// Seal fulfills the crypto.AEAD interface
func (c AesCbcHmac) Seal(dst, nonce, plaintext, data []byte) []byte {
ctlen := len(plaintext)
ciphertext := make([]byte, ctlen+c.Overhead())[:ctlen]
copy(ciphertext, plaintext)
ciphertext = padbuf.PadBuffer(ciphertext).Pad(c.blockCipher.BlockSize())
cbc := cipher.NewCBCEncrypter(c.blockCipher, nonce)
cbc.CryptBlocks(ciphertext, ciphertext)
authtag := c.ComputeAuthTag(data, nonce, ciphertext)
retlen := len(dst) + len(ciphertext) + len(authtag)
ret := ensureSize(dst, retlen)
out := ret[len(dst):]
n := copy(out, ciphertext)
n += copy(out[n:], authtag)
if debug.Enabled {
debug.Printf("Seal: ciphertext = %x (%d)\n", ciphertext, len(ciphertext))
debug.Printf("Seal: authtag = %x (%d)\n", authtag, len(authtag))
debug.Printf("Seal: ret = %x (%d)\n", ret, len(ret))
}
return ret
}
// Open fulfills the crypto.AEAD interface
func (c AesCbcHmac) Open(dst, nonce, ciphertext, data []byte) ([]byte, error) {
if len(ciphertext) < c.keysize {
return nil, errors.New("invalid ciphertext (too short)")
}
tagOffset := len(ciphertext) - c.tagsize
if tagOffset%c.blockCipher.BlockSize() != 0 {
return nil, fmt.Errorf(
"invalid ciphertext (invalid length: %d %% %d != 0)",
tagOffset,
c.blockCipher.BlockSize(),
)
}
tag := ciphertext[tagOffset:]
ciphertext = ciphertext[:tagOffset]
expectedTag := c.ComputeAuthTag(data, nonce, ciphertext)
if subtle.ConstantTimeCompare(expectedTag, tag) != 1 {
if debug.Enabled {
debug.Printf("provided tag = %x\n", tag)
debug.Printf("expected tag = %x\n", expectedTag)
}
return nil, errors.New("invalid ciphertext (tag mismatch)")
}
cbc := cipher.NewCBCDecrypter(c.blockCipher, nonce)
buf := make([]byte, tagOffset)
cbc.CryptBlocks(buf, ciphertext)
plaintext, err := padbuf.PadBuffer(buf).Unpad(c.blockCipher.BlockSize())
if err != nil {
return nil, err
}
ret := ensureSize(dst, len(plaintext))
out := ret[len(dst):]
copy(out, plaintext)
return ret, nil
}