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=head1 NAME

SM2 - Chinese SM2 signature and encryption algorithm support

=head1 DESCRIPTION

The B<SM2> algorithm was first defined by the Chinese national standard GM/T
0003-2012 and was later standardized by ISO as ISO/IEC 14888. B<SM2> is actually
an elliptic curve based algorithm. The current implementation in OpenSSL supports
both signature and encryption schemes via the 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> signatures can be generated by using the 'DigestSign' series of APIs, for
instance, EVP_DigestSignInit(), EVP_DigestSignUpdate() and EVP_DigestSignFinal().
Ditto for the verification process by calling the 'DigestVerify' series of APIs.

There are several special steps that need to be done before computing an B<SM2>
signature.

The B<EVP_PKEY> structure will default to using ECDSA for signatures when it is
created. It 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_DigestSignInit() or EVP_DigestVerifyInit() functions, a
preallocated 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_DigestSignInit()
or EVP_DigestVerifyInit() in such a scenario.

=head1 EXAMPLES

This example demonstrates the calling sequence for using an B<EVP_PKEY> to verify
a message with the SM2 signature algorithm and the SM3 hash algorithm:

 #include <openssl/evp.h>

 /* obtain an EVP_PKEY using 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 2018-2020 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>.

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