Introduction

Welcome to the user manual for GPG Symmetric Encrypt, a free Android app for password-based (symmetric) PGP encryption and decryption. The app protects your files and text using the well-established OpenPGP standard (RFC 4880) with the AES-256 cipher, powered by the Bouncy Castle crypto library.

1. What the App Does

GPG Symmetric Encrypt lets you protect sensitive information with a single password. Unlike „asymmetric“ PGP that uses a public/private key pair, symmetric encryption uses one password to both lock (encrypt) and unlock (decrypt) your data – just like a lock with one key.

The encrypted output is a standard OpenPGP message. Because it follows the OpenPGP standard, encrypted files can be decrypted by other OpenPGP-compatible software (for example GnuPG, also known as gpg, on desktop systems), and vice versa.

You can encrypt and decrypt:

• Files – one file at a time, or many files in a batch.
• Text – copied from your clipboard.

Results can be written to a file, copied to the clipboard, or shared with another app.

2. Key Features

• Password-based OpenPGP encryption using AES-256 (RFC 4880).
• Encrypt or decrypt files – single or multiple files.
• Encrypt or decrypt text directly from the clipboard.
• Automatic action detection – the app guesses whether to encrypt or decrypt based on what you load.
• Live password strength meter measured in bits of entropy (Weak / Medium / Strong).
• ASCII Armor option to create human-readable, copy-and-paste friendly output (.asc).
• Three output targets: File, Clipboard, or Share.
• Progress indicator while processing large files.
• Deep integration with Android – open .gpg/.pgp/.asc files directly, or share text/files into the app from other apps.
• Screen-capture protection – the app window is flagged so sensitive content is not shown in screenshots or the app switcher.

3. Installation

1. Install the GPG Symmetric Encrypt app from your app store, or sideload the APK.
2. Open the app from your app launcher. You will land on the main screen, which is a single, three-step form.

No account, registration, or network connection is required. All encryption happens on your device.

4. Understanding the Interface

The main screen is organised into three clearly numbered sections:

1. Source (Input) – choose what you want to encrypt or decrypt: File(s) or Clipboard. A status line shows what is currently selected.
2. Action – choose Encrypt or Decrypt, enter your password, and optionally enable ASCII Armor (.asc).
3. Target (Output) – choose where the result goes: File, Clipboard, or Share.

When you are ready, press the round Execute button (the floating action button at the bottom right) to run the operation. A progress bar appears while the operation runs.

5. Encrypting Files

1. In the 1. Source section, tap File(s) and pick one or more files from your device or cloud storage.
2. In the 2. Action section, make sure Encrypt is selected.
3. Enter a password. The password strength meter updates as you type – aim for a long, unpredictable password (see section 9).
4. Optionally switch on ASCII Armor (.asc) if you want human-readable output instead of a binary file (see section 10).
5. In the 3. Target section choose where to save the result:
   • File – you will be asked where to save. For a single source file the app suggests a matching name automatically. For multiple files you select a destination folder and each file is written there.
   • Clipboard – only makes sense with ASCII Armor enabled; the encrypted text is copied to your clipboard.
   • Share – the app creates the encrypted file and opens the Android share sheet so you can send it via email, messaging, cloud storage, and so on.
6. Tap the Execute button. When the operation finishes you will see a confirmation message.

Tip: When saving a single encrypted file, the app suggests originalname.gpg (binary) or originalname.asc (armored). You can change the name and location to your liking.

6. Decrypting Files

1. In the 1. Source section, tap File(s) and pick the encrypted file(s) you want to unlock. The app recognises the file type and, if you have not chosen otherwise, automatically selects Decrypt for you.
2. Enter the password that was used to encrypt the data.
3. In the 3. Target section choose where to put the decrypted result: File, Clipboard, or Share.
4. Tap the Execute button.

When decrypting, the app restores the original file name where possible (it is stored inside the OpenPGP message). If the encrypted data contains a file name, the decrypted output is saved under that name. Note that the ASCII Armor and password strength options are only shown while encrypting, because they do not apply to decryption.

Note: The app decrypts files that were encrypted with a password (symmetric encryption) in OpenPGP format. If you open a file that is not a valid encrypted message, or enter the wrong password, you will see an error message.

7. Encrypting and Decrypting Text

1. Copy the text you want to protect to your clipboard.
2. In the 1. Source section, tap Clipboard. The app reads the text from your clipboard and shows how many characters were loaded.
3. Choose Encrypt or Decrypt, enter your password, and pick a target.

For text, the Clipboard target is the most convenient choice: the result is copied straight back to your clipboard. When you select the Clipboard target, the app automatically turns on ASCII Armor, because armored output is the only text-friendly OpenPGP format.

If your clipboard is empty, you will see a „Clipboard is empty“ message.

8. Receiving Files from Other Apps and Sharing Results

Opening encrypted files from a file manager

The app registers itself as a handler for .gpg, .pgp, and .asc files. From a file manager, tap an encrypted file and choose GPG Symmetric Encrypt from the list. The file is loaded as the source and the app automatically switches to Decrypt.

Sharing text or files into the app

You can also share content into the app from any other app:

• Text: In any app, use Share and select GPG Symmetric Encrypt. The shared text becomes the source.
• One file: In a file manager or app, use Share and select GPG Symmetric Encrypt. The file becomes the source.
• Multiple files: Use Share with several files selected; all of them become the source.

The app then proceeds exactly as if you had picked the source manually: confirm the action, enter your password, choose a target, and tap Execute.

Sharing results

Choosing the Share target opens the Android share sheet with your freshly created file attached, so you can hand it to any compatible app.

9. Password Strength Indicator

While encrypting, the app shows a live estimate of your password’s strength, measured in bits of entropy:

• Weak – below 40 bits (shown in red).
• Medium – 40 to 79 bits (shown in orange).
• Strong – 80 bits or more (shown in green).

The estimate considers the character classes used (lowercase, uppercase, digits, symbols) and the length of the password. A higher number of bits means a stronger password.

Recommendations:

• Use a long passphrase – length matters more than clever substitutions. A passphrase of several random words or 16+ random characters is a good target.
• Never reuse a password across multiple files if you want them to be independently secure.
• You cannot recover a forgotten password – there is no backdoor. Keep your passwords safe, for example in a password manager.

10. ASCII Armor (.asc)

ASCII Armor is an option when encrypting. It converts the binary encrypted output into a plain-text representation that starts with -----BEGIN PGP MESSAGE-----. Armored output:

• Can be copied, pasted, and sent through channels that only handle text (email bodies, chat apps, forums).
• Is automatically enabled when you choose the Clipboard target, because binary data cannot be pasted into a text clipboard.

Encrypted files with armor use the .asc extension; without armor they use .gpg. Both decrypt the same way, and the app handles both automatically.

11. File Formats and Naming

Operation	Output extension	Description
Encrypt + ASCII Armor	.asc	Human-readable armored text
Encrypt (binary)	.gpg	Compact binary output
Decrypt	restored original name	Strips the .gpg/.asc extension
Decrypt (no original name)	.dec	Falls back to name.dec

When you choose the File target, the app suggests a name using these rules, and you can change it before saving.

12. Security Notes

• Everything stays on your device. Encryption and decryption happen locally; no data is uploaded anywhere.
• AES-256 is the encryption algorithm, with an integrity packet so corrupted or tampered-with data is detected during decryption.
• Screen protection: the app window is flagged with the secure flag, so it is hidden from screenshots and from the recent-apps preview on most devices.
• Password handling: your password is used only in memory and is cleared after the operation finishes.
• The password is the secret. Anyone with the password can decrypt your data. Anyone without it cannot – even if they obtain the encrypted file. If you lose the password, the data cannot be recovered.
• Decrypting with the wrong password fails with a „Wrong password or corrupted file“ message rather than producing garbage output.

13. Troubleshooting

„No source selected“

You pressed Execute without loading any input. Tap File(s) or Clipboard in the Source section first.

„Password cannot be empty“

You must enter a password. The password field is highlighted until you type one.

„Wrong password or corrupted file“

The password does not match the encrypted data, or the file is not a valid, intact OpenPGP message. Double-check the password and that the file was not truncated or modified after encryption.

„Clipboard is empty“

You tapped Clipboard as a source, but nothing is copied. Copy some text first and try again.

Other errors

Other failures are shown with an „Error:“ prefix, followed by a technical detail. If you see this, the input file may be in a format the app cannot handle.

The app does not appear when I open a .gpg file

Some file managers only show apps they consider „document“ apps. Try using Share from the file manager instead, or open the app and choose the file via File(s).

Screenshots seem blocked

This is intentional: the app enables screen-capture protection for your privacy. It cannot be turned off.

14. Frequently Asked Questions

Can files encrypted by this app be decrypted on a computer?

Yes. The output is a standard OpenPGP password-encrypted message. You can decrypt it with GnuPG (gpg) on Linux, macOS, or Windows using a symmetric decryption command such as gpg --decrypt file.gpg, and it will ask you for the password.

Can this app decrypt files created by GnuPG?

Yes, as long as the file was encrypted with a passphrase (symmetric encryption) using OpenPGP. Files encrypted to a public key (asymmetric encryption) cannot be decrypted with this app, because that would require the matching private key.

Is my data uploaded to a server?

No. The app performs all encryption and decryption locally on your device.

Can I encrypt several files at once?

Yes. Select multiple files as the source. When saving to File, you pick a destination folder and each file is encrypted (or decrypted) and written there under its matching name.

What happens if I rotate my device during a long operation?

The operation continues in the background and the app restores your selections after the rotation. Nothing is lost or cancelled.

Does the app require an internet connection?

No. All functionality works offline.

Which Android versions are supported?

The app runs on Android 7.0 (API 24) and newer.

15. Technical Details and About

You can view the app’s details at any time via the About entry in the app’s menu. The about dialog shows:

• Author: Stephan Fuhrmann
• Crypto library: Bouncy Castle
• Algorithm: AES-256 (symmetric encryption)
• Format: OpenPGP (RFC 4880)

Current app version: 1.1.

The app uses standard OpenPGP building blocks – the AES-256 cipher, an integrity packet, and ZIP compression inside the message – so its output is interoperable with the broader OpenPGP ecosystem while staying simple and password-based for everyday use.