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This question is a bit different from other questions here, but I think it is suitable to correctly understand the terminology of cryptography.

Consider the following two sets of terms:

  • Encryption scheme, signature scheme
  • Identification scheme, commitment scheme, secret-sharing scheme

The latter are actually "protocols", while we usually do not consider encryption/signature as (interactive) "protocols". Moreover, the word "scheme" is not usually applied to terms like "hash functions", "pseudo-random generators", "one-way functions", etc.

What does the term "scheme" mean? When is it meaningful to apply this term to a cryptographic construct? How does it relate to "protocols"?

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  • $\begingroup$ Honestly, I doubt there is a clear distinction. I know I use a lot of these types of terms interchangeably. $\endgroup$ – mikeazo Oct 17 '12 at 17:45
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Protocol refers to a highly structured mode of operation. A protocol acts in a very specific way which is laid out, it is not possible to deviate from a protocol without failure.

Scheme refers to a more generalised set of solutions to a problem, not defining any particular way that it should be done.

For example, A cryptographic communication scheme which one solution is the SSL protocol.

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As @mikeazo correctly states, I don't think there is any formal or precise definition for the term "scheme". It is probably used loosely in a way that might well mean a "protocol", "mode of operation", "cryptosystem", "cryptographic algorithm", or even something else entirely.

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Scheme would be more like the plan, design, or program of action to be followed. It differs from protocol in this context because it might not effect or manipulate any data or other tangible asset.

Protocol relates more to the conventions and treatment surrounding the formatting of the data used in electronic communications systems

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