1 files changed, 144 insertions, 0 deletions

diff --git a/crypto.go b/crypto.go
new file mode 100644
index 0000000..402d06b
--- /dev/null
+++ b/crypto.go
@@ -0,0 +1,144 @@
+package axon
+
+import (
+        "crypto/ed25519"
+        "crypto/rand"
+        "crypto/sha512"
+        "errors"
+        "fmt"
+        "math/big"
+
+        "golang.org/x/crypto/chacha20poly1305"
+        "golang.org/x/crypto/curve25519"
+)
+
+// p is the prime for Curve25519: 2^255 - 19.
+var curveP = new(big.Int).Sub(new(big.Int).Lsh(big.NewInt(1), 255), big.NewInt(19))
+
+// Ed25519PrivKeyToX25519 converts an Ed25519 private key to an X25519 scalar
+// by clamping the SHA-512 hash of the 32-byte seed (per RFC 8032 §5.1.5).
+func Ed25519PrivKeyToX25519(privKey ed25519.PrivateKey) [32]byte {
+        seed := privKey.Seed()
+        h := sha512.Sum512(seed)
+        // Clamp per RFC 8032.
+        h[0] &= 248
+        h[31] &= 127
+        h[31] |= 64
+        var scalar [32]byte
+        copy(scalar[:], h[:32])
+        return scalar
+}
+
+// Ed25519PubKeyToX25519 converts an Ed25519 public key (compressed Edwards
+// y-coordinate) to the corresponding Curve25519 Montgomery u-coordinate via
+// the birational equivalence: u = (1 + y) / (1 - y) mod p.
+func Ed25519PubKeyToX25519(pubKey ed25519.PublicKey) ([32]byte, error) {
+        if len(pubKey) != 32 {
+                return [32]byte{}, fmt.Errorf("axon: ed25519 pubkey must be 32 bytes, got %d", len(pubKey))
+        }
+
+        // Extract the y coordinate: clear the sign bit in byte 31.
+        yBytes := make([]byte, 32)
+        copy(yBytes, pubKey)
+        yBytes[31] &= 0x7f
+
+        // big.Int expects big-endian; reverse the little-endian bytes.
+        reversedY := make([]byte, 32)
+        for i, b := range yBytes {
+                reversedY[31-i] = b
+        }
+        y := new(big.Int).SetBytes(reversedY)
+
+        p := curveP
+
+        // u = (1 + y) * modInverse(1 - y, p) mod p
+        one := big.NewInt(1)
+
+        num := new(big.Int).Add(one, y)
+        num.Mod(num, p)
+
+        den := new(big.Int).Sub(one, y)
+        den.Mod(den, p)
+        den.ModInverse(den, p)
+
+        u := new(big.Int).Mul(num, den)
+        u.Mod(u, p)
+
+        // Encode as little-endian 32 bytes.
+        uBE := u.Bytes() // big-endian, variable length
+        var out [32]byte
+        for i, b := range uBE {
+                out[len(uBE)-1-i] = b
+        }
+        return out, nil
+}
+
+// DHSharedSecret computes the X25519 Diffie-Hellman shared secret between a
+// local Ed25519 private key and a remote Ed25519 public key. Both keys are
+// converted to their X25519 equivalents before the scalar multiplication.
+func DHSharedSecret(localPriv ed25519.PrivateKey, remotePub ed25519.PublicKey) ([32]byte, error) {
+        scalar := Ed25519PrivKeyToX25519(localPriv)
+        point, err := Ed25519PubKeyToX25519(remotePub)
+        if err != nil {
+                return [32]byte{}, fmt.Errorf("axon: convert remote pubkey to x25519: %w", err)
+        }
+        shared, err := curve25519.X25519(scalar[:], point[:])
+        if err != nil {
+                return [32]byte{}, fmt.Errorf("axon: x25519: %w", err)
+        }
+        var out [32]byte
+        copy(out[:], shared)
+        return out, nil
+}
+
+// EncryptDM encrypts plaintext for a DM using ChaCha20-Poly1305. The
+// encryption key is the X25519 shared secret derived from senderPriv and
+// recipientPub. The returned blob is: nonce (12 bytes) || ciphertext.
+func EncryptDM(senderPriv ed25519.PrivateKey, recipientPub ed25519.PublicKey, plaintext []byte) ([]byte, error) {
+        shared, err := DHSharedSecret(senderPriv, recipientPub)
+        if err != nil {
+                return nil, fmt.Errorf("axon: dh: %w", err)
+        }
+
+        aead, err := chacha20poly1305.New(shared[:])
+        if err != nil {
+                return nil, fmt.Errorf("axon: create aead: %w", err)
+        }
+
+        nonce := make([]byte, aead.NonceSize()) // 12 bytes
+        if _, err := rand.Read(nonce); err != nil {
+                return nil, fmt.Errorf("axon: generate nonce: %w", err)
+        }
+
+        ct := aead.Seal(nil, nonce, plaintext, nil)
+        return append(nonce, ct...), nil
+}
+
+// DecryptDM decrypts a blob produced by EncryptDM. recipientPriv is the
+// recipient's Ed25519 private key; senderPub is the sender's Ed25519 public
+// key. The blob must be nonce (12 bytes) || ciphertext.
+func DecryptDM(recipientPriv ed25519.PrivateKey, senderPub ed25519.PublicKey, blob []byte) ([]byte, error) {
+        shared, err := DHSharedSecret(recipientPriv, senderPub)
+        if err != nil {
+                return nil, fmt.Errorf("axon: dh: %w", err)
+        }
+
+        aead, err := chacha20poly1305.New(shared[:])
+        if err != nil {
+                return nil, fmt.Errorf("axon: create aead: %w", err)
+        }
+
+        nonceSize := aead.NonceSize()
+        if len(blob) < nonceSize {
+                return nil, errors.New("axon: ciphertext too short to contain nonce")
+        }
+
+        nonce := blob[:nonceSize]
+        ct := blob[nonceSize:]
+
+        pt, err := aead.Open(nil, nonce, ct, nil)
+        if err != nil {
+                return nil, fmt.Errorf("axon: decrypt: authentication failed")
+        }
+        return pt, nil
+}