BN_num_bytes.pod 1.6 KB

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  1. =pod
  2. =head1 NAME
  3. BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size
  4. =head1 SYNOPSIS
  5. #include <openssl/bn.h>
  6. int BN_num_bytes(const BIGNUM *a);
  7. int BN_num_bits(const BIGNUM *a);
  8. int BN_num_bits_word(BN_ULONG w);
  9. =head1 DESCRIPTION
  10. BN_num_bytes() returns the size of a B<BIGNUM> in bytes.
  11. BN_num_bits_word() returns the number of significant bits in a word.
  12. If we take 0x00000432 as an example, it returns 11, not 16, not 32.
  13. Basically, except for a zero, it returns I<floor(log2(w))+1>.
  14. BN_num_bits() returns the number of significant bits in a B<BIGNUM>,
  15. following the same principle as BN_num_bits_word().
  16. BN_num_bytes() is a macro.
  17. =head1 RETURN VALUES
  18. The size.
  19. =head1 NOTES
  20. Some have tried using BN_num_bits() on individual numbers in RSA keys,
  21. DH keys and DSA keys, and found that they don't always come up with
  22. the number of bits they expected (something like 512, 1024, 2048,
  23. ...). This is because generating a number with some specific number
  24. of bits doesn't always set the highest bits, thereby making the number
  25. of I<significant> bits a little lower. If you want to know the "key
  26. size" of such a key, either use functions like RSA_size(), DH_size()
  27. and DSA_size(), or use BN_num_bytes() and multiply with 8 (although
  28. there's no real guarantee that will match the "key size", just a lot
  29. more probability).
  30. =head1 SEE ALSO
  31. L<bn(3)|bn(3)>, L<DH_size(3)|DH_size(3)>, L<DSA_size(3)|DSA_size(3)>,
  32. L<RSA_size(3)|RSA_size(3)>
  33. =head1 HISTORY
  34. BN_num_bytes(), BN_num_bits() and BN_num_bits_word() are available in
  35. all versions of SSLeay and OpenSSL.
  36. =cut