[PATCH] embeddable-dll-service: add ability to derive public key from private key

[dotneutron <dotneutron@protonmail.ch>]

Developers using the embeddable service might find it useful to be able
to derive the public key from a given private key.

There was no apparent explanation for the bit setting/unsetting when
generating a private key. The reading material should help developers
understand the reasoning.

Signed-off-by: Neutron <dotneutron@protonmail.ch>
---
 embeddable-dll-service/main.go | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/embeddable-dll-service/main.go b/embeddable-dll-service/main.go
index b2e02fa4..a9b9dacd 100644
--- a/embeddable-dll-service/main.go
+++ b/embeddable-dll-service/main.go
@@ -53,10 +53,20 @@ func WireGuardGenerateKeypair(publicKey *byte, privateKey *byte) {
 	if err != nil || n != len(privateKeyArray) {
 		panic("Unable to generate random bytes")
 	}
+
+	// See https://www.jcraige.com/an-explainer-on-ed25519-clamping.
 	privateKeyArray[0] &= 248
 	privateKeyArray[31] = (privateKeyArray[31] & 127) | 64

 	curve25519.ScalarBaseMult(publicKeyArray, privateKeyArray)
 }

+//export WireGuardDerivePublicKeyFromPrivateKey
+func WireGuardDerivePublicKeyFromPrivateKey(publicKey *byte, privateKey *byte) {
+	publicKeyArray := (*[32]byte)(unsafe.Pointer(publicKey))
+	privateKeyArray := (*[32]byte)(unsafe.Pointer(privateKey))
+
+	curve25519.ScalarBaseMult(publicKeyArray, privateKeyArray)
+}
+
 func main() {}
--
2.32.0.windows.2