diff --git a/nse_openssl.cc b/nse_openssl.cc
index 808c11806..cc94472a4 100644
--- a/nse_openssl.cc
+++ b/nse_openssl.cc
@@ -214,6 +214,18 @@ static int l_bignum_bn2bin( lua_State *L ) /** bignum_bn2bin( BIGNUM bn ) */
   return 1;
 }
 
+static int l_bignum_bn2mpi( lua_State *L ) /** bignum_bn2mpi( BIGNUM bn ) */
+{
+  bignum_data_t * userdata = (bignum_data_t *) luaL_checkudata(L, 1, "BIGNUM");
+  unsigned char * result = (unsigned char *) malloc( BN_bn2mpi( userdata->bn, NULL ) );
+  if (!result) return luaL_error( L, "Couldn't allocate memory.");
+
+  int len = BN_bn2mpi( userdata->bn, result );
+  lua_pushlstring( L, (char *) result, len );
+  free( result );
+  return 1;
+}
+
 static int l_bignum_bn2dec( lua_State *L ) /** bignum_bn2dec( BIGNUM bn ) */
 {
   bignum_data_t * userdata = (bignum_data_t *) luaL_checkudata(L, 1, "BIGNUM");
@@ -515,6 +527,7 @@ static const struct luaL_Reg bignum_methods[] = {
   { "num_bits", l_bignum_num_bits },
   { "num_bytes", l_bignum_num_bytes },
   { "tobin", l_bignum_bn2bin },
+  { "tompi", l_bignum_bn2mpi },
   { "todec", l_bignum_bn2dec },
   { "tohex", l_bignum_bn2hex },
   { "is_bit_set", l_bignum_is_bit_set },
@@ -541,6 +554,7 @@ static const struct luaL_Reg openssllib[] = {
   { "bignum_rand", l_bignum_rand },
   { "bignum_pseudo_rand", l_bignum_rand },
   { "bignum_bn2bin", l_bignum_bn2bin },
+  { "bignum_bn2mpi", l_bignum_bn2mpi },
   { "bignum_bn2dec", l_bignum_bn2dec },
   { "bignum_bn2hex", l_bignum_bn2hex },
   { "bignum_add", l_bignum_add },
diff --git a/nselib/ssh2.lua b/nselib/ssh2.lua
index da2ab3eae..e8707c880 100644
--- a/nselib/ssh2.lua
+++ b/nselib/ssh2.lua
@@ -53,11 +53,11 @@ end
 --- Pack a multiprecision integer for sending.
 -- @param bn <code>openssl</code> bignum.
 -- @return Packed multiprecision integer.
-transport.pack_mpint = function( bn )
+transport.pack_mpint = openssl.bignum_bn2mpi or function( bn )
   local bytes, packed
   bytes = bn:num_bytes()
   packed = bn:tobin()
-  if bn:num_bits() % 8 == 0 then
+  if bytes > 0 and bn:num_bits() % 8 == 0 then
     bytes = bytes + 1
     packed = '\0' .. packed
   end
@@ -378,4 +378,26 @@ SSH2 = {
 }
 
 
+local unittest = require "unittest"
+if not unittest.testing() then
+  return _ENV
+end
+
+test_suite = unittest.TestSuite:new()
+
+local mpints = {
+  {"0",               "\x00\x00\x00\x00"},
+  {"9a378f9b2e332a7", "\x00\x00\x00\x08\x09\xa3\x78\xf9\xb2\xe3\x32\xa7"},
+  {"80",              "\x00\x00\x00\x02\x00\x80"},
+  --[[ RFC 4251 says negative numbers are 2's complement, but OpenSSL doesn't do that
+  {"-1234",           "\x00\x00\x00\x02\xed\xcc"},
+  {"-deadbeef",       "\x00\x00\x00\x05\xff\x21\x52\x41\x11"},
+  ]]--
+}
+
+for _, t in ipairs(mpints) do
+  local bn = openssl.bignum_hex2bn(t[1])
+  test_suite:add_test(unittest.equal(transport.pack_mpint(bn), t[2]), ("pack mpint 0x%s"):format(t[1]))
+end
+
 return _ENV;