x.crypto: initial addition of curve25519 module (#24748)

Author: blackshirt

vlib/x/crypto/curve25519/README.md

@@ -0,0 +1,53 @@
+# curve25519
+------------
+This module implement Diffie-Hellman key exchange mechanism (ECDHA) based on elliptic curve 
+known as Curve25519 in pure V.
+
+## About curve25519
+
+From wikipedia, Curve25519 is an elliptic curve used in elliptic-curve cryptography (ECC) 
+offering 128 bits of security (256-bit key size) and designed for use with the elliptic 
+curve Diffie–Hellman (ECDH) key agreement scheme. 
+It is one of the fastest curves in ECC, and is not covered by any known patents. 
+The reference implementation is public domain software. The original Curve25519 paper defined it as 
+a Diffie–Hellman (DH) function. 
+Daniel J. Bernstein has since proposed that the name "Curve25519" be used for the underlying curve,
+and the name "X25519" for the DH function Curve25519 is an elliptic curve 
+that offers 128 security bits 
+and is designed for use in the Elliptic Curve Diffie-Hellman (ECDH) key agreement key design scheme.
+
+Examples
+--------
+```v
+import x.crypto.curve25519
+
+fn main() {
+	// For example, two peers, Alice and Bob, want to create a shared secret together
+	//
+	// Alice generates a private key
+	mut alice_pvkey := curve25519.PrivateKey.new()!
+	// Alice's PublicKey to be shared with Bob
+	alice_pbkey := alice_pvkey.public_key()!
+
+	// The other peer, Bob, has a different private key
+	mut bob_pvkey := curve25519.PrivateKey.new()!
+	// Bob's public key to be shared with Alice
+	bob_pbkey := bob_pvkey.public_key()!
+
+	// Let the two peers exchange their respective public keys
+	//
+	// Alice derives the shared secret, using her own private key, and the public key that Bob shared
+	alice_shared_sec := curve25519.derive_shared_secret(mut alice_pvkey, bob_pbkey)!
+
+	// Bob derives the shared secret, using his own private key, and the public key that Alice shared
+	bob_shared_sec := curve25519.derive_shared_secret(mut bob_pvkey, alice_pbkey)!
+
+	// the two shared secrets (derived by Alice, and derived by Bob), should be the same
+	//
+	println(alice_shared_sec.hex()) // 49fd4a4d0637d2413cd501b20111fc50a592dc21460e45f451c03d1fd3cef900
+	println(bob_shared_sec.hex()) // 49fd4a4d0637d2413cd501b20111fc50a592dc21460e45f451c03d1fd3cef900
+	dump(alice_shared_sec == bob_shared_sec) // alice_shared_sec == bob_shared_sec: true
+
+	assert alice_shared_sec == bob_shared_sec
+}
+```