[#BN_mod_sqrt]
= BN_mod_sqrt
:mrdocs:

Compute a modular square root: retˆ2 ≡ a (mod n) when a quadratic residue.

== Synopsis

Declared in `<openssl/bn.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
xref:BIGNUM.adoc[BIGNUM]*
BN_mod_sqrt(
    xref:BIGNUM.adoc[BIGNUM]* ret,
    xref:BIGNUM.adoc[BIGNUM] const* a,
    xref:BIGNUM.adoc[BIGNUM] const* n,
    xref:BN_CTX.adoc[BN_CTX]* ctx);
----

== Return Value

Result BIGNUM (possibly `ret),` or NULL if no root exists / on error.

== Parameters

[cols="1,4"]
|===
| Name| Description
| *ret*
| Destination for the root, or NULL to allocate.
| *a*
| Value whose square root modulo `n` is requested.
| *n*
| Odd prime modulus (Tonelli–Shanks style algorithms).
| *ctx*
| BN_CTX scratch space.
|===


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