How can you transmit a sensitive file to someone securely?

Encrypt it end-to-end (S/MIME, PGP/GPG, or a strong AES archive) and share the password over a different channel.

* Send ciphertext and key on separate channels — intercepting the email yields only the locked box, not the key. *

Options from the source:

• S/MIME or PGP/GPG for encrypted email/files
• Encrypt the file into an archive with 7-Zip using AES-256, protected by a strong password

The critical operational detail: send the password over a separate channel — phone, SMS, fax — never in the same email as the encrypted file.

Why the separate channel matters: if an attacker intercepts the email, they get the ciphertext but not the key. Putting the password in the same message would hand them both — like taping the key to the locked box.

Tip: This "out-of-band key exchange" is the same principle behind MFA's second channel: security comes from the two pieces travelling independently.

Go deeper:

• Email encryption — Wikipedia — S/MIME vs PGP and how end-to-end email encryption works.
• GNU Privacy Guard (GnuPG) — Wikipedia — the free PGP implementation for encrypting files and mail.