SKALE-2844 write backup key to file on the disk

SEKManager.cpp

@@ -26,6 +26,7 @@
 #include "BLSCrypto.h"
 #include "LevelDB.h"
 
+#include <fstream>
 #include <iostream>
 #include <algorithm>
 
@@ -130,8 +131,12 @@ void gen_SEK(){
 
   carray2Hex(encr_SEK.data(), enc_len, hexEncrKey.data());
 
-  cout << "ATTENTION! THIS IS YOUR KEY FOR BACK UP. PLEASE COPY IT TO THE SAFE PLACE" << endl;
-  cout << "YOUR KEY IS " << SEK << endl;
+  std::ofstream sek_file("backup_key.txt");
+  sek_file.clear();
+
+  cout << "ATTENTION! YOUR BACKUP KEY WILL BE WROTE INTO backup_key.txt.\n" <<
+          "PLEASE COPY IT TO THE SAFE PLACE AND THEN DELETE THE FILE MANUALLY" << endl;
+  sek_file << SEK;
 
   if (!autoconfirm) {
     std::string confirm_str = "I confirm";

testw.cpp

@@ -580,6 +580,18 @@ TEST_CASE_METHOD(TestFixture, "DKG_BLS test", "[dkg-bls]") {
 
 }
 
+TEST_CASE_METHOD(TestFixture, "Backup Key", "[backup-key]") {
+    HttpClient client(RPC_ENDPOINT);
+    StubClient c(client, JSONRPC_CLIENT_V2);
+    std::ifstream sek_file("backup_key.txt");
+    REQUIRE(sek_file.good());
+
+    std::string sek;
+    sek_file >> sek;
+
+    REQUIRE(sek.size() == 32);
+}
+
 TEST_CASE_METHOD(TestFixture, "Get ServerStatus", "[get-server-status]") {
     HttpClient client(RPC_ENDPOINT);
     StubClient c(client, JSONRPC_CLIENT_V2);

testw.py

@@ -32,6 +32,7 @@ print("Top directory is:" + topDir)
 testList = [ "[cert-sign]",
             "[get-server-status]",
             "[get-server-version]",
+            "[backup-key]",
             "[ecdsa-key-gen]",
             "[ecdsa-aes-key-gen]",
             "[ecdsa-key-sig-gen]",