How to encrypt a PDF with AES in C#

The shortest working example

If you want AES-256 output, set the algorithm explicitly before saving.

using ZingPDF;

using var input = File.OpenRead("input.pdf");
using var output = File.Create("protected.pdf");
using var pdf = Pdf.Load(input);

await pdf.EncryptAsync(
    "user-password",
    "owner-password",
    PdfEncryptionAlgorithm.Aes256);

await pdf.SaveAsync(output);

That writes an AES-256 protected output file. Use Aes128 instead if that is the level you need.

AES-128 vs AES-256

The API supports both AES-128 and AES-256 through the standard security handler.

await pdf.EncryptAsync(
    "user-password",
    "owner-password",
    PdfEncryptionAlgorithm.Aes128);

If you do not pass an algorithm, ZingPDF keeps the default RC4-128 writer behavior for compatibility. For AES output, always pass the algorithm explicitly.

User password and owner password

The first password controls access to the document. The second is the owner password.

If you omit the owner password, ZingPDF uses the same value for both.

await pdf.EncryptAsync(
    "shared-password",
    algorithm: PdfEncryptionAlgorithm.Aes256);

Encrypting an existing PDF

You can encrypt a plain PDF that you just loaded, or one you have already modified in memory.

If the source PDF is already encrypted, authenticate it first so the document can be rewritten before you save the new protected output.

using var pdf = Pdf.Load(File.OpenRead("already-encrypted.pdf"));

await pdf.AuthenticateAsync("old-password");
await pdf.EncryptAsync(
    "new-user-password",
    "new-owner-password",
    PdfEncryptionAlgorithm.Aes256);

await pdf.SaveAsync(File.Create("re-encrypted.pdf"));