Separate the window sentence from the wrapped error so a bad certificate file is not reported as a key mismatch.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/gcm64/gcm.go

@@ -0,0 +1,63 @@
+// Package gcm64 seals and opens AES-GCM with the 8-byte tag this protocol uses.
+package gcm64
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/subtle"
+	"encoding/binary"
+	"errors"
+)
+
+// IV is the 12-byte nonce: a 6-byte prefix, the client id, and the counter.
+func IV(prefix []byte, clientID uint16, counter uint32) []byte {
+	iv := make([]byte, 12)
+	copy(iv, prefix)
+	binary.BigEndian.PutUint16(iv[6:8], clientID)
+	binary.BigEndian.PutUint32(iv[8:12], counter)
+	return iv
+}
+
+// Seal returns an 8-byte tag and the ciphertext. The tag is the leading 8
+// bytes of the ordinary 16-byte GCM tag, because crypto/cipher only builds
+// tags of 12 to 16 bytes.
+func Seal(key, nonce, plain, aad []byte) (tag, ct []byte, err error) {
+	g, err := full(key)
+	if err != nil {
+		return nil, nil, err
+	}
+	out := g.Seal(nil, nonce, plain, aad)
+	ct = append([]byte(nil), out[:len(plain)]...)
+	tag = append([]byte(nil), out[len(plain):len(plain)+8]...)
+	return tag, ct, nil
+}
+
+// Open checks the 8-byte tag and returns the plaintext.
+func Open(key, nonce, tag, ct, aad []byte) ([]byte, error) {
+	if len(tag) != 8 {
+		return nil, errors.New("tag")
+	}
+	g, err := full(key)
+	if err != nil {
+		return nil, err
+	}
+	stream := g.Seal(nil, nonce, make([]byte, len(ct)), aad)
+	plain := make([]byte, len(ct))
+	subtle.XORBytes(plain, ct, stream[:len(ct)])
+	check := g.Seal(nil, nonce, plain, aad)
+	var got, want [8]byte
+	copy(got[:], tag)
+	copy(want[:], check[len(plain):len(plain)+8])
+	if subtle.ConstantTimeCompare(got[:], want[:]) != 1 {
+		return nil, errors.New("tag")
+	}
+	return plain, nil
+}
+
+func full(key []byte) (cipher.AEAD, error) {
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+	return cipher.NewGCM(block)
+}