Major Refactoring, Client can now be used as a library

common/crypto.go

@@ -0,0 +1,98 @@
+package common
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+
+	"golang.org/x/crypto/chacha20poly1305"
+)
+
+// GenerateKey returns a base64-encoded 32-byte random key suitable to use as the
+// symmetric passphrase for age scrypt mode. Store this securely (never in Git).
+func GenerateKey() (string, error) {
+	k := make([]byte, 32)
+	if _, err := rand.Read(k); err != nil {
+		return "", err
+	}
+	out := make([]byte, base64.StdEncoding.EncodedLen(len(k)))
+	base64.StdEncoding.Encode(out, k)
+	return string(out), nil
+}
+
+func decodeKey(b64 string) ([]byte, error) {
+	key, err := base64.StdEncoding.DecodeString(b64) // standard padded
+	if err != nil {
+		return nil, err
+	}
+	if len(key) != chacha20poly1305.KeySize {
+		return nil, fmt.Errorf("bad key length: got %d, want %d", len(key), chacha20poly1305.KeySize)
+	}
+	return key, nil
+}
+
+func EncryptFileXChaCha(keyB64, inPath, outPath string, aad []byte) error {
+	key, err := decodeKey(keyB64)
+	if err != nil {
+		return err
+	}
+
+	aead, err := chacha20poly1305.NewX(key)
+	if err != nil {
+		return fmt.Errorf("new aead: %w", err)
+	}
+
+	plaintext, err := os.ReadFile(inPath)
+	if err != nil {
+		return fmt.Errorf("read input: %w", err)
+	}
+
+	nonce := make([]byte, chacha20poly1305.NonceSizeX) // 24 bytes
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return fmt.Errorf("nonce: %w", err)
+	}
+
+	ciphertext := aead.Seal(nil, nonce, plaintext, aad)
+
+	// Write: nonce || ciphertext
+	out := make([]byte, 0, len(nonce)+len(ciphertext))
+	out = append(out, nonce...)
+	out = append(out, ciphertext...)
+
+	if err := os.WriteFile(outPath, out, 0600); err != nil {
+		return fmt.Errorf("write output: %w", err)
+	}
+	return nil
+}
+
+func DecryptFileFromBytes(keyB64 string, inBytes []byte, outPath string, aad []byte) error {
+	key, err := decodeKey(keyB64)
+	if err != nil {
+		return err
+	}
+
+	aead, err := chacha20poly1305.NewX(key)
+	if err != nil {
+		return fmt.Errorf("new aead: %w", err)
+	}
+
+	if len(inBytes) < chacha20poly1305.NonceSizeX {
+		return errors.New("ciphertext too short")
+	}
+
+	nonce := inBytes[:chacha20poly1305.NonceSizeX]
+	ciphertext := inBytes[chacha20poly1305.NonceSizeX:]
+
+	plaintext, err := aead.Open(nil, nonce, ciphertext, aad)
+	if err != nil {
+		return fmt.Errorf("decrypt/auth failed: %w", err)
+	}
+
+	if err := os.WriteFile(outPath, plaintext, 0640); err != nil {
+		return fmt.Errorf("write output: %w", err)
+	}
+	return nil
+}