Mechanisms

• A step by step procedure for solving a problem

• An algorithm is a sequence of well-defined steps required to carry out a technique - it could be written in plain English.

• Detecting/correcting errors is primarily an algorithm (e.g. Luhn algorithm for credit cards - there's a list here)

• Cryptography is a strong mixture of algorithms (e.g. RSA , Diffie-Hellman, DES, AES...)

• An algorithm takes some input and uses mathematics and logic to produce the output

• A method for accomplishing a desired aim

• A technique is a way of doing something

• A way of carrying out a particular task, especially the execution or performance of an artistic work or a scientific procedure. 

Protocols are followed by sites to ensure that they are safe and trustworthy.

• A set of conventions or rules

• Protocols are the instructions for two devices to talk to each other.

Protocol (object-oriented programming), a common means for unrelated objects to communicate with each other (sometimes also called interfaces; related to duck typing).

Communication protocol, a defined set of rules and regulations that determine how data is transmitted in telecommunications and computer networking

• • Cryptographic protocol, a protocol for encrypting messages

A series of steps followed in a regular order, an established way of doing things

A procedure is the computer code that carries out the algorithm or part of an algorithm (and can have different meanings in terms of function, subroutine etc).