BN_SWAP(3) | Library Functions Manual | BN_SWAP(3)

# NAME

`BN_swap`, `BN_consttime_swap` — exchange BIGNUMs

# SYNOPSIS

library “libcrypto”
`#include <openssl/bn.h>`

void
`BN_swap`(BIGNUM *a, BIGNUM *b);

void
`BN_consttime_swap`(BN_ULONG condition, BIGNUM *a, BIGNUM *b, int nwords);

# DESCRIPTION

`BN_swap`() and `BN_consttime_swap`() exchange the values of a and b.

`BN_swap`() implements this by exchanging the pointers to the data buffers of a and b and also exchanging the values of the `BN_FLG_STATIC_DATA` bits. Consequently, the operation is fast and execution time does not depend on any properties of the two numbers. However, execution time obviously differs between swapping (by calling this function) and not swapping (by not calling this function).

`BN_consttime_swap`() only performs the exchange if the condition is non-zero; otherwise, it has no effect. It implements the exchange by exchanging the contents of the data buffers rather than the pointers to the data buffers. This is slower, but implemented in such a way that the execution time is not only independent of the properties of the two numbers, but also independent of the condition argument, i.e. the same for swapping or not swapping. Execution time does however grow in an approximately linear manner with the nwords argument.

`BN_consttime_swap`() calls abort(3) if at least one of a or b has fewer than nwords data words allocated or more than nwords data words are currently in use in at least one of them.

# SEE ALSO

BN_new(3), BN_set_flags(3)

# HISTORY

`BN_swap`() first appeared in OpenSSL 0.9.7 and has been available since OpenBSD 3.2.

`BN_consttime_swap`() first appeared in OpenSSL 1.0.1g and has been available since OpenBSD 5.6.

June 8, 2025 | Debian