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Two-channel transfer

Never send the data and the key over the same channel. Whoever intercepts the envelope then has no password, and whoever reads the email along has no medium. It is the same idea as two-factor authentication, and that is how I explain it to a court clerk’s office or a lawyer.

Two separate channels The data travels by post, the key by phone or email at a separate moment; both reach the recipient over physically different channels. Data Key Recipient post, registered phone or email, separate moment Physically different channels: whoever intercepts one does not yet have the other.

What are not two channels

An encrypted archive in one email and the password in the next. A share link with the password in the same email. A stick with a note in the same envelope. Each time, whoever intercepts that one channel has everything.

The second channel has to be genuinely different: paper against network, or phone against email. A second email address of the same recipient does not count. It runs over the same mail server and usually to the same device.

Three ways

Which one you choose depends on the recipient and on the amount of data.

Download link by email, PIN by text message

The fastest way, if the recipient agrees to an online transfer. The data sits on a service that asks for a PIN, see Boomerang. The link goes by email, the PIN by text message to a mobile number the recipient supplied in writing for that purpose.

Medium by post, password separately

Put the data in a password-protected 7-Zip archive, see below, and send the USB sticks by registered post, with acknowledgement of receipt. The password follows by phone or by email.

Put a short letter in the envelope explaining that the password comes separately, and why, and that the recipient needs 7-Zip. Otherwise they will think the archive is broken.

Rates at bpost from 1 January 2026: registered post costs €7.33 with stamps or labels and €7.14 with a franking machine, a PB number or Collect&Stamp, on top of the ordinary PRIOR rate. Advice of receipt costs €1.64 extra. Above 2 kg a flat rate of €33.00 applies, up to 10 kg. The current tables are on bpost’s rates page (in Dutch).

Email the link to a secure shared folder, see Tresorit, and send the password the same day by ordinary letter. Someone who receives a link that does not open and has to wait days for a letter ends up asking for the password by email anyway.

A short letter on letterhead is enough: docket number, parties, your details, and the password in a box. That way nobody sees the password before a reference.

Encryption the recipient can open

With a medium sent by registered post or handed over in person, you can assume it ends up in the right hands. What matters more is that the data are authentic, and you show that with the hashes in the report. The password only protects against prying eyes. So choose an encryption the recipient is sure to open, because you rarely know which system the court clerk’s office, the law firm or the technical adviser works on.

7-Zip. Encrypt the archive, not the medium: 7z format, AES-256, with the option Encrypt file names on (-mhe=on on the command line). Without that option the list of file names stays readable, and that alone already tells you what a case is about. 7-Zip is free and also runs on old Windows versions. The recipient does need it, though, and on a managed workstation not everyone may install software. So ask beforehand. Put the hash of the archive and of the files in it in the report.

An ordinary password-protected ZIP opens in Windows without extra software, but that encryption is weak and the file names stay readable.

No BitLocker To Go. Windows encrypts removable media by default with XTS-AES 128. Windows 7 cannot read that, and Windows 8.1 only with the necessary update. Those are exactly the machines still running in clerk’s offices and small firms. The security you gain does not outweigh a medium the recipient cannot open.

Working rules

Use two brands of media. That is simply extra redundancy. With one brand you depend on, say, a bad production batch, and you want to avoid that.

Make a second copy. If a medium is defective, you fall back on the second without having to extract everything again. An extra stick costs a few tens of euro.

Label every medium. A volume label that appears as soon as the medium is connected, a colour per recipient and an overview sheet that links medium and recipient. That way party A does not get the copy meant for party B.

Have the handover confirmed in writing. Have the clerk’s office stamp what you hand over, or have the registered mail signed for. Without that paper you cannot show the delivery.

Keep the password outside the case folder. A password in the case folder, certainly if it syncs with the cloud, makes the encryption pointless. Keep it in a password manager or offline.

Do not put the password in the file of your letter. That document sits in the same case folder as the data, and then key and data are together again.

If it is highly sensitive, deliver the media yourself. Note that choice and why. It accounts for the time and travel costs in your statement of costs.

In the report

Which media, which encryption, how many copies, over which channel each part was sent, and to whom. Put it in a table per recipient. Together with the receipt of the registered mail, that is your proof of delivery.

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