Hash Functions and Checksums¶
Hash functions are oneway functions, which map data of arbitrary size to a fixed output length. Most of the hash functions in Botan are designed to be cryptographically secure, which means that it is computationally infeasible to create a collision (finding two inputs with the same hash) or preimages (given a hash output, generating an arbitrary input with the same hash). But note that not all such hash functions meet their goals, in particular MD4 and MD5 are trivially broken. However they are still included due to their wide adoption in various protocols.
The class HashFunction
is defined in botan/hash.h.
Using a hash function is typically split into three stages: initialization,
update, and finalization (often referred to as a IUF interface). The
initialization stage is implicit: after creating a hash function object, it is
ready to process data. Then update is called one or more times. Calling update
several times is equivalent to calling it once with all of the arguments
concatenated. After completing a hash computation (eg using final
), the
internal state is reset to begin hashing a new message.

class
HashFunction
¶ 
size_t
output_length
()¶ Return the size (in bytes) of the output of this function.

void
update
(const uint8_t *input, size_t length)¶ Updates the computation with input.

void
update
(uint8_t input)¶ Updates the computation with input.

void
update
(const std::vector<uint8_t> &input)¶ Updates the computation with input.

void
update
(const std::string &input)¶ Updates the computation with input.

void
final
(uint8_t *out)¶ Finalize the calculation and place the result into
out
. For the argument taking an array, exactlyoutput_length
bytes will be written. After you callfinal
, the algorithm is reset to its initial state, so it may be reused immediately.

secure_vector<uint8_t>
final
()¶ Similar to the other function of the same name, except it returns the result in a newly allocated vector.

secure_vector<uint8_t>
process
(const uint8_t in[], size_t length)¶ Equivalent to calling
update
followed byfinal
.

secure_vector<uint8_t>
process
(const std::string &in)¶ Equivalent to calling
update
followed byfinal
.

size_t
Code Example¶
Assume we want to calculate the SHA256, SHA384, and SHA3 hash digests of the STDIN stream using the Botan library.
#include <botan/hash.h>
#include <botan/hex.h>
#include <iostream>
int main ()
{
std::unique_ptr<Botan::HashFunction> hash1(Botan::HashFunction::create("SHA256"));
std::unique_ptr<Botan::HashFunction> hash2(Botan::HashFunction::create("SHA384"));
std::unique_ptr<Botan::HashFunction> hash3(Botan::HashFunction::create("SHA3"));
std::vector<uint8_t> buf(2048);
while(std::cin.good())
{
//read STDIN to buffer
std::cin.read(reinterpret_cast<char*>(buf.data()), buf.size());
size_t readcount = std::cin.gcount();
//update hash computations with read data
hash1>update(buf.data(),readcount);
hash2>update(buf.data(),readcount);
hash3>update(buf.data(),readcount);
}
std::cout << "SHA256: " << Botan::hex_encode(hash1>final()) << std::endl;
std::cout << "SHA384: " << Botan::hex_encode(hash2>final()) << std::endl;
std::cout << "SHA3: " << Botan::hex_encode(hash3>final()) << std::endl;
return 0;
}
Available Hash Functions¶
The following cryptographic hash functions are implemented. If in doubt, any of SHA384, SHA3, BLAKE2b, or Skein512 are fine choices.
BLAKE2b¶
Available if BOTAN_HAS_BLAKE2B
is defined.
A recently designed hash function. Very fast on 64bit processors. Can output a hash of any length between 1 and 64 bytes, this is specified by passing a value to the constructor with the desired length.
GOST34.11¶
Available if BOTAN_HAS_GOST_34_11
is defined.
Russian national standard hash. It is old, slow, and has some weaknesses. Avoid it unless you must.
Keccak1600¶
Available if BOTAN_HAS_KECCAK
is defined.
An older (and incompatible) variant of SHA3, but sometimes used. Prefer SHA3 in new code.
MD4¶
Available if BOTAN_HAS_MD4
is defined.
An old hash function that is now known to be trivially breakable. It is very fast, and may still be suitable as a (noncryptographic) checksum.
RIPEMD160¶
Available if BOTAN_HAS_RIPEMD160
is defined.
A 160 bit hash function, quite old but still thought to be secure (up to the limit of 2**80 computation required for a collision which is possible with any 160 bit hash function). Somewhat deprecated these days.
SHA1¶
Available if BOTAN_HAS_SHA1
is defined.
Widely adopted NSA designed hash function. Starting to show significant signs of weakness, and collisions can now be generated. Avoid in new designs.
SHA256¶
Available if BOTAN_HAS_SHA2_32
is defined.
Relatively fast 256 bit hash function, thought to be secure.
Also includes the variant SHA224. There is no real reason to use SHA224.
SHA512¶
Available if BOTAN_HAS_SHA2_64
is defined.
SHA512 is faster than SHA256 on 64bit processors. Also includes the truncated variants SHA384 and SHA512/256.
SHAKE (SHAKE128, SHAKE256)¶
Available if BOTAN_HAS_SHAKE
is defined.
These are actually XOFs (extensible output functions) based on SHA3, which can output a value of any length.
SM3¶
Available if BOTAN_HAS_SM3
is defined.
Chinese national hash function, 256 bit output. Widely used in industry there. Fast and seemingly secure.
Skein512¶
Available if BOTAN_HAS_SKEIN_512
is defined.
A contender for the NIST SHA3 competition. Very fast on 64bit systems. Can output a hash of any length between 1 and 64 bytes. It also accepts a “personalization string” which can create variants of the hash. This is useful for domain separation.
Streebog (Streebog256, Streebog512)¶
Available if BOTAN_HAS_STREEBOG
is defined.
Newly designed Russian national hash function. Due to use of inputdependent table lookups, it is vulnerable to side channels. There is no reason to use it unless compatibility is needed.
Tiger¶
Available if BOTAN_HAS_TIGER
is defined.
An older 192bit hash function, optimized for 64bit systems. Possibly vulnerable to side channels due to its use of table lookups. Prefer Skein512 or BLAKE2b in new code.
Whirlpool¶
Available if BOTAN_HAS_WHIRLPOOL
is defined.
A 512bit hash function standardized by ISO and NESSIE. Relatively slow, and due to the table based implementation it is (unlike almost all other hashes) potentially vulnerable to cache based side channels. Prefer Skein512 or BLAKE2b in new code.
Hash Function Combiners¶
These are functions which combine multiple hash functions to create a new hash function. They are typically only used in specialized applications.
Parallel¶
Available if BOTAN_HAS_PARALLEL_HASH
is defined.
Parallel simply concatenates multiple hash functions. For example “Parallel(SHA256,SHA512)” outputs a 256+512 bit hash created by hashing the input with both SHA256 and SHA512 and concatenating the outputs.
Note that due to the “multicollision attack” it turns out that generating a collision for multiple parallel hash functions is no harder than generating a collision for the strongest hash function.
Comp4P¶
Available if BOTAN_HAS_COMB4P
is defined.
This combines two cryptographic hashes in such a way that preimage and collision attacks are provably at least as hard as a preimage or collision attack on the strongest hash.
Checksums¶
Note
Checksums are not suitable for cryptographic use, but can be used for error checking purposes.
Adler32¶
Available if BOTAN_HAS_ADLER32
is defined.
The Adler32 checksum is used in the zlib format. 32 bit output.
CRC24¶
Available if BOTAN_HAS_CRC24
is defined.
This is the CRC function used in OpenPGP. 24 bit output.
CRC32¶
Available if BOTAN_HAS_CRC32
is defined.
This is the 32bit CRC used in protocols such as Ethernet, gzip, PNG, etc.