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diff --git a/doc/internal/man3/x509v3_cache_extensions.pod b/doc/internal/man3/x509v3_cache_extensions.pod
new file mode 100644
index 000000000000..cc0aeb6079a8
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+++ b/doc/internal/man3/x509v3_cache_extensions.pod
@@ -0,0 +1,41 @@
+=pod
+
+=head1 NAME
+
+x509v3_cache_extensions
+- cache info on various X.509v3 extensions and further derived certificate data
+
+=head1 SYNOPSIS
+
+ #include <openssl/x509v3.h>
+
+ int x509v3_cache_extensions(X509 *x, OSSL_LIB_CTX *libctx, const char *propq);
+
+=head1 DESCRIPTION
+
+This function processes any X509v3 extensions present in an X509 object I<x>
+and caches the result of that processing as well as further derived info,
+for instance whether the certificate is self-issued or has version X.509v1.
+It computes the SHA1 digest of the certificate using the default library context
+and property query string and stores the result in x->sha1_hash,
+or on failure sets B<EXFLAG_NO_FINGERPRINT> in x->flags.
+It sets B<X509_SIG_INFO_VALID> in x->flags if x->siginf was filled successfully,
+which may not be possible if a referenced algorithm is unknown or not available.
+Many OpenSSL functions that use an X509 object call this function implicitly.
+
+=head1 RETURN VALUES
+
+This function returns 0 if the extensions or other portions of the certificate
+are invalid or an error occurred.
+Otherwise it returns 1.
+
+=head1 COPYRIGHT
+
+Copyright 2020-2021 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the Apache License 2.0 (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut