The Low-Level Interface¶
Botan has two different interfaces. The one documented in this section is meant more for implementing higher-level types (see the section on filters, earlier in this manual) than for use by applications. Using it safely requires a solid knowledge of encryption techniques and best practices, so unless you know, for example, what CBC mode and nonces are, and why PKCS #1 padding is important, you should avoid this interface in favor of something working at a higher level.
Basic Algorithm Abilities¶
There are a small handful of functions implemented by most of Botan’s algorithm objects. Among these are:
-
std::string Algorithm::name()¶
Returns a human-readable string of the name of this
algorithm. Examples of names returned are “AES-128” and
“HMAC(SHA-512)”. You can turn names back into algorithm objects using
the functions in lookup.h
.
-
void Algorithm::clear()¶
Clear out the algorithm’s internal state. A block cipher object will “forget” its key, a hash function will “forget” any data put into it, etc. The object will look and behave as it did when you initially allocated it.
-
T *Algorithm::clone()¶
This function is central to Botan’s name-based interface. The
clone
has many different return types, such as BlockCipher
*
and HashFunction
*, depending on what kind of object it is called
on. Note that unlike Java’s clone, this returns a new object in a
“pristine” state; that is, operations done on the initial object
before calling clone
do not affect the initial state of the new
clone.
Cloned objects can (and should) be deallocated with the C++ delete
operator.
Keys and IVs¶
Both symmetric keys and initialization values can be considered byte (or octet) strings. These are represented by
-
class OctetString¶
Also known as
SymmetricKey
andInitializationVector
, when you want to express intent.-
OctetString(RandomNumberGenerator &rng, size_t length)¶
This constructor creates a new random key length bytes long using the random number generator.
-
OctetString(std::string str)¶
The argument str is assumed to be a hex string; it is converted to binary and stored. Whitespace is ignored.
-
OctetString(const byte *input, size_t length)¶
This constructor copies its input.
-
as_string() const¶
Returns the hex representation of the key or IV
-
OctetString(RandomNumberGenerator &rng, size_t length)¶
Symmetrically Keyed Algorithms¶
Block ciphers, stream ciphers, and MACs are all keyed operations; to be useful, they have to be set to use a particular key, which is a randomly chosen string of bits of a specified length. The length required by any particular algorithm may vary, depending on both the algorithm specification and the implementation. You can query any botan object to find out what key length(s) it supports.
To make this similarity in terms of keying explicit, all algorithms of those types are derived from the :cpp:class`SymmetricAlgorithm` base. This type provides functions for setting the key, and querying restrictions on the size of the key.
-
class SymmetricAlgorithm¶
-
void set_key(const byte *key, size_t length)¶
-
void set_key(const SymmetricKey &key)¶
This sets the key to the value specified. Most algorithms only accept keys of certain lengths. If you attempt to call
set_key
with a key length that is not supported, the exceptionInvalid_Key_Length
will be thrown.In all cases,
set_key
must be called on an object before any data processing (encryption, decryption, etc) is done by that object. If this is not done, the results are undefined.
-
bool valid_keylength(size_t length) const¶
This function returns true if and only if length is a valid keylength for the algorithm.
-
size_t minimum_keylength() const¶
Return the smallest key length (in bytes) that is acceptible for the algorithm.
-
size_t maximum_keylength() const¶
Return the largest key length (in bytes) that is acceptible for the algorithm
-
void set_key(const byte *key, size_t length)¶
Block Ciphers¶
All block ciphers classes in botan are subclasses of
-
class BlockCipher¶
Which subclasses the
SymmetricAlgorithm
interface.-
size_t block_size() const¶
Returns the block size of the cipher in bytes
-
void encrypt_n(const byte *in, byte *out, size_t n) const¶
Encrypt n blocks of data, taking the input from the array in and placing the ciphertext into out. The two pointers may be identical, but should not overlap ranges.
-
void encrypt(const byte *in, byte *out) const¶
Encrypt a single block, taking the input from in and placing it in out. Acts like
encrypt_n
(in, out, 1).
-
void decrypt_n(const byte *in, byte out, size_t n) const¶
Decrypt n blocks of data, taking the input from in and placing the plaintext in out. The two pointers may be identical, but should not overlap ranges.
-
size_t block_size() const¶
Stream Ciphers¶
Stream ciphers are somewhat different from block ciphers, in that encrypting data results in changing the internal state of the cipher. Also, you may encrypt any length of data in one go (in byte amounts).
-
void StreamCipher::encrypt(const byte *in, byte *out, size_t length)¶
-
void StreamCipher::encrypt(byte *data, size_t length)¶
Stream ciphers implement the SymmetricAlgorithm
interface.
Hash Functions / Message Authentication Codes¶
Hash functions take their input without producing any output, only
producing anything when all input has already taken place. MACs are
very similar, but are additionally keyed. Both of these are derived
from the base class BufferedComputation
, which has the following
functions.
-
size_t BufferedComputation::output_length()¶
Return the size of the output of this function.
-
void BufferedComputation::update(const byte *input, size_t length)¶
-
void BufferedComputation::update(byte input)¶
-
void BufferedComputation::update(const std::string &input)¶
Updates the hash/mac calculation with input.
-
void BufferedComputation::final(byte *out)¶
-
SecureVector<byte> BufferedComputation::final()¶
Complete the hash/MAC calculation and place the result into out
.
For the argument taking an array, exactly output_length
bytes will
be written. After you call final
, the hash function is reset to
its initial state, so it may be reused immediately.
The second method of using final is to call it with no arguments at all, as shown in the second prototype. It will return the hash/mac value in a memory buffer.
There is also a pair of functions called process
. They are a
combination of a single update
, and final
. Both versions
return the final value, rather than placing it an array. Calling
process
with a single byte value isn’t available, mostly because
it would rarely be useful.
A MAC can be viewed (in most cases) as a keyed hash function, so
classes that are derived from MessageAuthenticationCode
have
update
and final
classes just like a HashFunction
(and
like a HashFunction
, after final
is called, it can be used to
make a new MAC right away; the key is kept around).
A MAC has the SymmetricAlgorithm
interface in addition to the
BufferedComputation
interface.
Checksums¶
Checksums are very similar to hash functions, and in fact share the same interface. But there are some significant differences, the major ones being that the output size is very small (usually in the range of 2 to 4 bytes), and is not cryptographically secure. But for their intended purpose (error checking), they perform very well. Some examples of checksums included in Botan are the Adler32 and CRC32 checksums.