Warn about short RSA/DSA keys in PuTTYgen.

It's only a warning; Windows PuTTYgen puts it up as a message box, and
will still generate the key if you click yes, and Unix PuTTYgen just
prints the warning and gets on with generation anyway. But it might
help encourage people to move away from 1024-bit keys, if they're
still using them.

cmdgen.c

@@ -223,6 +223,8 @@ static char *readpassphrase(const char *filename)
     return line;
 }
 
+#define DEFAULT_RSADSA_BITS 2048
+
 int main(int argc, char **argv)
 {
     char *infile = NULL;
@@ -500,7 +502,7 @@ int main(int argc, char **argv)
             bits = 256;
             break;
           default:
-            bits = 2048;
+            bits = DEFAULT_RSADSA_BITS;
             break;
         }
     }
@@ -520,6 +522,11 @@ int main(int argc, char **argv)
             fprintf(stderr, "puttygen: cannot generate %s keys shorter than"
                     " 256 bits\n", (keytype == DSA ? "DSA" : "RSA"));
             errs = TRUE;
+        } else if (bits < DEFAULT_RSADSA_BITS) {
+            fprintf(stderr, "puttygen: warning: %s keys shorter than"
+                    " %d bits are probably not secure\n",
+                    (keytype == DSA ? "DSA" : "RSA"), DEFAULT_RSADSA_BITS);
+            /* but this is just a warning, so proceed anyway */
         }
     }
 

windows/winpgen.c

@@ -1115,6 +1115,7 @@ static INT_PTR CALLBACK MainDlgProc(HWND hwnd, UINT msg,
                 } else if (IsDlgButtonChecked(hwnd, IDC_KEYSSH2ED25519)) {
                     state->keytype = ED25519;
                 }
+
 		if ((state->keytype == RSA || state->keytype == DSA) &&
                     state->key_bits < 256) {
                     char *message = dupprintf
@@ -1128,7 +1129,18 @@ static INT_PTR CALLBACK MainDlgProc(HWND hwnd, UINT msg,
 			break;
 		    state->key_bits = DEFAULT_KEY_BITS;
 		    SetDlgItemInt(hwnd, IDC_BITS, DEFAULT_KEY_BITS, FALSE);
+		} else if ((state->keytype == RSA || state->keytype == DSA) &&
+                           state->key_bits < DEFAULT_KEY_BITS) {
+                    char *message = dupprintf
+                        ("Keys shorter than %d bits are not recommended. "
+                         "Really generate this key?", DEFAULT_KEY_BITS);
+		    int ret = MessageBox(hwnd, message, "PuTTYgen Warning",
+					 MB_ICONWARNING | MB_OKCANCEL);
+                    sfree(message);
+		    if (ret != IDOK)
+			break;
                 }
+
 		ui_set_state(hwnd, state, 1);
 		SetDlgItemText(hwnd, IDC_GENERATING, entropy_msg);
 		state->key_exists = FALSE;