How the Czechoslovak secret service ŠtB breaks the encryption machine Gretag TC 803 at the Spanish embassy in Prague: https://www.info.cz/zpravodajstvi/cesko/akce-sumperk?odemknout=YAYLEWR8KH
Detailed description of the encryption machine Gretag TC 803: https://www.cryptomuseum.com/crypto/gretag/803/index.htm
"...Another break-in at the embassy took place on June 27, 1980. In this case, the State Security encountered a problem that it could not deal with. The Spanish started using the Gretag TC 803 encryption machine. ... Daily slips of paper with an encryption key were inserted into it. These were stored in a safe and bound with bookbinding cloth on three sides and in the middle. The canvas thus formed a kind of letter E with four arms. Between the canvas arms were perforated slips of metal foil that were inserted into the encryption machine. One part was designated for each day. Used and unused keys were shredded the next day. Without knowledge of this key, encrypted telexes could not be decrypted. Not even a specialist bookbinder from the Department of Intelligence could devise a way to bypass this security in a way that would not be noticed by the Spanish embassy cipher clerk. ... Although the StB found out the settings of the encryption machine's commutators for the second half of 1981, without knowledge of the day's encryption key, it was unable to decipher the dispatches sent. "An attempt was made to disassemble the block. A part of the light blue cardboard that covers the end of the binding cloth strips was peeled off by steaming. A high risk of exposure was found during this operation, as it is an area of 20 x 15 cm. (…) On the basis of the knowledge gained from the said inspection, I recommend consulting the problem through cooperation with the relevant workplace of experts in the issue of specially secured shipments in the USSR." Another inspection of the embassy took place on September 22, 1981, the StB copied the paper documents, but it was again unable to break the encrypted communication failed. ... The direct participation of four KGB workers in breaking into the Spanish embassy in Prague took place for the first time on March 17, 1983. The KGB had evidently managed to find a way to break the cipher blocks and copy the daily cipher for the Gretag cipher machine. It was not until four years after the installation of the "security" door to the encryption room that an employee of the Grany company, which manufactured the door, visited the embassy and removed the "defect". Between August 7 and 10, 1984, State Security verified that the door to the code room was recoded and was unable to discover the new password. The new lock was only breached during another break-in at the embassy on July 3, 1985, when Soviet specialists managed to copy all the encryption keys necessary for decrypting the communications of the Spanish embassy. Another break-in at the embassy took place in June 1986, when it was again possible to break into the encryption room and copy everything that the StB and KGB were interested in."