# Diffie-Hellman functions

## Functions
const DH_METHOD * | DH_ltm_method (void)
DH * | DH_new (void)
DH * | DH_new_method (ENGINE *engine)
void | DH_free (DH *dh)
int | DH_up_ref (DH *dh)
int | DH_size (const DH *dh)
int | DH_set_ex_data (DH *dh, int idx, void *data)
void * | DH_get_ex_data (DH *dh, int idx)
int | DH_generate_parameters_ex (DH *dh, int prime_len, int generator, BN_GENCB *cb)
int | DH_check_pubkey (const DH *dh, const BIGNUM *pub_key, int *codes)
int | DH_generate_key (DH *dh)
int | DH_compute_key (unsigned char *shared_key, const BIGNUM *peer_pub_key, DH *dh)
int | DH_set_method (DH *dh, const DH_METHOD *method)
const DH_METHOD * | DH_null_method (void)
void | DH_set_default_method (const DH_METHOD *meth)
const DH_METHOD * | DH_get_default_method (void)

## Detailed Description

See the DH - Diffie-Hellman key exchange for description and examples.

## Function Documentation

int DH_check_pubkey | ( | const DH * | dh,
|  | const BIGNUM * | pub_key,
|  | int * | codes |
| ) |  |  |

Check that the public key is sane.

Parameters:

  | dh | the local peer DH parameters.
  | pub_key | the remote peer public key parameters.
  | codes | return that the failures of the pub_key are.

Returns:
  1 on success, 0 on failure and *codes is set the the combined fail check for the public key

Checks that the function performs are:

- pub_key is not negative

- pub_key > 1 and pub_key < p - 1, to avoid small subgroups attack.

- if g == 2, pub_key have more then one bit set, if bits set is 1, log_2(pub_key) is trival

int DH_compute_key | ( | unsigned char * | shared_key,
|  | const BIGNUM * | peer_pub_key,
|  | DH * | dh |
| ) |  |  |

Complute the shared secret key.

Parameters:

  | shared_key | the resulting shared key, need to be at least DH_size() large.
  | peer_pub_key | the peer's public key.
  | dh | the dh key pair.

Returns:
  1 on success.

Checks that the pubkey passed in is valid using DH_check_pubkey().

void DH_free | ( | DH * | dh | ) |

Free a DH object and release related resources, like ENGINE, that the object was using.

Parameters:

  | dh | object to be freed.

int DH_generate_key | ( | DH * | dh | ) |

Generate a new DH private-public key pair. The dh parameter must be allocted first with DH_new(). dh->p and dp->g must be set.

Parameters:

  | dh | dh parameter.

Returns:
  1 on success.

int DH_generate_parameters_ex | ( | DH * | dh,
|  | int | prime_len,
|  | int | generator,
|  | BN_GENCB * | cb |
| ) |  |  |

Generate DH parameters for the DH object give parameters.

Parameters:

  | dh | The DH object to generate parameters for.
  | prime_len | length of the prime
  | generator | generator, g
  | cb | Callback parameters to show progress, can be NULL.

Returns:
  the maximum size in bytes of the out data.

const DH_METHOD* DH_get_default_method | ( | void |  | ) |

Return the default DH implementation.

Returns:
  pointer to a DH_METHOD.

void* DH_get_ex_data | ( | DH * | dh,
|  | int | idx |
| ) |  |  |

Get the data for index idx in the DH object.

Parameters:

  | dh | DH object.
  | idx | index to get the data for.

Returns:
  the object store in index idx

const DH_METHOD* DH_ltm_method | ( | void |  | ) |

DH implementation using libtommath.

Returns:
  the DH_METHOD for the DH implementation using libtommath.

DH* DH_new | ( | void |  | ) |

Create a new DH object using DH_new_method(NULL), see DH_new_method().

Returns:
  a newly allocated DH object.

DH* DH_new_method | ( | ENGINE * | engine | ) |

Create a new DH object from the given engine, if the NULL is used, the default engine is used. Free the DH object with DH_free().

Parameters:

  | engine | The engine to use to allocate the DH object.

Returns:
  a newly allocated DH object.

const DH_METHOD* DH_null_method | ( | void |  | ) |

Return the dummy DH implementation.

Returns:
  pointer to a DH_METHOD.

void DH_set_default_method | ( | const DH_METHOD * | meth | ) |

Set the default DH implementation.

Parameters:

  | meth | pointer to a DH_METHOD.

int DH_set_ex_data | ( | DH * | dh,
|  | int | idx,
|  | void * | data |
| ) |  |  |

Set the data index idx in the DH object to data.

Parameters:

  | dh | DH object.
  | idx | index to set the data for.
  | data | data to store for the index idx.

Returns:
  1 on success.

int DH_set_method | ( | DH * | dh,
|  | const DH_METHOD * | method |
| ) |  |  |

Set a new method for the DH keypair.

Parameters:

  | dh | dh parameter.
  | method | the new method for the DH parameter.

Returns:
  1 on success.

int DH_size | ( | const DH * | dh | ) |

The maximum output size of the DH_compute_key() function.

Parameters:

  | dh | The DH object to get the size from.

Returns:
  the maximum size in bytes of the out data.

int DH_up_ref | ( | DH * | dh | ) |

Add a reference to the DH object. The object should be free with DH_free() to drop the reference.

Parameters:

  | dh | the object to increase the reference count too.

Returns:
  the updated reference count, can't safely be used except for debug printing.