While reading this article: Does Skype use encryption? I came to know that PSTN do not have encryption.

Why it is like that? More so since we have our PSTN phones now more powerful (in terms of hardware) and cheaper. Why not write a encryption application at the transmitter and receiver end?

  • Mobile phones are normally considered part of the PSTN and they do use encryption, of varying strength. You're right that land lines do not use encryption. – paj28 Feb 19 '14 at 12:04
  • Note that just because it's encrypted doesn't mean it's secure, you need to consider the key management too. Skype is compromised: schneier.com/blog/archives/2013/06/new_details_on.html – pjc50 Feb 19 '14 at 14:04

Telephones are not encrypted because back in the late 1800s, when telephone systems were being developed, any sort of encryption was far beyond the capability of the hardware, even if it had been something that people would have thought important. Since then, compatibility with existing hardware has been considered more valuable than protection against eavesdropping -- listening in on a PSTN conversation requires physical access to the wires involved, and is difficult for anybody other than the phone company to do.

That said, there are encryption-like devices for telephones known as scramblers, originally developed around the time of World War II. Modern scramblers are every bit as good as other encryption systems.

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    Plus through the 1960s multiple homes were hocked into a single party line with only the notification ring differing between parties. This popular option was the most common implementation and relied on shared 'evesdropping' ability. – zedman9991 Feb 19 '14 at 13:51
  • @zedman9991 - in fairness, a shared party line could potentially still utilize encryption to make it so that only the correct caller could get the call. That situation would probably have been the biggest argument for such a technology if it had existed at the time. – AJ Henderson Feb 19 '14 at 14:16
  • @AJ Yes your logic is sound but economically maximizing profits on existing racks of relays depended on managing expectations of privacy which were nil for telephones at the time. Those expectations and the revenue scheme are so changed today that it is hard for folks to understand legacy design decisions that got us here. – zedman9991 Feb 19 '14 at 14:54
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    @zedman9991 - yes, my point was just that a party line (using one line for multiple parties) did not depend on the eavesdropping ability, but rather the eavesdropping ability simply wasn't worth addressing economically because it wasn't a concern (or even possible) at the time. – AJ Henderson Feb 19 '14 at 14:56

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