Telex and the Prehistory of Interbank Messaging

Today a cross-border payment instruction is a structured message flying across a secure global network in seconds. Before the 1970s, it was a line of text clattering out of a machine called a Telex, authenticated by a number a clerk worked out by hand. Understanding this prehistory explains why SWIFT was such a leap, and why so much of modern payments messaging is about solving problems Telex could not.

What Telex actually was

Telex was a global network of teleprinters — essentially typewriters connected over telegraph and telephone lines. Each subscriber had a unique Telex number, and one machine could dial another anywhere in the world and exchange typed text in real time. From roughly the 1930s through the latter half of the twentieth century, Telex was the backbone of international business communication, and banks used it heavily to send payment instructions to their correspondents abroad.

A payment by Telex was, in essence, a typed message: pay this beneficiary this amount, debit our account with you, here are the details. It was faster than post and more durable than a phone call because it produced a printed record at both ends. But it had two deep problems: authentication and structure.

The test key: authentication by arithmetic

How does the receiving bank know a Telex message genuinely came from the sending bank and was not forged or altered? There was no cryptography in the modern sense. Instead, banks used test keys. A test key was a shared secret system — effectively a code book and a set of arithmetic rules agreed bilaterally between two banks. To send an authenticated payment, a clerk would compute a test number by combining values derived from the amount, the currency, the date, and a sequence, according to the agreed formula and secret tables.

The receiving bank's clerk would independently recompute the expected test number using the same secret tables. If the numbers matched, the message was considered authentic. If they did not, it was rejected or queried. This was labour-intensive, error-prone, and only as secure as the secrecy of the code books, which had to be exchanged and updated between every pair of correspondent banks.

Why Telex could not scale

• Weak, manual security. Test keys depended on shared secret books and human arithmetic. A leaked book or a calculation error compromised authentication.
• No standard structure. Messages were free-form text. Different banks wrote instructions differently, so processing could not be automated and misinterpretation was common.
• Bilateral overhead. Every pair of banks needed its own test-key arrangement, which did not scale as international banking exploded.
• Slow and manual. Human clerks composed, tested, sent, received, and verified each message.

Enter SWIFT

In 1973, a group of banks founded the Society for Worldwide Interbank Financial Telecommunication (SWIFT), going live in the late 1970s. SWIFT solved exactly the problems Telex could not. It provided standardised message types — the numbered MT formats — so instructions had a defined, machine-readable structure. It provided centralised, network-level authentication and security rather than bilateral code books. And it gave every institution a standard address, the BIC (Bank Identifier Code), so routing was consistent worldwide.

The shift from Telex to SWIFT is a clean illustration of a recurring theme in payments: moving from free-form, manually authenticated messages to structured, standardised, network-authenticated ones. The same theme drives today's migration from SWIFT MT to ISO 20022, which pushes even further toward rich structured data.

Echoes in the present

Telex is largely gone, but its fingerprints remain. The idea that a payment instruction must be authenticated, structured, and routable is unchanged; only the mechanisms have improved. When you see debates about data richness, sanctions screening, or straight-through processing, you are watching the industry keep solving, at higher fidelity, the very problems that test keys and free-text Telex messages exposed decades ago.

Key takeaways

• Before SWIFT, banks sent cross-border payment instructions as free-text messages over the global Telex teleprinter network.
• Authentication relied on test keys — shared secret code books and manual arithmetic to compute and verify a check number.
• Telex could not scale: weak manual security, no standard message structure, and bilateral overhead between every pair of banks.
• SWIFT, founded in 1973, introduced standardised MT message types, network-level security, and BIC addressing.
• The Telex-to-SWIFT shift mirrors today's SWIFT MT-to-ISO 20022 migration: the move toward structured, standardised, authenticated messaging.