crypto: Use helpers for size calculations

internal/cache/file.go

@@ -8,7 +8,6 @@ import (
 	"runtime"
 
 	"github.com/pkg/errors"
-	"github.com/restic/restic/internal/crypto"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/fs"
 	"github.com/restic/restic/internal/restic"
@@ -59,7 +58,7 @@ func (c *Cache) load(h restic.Handle, length int, offset int64) (io.ReadCloser,
 		return nil, errors.WithStack(err)
 	}
 
-	if fi.Size() <= crypto.Extension {
+	if fi.Size() <= int64(restic.CiphertextLength(0)) {
 		_ = f.Close()
 		_ = c.remove(h)
 		return nil, errors.Errorf("cached file %v is truncated, removing", h)
@@ -117,7 +116,7 @@ func (c *Cache) Save(h restic.Handle, rd io.Reader) error {
 		return errors.Wrap(err, "Copy")
 	}
 
-	if n <= crypto.Extension {
+	if n <= int64(restic.CiphertextLength(0)) {
 		_ = f.Close()
 		_ = fs.Remove(f.Name())
 		debug.Log("trying to cache truncated file %v, removing", h)

internal/repository/key.go

@@ -262,7 +262,7 @@ func AddKey(ctx context.Context, s *Repository, password, username, hostname str
 	}
 
 	nonce := crypto.NewRandomNonce()
-	ciphertext := make([]byte, 0, len(buf)+newkey.user.Overhead()+newkey.user.NonceSize())
+	ciphertext := make([]byte, 0, restic.CiphertextLength(len(buf)))
 	ciphertext = append(ciphertext, nonce...)
 	ciphertext = newkey.user.Seal(ciphertext, nonce, buf, nil)
 	newkey.Data = ciphertext

internal/restorer/filerestorer.go

@@ -195,7 +195,7 @@ func (r *fileRestorer) downloadPack(ctx context.Context, pack *packInfo) error {
 				if packID.Equal(pack.id) {
 					addBlob(blob, fileOffset)
 				}
-				fileOffset += int64(blob.Length) - crypto.Extension
+				fileOffset += int64(restic.PlaintextLength(int(blob.Length)))
 			})
 			if err != nil {
 				// restoreFiles should have caught this error before