flutter / third_party / openssl / 4b1b629725970384d6cf4dafe9e83e54859574cd / . / doc / man3 / BN_num_bytes.pod

=pod | |

=head1 NAME | |

BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size | |

=head1 SYNOPSIS | |

#include <openssl/bn.h> | |

int BN_num_bytes(const BIGNUM *a); | |

int BN_num_bits(const BIGNUM *a); | |

int BN_num_bits_word(BN_ULONG w); | |

=head1 DESCRIPTION | |

BN_num_bytes() returns the size of a B<BIGNUM> in bytes. | |

BN_num_bits_word() returns the number of significant bits in a word. | |

If we take 0x00000432 as an example, it returns 11, not 16, not 32. | |

Basically, except for a zero, it returns I<floor(log2(w))+1>. | |

BN_num_bits() returns the number of significant bits in a B<BIGNUM>, | |

following the same principle as BN_num_bits_word(). | |

BN_num_bytes() is a macro. | |

=head1 RETURN VALUES | |

The size. | |

=head1 NOTES | |

Some have tried using BN_num_bits() on individual numbers in RSA keys, | |

DH keys and DSA keys, and found that they don't always come up with | |

the number of bits they expected (something like 512, 1024, 2048, | |

...). This is because generating a number with some specific number | |

of bits doesn't always set the highest bits, thereby making the number | |

of I<significant> bits a little lower. If you want to know the "key | |

size" of such a key, either use functions like RSA_size(), DH_size() | |

and DSA_size(), or use BN_num_bytes() and multiply with 8 (although | |

there's no real guarantee that will match the "key size", just a lot | |

more probability). | |

=head1 SEE ALSO | |

L<DH_size(3)>, L<DSA_size(3)>, | |

L<RSA_size(3)> | |

=head1 COPYRIGHT | |

Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved. | |

Licensed under the Apache License 2.0 (the "License"). You may not use | |

this file except in compliance with the License. You can obtain a copy | |

in the file LICENSE in the source distribution or at | |

L<https://www.openssl.org/source/license.html>. | |

=cut |