ENHANCED: Use crrl for Ed25519 signing and signature verification.

The main motivation for this change is the introduction of the newly
available predicate ed25519_seed_keypair/2, allowing to generate a key
pair from a given seed. In this way, a key pair can be dynamically
generated from (for example) a password, using crypto_password_hash/3
in combination with crypto_data_hkdf/4 to generate the seed. The
advantage of this method is that the private key need not be stored at
all anywhere.

It is not possible to add a corresponding feature to ring, since it is
closed as "not planned": https://github.com/briansmith/ring/issues/1003

I also used this opportunity to move more of the logic to Prolog. We
now have total control of the key pair representation, and I also
changed the representation to conform to the PKCS#8 v2 standard,
something that only later ring versions do, while still being
backwards compatible with tools that produce a wrong representation
including earlier ring versions.

Another great advantage we get from this change is that the Ed25519
predicates now also run on the 32-bit and WASM versions of Scryer.

src/lib/crypto.pl

@@ -25,6 +25,7 @@
            crypto_password_hash/3,       % +Password, -Hash, +Options
            crypto_data_encrypt/6,        % +PlainText, +Algorithm, +Key, +IV, -CipherText, +Options
            crypto_data_decrypt/6,        % +CipherText, +Algorithm, +Key, +IV, -PlainText, +Options
+           ed25519_seed_keypair/2,       % +Seed, -KeyPair
            ed25519_new_keypair/1,        % -KeyPair
            ed25519_keypair_public_key/2, % +KeyPair, +PublicKey
            ed25519_sign/4,               % +KeyPair, +Data, -Signature, +Options
@@ -612,6 +613,34 @@ encoding_chars(utf8, Cs, Cs) :-
    ===============================
 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
 
+%% ed25519_seed_keypair(+Seed, -Pair)
+%
+% Use Seed to deterministically generate an Ed25519 key pair Pair, a
+% list of characters. Seed must be a list of 32 bytes.  It can be
+% chosen at random (using for example `crypto_n_random_bytes/2`) or
+% derived from input keying material (IKM) using for example
+% `crypto_data_hkdf/4`.  The pair contains the private key and must be
+% kept absolutely secret.  Pair can be used for signing. Its public
+% key can be obtained with `ed25519_keypair_public_key/2`.
+
+ed25519_seed_keypair(Seed, Pair) :-
+        must_be_bytes(Seed, ed25519_keypair_from_seed/2),
+        length(Seed, 32),
+        '$ed25519_seed_to_public_key'(Seed, Public),
+        maplist(char_code, Public, PublicBytes),
+        phrase(([0x30,81], % a sequence of 81 bytes follows
+                [2,1],     % the integer 1 denoting version 2 (awesome design!)
+                [1],       % the public key is also present
+                [48,5],    % an element of 5 bytes follows
+                [6,3,43,101,112],   % OID of Ed25519
+                [4,34],    % an octet string of 34 bytes follows
+                [4,32],    % an octet string of 32 bytes follows
+                seq(Seed), % the seed is the private key
+                [129,33],
+                [0],       % 32 bytes is divisible by 8
+                seq(PublicBytes)), ASN1),
+        maplist(char_code, Pair, ASN1).
+
 %% ed25519_new_keypair(-Pair)
 %
 %  Yields a new Ed25519 key pair Pair, a list of characters. The
@@ -620,7 +649,8 @@ encoding_chars(utf8, Cs, Cs) :-
 %  with `ed25519_keypair_public_key/2`.
 
 ed25519_new_keypair(Pair) :-
-        '$ed25519_new_keypair'(Pair).
+        crypto_n_random_bytes(32, Bytes),
+        ed25519_seed_keypair(Bytes, Pair).
 
 %% ed25519_keypair_public_key(+Pair, -PublicKey)
 %
@@ -629,8 +659,11 @@ ed25519_new_keypair(Pair) :-
 %  The public key is represented as a list of characters.
 
 ed25519_keypair_public_key(Pair, PublicKey) :-
-        must_be_octet_chars(Pair, ed25519_keypair_public_key),
-        '$ed25519_keypair_public_key'(Pair, PublicKey).
+        must_be_octet_chars(Pair, ed25519_keypair_public_key/2),
+        reverse(Pair, RPs),
+        length(RPublicKey, 32),
+        phrase((seq(RPublicKey),...), RPs),
+        reverse(RPublicKey, PublicKey).
 
 %% ed25519_sign(+Key, +Data, -Signature, +Options)
 %
@@ -638,10 +671,14 @@ ed25519_keypair_public_key(Pair, PublicKey) :-
 %  PKCS#8 v2 format as generated by `ed25519_new_keypair/1`. Sign Data
 %  with Key, yielding Signature as a list of hexadecimal characters.
 
-ed25519_sign(Key, Data0, Signature, Options) :-
-        must_be_octet_chars(Key, ed25519_sign),
+ed25519_sign(KeyPair, Data0, Signature, Options) :-
+        must_be_octet_chars(KeyPair, ed25519_sign/4),
+        length(Prefix, 16),
+        length(PrivateKeyChars, 32),
+        phrase((seq(Prefix),seq(PrivateKeyChars),...), KeyPair),
+        maplist(char_code, PrivateKeyChars, PrivateKey),
         options_data_chars(Options, Data0, Data, Encoding),
-        '$ed25519_sign'(Key, Data, Encoding, Signature0),
+        '$ed25519_sign_raw'(PrivateKey, Data, Encoding, Signature0),
         hex_bytes(Signature, Signature0).
 
 %% ed25519_verify(+Key, +Data, +Signature, +Options)
@@ -658,10 +695,10 @@ ed25519_sign(Key, Data0, Signature, Options) :-
 %    which treats Data as a list of raw bytes.
 
 ed25519_verify(Key, Data0, Signature0, Options) :-
-        must_be_octet_chars(Key, ed25519_verify),
+        must_be_octet_chars(Key, ed25519_verify/4),
         options_data_chars(Options, Data0, Data, Encoding),
         hex_bytes(Signature0, Signature),
-        '$ed25519_verify'(Key, Data, Encoding, Signature).
+        '$ed25519_verify_raw'(Key, Data, Encoding, Signature).
 
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    X25519: ECDH key exchange over Curve25519