Add a SM2(7) man page

Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7113)

doc/man7/SM2.pod

+=pod
+
+=head1 NAME
+
+SM2 - Chinese SM2 signature and encryption algorithm support
+
+=head1 DESCRIPTION
+
+B<SM2> algorithm is first defined by the Chinese national standard GM/T 0003-2012
+and is standardized by ISO as ISO/IEC 14888. B<SM2> is actually an elliptic curve
+based algorithm. Currnet implementation in OpenSSL supports both signature and
+encryption schemes via EVP interface.
+
+When doing the B<SM2> signature algorithm, it requires a distinguishing identifier
+to form the message prefix which is hashed before the real message is hashed.
+
+=head1 NOTES
+
+B<SM2> signature can be generated by using the 'DigestSign' series APIs, for instance,
+EVP_DigestSignInit(), EVP_DigestSignUpdate() and EVP_DigestSignFinal(). Ditto
+for the verification process by calling the 'DigestVerify' series APIs.
+
+There are several special steps need to be done before computing an B<SM2> signature.
+
+The B<EVP_PKEY> structure should be set to B<EVP_PKEY_SM2> by calling:
+
+ EVP_PKEY_set_alias_type(pkey, EVP_PKEY_SM2);
+
+Then an ID should be set by calling:
+
+ EVP_PKEY_CTX_set1_id(pctx, id, id_len);
+
+When calling the EVP_DeigestSignInit() or EVP_DigestVerifyInit() function, a
+pre-allocated B<EVP_PKEY_CTX> should be assigned to the B<EVP_MD_CTX>. This is
+done by calling:
+
+ EVP_MD_CTX_set_pkey_ctx(mctx, pctx);
+
+And normally there is no need to pass a B<pctx> parameter to EVP_DeigestSignInit()
+or EVP_DigestVerifyInit() in such scenario.
+
+=head1 EXAMPLE
+
+This example demonstrates the calling sequence on how to use an B<EVP_PKEY> to
+sign a message with SM2 signature algorithm and SM3 hash algorithm:
+
+ #include <openssl/evp.h>
+
+ /* obtain an EVP_PKEY from whatever methods... */
+ EVP_PKEY_set_alias_type(pkey, EVP_PKEY_SM2);
+ mctx = EVP_MD_CTX_new();
+ pctx = EVP_PKEY_CTX_new(pkey, NULL);
+ EVP_PKEY_CTX_set1_id(pctx, id, id_len);
+ EVP_MD_CTX_set_pkey_ctx(mctx, pctx);;
+ EVP_DigestVerifyInit(mctx, NULL, EVP_sm3(), NULL, pkey);
+ EVP_DigestVerifyUpdate(mctx, msg, msg_len);
+ EVP_DigestVerifyFinal(mctx, sig, sig_len)
+
+=head1 SEE ALSO
+
+L<EVP_PKEY_CTX_new(3)>,
+L<EVP_PKEY_set_alias_type(3)>,
+L<EVP_DigestSignInit(3)>,
+L<EVP_DigestVerifyInit(3)>,
+L<EVP_PKEY_CTX_set1_id(3)>,
+L<EVP_MD_CTX_set_pkey_ctx(3)>
+
+=head1 COPYRIGHT
+
+Copyright 2018 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (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