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diff --git a/doc/lowlevel.txt b/doc/lowlevel.txt index 4f63948da..d6bf071bc 100644 --- a/doc/lowlevel.txt +++ b/doc/lowlevel.txt @@ -216,116 +216,6 @@ away; the key is kept around). A MAC has the ``SymmetricAlgorithm`` interface in addition to the ``BufferedComputation`` interface. -Random Number Generators ---------------------------------- - -The random number generators provided in Botan are meant for creating -keys, IVs, padding, nonces, and anything else that requires 'random' -data. It is important to remember that the output of these classes -will vary, even if they are supplied with ethe same seed (ie, two -``Randpool`` objects with similar initial states will not produce -the same output, because the value of high resolution timers is added -to the state at various points). - -To ensure good quality output, a PRNG needs to be seeded with truly -random data (such as that produced by a hardware RNG). Typically, you -will use an ``EntropySource`` (see below). To add some (typically -application specific) entropy to a PRNG, you can use - -.. cpp:function:: void add_entropy(const byte* data, size_t length) - -Once a PRNG has been initialized, you can get a single byte of random -data by - -.. cpp:function:: byte random() - -or get a large block by calling - -.. cpp:function:: void randomize(byte* data, size_t length) - -Randpool -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ - -``Randpool`` is the primary PRNG within Botan. In recent versions all -uses of it have been wrapped by an implementation of the X9.31 PRNG -(see below). If for some reason you should have cause to create a PRNG -instead of using the "global" one owned by the library, it would be -wise to consider the same on the grounds of general caution; while -``Randpool`` is designed with known attacks and PRNG weaknesses in -mind, it is not an standard/official PRNG. The remainder of this -section is a (fairly technical, though high-level) description of the -algorithms used in this PRNG. Unless you have a specific interest in -this subject, the rest of this section might prove somewhat -uninteresting. - -``Randpool`` has an internal state called pool, which is 512 bytes -long. This is where entropy is mixed into and extracted from. There is also a -small output buffer (called buffer), which holds the data which has already -been generated but has just not been output yet. - -It is based around a MAC and a block cipher (which are currently -HMAC(SHA-256) and AES-256). Where a specific size is mentioned, it -should be taken as a multiple of the cipher's block size. For example, -if a 256-bit block cipher were used instead of AES, all the sizes -internally would double. Every time some new output is needed, we -compute the MAC of a counter and a high resolution timer. The -resulting MAC is XORed into the output buffer (wrapping as needed), -and the output buffer is then encrypted with AES, producing 16 bytes -of output. - -After 8 blocks (or 128 bytes) have been produced, we mix the pool. To -do this, we first rekey both the MAC and the cipher; the new MAC key -is the MAC of the current pool under the old MAC key, while the new -cipher key is the MAC of the current pool under the just-chosen MAC -key. We then encrypt the entire pool in CBC mode, using the current -(unused) output buffer as the IV. We then generate a new output -buffer, using the mechanism described in the previous paragraph. - -To add randomness to the PRNG, we compute the MAC of the input and XOR -the output into the start of the pool. Then we remix the pool and -produce a new output buffer. The initial MAC operation should make it -very hard for chosen inputs to harm the security of ``Randpool``, and -as HMAC should be able to hold roughly 256 bits of state, it is -unlikely that we are wasting much input entropy (or, if we are, it -doesn't matter, because we have a very abundant supply). - -ANSI X9.31 -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ - -``ANSI_X931_PRNG`` is the standard issue X9.31 Appendix A.2.4 PRNG, -though using AES-256 instead of 3DES as the block cipher. This PRNG -implementation has been checked against official X9.31 test vectors. - -Internally, the PRNG holds a pointer to another PRNG (typically -Randpool). This internal PRNG generates the key and seed used by the -X9.31 algorithm, as well as the date/time vectors. Each time an X9.31 -PRNG object receives entropy, it passes it along to the PRNG it is -holding, and then pulls out some random bits to generate a new key and -seed. This PRNG considers itself seeded as soon as the internal PRNG -is seeded. - - -Entropy Sources ---------------------------------- - -An ``EntropySource`` is an abstract representation of some method of -gather "real" entropy. This tends to be very system dependent. The -*only* way you should use an ``EntropySource`` is to pass it to a PRNG -that will extract entropy from it -- never use the output directly for -any kind of key or nonce generation! - -``EntropySource`` has a pair of functions for getting entropy from -some external source, called ``fast_poll`` and ``slow_poll``. These -pass a buffer of bytes to be written; the functions then return how -many bytes of entropy were gathered. - -Note for writers of ``EntropySource`` subclasses: it isn't necessary -to use any kind of cryptographic hash on your output. The data -produced by an EntropySource is only used by an application after it -has been hashed by the ``RandomNumberGenerator`` that asked for the -entropy, thus any hashing you do will be wasteful of both CPU cycles -and entropy. - User Interfaces --------------------------------- @@ -384,154 +274,6 @@ wxWidgets, FOX, etc), and would like to make it available to users in general (ideally under a permissive license such as public domain or MIT/BSD), feel free to send in a copy. -PBKDF Algorithms ---------------------------------- - -There are various procedures (usually ad-hoc) for turning a -passphrase into a (mostly) arbitrary length key for a symmetric -cipher. A general interface for such algorithms is presented in -``pbkdf.h``. The main function is ``derive_key``, which -takes a passphrase, a salt, an iteration count, and the desired length -of the output key, and returns a key of that length, deterministically -produced from the passphrase and salt. If an algorithm can't produce a -key of that size, it will throw an exception (most notably, PKCS #5's -PBKDF1 can only produce strings between 1 and $n$ bytes, where $n$ is -the output size of the underlying hash function). - -The purpose of the iteration count is to make the algorithm take -longer to compute the final key (reducing the speed of brute-force -attacks of various kinds). Most standards recommend an iteration count -of at least 10000. Currently defined PBKDF algorithms are -"PBKDF1(digest)", "PBKDF2(digest)", and "OpenPGP-S2K(digest)"; you can -retrieve any of these using the ``get_pbkdf``, found in -``lookup.h``. As of this writing, "PBKDF2(SHA-256)" with 10000 -iterations and a 16 byte salt is recommend for new applications. - -OpenPGP S2K -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ - -There are some oddities about OpenPGP's S2K algorithms that are -documented here. For one thing, it uses the iteration count in a -strange manner; instead of specifying how many times to iterate the -hash, it tells how many *bytes* should be hashed in total -(including the salt). So the exact iteration count will depend on the -size of the salt (which is fixed at 8 bytes by the OpenPGP standard, -though the implementation will allow any salt size) and the size of -the passphrase. - -To get what OpenPGP calls "Simple S2K", set iterations to 0, and do -not specify a salt. To get "Salted S2K", again leave the iteration -count at 0, but give an 8-byte salt. "Salted and Iterated S2K" -requires an 8-byte salt and some iteration count (this should be -significantly larger than the size of the longest passphrase that -might reasonably be used; somewhere from 1024 to 65536 would probably -be about right). Using both a reasonably sized salt and a large -iteration count is highly recommended to prevent password guessing -attempts. - -Password Hashing ---------------------------------- - -Storing passwords for user authentication purposes in plaintext is the -simplest but least secure method; when an attacker compromises the -database in which the passwords are stored, they immediately gain -access to all of them. Often passwords are reused among multiple -services or machines, meaning once a password to a single service is -known an attacker has a substantial head start on attacking other -machines. - -The general approach is to store, instead of the password, the output -of a one way function of the password. Upon receiving an -authentication request, the authenticator can recompute the one way -function and compare the value just computed with the one that was -stored. If they match, then the authentication request succeeds. But -when an attacker gains access to the database, they only have the -output of the one way function, not the original password. - -Common hash functions such as SHA-256 are one way, but used alone they -have problems for this purpose. What an attacker can do, upon gaining -access to such a stored password database, is hash common dictionary -words and other possible passwords, storing them in a list. Then he -can search through his list; if a stored hash and an entry in his list -match, then he has found the password. Even worse, this can happen -*offline*: an attacker can begin hashing common passwords days, -months, or years before ever gaining access to the database. In -addition, if two users choose the same password, the one way function -output will be the same for both of them, which will be visible upon -inspection of the database. - -There are two solutions to these problems: salting and -iteration. Salting refers to including, along with the password, a -randomly chosen value which perturbs the one way function. Salting can -reduce the effectivness of offline dictionary generation (because for -each potential password, an attacker would have to compute the one way -function output for all possible salts - with a large enough salt, -this can make the problem quite difficult). It also prevents the same -password from producing the same output, as long as the salts do not -collide. With a large salt (say 80 to 128 bits) this will be quite -unlikely. Iteration refers to the general technique of forcing -multiple one way function evaluations when computing the output, to -slow down the operation. For instance if hashing a single password -requires running SHA-256 100,000 times instead of just once, that will -slow down user authentication by a factor of 100,000, but user -authentication happens quite rarely, and usually there are more -expensive operations that need to occur anyway (network and database -I/O, etc). On the other hand, an attacker who is attempting to break a -database full of stolen password hashes will be seriously -inconvenienced by a factor of 100,000 slowdown; they will be able to -only test at a rate of .0001% of what they would without iterations -(or, equivalently, will require 100,000 times as many zombie botnet -hosts). - -There are many different ways of doing this password hashing -operation, with common ones including Unix's crypt (which is based on -DES) and OpenBSD's bcrypt (based on Blowfish). Other variants using -MD5 or SHA-256 are also in use on various systems. - -Botan provides two techniques, passhash9 and bcrypt - -Passhash9 -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ - -Botan provides a password hashing technique called passhash9, in -``passhash9.h``, which is based on PBKDF2. A usage example can be -found in ``doc/examples/passhash.cpp``. Three functions are provided -in this header: - -.. cpp:function:: std::string generate_passhash9(const std::string& password, RandomNumberGenerator& rng, u16bit work_factor = 10) - - Takes the password to hash, a rng, and a work factor, which tells - how many iterations to compute. The default work factor is 10 - (which means 100,000 iterations), but any non-zero value is - accepted. - -.. cpp:function:: std::string generate_passhash9(const std::string& password, byte alg_id, RandomNumberGenerator& rng, u16bit work_factor = 10) - - Like the other ``generate_passhash9``, but taking a parameter that - specifies which PRF to use. Currently defined values are 0 - ("HMAC(SHA-1)"), 1 ("HMAC(SHA-256)"), and 2 ("CMAC(Blowfish)"). - -.. cpp:function:: bool check_passhash9(const std::string& password, const std::string& hash) - - Takes a password and a passhash9 output and checks if the password - is the same as the one that was used to generate the passhash9 - output, returning a boolean true (same) or false (not same). - -Bcrypt -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ - -Bcrypt is a password hashing scheme originally designed for use in -OpenBSD, but numerous other implementations exist. It is made -available by including ``bcrypt.h``, and provides the functions - -.. cpp:function:: std::string generate_bcrypt(const std::string& password, RandomNumberGenerator& rng, u16bit work_factor = 10) - -and - -.. cpp:function:: bool check_bcrypt(const std::string& password, const std::string& hash) - -These work in exactly the same way as the passhash9 password hashing -functions. Checksums --------------------------------- |