Authentication Table Guide: How to Verify Identity Over Radio or Phone

An authentication table is a grid of random letters and numbers that everyone in your group fills out identically ahead of time. To check who you're talking to over a radio or phone, you name two coordinates on the grid, the other person reads back the character at that intersection, and you compare it to your copy. Each code is used once and then crossed off.

What is an authentication table?

It's a challenge and response system for verifying identity, which works like an offline version of two-factor authentication. Anyone can claim to be your spouse on a bad phone line or an unfamiliar radio. Only someone holding the same table can answer the challenge correctly.

The Authentication Table sheet in the Contingency Planner has three grids. The main grid is for letters, with columns labeled A through M across the top and rows labeled N through Z down the side. You fill every square where a row and column meet with a random letter. Below it are two smaller number grids, each labeled with digits across the top and down the side, and you fill those squares with random numbers. Together these grids are the shared secret your group uses to prove who's on the other end.

How do you set one up and share it?

Fill the grids with characters that are actually random. Avoid patterns, words, keyboard runs and anything someone could guess from knowing you. Then make an exact copy for each person in your group, and check every square against the original, because one copying error means a correct response looks wrong.

Hand the copies over in person before you need them. Sending the table over the same phone or radio channel you plan to verify defeats the purpose. Treat each copy like a key, keep it private, and know where it is. The planner's sheets are stone paper, so they're waterproof and tear-resistant enough to live in a go-bag or glovebox.

How does a challenge and response work?

The person asking for proof is the challenger. They pick an unused square and name its column and row in the phonetic alphabet. The responder finds that square on their own table and reads back what's there, and the challenger checks it against their copy. Because the top letters (A to M) and side letters (N to Z) never overlap, nobody can mix up which is the row and which is the column. A letter challenge sounds like this.

• The challenger says, "Authenticate Bravo Papa."
• The responder finds column B, row P on their table and answers, "I authenticate Bravo Papa with Kilo."
• The challenger checks column B, row P on their own table. If it says K, the responder is verified.

The number grids work the same way with digits. Say which grid you're using, then the row and column, and spell the numbers out. Since the two number grids share digits, agree ahead of time on the order you'll say them in, row then column for example, so "three eight" always points to the same square. A number exchange might go "Authenticate grid two, three eight," answered with "I authenticate three eight with seven."

Why do you strike out used codes?

Anyone listening can hear both the challenge and the answer. If you use the same square again, a listener can simply repeat the answer they overheard. The guide on the back of the planner's Authentication Table sheet makes the point directly. Use each code only one time.

So after every successful exchange, both people strike out that square on their tables, whether it came from the letter grid or a number grid. When a table starts running low, fill out a new one and get copies to everyone in person. The Security expansion pack in the Contingency Planner Complete Kit includes three more Authentication Table sheets for exactly this.

What habits make authentication work?

• Authenticate before you share anything sensitive, such as a location, plans or who's home.
• Use the phonetic alphabet for letters and spell out numbers so nothing gets misheard.
• Run more than one challenge, letter and number, when the stakes are higher.
• Always say which number grid you mean.
• Strike out every used code on both tables right away.
• Practice with your group on an ordinary day so it isn't new when you need it.

If an answer comes back wrong, allow one retry with a fresh square in case of a misread. After that, treat the other party as unverified and share nothing until you can confirm who they are another way. An authentication table proves identity but doesn't hide what you say afterward. To keep the content of a message private, pair it with the planner's Card of the Day or Cipher Table.

Common questions

Is an authentication table the same as encryption?

No. It verifies who you're talking to, and the rest of the conversation is still in the clear for anyone listening. Use it alongside a code system like the Card of the Day or a cipher when the message itself needs protecting.

How many times can I use the same code?

Once. A listener who overhears a challenge and its answer can repeat that answer later, so both people strike out each square as soon as it's used, and the group switches to a fresh table when the old one runs low.

Does this work over phone calls and texts, or only radio?

It works over any channel where you can say or type a short challenge, including phone calls, text messages, GMRS or ham radio, and Meshtastic. It's most useful on channels where you can't see the other person or trust caller ID.

Where can I get a blank authentication table?

The Contingency Planner binder includes an Authentication Table sheet with a how-to guide and worked example on the back, and the Complete Kit's Security expansion pack adds three more along with Cipher Table and Card of the Day sheets.

The Contingency Planner Complete Kit includes the binder and all four expansion packs, so your group has enough authentication tables to rotate.