Replace case with a simpler assertion

src/hz_key_master.erl

@@ -1,17 +1,43 @@
 %%% @doc
-%%% Key functions
+%%% Hakuzaru Key Functions
 %%%
-%%% The main reason this is a module of its own is that in the original architecture
-%%% it was a process rather than just a library of functions. Now that it exists, though,
-%%% there is little motivation to cram everything here into the controller process's
-%%% code.
+%%% The Gajumaru's default key type is based on Elliptical Curve Cryptography (ECC).
+%%% The specific curve used is 25519, and the typical key representation is Ed25519.
+%%%
+%%% The "Ed" in "Ed25519" stands for Harold Edwards. This form represents
+%%% a coordinate on a "Twisted Edwards Curve".
+%%%
+%%% The "X" in "X25519" stands for the X-coordinate, also known as the
+%%% "Montgomery u-coordinate" on a "Montgomery Curve".
+%%%
+%%% The two are equivalent, but have meaningfully different properties.
 %%% @end
 
 -module(hz_key_master).
 -vsn("0.9.2").
 
--export([make_key/1, encode/1, decode/1]).
--export([lcg/1]).
+-export([make_key/0, make_key/1, encode/1, decode/1]).
+-export([shared_secret_a/6, shared_secret_b/6,
+         ed25519_pk_to_x25519/1, ed25519_sk_to_x25519/1,
+         hkdf/4, hkdf/5]).
+
+
+-spec make_key() -> {ID, KeyPair}
+    when ID      :: string(),
+         KeyPair :: #{secret => binary(), public => binary()}.
+%% @doc
+%% @equiv make_key(<<>>)
+
+make_key() ->
+    make_key(<<>>).
+
+
+-spec make_key(Secret) -> {ID, KeyPair}
+    when Secret  :: <<>> | <<_:32*8>>,
+         ID      :: string(),
+         KeyPair :: #{secret => binary(), public => binary()}.
+%% @doc
+%% Generate a Ed25519 keypair tagged with the corresponding Gajumaru ID.
 
 make_key(<<>>) ->
     Pair = #{public := Public} = ecu_eddsa:sign_keypair(),
@@ -125,28 +151,212 @@ sumcheck(Width, Bits) ->
     end.
 
 
-
--spec lcg(integer()) -> integer().
-%% A simple PRNG that fits into 32 bits and is easy to implement anywhere (Kotlin).
-%% Specifically, it is a "linear congruential generator" of the Lehmer variety.
-%% The constants used are based on recommendations from Park, Miller and Stockmeyer:
-%% https://www.firstpr.com.au/dsp/rand31/p105-crawford.pdf#page=4
+-spec shared_secret_a(A_E_E_SK, B_P_E_PK, B_E_E_PK, Protocol, Version, Salt) -> SS
+    when A_E_E_SK :: binary(),
+         B_P_E_PK :: <<_:32*8>>,
+         B_E_E_PK :: <<_:32*8>>,
+         Protocol :: binary(),
+         Version  :: binary(),
+         Salt     :: binary(),
+         SS       :: <<_:32*8>>.
+%% @doc
+%% Alice's side of a shared key derivation based on ed25519 keys as generated by this module.
 %%
-%% The input value should be between 1 and 2^31-1.
-%%
-%% The purpose of this PRNG is for password-based dictionary shuffling.
+%% Typically Alice would be providing an ephemeral key to establish
+%% a shared secret while remaining (at least initially) anonymous from Bob. Bob,
+%% on the other hand, is providing a permanent key and also an ephemeral key,
+%% proving identity without exposing the shared secret in the future were one of
+%% the secrets to be compromised.
+%% <ul>
+%%  <li>`A_E_E_SK' Alice's Ephemeral Ed25519 Secret Key.</li>
+%%  <li>`B_P_E_PK' Bob's Permanent Ed25519 Public Key.</li>
+%%  <li>`B_E_E_PK' Bob's Ephemeral Ed25519 Public Key.</li>
+%%  <li>`Protocol' is an arbitrary binary string, typically a protocol name in UTF-8.</li>
+%%  <li>`Version' is another arbitrary binary string, typically a protocol version in UTF-8.</li>
+%%  <li>`Salt' is a binary salt, which if empty will be replaced by a binary string of zeroes.</li>
+%%  <li>`SS' is the resulting 32-byte shared secret.</li>
+%% </ul>
 
-lcg(N) ->
-    M = 16#7FFFFFFF,
-    A = 48271,
-    Q = 44488,  %  M div A
-    R = 3399,   %  M rem A
-    Div = N div Q,
-    Rem = N rem Q,
-    S = Rem * A,
-    T = Div * R,
-    Result = S - T,
-    case Result < 0 of
-        false -> Result;
-        true  -> Result + M
+shared_secret_a(A_E_E_SK, B_P_E_PK, B_E_E_PK, Protocol, Version, Salt) ->
+    A_E_X_SK = ed25519_sk_to_x25519(A_E_E_SK),
+    B_P_X_PK = ed25519_pk_to_x25519(B_P_E_PK),
+    B_E_X_PK = ed25519_pk_to_x25519(B_E_E_PK),
+    DH_Permanent = crypto:compute_key(ecdh, B_P_X_PK, A_E_X_SK, x25519),
+    DH_Ephemeral = crypto:compute_key(ecdh, B_E_X_PK, A_E_X_SK, x25519),
+    finalize_hkdf(DH_Permanent, DH_Ephemeral, Protocol, Version, Salt).
+
+
+-spec shared_secret_b(B_P_E_SK, B_E_E_SK, A_E_E_PK, Protocol, Version, Salt) -> SS
+    when B_P_E_SK :: binary(),
+         B_E_E_SK :: binary(),
+         A_E_E_PK :: <<_:32*8>>,
+         Protocol :: binary(),
+         Version  :: binary(),
+         Salt     :: binary(),
+         SS       :: <<_:32*8>>.
+%% @doc
+%% Bobs's side of a shared key derivation based on ed25519 keys as generated by this module.
+%%
+%% Typically Alice would be providing an ephemeral key to establish
+%% a shared secret while remaining (at least initially) anonymous from Bob. Bob,
+%% on the other hand, is providing a permanent key and also an ephemeral key,
+%% proving identity without exposing the shared secret in the future were one of
+%% the secrets to be compromised.
+%% <ul>
+%%  <li>`B_P_E_SK' Bob's Permanent Ed25519 Secret Key.</li>
+%%  <li>`B_E_E_SK' Bob's Ephemeral Ed25519 Secret Key.</li>
+%%  <li>`A_E_E_PK' Alice's Ephemeral Ed25519 Public Key.</li>
+%%  <li>`Protocol' is an arbitrary binary string, typically a protocol name in UTF-8.</li>
+%%  <li>`Version' is another arbitrary binary string, typically a protocol version in UTF-8.</li>
+%%  <li>`Salt' is a binary salt, which if empty will be replaced by a binary string of zeroes.</li>
+%%  <li>`SS' is the resulting 32-byte shared secret.</li>
+%% </ul>
+
+shared_secret_b(B_P_E_SK, B_E_E_SK, A_E_E_PK, Protocol, Version, Salt) ->
+    B_P_X_SK = ed25519_sk_to_x25519(B_P_E_SK),
+    B_E_X_SK = ed25519_sk_to_x25519(B_E_E_SK),
+    A_E_X_PK = ed25519_pk_to_x25519(A_E_E_PK),
+    DH_Permanent = crypto:compute_key(ecdh, A_E_X_PK, B_P_X_SK, x25519),
+    DH_Ephemeral = crypto:compute_key(ecdh, A_E_X_PK, B_E_X_SK, x25519),
+    finalize_hkdf(DH_Permanent, DH_Ephemeral, Protocol, Version, Salt).
+
+finalize_hkdf(DH_Permanent, DH_Ephemeral, Protocol, Version, Salt) ->
+    MixedInput = <<DH_Permanent/binary, DH_Ephemeral/binary>>,
+    Info       = <<Protocol/binary, ":", Version/binary, ":">>,
+    hkdf(sha256, MixedInput, Salt, Info).
+
+
+%% Curve25519 Prime Field Constant: 2^255 - 19
+%% Yes, in hex it reads kind of like "lucky fed"
+p() -> 16#7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFED.
+
+
+-spec ed25519_pk_to_x25519(ED25519_PubKey) -> X25519_PubKey
+    when ED25519_PubKey :: <<_:32*8>>,
+         X25519_PubKey  :: <<_:32*8>>.
+%% @doc
+%% Convert a curve 25519 public key from Edwards representation to X-coordinate
+%% representation.
+
+ed25519_pk_to_x25519(<<ED25519_PK:32/binary>>) ->
+    <<CompressedInt:256/little-integer>> = ED25519_PK,
+    % Clear the sign bit (MSB) to get the raw y-coordinate
+    Y = CompressedInt band ((1 bsl 255) - 1),
+
+    % Compute u = (1 + y) / (1 - y) mod P
+    Num = (1 + Y) rem p(),
+    Den = (1 - Y + p()) rem p(),
+    case Den =:= 0 of
+        true -> 
+            % If y == 1, the point maps to the point at infinity.
+            % On X25519, this translates to u = 0.
+            <<0:256/little-integer>>;
+        false ->
+            U = (Num * mod_inv(Den, p())) rem p(),
+            <<U:256/little-integer>>
     end.
+
+
+-spec ed25519_sk_to_x25519(ED25519_SecKey) -> X25519_SecKey
+    when ED25519_SecKey :: binary(),
+         X25519_SecKey  :: <<_:32*8>>.
+%% @doc
+%% Convert a curve 25519 secret key from Edwards representation to X-coordinate
+%% representation.
+
+ed25519_sk_to_x25519(<<ED25519_SK_Secret:32/binary, _/binary>>) ->
+    <<X25519_SK:32/binary, _/binary>> = crypto:hash(sha512, ED25519_SK_Secret),
+    X25519_SK.
+
+mod_inv(A, M) ->
+    {1, X, _} = ext_gcd(A, M),
+    (X + M) rem M.
+
+ext_gcd(A, 0) ->
+    {A, 1, 0};
+ext_gcd(A, B) ->
+    {G, X1, Y1} = ext_gcd(B, A rem B),
+    {G, Y1, X1 - (A div B) * Y1}.
+
+
+-spec hkdf(Hash, IKM, Salt, Info) -> DerivedKey
+    when Hash       :: md5 | sha | sha224 | sha256 | sha384 | sha512,
+         IKM        :: binary(),
+         Salt       :: binary(),
+         Info       :: binary(),
+         DerivedKey :: <<_:32*8>>.
+%% @doc
+%% 32-byte HMAC-Based Extract-and-Expand Key Derivation
+%% @equiv hkdf(Hash, IKM, Salt, Info, 32)
+
+hkdf(Hash, IKM, Salt, Info) ->
+    hkdf(Hash, IKM, Salt, Info, 32).
+
+
+-spec hkdf(Hash, IKM, Salt, Info, Length) -> DerivedKey
+    when Hash       :: md5 | sha | sha224 | sha256 | sha384 | sha512,
+         IKM        :: binary(),
+         Salt       :: binary(),
+         Info       :: binary(),
+         Length     :: 16 | 20 | 28 | 32 | 48 | 64,
+         DerivedKey :: binary().
+%% @doc
+%% RFC-5869 compliant HMAC-Based Extract-and-Expand Key Derivation
+%%
+%% RFC-5869:
+%% <a href="https://datatracker.ietf.org/doc/html/rfc5869">https://datatracker.ietf.org/doc/html/rfc5869</a>
+%%
+%% The purpose of HKDF is to take an initial, raw secret input that might
+%% be mathematically strong but structurally "clumpy" and transform it into one
+%% or more uniform, high-entropy keys suitable for use in cryptography.
+%% 
+%% The problem is that when Alice and Bob compute a Diffie-Hellman shared secret
+%% over X25519, the resulting bytes are mathematically secure, but they are not
+%% evenly distributed as random noise. Cryptographic ciphers expect keys where
+%% every single bit has an exactly 50% chance of being a 0 or a 1. Passing raw
+%% DH outputs straight into a cipher can introduce subtle, exploitable patterns.
+%%
+%% HKDF "smooths out" the entropy.
+%%
+%% HMAC stands for "Keyed-Hash Message Authentication Code", but without the
+%% leading "K" just to keep us on our toes. The problem it solves is that simply
+%% concatenating a secret and some target data and hashing them together to produce
+%% a message authentication hash leaves the resulting hash vulnerable to a "length
+%% extension attack". An attacker can append additional data to the end of the
+%% message and arrive at a valid new hash without ever knowing the secret.
+%%
+%% RFC-2104 provides good background information on the technique:
+%% <a href="https://datatracker.ietf.org/doc/html/rfc2104">https://datatracker.ietf.org/doc/html/rfc2104</a>
+
+hkdf(Hash, IKM, Salt, Info, Length) ->
+    PRK = extract(Hash, Salt, IKM),
+    expand(Hash, PRK, Info, Length).
+
+extract(Hash, <<>>, IKM) ->
+    %% If salt is empty RFC 5869 requires a string of zeros equal to hash size
+    Salt = binary:copy(<<0>>, hash_size(Hash)),
+    extract(Hash, Salt, IKM);
+extract(Hash, Salt, IKM) ->
+    crypto:mac(hmac, Hash, Salt, IKM).
+
+expand(Hash, PRK, Info, OutLen) ->
+    HashLen = hash_size(Hash),
+    BlockCount = (OutLen + HashLen - 1) div HashLen,
+    true = BlockCount =< 255,
+    FullBlocks = expand_loop(Hash, PRK, Info, BlockCount, 1, <<>>, <<>>),
+    <<Output:OutLen/binary, _/binary>> = FullBlocks,
+    Output.
+
+expand_loop(Hash, PRK, Info, N, Counter, PrevT, Acc) when Counter =< N ->
+    Payload = <<PrevT/binary, Info/binary, Counter:8>>,
+    T = crypto:mac(hmac, Hash, PRK, Payload),
+    expand_loop(Hash, PRK, Info, N, Counter + 1, T, <<Acc/binary, T/binary>>);
+expand_loop(_, _, _, _, _, _, Acc) ->
+    Acc.
+
+hash_size(md5)    -> 16;
+hash_size(sha)    -> 20;
+hash_size(sha224) -> 28;
+hash_size(sha256) -> 32;
+hash_size(sha384) -> 48;
+hash_size(sha512) -> 64.