[#CRYPTO_gcm128_finish]
= CRYPTO_gcm128_finish
:mrdocs:

Finalise GCM and optionally verify a received authentication tag.

== Synopsis

Declared in `<openssl/modes.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
int
CRYPTO_gcm128_finish(
    xref:GCM128_CONTEXT-04.adoc[GCM128_CONTEXT]* ctx,
    unsigned char const* tag,
    size_t len);
----

== Return Value

1 on success (and tag match when verifying), or 0 on failure.

== Parameters

[cols="1,4"]
|===
| Name| Description
| *ctx*
| GCM context after encrypt/decrypt (and AAD) processing.
| *tag*
| Expected tag to verify, or NULL to skip verification (encrypt path).
| *len*
| Length of `tag` in bytes when `tag` is non‐NULL.
|===


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