[#BN_nist_mod_521]
= BN_nist_mod_521
:mrdocs:

Fast reduction of `a` modulo the NIST P‐521 prime.

== Synopsis

Declared in `<openssl/bn.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
int
BN_nist_mod_521(
    xref:BIGNUM.adoc[BIGNUM]* r,
    xref:BIGNUM.adoc[BIGNUM] const* a,
    xref:BIGNUM.adoc[BIGNUM] const* p,
    xref:BN_CTX.adoc[BN_CTX]* ctx);
----

== Return Value

1 on success, or 0 on error.

== Parameters

[cols="1,4"]
|===
| Name| Description
| *r*
| Destination (may equal `a).`
| *a*
| Value to reduce (non‐negative).
| *p*
| Must be the NIST P‐521 prime (unused for the fast path but retained for API shape).
| *ctx*
| BN_CTX scratch space (may be unused).
|===


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