Update demos

To build demos, run
`cmake .. -DENABLE_DEMOS=ON`

demos/src/sm4_demo.c

@@ -0,0 +1,66 @@
+/*
+ *  Copyright 2014-2023 The GmSSL Project. All Rights Reserved.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the License); you may
+ *  not use this file except in compliance with the License.
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ */
+
+// sm4 demo1: encrypt and decrypt a block of message (16 bytes)
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <gmssl/sm4.h>
+
+
+int main(void)
+{
+	SM4_KEY sm4_key;
+	unsigned char key[16] = {
+		0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,
+		0x01,0xf2,0x03,0x04,0x05,0x06,0x07,0x08,
+	};
+	unsigned char mbuf[16] = {
+		0x31,0x32,0x33,0x34,0x35,0x36,0x37,0x38,
+		0x31,0x32,0x33,0x34,0x35,0x36,0x37,0x38,
+	};
+	unsigned char cbuf[16];
+	unsigned char pbuf[16];
+	int i;
+
+	printf("key: ");
+	for (i = 0; i < sizeof(key); i++) {
+		printf("%02X", key[i]);
+	}
+	printf("\n");
+
+	printf("plaintext : ");
+	for (i = 0; i < sizeof(mbuf); i++) {
+		printf("%02X", mbuf[i]);
+	}
+	printf("\n");
+
+	// SM4 encrypt a block
+	sm4_set_encrypt_key(&sm4_key, key);
+	sm4_encrypt(&sm4_key, mbuf, cbuf);
+
+	printf("ciphertext: ");
+	for (i = 0; i < sizeof(cbuf); i++) {
+		printf("%02X", cbuf[i]);
+	}
+	printf("\n");
+
+	// SM4 decrypt a block
+	sm4_set_decrypt_key(&sm4_key, key);
+	sm4_decrypt(&sm4_key, cbuf, pbuf);
+
+	printf("decrypted : ");
+	for (i = 0; i < sizeof(pbuf); i++) {
+		printf("%02X", pbuf[i]);
+	}
+	printf("\n");
+
+	return 0;
+}