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= Usage of blockciphers =
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Author: Daniel Otte
email: daniel.otte@rub.de
0. Foreword
This file will describe how to use the blockcipher implementations provided by
this library. It will not only show how to call the cryptographic functions
but also discuss a little how to build security mechanisms from that.
So you will also be introduced to the basic "modes of operation".
1. What a blockcipher does
A blockcipher is a algorithm which turn an input of fixed length into an
output of the same length (enciphering or encrypting). The transformation is
specified by a key which has to be of a fixed length, or a length of a given
set or range.
Generally there is also an algorithm which turns the output back to the
previous input (deciphering or decrypting) when supplied with te same key.
1.1. high frequent parameters:
block size: 64 bits, 128 bits
key size: 64 bits, 80 bits, 128 bits, 192 bits, 256 bits
(note that some blockciphers use different sizes)
2. Parts of a blockcipher
* encryption algorithm
* decryption algorithm
* mostly a set of subkeys
* mostly a keyschedule which generates the subkeys from the supplied key.
As we can see here a blockcipher normally has an algorithm besides the
encryption and decryption algorithm, which we call keyschedule.
Mostly the encryption and decryption algorithm consist of multiple rounds,
where each round (and sometimes between rounds) subkeys are needed to modify
the data. This subkeys are generated by the keyschedule and stored in a state
or context variable.
Note that not all algorithms need a pregenerated context, sometimes it is easy
to generate the subkeys "on the fly" so there is not always the need of a
context variable.
3. blockcipher API
The API is not always consistent due to the fact that we tried to optimize the
code for size (flash, heap and stack) and speed (runtime of the different
components).
Generally the API of the implemented blockciphers consists of:
*_init function, which implements the keyschedule
*_enc function, which implements the encryption algorithm
*_dec function, which implements the decryption algorithm
*_free function, which frees memory allocated for the keyschedule
*_ctx_t context type, which can contain a keyschedule and other information
3.1 look at the prototypes
Generally the prototypes (defined in the *.h files) will tell you what
parameter means what.
3.1.2 sizes in bits and bytes
Working with cryptographical functions involves working with different
lengths. Some times you want to know it in bits and sometimes in bytes. To
reduce frustration and to avoid bugs we suffix a length parameter with either
_b or _B depending on the meaning. _b means in bits and _B means in bytes
(big b big word).
3.2. *_init function
The *_init function generally takes a pointer to the key as first parameter.
For ciphers where the keysize is not fixed the second parameter gives the
keysize (in bits regularly) and the last parameter points to the context
variable to fill.
For some ciphers there are additional parameters like the number of rounds,
these parameters generally occur before the context pointer.
3.3. *_enc and *_dec functions
The encryption and decryption function of a specific algorithm normally do not
differ in their parameters. Generally these functions take a pointer to the
block to operate on. Some ciphers allow to specify two blocks, where the first
one will be written to and the second will contain the source block. The two
blocks may overlap or be the same. The last parameter specifies either the key
direct (with a pointer to it) or is a pointer to a context created with the
*_init function.
3.4. *_free function
A *_free function is only provided where needed (so most ciphers do not have
it). It is used to free memory dynamically allocated by the *_init function.
4. modes of operation
The usage of cryptographic algorithms is usually motivated by the intend to
fight potential threads. Blockciphers are generally good building blocks.
There are different attacks to the cipher itself, but this is work to be done
by cryptographers, but what stays up to you is using this building blocks in a
secure manner.
You may read http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation to
learn more.
4.1. ECB (electronic codebook mode)
Electronic codebook mode is the simplest mode of operation and its usages is
generally not suggested. In ECB-mode a message which is to encrypt is simply
split up in blocks and each block gets independently encrypted. The problem
with this mode is that, for example same data produces the same ciphertext,
which may also allows an attack to inject selected data.
+----+ +----+ +----+ +----+ +----+ +----+
| P1 | | P2 | | P3 | | C1 | | C2 | | C3 |
+----+ +----+ +----+ +----+ +----+ +----+
| | | | | |
V V V V V V
o---o o---o o---o o---o o---o o---o
| E | | E | | E | | D | | D | | D |
o---o o---o o---o o---o o---o o---o
| | | | | |
V V V V V V
+----+ +----+ +----+ +----+ +----+ +----+
| C1 | | C2 | | C3 | | P1 | | P2 | | P3 |
+----+ +----+ +----+ +----+ +----+ +----+
4.2. CBC (chipher-block-chaining mode)
CBC-mode is a more advanced mode of operation. It solves most problems of
ECB-mode. It again works by split ing up the message into blocks and
introducing a initialization vector (IV) at the beginning. The IV should be
randomly generated and is not required to be kept secret. The plaintext of
each block is XORed with the ciphertext of the previous block (the first block
is XORed with the IV) and then gets encrypted producing the ciphertext block.
For decryption of a block simply decrypt the block an XOR it with the previous
ciphertext block (or the IV in the case of the first block).
CBC-mode has some properties which make it quite useless for some application.
For example if you want to store a large amount of data, and you want to make
a change in one block you would have to decrypt and re-encrypt all following
blocks. If you have such a case read more about block cipher modes.
The wikipedia article http://en.wikipedia.org/wiki/Block_cipher_modes_of_
operation#Other_modes_and_other_cryptographic_primitives would make a good
start.
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
| IV | | P1 | | P2 | | P3 | | IV | | C1 | | C2 | | C3 |
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
| | | | | | | |
+------> X +--> X +--> X | +---+ +---+ |
| | | | | | | | | | | |
| V | V | V | V | V | V
| o---o | o---o | o---o | o---o | o---o | o---o
| | E | | | E | | | E | | | D | | | D | | | D |
| o---o | o---o | o---o | o---o | o---o | o---o
| | | | | | | | | | | |
| +---+ +---+ + +------> X +--> X +--> X
| | | | | | | |
V V V V V V V V
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
| IV | | C1 | | C2 | | C3 | | IV | | P1 | | P2 | | P3 |
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
4.3. stream cipher modes
The following modes of operation turn the blockcipher in something better
described as stream cipher. So you may consider reading USAGE.streamciphers
or anything else about streamcipher if you wish to use this modes.
4.3.1. CTR (counter mode)
This is quite simple. You use a counter which gets encrypted to produce a
key stream. This key stream may be used to encrypt data by XOR-ing the
plaintext with the key stream. Decrypting is exactly the same then encrypting
BE WARNED, an attacker might flip a bit in the ciphertext and the
corresponding bit in the plaintext gets flipped.
+---------+ o--o +---------+ o--o +---------+ o--o +---------+
| counter |-|+1|->| counter |-|+1|->| counter |-|+1|->| counter |
+---------+ o--o +---------+ o--o +---------+ o--o +---------+
| | | |
V V V V
o---o o---o o---o o---o
| E | | E | | E | | E |
o---o o---o o---o o---o
| | | |
V V V V
+--------+ +--------+ +--------+ +--------+
| key | | key | | key | | key |
| stream | | stream | | stream | | stream |
+--------+ +--------+ +--------+ +--------+
4.3.2 OFB (output-feedback mode)
OFB-mode is much like CTR-mode. In fact the only difference is that you do not
increment a counter, but use the output of the encryption operation before as
input.
+-------+ +-------+ +-------+
| IV | +---->| input | +---->| input |
+-------+ | +-------+ | +-------+
| | | | |
V | V | V
o---o | o---o | o---o
| E | | | E | | | E |
o---o | o---o | o---o
| | | | |
V | V | V
+--------+ | +--------+ | +--------+
| output |--+ | output |--+ | output |
+--------+ +--------+ +--------+
| | |
V V V
+--------+ +--------+ +--------+
| key | | key | | key |
| stream | | stream | | stream |
+--------+ +--------+ +--------+
4.3.2 CFB (cipher-feedback mode)
CFB-mode looks much like OFB-mode, but it has a lot of different properties.
Instead of using the previous output block as input the resulting ciphertext
is used as input. Due to the fact that not the entire output-block needs to be
used, the ciphertext does not form the entire input block for the next
operation but it is shifted in the input block.
The resulting cipher is something known as self synchronizing stream cipher.
This means that a manipulation of a single bit in the ciphertext will result
in this bit flipped in the corresponding plaintext but the following blocks
will be "destroyed" until the cipher "heald" itself, meaning the manipulated
ciphertext block gets shift out of the input block.
+-------+ +-------+ +-------+
| IV | +--------->>| input | +--------->>| input |
+-------+ | +-------+ | +-------+
| | | | |
V | V | V
o---o | o---o | o---o
| E | | | E | | | E |
o---o | o---o | o---o
| | | | |
V | V | V
+--------+ | +--------+ | +--------+
| output | | | output | | | output |
+--------+ | +--------+ | +--------+
| | | | |
+----+ V +----+ +----+ V +----+ +----+ V +----+
| P1 |-->X-->| C1 | | P2 |-->X-->| C2 | | P3 |-->X-->| C3 |
+----+ +----+ +----+ +----+ +----+ +----+
+-------------+ +-------------+
| +-------+ | | +-------+ | +-------+
| | IV | +---------|>>| input | +-------->>| input |
| +-------+ | +-------+ +-------+
| | | | |
| V | V V
| o---o | o---o o---o
| | E | | | E | | E |
| o---o | o---o o---o
| | | | |
| V | V V
| +--------+ | +--------+ +--------+
| | output | | | output | | output |
| +--------+ | +--------+ +--------+
| | | | |
+----+ V +----+ +----+ V +----+ +----+ V +----+
| C1 |-->X-->| P1 | | C2 |-->X-->| P2 | | C3 |-->X-->| P3 |
+----+ +----+ +----+ +----+ +----+ +----+
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