Document the RSA library.

doc/crypto/RSA_check_key.pod

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+=pod
+
+=head1 NAME
+
+RSA_check_key - Validate RSA keys
+
+=head1 SYNOPSIS
+
+ #include <openssl/rsa.h>
+
+ int RSA_check_key(RSA *rsa);
+
+=head1 DESCRIPTION
+
+This function validates RSA keys. It checks that B<p> and B<q> are
+in fact prime, and that B<n = p*q>.
+
+In the case of private keys, it also checks that B<d*e = 1 mod (p-1*q-1)>,
+and that B<dmp1>, B<dmq1> and B<iqmp> are set correctly or are B<NULL>.
+
+The key's public components may not be B<NULL>.
+
+=head1 RETURN VALUE
+
+RSA_check_key() returns 1 if B<rsa> is a valid RSA key, and 0 otherwise.
+-1 is returned if an error occurs while checking the key.
+
+If the key is invalid or an error occurred, the reason code can be
+obtained using ERR_get_error(3).
+
+=head1 SEE ALSO
+
+rsa(3), err(3)
+
+=head1 HISTORY
+
+RSA_check() appeared in OpenSSL 0.9.4.
+
+=cut