.. _cipher_modes:
Cipher Modes
=====================
A block cipher by itself, is only able to securely encrypt a single data block.
To be able to securely encrypt data of arbitrary length, a mode of operation
applies the block cipher's single block operation repeatedly to encrypt
an entire message.
All cipher mode implementations are are derived from the base class
:cpp:class:`Cipher_Mode`, which is declared in ``botan/cipher_mode.h``.
.. warning::
Using an unauthenticted cipher mode without combining it with a
:ref:`mac` is insecure. Prefer using an :ref:`aead`.
.. cpp:class:: Cipher_Mode
.. cpp:function:: void set_key(const uint8_t* key, size_t length)
Set the symmetric key to be used.
.. cpp:function:: bool valid_keylength(size_t length) const
This function returns true if and only if *length* is a valid
keylength for the algorithm.
.. cpp:function:: size_t minimum_keylength() const
Return the smallest key length (in bytes) that is acceptable for the
algorithm.
.. cpp:function:: size_t maximum_keylength() const
Return the largest key length (in bytes) that is acceptable for the
algorithm.
.. cpp:function:: size_t default_nonce_length() const
Return the default (preferable) nonce size for this cipher mode.
.. cpp:function:: bool valid_nonce_length(size_t nonce_len) const
Return true if *nonce_len* is a valid length for a nonce with this
algorithm.
.. cpp:function:: bool authenticated() const
Return true if this cipher mode is authenticated
.. cpp:function:: size_t tag_size() const
Return the length in bytes of the authentication tag this algorithm
generates. If the mode is not authenticated, this will return 0. If the mode
is authenticated, it will return some positive value (typically somewhere
between 8 and 16).
.. cpp:function:: void clear()
Clear all internal state. The object will act exactly like one which was
just allocated.
.. cpp:function:: void reset()
Reset all message state. For example if you called :cpp:func:`start_msg`,
then :cpp:func:`process` to process some ciphertext, but then encounter an
IO error and must abandon the current message, you can call `reset`. The
object will retain the key (unlike calling :cpp:func:`clear` which also
resets the key) but the nonce and current message state will be erased.
.. cpp:function:: void start_msg(const uint8_t* nonce, size_t nonce_len)
Set up for processing a new message. This function must be called with a new
random value for each message. For almost all modes (excepting SIV), if the
same nonce is ever used twice with the same key, the encryption scheme loses
its confidentiality and/or authenticity properties.
.. cpp:function:: void start(const std::vector<uint8_t> nonce)
Acts like :cpp:func:`start_msg`\ (nonce.data(), nonce.size()).
.. cpp:function:: void start(const uint8_t* nonce, size_t nonce_len)
Acts like :cpp:func:`start_msg`\ (nonce, nonce_len).
.. cpp:function:: virtual size_t update_granularity() const
The :cpp:class:`Cipher_Mode` interface requires message processing in multiples of the block size.
Returns size of required blocks to update and 1, if the mode can process messages of any length.
.. cpp:function:: virtual size_t process(uint8_t* msg, size_t msg_len)
Process msg in place and returns the number of bytes written. *msg* must
be a multiple of :cpp:func:`update_granularity`.
.. cpp:function:: void update(secure_vector<uint8_t>& buffer, size_t offset = 0)
Continue processing a message in the buffer in place. The passed buffer's
size must be a multiple of :cpp:func:`update_granularity`. The first
*offset* bytes of the buffer will be ignored.
.. cpp:function:: size_t minimum_final_size() const
Returns the minimum size needed for :cpp:func:`finish`.
.. cpp:function:: void finish(secure_vector<uint8_t>& final_block, size_t offset = 0)
Finalize the message processing with a final block of at least :cpp:func:`minimum_final_size` size.
The first *offset* bytes of the passed final block will be ignored.
Code Example
---------------------
The following code encrypts the specified plaintext using AES-128/CBC
with PKCS#7 padding.
.. warning::
This example ignores the requirement to authenticate the ciphertext
.. note::
Simply replacing the string "AES-128/CBC/PKCS7" string in the example below
with "AES-128/GCM" suffices to use authenticated encryption.
.. code-block:: cpp
#include <botan/rng.h>
#include <botan/auto_rng.h>
#include <botan/cipher_mode.h>
#include <botan/hex.h>
#include <iostream>
int main()
{
Botan::AutoSeeded_RNG rng;
const std::string plaintext("Your great-grandfather gave this watch to your granddad for good luck. Unfortunately, Dane's luck wasn't as good as his old man's.");
const std::vector<uint8_t> key = Botan::hex_decode("2B7E151628AED2A6ABF7158809CF4F3C");
std::unique_ptr<Botan::Cipher_Mode> enc = Botan::Cipher_Mode::create("AES-128/CBC/PKCS7", Botan::ENCRYPTION);
enc->set_key(key);
//generate fresh nonce (IV)
Botan::secure_vector<uint8_t> iv = rng.random_vec(enc->default_nonce_length());
// Copy input data to a buffer that will be encrypted
Botan::secure_vector<uint8_t> pt(plaintext.data(), plaintext.data()+plaintext.length());
enc->start(iv);
enc->finish(pt);
std::cout << enc->name() << " with iv " << Botan::hex_encode(iv) << " " << Botan::hex_encode(pt) << "\n";
return 0;
}
Available Unauthenticated Cipher Modes
-----------------------------------------
.. note::
CTR and OFB modes are also implemented, but these are treated as
:cpp:class:`Stream_Cipher`\s instead.
CBC
~~~~~~~~~~~~
Available if ``BOTAN_HAS_MODE_CBC`` is defined.
CBC requires the plaintext be padded using a reversible rule. The following
padding schemes are implemented
PKCS#7 (RFC5652)
The last byte in the padded block defines the padding length p, the remaining padding bytes are set to p as well.
ANSI X9.23
The last byte in the padded block defines the padding length, the remaining padding is filled with 0x00.
OneAndZeros (ISO/IEC 7816-4)
The first padding byte is set to 0x80, the remaining padding bytes are set to 0x00.
Ciphertext stealing (CTS) is also implemented. This scheme allows the
ciphertext to have the same length as the plaintext, however using CTS
requires the input be at least one full block plus one byte. It is
also less commonly implemented.
CFB
~~~~~~~~~~~~
Available if ``BOTAN_HAS_MODE_CFB`` is defined.
CFB uses a block cipher to create a self-synchronizing stream cipher. It is used
for example in the OpenPGP protocol. There is no reason to prefer it, as it has
worse performance characteristics than modes such as CTR or CBC.
XTS
~~~~~~~~~
Available if ``BOTAN_HAS_MODE_XTS`` is defined.
XTS is a mode specialized for encrypting disk or database storage
where ciphertext expansion is not possible. XTS requires all inputs be
at least one full block (16 bytes for AES), however for any acceptable
input length, there is no ciphertext expansion.
.. _aead:
AEAD Mode
---------------------------
AEAD (Authenticated Encryption with Associated Data) modes provide message
encryption, message authentication, and the ability to authenticate additional
data that is not included in the ciphertext (such as a sequence number or
header). It is a subclass of :cpp:class:`Cipher_Mode`.
.. cpp:class:: AEAD_Mode
.. cpp:function:: void set_key(const SymmetricKey& key)
Set the key
.. cpp:function:: Key_Length_Specification key_spec() const
Return the key length specification
.. cpp:function:: void set_associated_data(const uint8_t ad[], size_t ad_len)
Set any associated data for this message. For maximum portability between
different modes, this must be called after :cpp:func:`set_key` and before
:cpp:func:`start`.
If the associated data does not change, it is not necessary to call this
function more than once, even across multiple calls to :cpp:func:`start`
and :cpp:func:`finish`.
.. cpp:function:: void start(const uint8_t nonce[], size_t nonce_len)
Start processing a message, using *nonce* as the unique per-message
value. It does not need to be random, simply unique (per key).
.. warning::
With almost all AEADs, if the same nonce is ever used to encrypt two
different messages under the same key, all security is lost. If
reliably generating unique nonces is difficult in your environment,
use SIV mode which retains security even if nonces are repeated.
.. cpp:function:: void update(secure_vector<uint8_t>& buffer, size_t offset = 0)
Continue processing a message. The *buffer* is an in/out parameter and
may be resized. In particular, some modes require that all input be
consumed before any output is produced; with these modes, *buffer* will
be returned empty.
On input, the buffer must be sized in blocks of size
:cpp:func:`update_granularity`. For instance if the update granularity
was 64, then *buffer* could be 64, 128, 192, ... bytes.
The first *offset* bytes of *buffer* will be ignored (this allows in
place processing of a buffer that contains an initial plaintext header)
.. cpp:function:: void finish(secure_vector<uint8_t>& buffer, size_t offset = 0)
Complete processing a message with a final input of *buffer*, which is
treated the same as with :cpp:func:`update`. It must contain at least
:cpp:func:`final_minimum_size` bytes.
Note that if you have the entire message in hand, calling finish without
ever calling update is both efficient and convenient.
.. note::
During decryption, if the supplied authentication tag does not
validate, finish will throw an instance of Invalid_Authentication_Tag
(aka Integrity_Failure, which was the name for this exception in
versions before 2.10, a typedef is included for compatability).
If this occurs, all plaintext previously output via calls to update
must be destroyed and not used in any way that an attacker could
observe the effects of. This could be anything from echoing the
plaintext back (perhaps in an error message), or by making an external
RPC whose destination or contents depend on the plaintext. The only
thing you can do is buffer it, and in the event of an invalid tag,
erase the previously decrypted content from memory.
One simply way to assure this could never happen is to never
call update, and instead always marshal the entire message
into a single buffer and call finish on it when decrypting.
.. cpp:function:: size_t update_granularity() const
The AEAD interface requires :cpp:func:`update` be called with blocks of
this size. This will be 1, if the mode can process any length inputs.
.. cpp:function:: size_t final_minimum_size() const
The AEAD interface requires :cpp:func:`finish` be called with at least
this many bytes (which may be zero, or greater than
:cpp:func:`update_granularity`)
.. cpp:function:: bool valid_nonce_length(size_t nonce_len) const
Returns true if *nonce_len* is a valid nonce length for this scheme. For
EAX and GCM, any length nonces are allowed. OCB allows any value between
8 and 15 bytes.
.. cpp:function:: size_t default_nonce_length() const
Returns a reasonable length for the nonce, typically either 96
bits, or the only supported length for modes which don't
support 96 bit nonces.
Available AEAD Modes
-------------------------
If in doubt about what to use, pick ChaCha20Poly1305, AES-256/GCM, or AES-256/SIV.
Both ChaCha20Poly1305 and AES with GCM are widely implemented. SIV is somewhat
more obscure (and is slower than either GCM or ChaCha20Poly1305), but has
excellent security properties.
ChaCha20Poly1305
~~~~~~~~~~~~~~~~~~
Available if ``BOTAN_HAS_AEAD_CHACHA20_POLY1305`` is defined.
Unlike the other AEADs which are based on block ciphers, this mode is based on
the ChaCha stream cipher and the Poly1305 authentication code. It is very fast
on all modern platforms.
ChaCha20Poly1305 supports 64-bit, 96-bit, and (since 2.8) 192-bit nonces. 64-bit nonces
are the "classic" ChaCha20Poly1305 design. 96-bit nonces are used by the IETF standard
version of ChaCha20Poly1305. And 192-bit nonces is the XChaCha20Poly1305 construction,
which is somewhat less common.
For best interop use the IETF version with 96-bit nonces. However 96 bits is small enough
that it can be dangerous to generate nonces randomly if more than ~ 2^32 messages are
encrypted under a single key, since if a nonce is ever reused ChaCha20Poly1305 becomes
insecure. It is better to use a counter for the nonce in this case.
If you are encrypting many messages under a single key and cannot maintain a counter for
the nonce, prefer XChaCha20Poly1305 since a 192 bit nonce is large enough that randomly
chosen nonces are extremely unlikely to repeat.
GCM
~~~~~
Available if ``BOTAN_HAS_AEAD_GCM`` is defined.
NIST standard, commonly used. Requires a 128-bit block cipher. Fairly slow,
unless hardware support for carryless multiplies is available.
OCB
~~~~~
Available if ``BOTAN_HAS_AEAD_OCB`` is defined.
A block cipher based AEAD. Supports 128-bit, 256-bit and 512-bit block ciphers.
This mode is very fast and easily secured against side channels. Adoption has
been poor because it is patented in the United States, though a license is
available allowing it to be freely used by open source software.
EAX
~~~~~
Available if ``BOTAN_HAS_AEAD_EAX`` is defined.
A secure composition of CTR mode and CMAC. Supports 128-bit, 256-bit and 512-bit
block ciphers.
SIV
~~~~~~
Available if ``BOTAN_HAS_AEAD_SIV`` is defined.
Requires a 128-bit block cipher. Unlike other AEADs, SIV is "misuse resistant";
if a nonce is repeated, SIV retains security, with the exception that if the
same nonce is used to encrypt the same message multiple times, an attacker can
detect the fact that the message was duplicated (this is simply because if both
the nonce and the message are reused, SIV will output identical ciphertexts).
CCM
~~~~~
Available if ``BOTAN_HAS_AEAD_CCM`` is defined.
A composition of CTR mode and CBC-MAC. Requires a 128-bit block cipher. This is
a NIST standard mode, but that is about all to recommend it. Prefer EAX.