# Botan

# Hash Functions and Checksums

Hash functions are one-way 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
  static std::unique_ptr<HashFunction> create(const std::string &name)

    Return a newly allocated hash function object, or nullptr if the name is not recognized.

  static std::unique_ptr<HashFunction> create_or_throw(const std::string &name)

    Like `create` except that it will throw an exception instead of returning nullptr.

  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, exactly `output_length` bytes will be written. After you call `final`, 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 by `final`.

  secure_vector<uint8_t> process(const std::string &in)

    Equivalent to calling `update` followed by `final`.

  std::unique_ptr<HashFunction> new_object()

    Return a newly allocated HashFunction object of the same type as this one.

  std::unique_ptr<HashFunction> copy_state()

    Return a newly allocated HashFunction object of the same type as this one, whose internal state matches the current state of this.

## Code Example

Assume we want to calculate the SHA-256, SHA-384, and SHA-3 hash digests of the STDIN stream using the Botan library.

    #include <botan/hash.h>
    #include <botan/hex.h>

    #include <iostream>

    int main() {
       const auto hash1 = Botan::HashFunction::create_or_throw("SHA-256");
       const auto hash2 = Botan::HashFunction::create_or_throw("SHA-384");
       const auto hash3 = Botan::HashFunction::create_or_throw("SHA-3");
       std::vector<uint8_t> buf(2048);

       while(std::cin.good()) {
          // read STDIN to buffer
          std::cin.read(reinterpret_cast<char*>(buf.data()), static_cast<std::streamsize>(buf.size()));
          const auto readcount = static_cast<size_t>(std::cin.gcount());
          // update hash computations with read data
          hash1->update(std::span{buf}.first(readcount));
          hash2->update(std::span{buf}.first(readcount));
          hash3->update(std::span{buf}.first(readcount));
       }
       std::cout << "SHA-256: " << Botan::hex_encode(hash1->final()) << '\n';
       std::cout << "SHA-384: " << Botan::hex_encode(hash2->final()) << '\n';
       std::cout << "SHA-3: " << Botan::hex_encode(hash3->final()) << '\n';
       return 0;
    }

## Available Hash Functions

The following cryptographic hash functions are implemented. If in doubt, any of SHA-384, SHA-3, or BLAKE2b are fine choices.

### Ascon-Hash256

Available if `BOTAN_HAS_ASCON_HASH256` is defined.

A hash function based on the Ascon permutation, specifically designed to allow small footprint implementations. Its main use case is in constrained environments, such as IoT devices where traditional cryptographic functions may be too resource intensive.

Unless you are interoperating with an existing device which due to resource constraints can only use Ascon, prefer more typical hashes such as SHA-256, SHA-512, or SHA-3.

This hash function is standardized by NIST in SP.800-232. It is not compatible with earlier versions of the Ascon specification.

Algorithm specification name: `Ascon-Hash256`

### Ascon-XOF128

Available if `BOTAN_HAS_ASCON_XOF128` is defined.

An eXtensible Output Functions (XOF) based on the Ascon permutation. Just like the described Ascon-Hash above, its main use case is in constrained environments, such as IoT devices where traditional cryptographic functions may be too resource intensive.

Unless you are interoperating with an existing device which due to resource constraints can only use Ascon, prefer the more typical XOF SHAKE-128, or SHAKE-512.

This XOF is standardized by NIST in SP.800-232. It is not compatible with earlier versions of the Ascon specification.

Algorithm specification name: `Ascon-XOF128`

### BLAKE2b

Available if `BOTAN_HAS_BLAKE2B` is defined.

A recently designed hash function. Very fast on 64-bit 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.

Named like “Blake2b” which selects default 512-bit output, or as “Blake2b(256)” to select 256 bits of output.

Algorithm specification name: `BLAKE2b(<optional output bits>)` (reported name) / `Blake2b(<optional output bits>)`

- Output bits defaults to 512.

- Examples: `BLAKE2b(256)`, `BLAKE2b(512)`, `BLAKE2b`

### BLAKE2s

Available if `BOTAN_HAS_BLAKE2S` is defined.

A recently designed hash function. Very fast on 32-bit processors. Can output a hash of any length between 1 and 32 bytes, this is specified by passing a value to the constructor with the desired length.

Named like “Blake2s” which selects default 256-bit output, or as “Blake2s(128)” to select 128 bits of output.

Algorithm specification name: `BLAKE2s(<optional output bits>)` (reported name) / `Blake2s(<optional output bits>)`

- Output bits defaults to 256.

- Examples: `BLAKE2s(128)`, `BLAKE2s(256)`, `BLAKE2s`

### GOST-34.11

Deprecated since version 2.11.

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.

Warning

As this hash function is no longer approved by the latest Russian standards, support for GOST 34.11 hash is deprecated and will be removed in a future major release.

Algorithm specification name: `GOST-R-34.11-94` (reported name) / `GOST-34.11`

### Keccak-1600

Available if `BOTAN_HAS_KECCAK` is defined.

An older (and incompatible) variant of SHA-3, but sometimes used. Prefer SHA-3 in new code.

Algorithm specification name: `Keccak-1600(<optional output bits>)`

- Output bits defaults to 512.

- Examples: `Keccak-1600(256)`, `Keccak-1600(512)`, `Keccak-1600`

### MD4

An old and now broken hash function. Available if `BOTAN_HAS_MD4` is defined.

Warning

MD4 collisions can be easily created. There is no safe cryptographic use for this function.

Warning

Support for MD4 is deprecated and will be removed in a future major release.

Algorithm specification name: `MD4`

### MD5

An old and now broken hash function. Available if `BOTAN_HAS_MD5` is defined.

Warning

MD5 collisions can be easily created. MD5 should never be used for signatures.

Algorithm specification name: `MD5`

### RIPEMD-160

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. Prefer SHA-2 or SHA-3 in new code.

Algorithm specification name: `RIPEMD-160`

### SHA-1

Available if `BOTAN_HAS_SHA1` is defined.

Widely adopted NSA designed hash function. Use SHA-2 or SHA-3 in new code.

Warning

SHA-1 collisions can now be created by moderately resourced attackers. It must never be used for signatures.

Algorithm specification name: `SHA-1`

### SHA-256

Available if `BOTAN_HAS_SHA2_32` is defined.

Relatively fast 256 bit hash function, thought to be secure.

Also includes the variant SHA-224. There is no real reason to use SHA-224.

Algorithm specification names:

- `SHA-224`

- `SHA-256`

### SHA-512

Available if `BOTAN_HAS_SHA2_64` is defined.

SHA-512 is faster than SHA-256 on 64-bit processors. Also includes the truncated variants SHA-384 and SHA-512/256, which have the advantage of avoiding message extension attacks.

Algorithm specification names:

- `SHA-384`

- `SHA-512`

- `SHA-512-256`

### SHA-3

Available if `BOTAN_HAS_SHA3` is defined.

The new NIST standard hash. Fairly slow.

Supports 224, 256, 384 or 512 bit outputs. SHA-3 is faster with smaller outputs. Use as “SHA-3(256)” or “SHA-3(512)”. Plain “SHA-3” selects default 512 bit output.

Algorithm specification name: `SHA-3(<optional output bits>)`

- Output bits defaults to 512.

- Examples: `SHA-3(256)`, `SHA-3(512)`, `SHA-3`

### SHAKE (SHAKE-128, SHAKE-256)

Available if `BOTAN_HAS_SHAKE` is defined.

These are actually XOFs (extensible output functions) based on SHA-3, which can output a value of any byte length. For example “SHAKE-128(1024)” will produce 1024 bits of output. The specified length must be a multiple of 8.

Algorithm specification names:

- `SHAKE-128(<output bits>)`, e.g. `SHAKE-128(128)`

- `SHAKE-256(<output bits>`, e.g. `SHAKE-256(256)`

### Skein-512

Available if `BOTAN_HAS_SKEIN_512` is defined.

A contender for the NIST SHA-3 competition. Very fast on 64-bit systems. Can output a hash of any length between 1 and 64 bytes. It also accepts an optional “personalization string” which can create variants of the hash. This is useful for domain separation.

To set a personalization string set the second param to any value, typically ASCII strings are used. Examples “Skein-512(256)” or “Skein-512(384,personalization_string)”.

Algorithm specification name:

- `Skein-512(<optional output bits>)`

  - Output bits defaults to 512.

  - Examples: `Skein-512(256)`, `Skein-512(512)`, `Skein-512`

- `Skein-512(<output bits>,<personalization>)`, e.g. `Skein-512(512,Test)`

### SM3

Available if `BOTAN_HAS_SM3` is defined.

Chinese national hash function, 256 bit output. Widely used in industry there. Fast and seemingly secure, but no reason to prefer it over SHA-2 or SHA-3 unless required.

Algorithm specification name: `SM3`

### Streebog (Streebog-256, Streebog-512)

Available if `BOTAN_HAS_STREEBOG` is defined.

Newly designed Russian national hash function. Due to use of input-dependent table lookups, it is vulnerable to side channels. There is no reason to use it unless compatibility is needed.

Warning

The Streebog Sbox has recently been revealed to have a hidden structure which interacts with its linear layer in a way which may provide a backdoor when used in certain ways. Avoid Streebog if at all possible.

Algorithm specification names:

- `Streebog-256`

- `Streebog-512`

### Whirlpool

Available if `BOTAN_HAS_WHIRLPOOL` is defined.

A 512-bit hash function standardized by ISO and NESSIE. Relatively slow, and due to the table based implementation it is potentially vulnerable to cache based side channels.

Algorithm specification name: `Whirlpool`

## Hash Function Combiners and Modifiers

These are functions which combine multiple hash functions, or modify the output of 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(SHA-256,SHA-512)” outputs a 256+512 bit hash created by hashing the input with both SHA-256 and SHA-512 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.

Algorithm specification name: `Parallel(<HashFunction>,<HashFunction>,...)`, e.g. `Parallel(SHA-256,SHA-512)`, `Parallel(MD5,SHA-1,SHA-256,SHA-512)`

### 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.

Algorithm specification name: `Comb4P(<HashFunction>,<HashFunction>)`, e.g. `Comb4P(SHA-1,RIPEMD-160)`

### Truncated

Available if `BOTAN_HAS_TRUNCATED_HASH` is defined.

Wrapper class to truncate underlying hash function output to a given number of bits. The leading bits are retained.

Algorithm specification name: `Truncated(<HashFunction>,<output bits>)`, e.g. `Truncated(SHAKE-128(256),42)`

## 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.

Algorithm specification name: `Adler32`

### CRC24

Available if `BOTAN_HAS_CRC24` is defined.

This is the CRC function used in OpenPGP. 24 bit output.

Algorithm specification name: `CRC32`

### CRC32

Available if `BOTAN_HAS_CRC32` is defined.

This is the 32-bit CRC used in protocols such as Ethernet, gzip, PNG, etc.

Algorithm specification name: `CRC32`