Рыба проекта. Минимальная функциональность

pkg/adapters/crypto/x25519/x25519.go

@@ -0,0 +1,196 @@
+package x25519
+
+import (
+	"crypto/ecdh"
+	"crypto/sha256"
+	"errors"
+	"fmt"
+	"io"
+
+	"git.tswf.io/infra/go-synapse-backupper/pkg/domain/crypto"
+)
+
+// suiteID is the scheme identifier for X25519 ECDH KEM.
+const suiteID uint16 = 0x0007
+
+// ErrDecapsulationFailed is returned when ciphertext decapsulation fails,
+// typically because the ciphertext is not a valid X25519 public key.
+var ErrDecapsulationFailed = errors.New("decapsulation failed")
+
+// DefaultRegistry is the package-level registry for X25519.
+var DefaultRegistry = crypto.NewRegistry()
+
+// kemAdapter wraps the Go stdlib crypto/ecdh X25519 implementation to satisfy
+// the pkg/domain/crypto.KEM interface.
+type kemAdapter struct{}
+
+// New creates a new KEM adapter instance.
+func New() crypto.KEM {
+	return &kemAdapter{}
+}
+
+// SchemeID returns the X25519 scheme identifier (0x0007).
+func (k *kemAdapter) SchemeID() uint16 {
+	return suiteID
+}
+
+// GenerateKeyPair generates a new X25519 key pair.
+func (k *kemAdapter) GenerateKeyPair(
+	rand io.Reader,
+) (
+	crypto.RecipientPub,
+	crypto.RecipientPriv,
+	error,
+) {
+	ecdhPriv, err := ecdh.X25519().GenerateKey(rand)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	rawPub := ecdhPriv.PublicKey().Bytes()
+	keyID := computeKeyID(rawPub)
+
+	pub := &pubKey{
+		key:   ecdhPriv.PublicKey(),
+		keyID: keyID,
+	}
+	priv := &privKey{
+		key:   ecdhPriv,
+		keyID: keyID,
+	}
+
+	return pub, priv, nil
+}
+
+// Encapsulate generates a shared secret and ciphertext for the given public key.
+// The ciphertext is the ephemeral public key (32 bytes).
+func (k *kemAdapter) Encapsulate(
+	pub crypto.RecipientPub,
+	rand io.Reader,
+) (
+	ciphertext []byte,
+	sharedSecret []byte,
+	err error,
+) {
+	p, ok := pub.(*pubKey)
+	if !ok {
+		raw := pub.Raw()
+		ek, parseErr := ecdh.X25519().NewPublicKey(raw)
+		if parseErr != nil {
+			return nil, nil, fmt.Errorf("invalid public key for X25519: %w", parseErr)
+		}
+		p = &pubKey{key: ek, keyID: computeKeyID(raw)}
+	}
+
+	ephPriv, err := ecdh.X25519().GenerateKey(rand)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	ct := ephPriv.PublicKey().Bytes()
+	ss, err := ephPriv.ECDH(p.key)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return ct, ss, nil
+}
+
+// LoadPriv loads an X25519 private key from raw bytes.
+func (k *kemAdapter) LoadPriv(
+	raw []byte,
+) (
+	crypto.RecipientPriv,
+	error,
+) {
+	dk, err := ecdh.X25519().NewPrivateKey(raw)
+	if err != nil {
+		return nil, fmt.Errorf("invalid X25519 private key: %w", err)
+	}
+	pubRaw := dk.PublicKey().Bytes()
+	return &privKey{key: dk, keyID: computeKeyID(pubRaw)}, nil
+}
+
+// Decapsulate recovers the shared secret from a ciphertext using the private key.
+// The ciphertext must be a valid 32-byte X25519 public key.
+func (k *kemAdapter) Decapsulate(
+	priv crypto.RecipientPriv,
+	ciphertext []byte,
+) (
+	sharedSecret []byte,
+	err error,
+) {
+	p, ok := priv.(*privKey)
+	if !ok {
+		return nil, fmt.Errorf("invalid private key type for X25519")
+	}
+
+	if len(ciphertext) != 32 {
+		return nil, errors.Join(ErrDecapsulationFailed, errors.New("invalid ciphertext length"))
+	}
+
+	// X25519 public keys are 255-bit Montgomery u-coordinates; bit 255 must be zero.
+	if ciphertext[31]&0x80 != 0 {
+		return nil, ErrDecapsulationFailed
+	}
+
+	// Reject the all-zero public key (identity point), which yields an all-zero shared secret.
+	allZero := true
+	for _, b := range ciphertext {
+		if b != 0 {
+			allZero = false
+			break
+		}
+	}
+	if allZero {
+		return nil, ErrDecapsulationFailed
+	}
+
+	ephPub, err := ecdh.X25519().NewPublicKey(ciphertext)
+	if err != nil {
+		return nil, errors.Join(ErrDecapsulationFailed, err)
+	}
+
+	ss, err := p.key.ECDH(ephPub)
+	if err != nil {
+		return nil, errors.Join(ErrDecapsulationFailed, err)
+	}
+
+	return ss, nil
+}
+
+// pubKey wraps *ecdh.PublicKey to satisfy crypto.RecipientPub.
+type pubKey struct {
+	key   *ecdh.PublicKey
+	keyID []byte
+}
+
+func (p *pubKey) SchemeID() uint16 { return suiteID }
+func (p *pubKey) KeyID() []byte    { return p.keyID }
+func (p *pubKey) Raw() []byte      { return p.key.Bytes() }
+
+// privKey wraps *ecdh.PrivateKey to satisfy crypto.RecipientPriv.
+type privKey struct {
+	key   *ecdh.PrivateKey
+	keyID []byte
+}
+
+func (p *privKey) SchemeID() uint16 { return suiteID }
+func (p *privKey) KeyID() []byte    { return p.keyID }
+func (p *privKey) Raw() []byte      { return p.key.Bytes() }
+
+// computeKeyID derives the first 8 bytes of SHA-256 over the raw public key.
+func computeKeyID(raw []byte) []byte {
+	h := sha256.Sum256(raw)
+	return h[:8]
+}
+
+// init registers the X25519 factory under suiteID 0x0007.
+func init() {
+	_ = DefaultRegistry.Register(
+		suiteID,
+		func() crypto.KEM {
+			return New()
+		},
+	)
+}