[Cryptography] Radio leakage from System 360/370 era machines

[Bob Wilson]

In Volume 2, Issue 8, Henry Baker comments on "listening" to old 
mainframes using a radio:
> BTW, IBM mainframes had powerful radio side-channels: in the early 
> 1960's, one of the programmers that worked with me used to put an 
> ordinary portable radio on top of the IBM CPU so that he could 
> "listen" to his program go through its paces. Any change in the audio 
> would indicate a problem with the software.
In fact that was not an accident, in some cases it was a documented 
feature. It was a part of other systems, but my experience with it was 
with CDC 1604 and 3600 machinery. There was a speaker in the operator's 
console, driven by the upper octal digit of the accumulator. DC current 
proportional to that digit was fed through the speaker. As an example, 
if you put 0707070707070707 (sixteen digits octal, to fill the 48 bit 
register) in the A register and then rotated it left or right, delay, 
rotate, delay, etc. you got a square wave sound out of the speaker. More 
creative strings of digits could create lots of different sounds. I 
remember a diagnostic that CDC's onsite engineers ran that was called 
"Bagpipe", because that is what it sounded like.

I remember one programmer whose intense numeric calculations would make 
the speaker go quiet, because the A register was changing at a rate 
above human hearing during his intense computations. Operators had 
learned that if programs in general shut up the speaker it meant the 
machine was in a small tight loop, so they would interrupt the run and 
kick his program off the machine. So the first thing his code would do 
was to modify system code (this was before there was much in the way of 
separation between user code and system code, there was one big address 
space for us all to play in) and if the operator changed a switch 
setting so as to kick him off, he printed out on the console typewriter 
a message to keep their (text suppressed in interests of decency) hands 
off the switch, and telling the operator another way to stop the program 
if it were really necessary.

So if you consider that audio information as leakage, it was endorsed by 
the system designers! Also, listening that way or using a radio to 
listen in on activity with other machines was used to play music, back 
in the day when we had to demonstrate the machine to visitors.

Bob W
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