Add SM2 demos

demos/sm2/sm2_encrypt_demo.c

@@ -0,0 +1,30 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <gmssl/sm2.h>
+#include <gmssl/error.h>
+
+
+int main(void)
+{
+	SM2_KEY sm2_key;
+	SM2_KEY pub_key;
+	unsigned char plaintext[SM2_MAX_PLAINTEXT_SIZE];
+	unsigned char ciphertext[SM2_MAX_CIPHERTEXT_SIZE];
+	size_t len;
+
+	sm2_key_generate(&sm2_key);
+	memcpy(&pub_key, &sm2_key, sizeof(SM2_POINT));
+
+	sm2_encrypt(&pub_key, (uint8_t *)"hello world", strlen("hello world"), ciphertext, &len);
+	format_bytes(stdout, 0, 0, "ciphertext", ciphertext, len);
+
+	if (sm2_decrypt(&sm2_key, ciphertext, len, plaintext, &len) != 1) {
+		fprintf(stderr, "error\n");
+		return 1;
+	}
+	plaintext[len] = 0;
+	printf("plaintext: %s\n", plaintext);
+
+	return 0;
+}