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diff --git a/doc/lowlevel.rst b/doc/lowlevel.rst new file mode 100644 index 000000000..cd343063a --- /dev/null +++ b/doc/lowlevel.rst @@ -0,0 +1,236 @@ + +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: + +.. cpp:function:: 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``. + +.. cpp:function:: 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. + +.. cpp:function:: 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 + +.. cpp:class:: OctetString + + Also known as ``SymmetricKey`` and ``InitializationVector``, when + you want to express intent. + + .. cpp:function:: OctetString(RandomNumberGenerator& rng, size_t length) + + This constructor creates a new random key *length* bytes long + using the random number generator. + + .. cpp:function:: OctetString(std::string str) + + The argument *str* is assumed to be a hex string; it is + converted to binary and stored. Whitespace is ignored. + + .. cpp:function:: OctetString(const byte* input, size_t length) + + This constructor copies its input. + + .. cpp:function:: as_string() 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 :cpp:class`SymmetricAlgorithm` base. +This type provides functions for setting the key, and querying +restrictions on the size of the key. + +.. cpp:class:: SymmetricAlgorithm + + .. cpp:function:: void set_key(const byte* key, size_t length) + + .. cpp:function:: 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 + exception ``Invalid_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. + + .. 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 acceptible for the + algorithm. + + .. cpp:function:: size_t maximum_keylength() const + + Return the largest key length (in bytes) that is acceptible for the + algorithm + +Block Ciphers +--------------------------------- + +All block ciphers classes in botan are subclasses of + +.. cpp:class:: BlockCipher + + Which subclasses the :cpp:class:`SymmetricAlgorithm` interface. + + .. cpp:function:: size_t block_size() const + + Returns the block size of the cipher in bytes + + .. cpp:function:: 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. + + .. cpp:function:: void encrypt(const byte* in, byte* out) const + + Encrypt a single block, taking the input from *in* and placing + it in *out*. Acts like :cpp:func:`encrypt_n`\ (in, out, 1). + + .. cpp:function:: void encrypt(byte* block) const + + Identical to :cpp:func:`encrypt`\ (block, block) + + .. cpp:function:: 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. + + .. cpp:function:: void decrypt(const byte* in, byte* out) const + + Decrypt a single block, taking the input from *in* and placing it + in *out*. Acts like :cpp:func:`decrypt_n`\ (in, out, 1). + + .. cpp:function:: void decrypt(byte* block) const + + Identical to :cpp:func:`decrypt`\ (block, block) + +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). + +.. cpp:function:: void StreamCipher::encrypt(const byte* in, byte* out, size_t length) + +.. cpp:function:: 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. + +.. cpp:function:: size_t BufferedComputation::output_length() + +Return the size of the output of this function. + +.. cpp:function:: void BufferedComputation::update(const byte* input, size_t length) + +.. cpp:function:: void BufferedComputation::update(byte input) + +.. cpp:function:: void BufferedComputation::update(const std::string& input) + +Updates the hash/mac calculation with *input*. + +.. cpp:function:: void BufferedComputation::final(byte* out) + +.. cpp:function:: secure_vector<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. |