These are notes on the type of espionage tradecraft used in the movie:
1. Communications
(a) Use of special "Morse Code" (in a German version) for secret communication between conspiring agents, even when others are listening. Example: Spy 1 at typewriter pretends to type (manual typewriter), but just keys in "X" characters, but in code pattern that purposefully is within hearing distance of nearby agent Spy 2.
(b) Use of short-wave radio. (but encrypted, and also "cleartext" during an emergency when there was no time to encrypt)
(c) Use of carrier pigeons (to fly hand-written messages from the UK to Germany (or to some pick up point))
(d) Code words. Use of secret code words, part 1 a "challenge" phrase, part 2 a "response" phrase. Agents used this within the context of ordinary conversation in order to identify each other without onlookers noticing.
2. Espionage Network
(a) A "chief of station" type (played by Karlov), who is disguised as a lowly butler.
(b) Even the other German agents do not know the true identity of the chief. They know he is part of the network, but he claims to be "taking orders" from the chief. He, of course, is actually the chief.
(c) Couriers. The milkman serves as a courier for information. Karlov as the "butler" each day gets fresh milk (delivered in those days by kart) and used the opportunity to pass on important information. Sometimes the information is spoken, sometimes passed along in a letter.
(d) Radio man. Separate person is assigned task of collecting information, encoding it, and transmitting it to Germany.
(e) Feigned death. One of the German agents, fakes his death in order to help cover up his activities, but is observed.
3. Counterintelligence
(a) Use of "deep plant" British agent who penetrates German intelligence then comes to the UK pretending to spy on the British, but with the single intention of helping locate the "chief of station".
(b) "Legend". The German agent is given a life story, completely false, that arouses sympathy, allowing them to be "taken in" (e.g. hired) by a targeted British family. (It is targeted because it is elite enough to be involved with British military or government.)
In general the story line is written in a way that only at the end are the true sympathies of the different agents and double agents known.
1. Communications
(a) Use of special "Morse Code" (in a German version) for secret communication between conspiring agents, even when others are listening. Example: Spy 1 at typewriter pretends to type (manual typewriter), but just keys in "X" characters, but in code pattern that purposefully is within hearing distance of nearby agent Spy 2.
(b) Use of short-wave radio. (but encrypted, and also "cleartext" during an emergency when there was no time to encrypt)
(c) Use of carrier pigeons (to fly hand-written messages from the UK to Germany (or to some pick up point))
(d) Code words. Use of secret code words, part 1 a "challenge" phrase, part 2 a "response" phrase. Agents used this within the context of ordinary conversation in order to identify each other without onlookers noticing.
2. Espionage Network
(a) A "chief of station" type (played by Karlov), who is disguised as a lowly butler.
(b) Even the other German agents do not know the true identity of the chief. They know he is part of the network, but he claims to be "taking orders" from the chief. He, of course, is actually the chief.
(c) Couriers. The milkman serves as a courier for information. Karlov as the "butler" each day gets fresh milk (delivered in those days by kart) and used the opportunity to pass on important information. Sometimes the information is spoken, sometimes passed along in a letter.
(d) Radio man. Separate person is assigned task of collecting information, encoding it, and transmitting it to Germany.
(e) Feigned death. One of the German agents, fakes his death in order to help cover up his activities, but is observed.
3. Counterintelligence
(a) Use of "deep plant" British agent who penetrates German intelligence then comes to the UK pretending to spy on the British, but with the single intention of helping locate the "chief of station".
(b) "Legend". The German agent is given a life story, completely false, that arouses sympathy, allowing them to be "taken in" (e.g. hired) by a targeted British family. (It is targeted because it is elite enough to be involved with British military or government.)
In general the story line is written in a way that only at the end are the true sympathies of the different agents and double agents known.