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Is there a difference between cryptography and cryptology, if any?

An internet search suggests that both terms can be used interchangeably.

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Today, indeed the the terms "Cryptography" and "Cryptology" can mostly be used interchangeably.

Historically things have been more interesting though, where Cryptology was the umbrella term for Cryptanalysis and (constructive) Cryptography. For example the Handbook of Applied Cryptography (chapter 1 PDF) has the following definition (page 15) of "Cryptology":

Cryptology is the study of cryptography (Definition 1.1) and cryptanalysis.

with "Cryptanalysis" being defined as (on page 15)

Cryptanalysis is the study of mathematical techniques for attempting to defeat cryptographic techniques, and, more generally, information security services.

and "Cryptography" being defined as (on page 4)

Cryptography is the study of mathematical techniques related to aspects of information security such as confidentiality, data integrity, entity authentication, and data origin authentication.

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    $\begingroup$ The "mostly" in the first sentence should be parsed as "everyone will understand what you mean even though there may be potentially pedants disagreeing" $\endgroup$ – SEJPM Mar 25 '18 at 13:26
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    $\begingroup$ I think there is a good reason that these terms are almost interchangeable nowadays: One cannot realistically be a good cryptographer without knowing cryptanalysis, and vice versa. Even though the goals are different, the required skill set is very similar, so cryptography and cryptanalysis can be seen as just two faces of the same medal. $\endgroup$ – yyyyyyy Mar 25 '18 at 14:01
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    $\begingroup$ I think "attempting to defeat" is a bit too broad in the definition within the book; you can perform cryptanalysis without attempting to break anything (although you need to keep that in mind to keep sharp). Certainly I would not find breaking information security services part of cryptanalysis - is a hacker perform crypt-analysis? Crypt-analysis could be part of hacking but that's not the same thing Probably it is better to just use the term "cryptography" as umbrella term so that the confusion cannot arise in the first place. Cryptology isn't used all that much; the site is cryptography :) $\endgroup$ – Maarten Bodewes Mar 25 '18 at 16:00
  • $\begingroup$ @yyyyyyy you could send a fairly secret message (crypto-graphic) without knowing about mathematical techniques (Ella's tattooed head example). $\endgroup$ – CPHPython Mar 26 '18 at 8:54
  • $\begingroup$ Somewhat related: "codes" and "ciphers" are often used interchangeably even though there is an important distinction between the two. For whatever reason, it is considered more pedantic to insist on the distinction between "cryptography" and "cryptology" than it is to insist on the distinction between "codes" and "ciphers". $\endgroup$ – John Coleman Mar 26 '18 at 12:29
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Word roots

The greek word root "crypto" means "hidden" or "secret".

The greek word root "ology" means "study of", "science of", or "theory of".

The greek word root "graph" means "to write" or "writing".

Combinations

Thus:

  • The literal interpretation of "cryptology" is the "science of secrets".

  • The literal interpetation of "cryptography" is "secret writing".

Interpretation

"Cryptography" used to include techniques that were not scientific/mathematical in the ways that we employ today.

For example, a long time ago, one way to send a secret message would have been to find a willing person (or slave), shave their head bald, tattoo your message to their head, then wait for them to grow their hair back. The goal being to keep the message secret except from the intended recipient. This fulfills the qualification of "secret writing".

"Cryptology", being a scientific/mathematical pursuit, incorporates algorithms and analysis.

In the modern world, it is difficult to separate the scientific/mathematical/analysis aspect from the secret-writing aspect, as analysis plays a key role in the design process. One could argue that if the design process does not include analysis, then it is not really design - You cannot construct a solution* if you don't understand the problem(s)

*Where "solution" means "something that actually works", as opposed to "something I believe/hope works"

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    $\begingroup$ @Q-Club Maybe, in that time, if I remember correctly, they were called "Romans" :) $\endgroup$ – Maarten Bodewes Mar 25 '18 at 19:55
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    $\begingroup$ I would say that the example of tattoing a slave qualifies more as an example of steganography. $\endgroup$ – Geoffroy Couteau Mar 25 '18 at 20:36
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    $\begingroup$ I'm not the downvoter, I upvoted the answer, I was just adding my two cents on a detail :) $\endgroup$ – Geoffroy Couteau Mar 26 '18 at 6:40
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    $\begingroup$ A QR code tattooed on the head though might just work nowadays :) $\endgroup$ – CPHPython Mar 26 '18 at 8:56
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    $\begingroup$ Well, I'll step up as one of the downvoters (but not the one from the earlier comment). I believe the answer's appeal to etymologies to argue for word usage norms is objectively wrong; language just doesn't work that way. First, etymology is not destiny (the usage of the roots in ancient Greek doesn't determine their usage in English). Second, you can't generally predict the meanings of derived or compound words from their parts. $\endgroup$ – Luis Casillas Mar 26 '18 at 23:11

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