BN_SET_FLAGS(3)

okay so bestie, this is BN_SET_FLAGS(3) from the Library Functions Manual, and we’re talking about flags on BIGNUM objects — lowkey important, highkey iconic ✨💅

💖 NAME 💖

BN_set_flags, BN_get_flags — enable and inspect flags on BIGNUM objects 🫶

✨ SYNOPSIS ✨

#include <openssl/bn.h>

void
BN_set_flags(BIGNUM *b, int flags);

int
BN_get_flags(const BIGNUM *b, int flags);

🦋 DESCRIPTION 🦋

BN_set_flags() enables the given flags on b. The flags argument can contain zero or more of the following constants OR’ed together, no cap:

• BN_FLG_CONSTTIME — if this flag is set on the divident a or the divisor d in BN_div(3), on the exponent p in BN_mod_exp(3), or on the divisor a or the modulus n in BN_mod_inverse(3), these functions select algorithms with an execution time independent of the respective numbers, to avoid exposing sensitive information to timing side-channel attacks. This flag is off by default for BIGNUM objects created with BN_new(3) ⏱️🛡️
• BN_FLG_MALLOCED — if this flag is set, BN_free(3) and BN_clear_free(3) will not only clear and free the components of b, but also b itself. This flag is set internally by BN_new(3). Setting it manually on an existing BIGNUM object is usually a bad idea and can cause calls to free(3) with bogus arguments 💀🙈
• BN_FLG_STATIC_DATA — if this flag is set, BN_clear_free(3) will neither clear nor free the memory used for storing the number. Consequently, setting it manually on an existing BIGNUM object is usually a terrible idea that can cause both disclosure of secret data and memory leaks. This flag is automatically set on the constant BIGNUM object returned by BN_value_one(3) 🚨🧨

BN_get_flags() interprets flags as a bitmask and returns those of the given flags that are set in b, OR’ed together, or 0 if none of the given flags is set. The flags argument has the same syntax as for BN_set_flags() 🔍✨

🌈 RETURN VALUES 🌈

BN_get_flags() returns zero or more of the above constants, OR’ed together. That’s the whole vibe check 💖

🎀 SEE ALSO 🎀

• BN_mod_exp(3)
• BN_mod_inverse(3)
• BN_new(3)
• BN_with_flags(3)

⭐ HISTORY ⭐

BN_set_flags() and BN_get_flags() first appeared in SSLeay 0.9.1 and have been available since OpenBSD 2.6. vintage but still serving 👑📜

💅 CAVEATS 💅

No public interface exists to clear a flag once it is set. So think twice before using BN_set_flags(), bestie — fr fr 🫶⚠️

💀 BUGS 💀

• Even if the BN_FLG_CONSTTIME flag is set on a or b, BN_gcd() neither fails nor operates in constant time, potentially allowing timing side-channel attacks 😭⏱️
• Even if the BN_FLG_CONSTTIME flag is set on p, if the modulus m is even, BN_mod_exp(3) does not operate in constant time, potentially allowing timing side-channel attacks 😬🕳️
• If BN_FLG_CONSTTIME is set on p, BN_exp() fails instead of operating in constant time 💥🚫

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okay bye bestie — this one’s from the Debian edition, dated April 27, 2023 💖🎀🫶