, Version, WriteState1, Epoch, Seq, Comp),
TLSVersion = dtls_v1:corresponding_tls_version(Version),
{CipherFragment, CipherS1} =
ssl_cipher:cipher(BulkCipherAlgo, CipherS0, MAC, Fragment, TLSVersion),
{CipherFragment, WriteState0#{cipher_state => CipherS1}}.
%%--------------------------------------------------------------------
decode_cipher_text(#ssl_tls{type = Type, version = Version,
epoch = Epoch,
sequence_number = Seq,
fragment = CipherFragment} = CipherText,
#{compression_state := CompressionS0,
cipher_state := CipherS0,
security_parameters :=
#security_parameters{
cipher_type = ?AEAD,
bulk_cipher_algorithm =
BulkCipherAlgo,
compression_algorithm = CompAlg}} = ReadState0,
ConnnectionStates0) ->
AAD = calc_aad(Type, Version, Epoch, Seq),
TLSVersion = dtls_v1:corresponding_tls_version(Version),
case ssl_cipher:decipher_aead(BulkCipherAlgo, CipherS0, Seq, AAD, CipherFragment, TLSVersion) of
{PlainFragment, CipherState} ->
{Plain, CompressionS1} = ssl_record:uncompress(CompAlg,
PlainFragment, CompressionS0),
ReadState0 = ReadState0#{compression_state => CompressionS1,
cipher_state => CipherState},
ReadState = update_replay_window(Seq, ReadState0),
ConnnectionStates = set_connection_state_by_epoch(ReadState, Epoch, ConnnectionStates0, read),
{CipherText#ssl_tls{fragment = Plain}, ConnnectionStates};
#alert{} = Alert ->
Alert
end;
decode_cipher_text(#ssl_tls{type = Type, version = Version,
epoch = Epoch,
sequence_number = Seq,
fragment = CipherFragment} = CipherText,
#{compression_state := CompressionS0,
security_parameters :=
#security_parameters{
compression_algorithm = CompAlg}} = ReadState0,
ConnnectionStates0) ->
{PlainFragment, Mac, ReadState1} = ssl_record:decipher(dtls_v1:corresponding_tls_version(Version),
CipherFragment, ReadState0, true),
MacHash = calc_mac_hash(Type, Version, ReadState1, Epoch, Seq, PlainFragment),
case ssl_record:is_correct_mac(Mac, MacHash) of
true ->
{Plain, CompressionS1} = ssl_record:uncompress(CompAlg,
PlainFragment, CompressionS0),
ReadState2 = ReadState1#{compression_state => CompressionS1},
ReadState = update_replay_window(Seq, ReadState2),
ConnnectionStates = set_connection_state_by_epoch(ReadState, Epoch, ConnnectionStates0, read),
{CipherText#ssl_tls{fragment = Plain}, ConnnectionStates};
false ->
?ALERT_REC(?FATAL, ?BAD_RECORD_MAC)
end.
%%--------------------------------------------------------------------
calc_mac_hash(Type, Version, #{mac_secret := MacSecret,
security_parameters := #security_parameters{mac_algorithm = MacAlg}},
Epoch, SeqNo, Fragment) ->
Length = erlang:iolist_size(Fragment),
mac_hash(Version, MacAlg, MacSecret, Epoch, SeqNo, Type,
Length, Fragment).
mac_hash({Major, Minor}, MacAlg, MacSecret, Epoch, SeqNo, Type, Length, Fragment) ->
Value = [<<?UINT16(Epoch), ?UINT48(SeqNo), ?BYTE(Type),
?BYTE(Major), ?BYTE(Minor), ?UINT16(Length)>>,
Fragment],
dtls_v1:hmac_hash(MacAlg, MacSecret, Value).
calc_aad(Type, {MajVer, MinVer}, Epoch, SeqNo) ->
<<?UINT16(Epoch), ?UINT48(SeqNo), ?BYTE(Type), ?BYTE(MajVer), ?BYTE(MinVer)>>.
%%--------------------------------------------------------------------
lowest_list_protocol_version(Ver, []) ->
Ver;
lowest_list_protocol_version(Ver1, [Ver2 | Rest]) ->
lowest_list_protocol_version(lowest_protocol_version(Ver1, Ver2), Rest).
highest_list_protocol_version(Ver, []) ->
Ver;
highest_list_protocol_version(Ver1, [Ver2 | Rest]) ->
highest_list_protocol_version(highest_protocol_version(Ver1, Ver2), Rest).
highest_protocol_version() ->
highest_protocol_version(supported_protocol_versions()).
lowest_protocol_version() ->
lowest_protocol_version(supported_protocol_versions()).
sufficient_dtlsv1_2_crypto_support() ->
CryptoSupport = crypto:supports(),
proplists:get_bool(sha256, proplists:get_value(hashs, CryptoSupport)).
