Tips on Safely Saving Card Details in Phone

you mentioned safely and then mentioned google which will copy everything on cloud

In Samsung phones, use Secure Folder with strong password. Move important files into Secure Folder.

Download the banking app for that respective CC and link the card to it.

Use a password manager. It would help you auto fill details as well. 

No safe method is convenient.

Every single ready made solution you can think of, like notes apps, password managers, etc. either store your data in cloud, or process you data in some form. All these can be hacked/breached and your data might be put in open. All risky.

Possible solutions:

1. Low security, but easy to use:
Use any open source app which is COMPLETELY OFFLINE. There are some apps (I forgot their names) which store encrypted data on device only, i.e. neither store data on cloud not collect any user data. They've only one con - if phone lost or formatted or app uninstalled, data is gone.

2. Good security, medium tough to use:
Samsung Knox based secure folder is one good option, since it makes use of hardware level security. If you use other brand phones, see if they provide similar functionality, though I would never trust Chinese brands here as they fake everything.

3. Top security, difficulty level high:
Create your own algorithm:
Save your card data or bank credentials in a slightly different pattern in a word file on your pc. By 'different pattern', I mean, if your card number is 1223 4556 7889 1234, then you can just add 1 to last digit of each block (underlined numbers), so the new card number will be 1224 4557 7880 1235. Here I added 1, you can choose your number between 1-9. You can do the same for expiry year too. And save that file as pdf and attach a password to it. You can simply do it while saving as pdf from word itself.

Now whenever you're filling your card details on any website, just subtract that number to recreate actual card number on the fly. This way, even if someone unlocks your file, still card numbers are not correct there, and only you know the key to correct them.

You can even choose to put this file in secure folder or something, but that'll add to difficulty to use.

Pro tip: Use quite large font size when you save file as pdf, so that when you open it in mobile, data is visible clearly and you don't have to zoom-in zoom-out too much.

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