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:

@property string 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 botan.libstate.lookup.

void 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 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 should be stored in a Unique! or a scoped! object to minimize GC collection cycles.

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 and InitializationVector, when you want to express intent.

this(RandomNumberGenerator rng, size_t length);

This constructor creates a new random key length bytes long using the random number generator.

this(string str);

The argument str is assumed to be a hex string; it is converted to binary and stored. Whitespace is ignored.

this(in byte* input, size_t length);

This constructor copies its input.

string toString() const

Returns the hex representation of the key or IV

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 SymmetricAlgorithm base class. This type provides functions for setting the key, and querying restrictions on the size of the key.

void setKey(in ubyte* key, size_t length);
void setKey(in SymmetricKey key);

This sets the key to the value specified. Most algorithms only accept keys of certain lengths. If you attempt to call setKey with a key length that is not supported, the exception InvalidKeyLength will be thrown.

In all cases, setKey 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 validKeylength(size_t length) const

This function returns true if and only if length is a valid keylength for the algorithm.

size_t minimumKeylength() const

Return the smallest key length (in bytes) that is acceptable for the algorithm.

size_t maximumKeylength() const

Return the largest key length (in bytes) that is acceptable for the algorithm

Block Ciphers

All block ciphers classes in botan are subclasses of BlockCipher, which subclasses the SymmetricAlgorithm interface.

@property size_t blockSize() const

Returns the block size of the cipher in bytes

void encryptN(in ubyte* input, ubyte* output, size_t n) const

Encrypt n blocks of data, taking the input from the array input and placing the ciphertext into output. The two pointers may be identical, but should not overlap ranges.

void encrypt(in ubyte* input, ubyte* output) const

Encrypt a single block, taking the input from input and placing it in output. Acts like a call to encryptN(input, output, 1).

void encrypt(ubyte* block) const

Identical to encrypt(block, block)

void decryptN(in ubyte* input, ubyte output, 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.

void decrypt(in ubyte* input, ubyte* output) const

Decrypt a single block, taking the input from input and placing it in output. Acts like a call to decryptN(input, output, 1).

void decrypt(ubyte* block) const

Identical to decrypt(block, block)

@property size_t parallelism() const

Returns the native parallelism of this implementation, ie how many blocks can be processed in parallel if sufficient data is passed to encryptN or decryptN.

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 encrypt(const ubyte* input, ubyte* output, size_t length);
void encrypt(ubyte* 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 outputLength()

Return the size of the output of this function.

void update(in ubyte* input, size_t length);
void update(ubyte input);
void update(in string input);

Updates the hash/mac calculation with input.

void flush(ubyte* output);
SecureVector!ubyte flush();

Complete the hash/MAC calculation and place the result into output. For the argument taking an array, exactly outputLength bytes will be written. After you call flush, 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 flush. 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 flush classes just like a HashFunction (and like a HashFunction, after flush 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.