init

aead.go

@@ -0,0 +1,44 @@
+package fumitok
+
+import (
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/binary"
+	"errors"
+)
+
+var (
+	ErrCipherTextTooShort = errors.New("ciphertext too short")
+)
+
+func encode(aead cipher.AEAD, additional uint16, b []byte) []byte {
+	nsz := aead.NonceSize()
+	// Accocate capacity for all the stuffs.
+	buf := make([]byte, 2+nsz+len(b)+aead.Overhead())
+	binary.LittleEndian.PutUint16(buf[:2], additional)
+	nonce := buf[2 : 2+nsz]
+	// Select a random nonce
+	_, err := rand.Read(nonce)
+	if err != nil {
+		panic(err)
+	}
+	// Encrypt the message and append the ciphertext to the nonce.
+	eb := aead.Seal(nonce[nsz:nsz], nonce, b, buf[:2])
+	return nonce[:nsz+len(eb)]
+}
+
+func decode(aead cipher.AEAD, additional uint16, b []byte) ([]byte, error) {
+	nsz := aead.NonceSize()
+	if len(b) < nsz {
+		return nil, ErrCipherTextTooShort
+	}
+	// Split nonce and ciphertext.
+	nonce, ciphertext := b[:nsz], b[nsz:]
+	if len(ciphertext) == 0 {
+		return nil, nil
+	}
+	// Decrypt the message and check it wasn't tampered with.
+	var buf [2]byte
+	binary.LittleEndian.PutUint16(buf[:], additional)
+	return aead.Open(nil, nonce, ciphertext, buf[:])
+}