Documentation for internal PUNYCODE-related functions

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from openssl#9654)

doc/internal/man3/ossl_punycode_decode.pod

@@ -0,0 +1,60 @@
+=pod
+
+=head1 NAME
+
+ossl_punycode_decode, ossl_a2ulabel, ossl_a2ucompare
+- internal punycode-related functions
+
+=head1 SYNOPSIS
+
+  #include "crypto/punycode.h"
+
+  int ossl_punycode_decode(const char *pEncoded, const size_t enc_len,
+                           unsigned int *pDecoded, unsigned int *pout_length);
+
+  int ossl_a2ulabel(const char *in, char *out, size_t *outlen);
+
+  int ossl_a2ucompare(const char *a, const char *u);
+
+=head1 DESCRIPTION
+
+PUNYCODE encoding introduced in RFCs 3490-3492 is widely used for
+representation of host names in ASCII-only format. Some specifications,
+such as RFC 8398, require comparison of host names encoded in UTF-8 charset.
+
+ossl_a2ulabel() decodes NULL-terminated hostname from PUNYCODE to UTF-8,
+using a provided buffer for output.
+
+ossl_a2ucompare() accepts two NULL-terminated hostnames, decodes the 1st
+from PUNYCODE to UTF-8 and compares it with the 2nd one as is.
+
+ossl_punycode_decode() decodes one label (one dot-separated part) from
+a hostname, with stripped PUNYCODE marker I<xn-->.
+
+=head1 RETURN VALUES
+
+ossl_a2ulabel() returns 1 on success, 0 on not enough buf passed,
+-1 on invalid PUNYCODE string passed. When valid string is provided, it sets the
+I<*outlen> to the length of required buffer to perform correct decoding.
+
+ossl_a2ucompare() returns 1 on non-equal strings, 0 on equal strings,
+-1 when invalid PUNYCODE string passed.
+
+ossl_punycode_decode() returns 1 on success, 0 on error. On success,
+*pout_length contains the number of codepoints decoded.
+
+=head1 HISTORY
+
+The functions described here were all added in OpenSSL 3.0.
+
+=head1 COPYRIGHT
+
+Copyright 2019 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
+