Initial public release of birdwatch-relay

internal/crypto/sealedbox.go

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// Sealed-box encryption: anonymous public-key encryption. Anyone holding the
+// recipient's public key can produce a ciphertext; only the holder of the
+// recipient's private key can decrypt it. Built on X25519 + XSalsa20-Poly1305.
+//
+// Wire format: ephemeral_pubkey (32 bytes) || box_ciphertext.
+// See https://pkg.go.dev/golang.org/x/crypto/nacl/box#SealAnonymous.
+//
+// On the Android side, the matching primitive is libsodium's
+// crypto_box_seal / crypto_box_seal_open. Lazysodium-android is a
+// drop-in dependency.
+
+package crypto
+
+import (
+	"crypto/rand"
+	"errors"
+
+	"golang.org/x/crypto/nacl/box"
+)
+
+// Seal encrypts plaintext to recipientPub. recipientPub must be 32 bytes (X25519).
+func Seal(plaintext []byte, recipientPub []byte) ([]byte, error) {
+	if len(recipientPub) != 32 {
+		return nil, errors.New("recipient public key must be 32 bytes")
+	}
+	var pub [32]byte
+	copy(pub[:], recipientPub)
+	return box.SealAnonymous(nil, plaintext, &pub, rand.Reader)
+}