package envelope

import (
	"bytes"
	"fmt"
)

// magic prefixes every sealed ciphertext, ahead of the version byte. The
// leading NUL guarantees that printable-text plaintext can never carry it, so a
// text secret accidentally stored in the ciphertext column fails the header
// check instead of passing as sealed data. It is a tripwire for mistakes, not a
// defense against forgery (the GCM auth tag is the integrity guarantee).
var magic = [4]byte{0x00, 'E', 'N', 'V'}

// version is the ciphertext framing version, following the magic.
const version = 1

// headerLen is the magic prefix plus the version byte.
const headerLen = len(magic) + 1

// IsSealed reports whether b carries the sealed-data header (magic + known
// version). Storage code checks this before persisting, so plaintext or a
// mis-sourced byte slice cannot be written to a ciphertext column.
func IsSealed(b []byte) bool {
	return validHeader(b) == nil
}

func validHeader(ct []byte) error {

	if len(ct) < headerLen {
		return fmt.Errorf("envelope: ciphertext too short")
	}
	if !bytes.Equal(ct[:len(magic)], magic[:]) {
		return fmt.Errorf("envelope: bad magic (not sealed data?)")
	}
	if ct[len(magic)] != version {
		return fmt.Errorf("envelope: unknown framing version %d", ct[len(magic)])
	}
	return nil
}