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A partial password is a mode of password authentication.
By asking the user to enter only a few specific characters from their password, rather than the whole password, partial passwords help to protect the user from password theft. As only part of the password is revealed at once it becomes more difficult to obtain the password using techniques such as keystroke logging or shoulder surfing.
There is a paper by David Aspinall and Mike Just  that describes partial password implementations and attacks in a detailed study.
Verifying partial passwords
It is good practice to not store passwords in cleartext. Instead when checking a whole password it is common to store the result of passing the password to a cryptographic hash function. As the user doesn't supply the whole password it cannot be verified against a stored digest of the whole password. Some have suggested storing the digest of each combination of letters that could be requested but they note that this results in generating and storing a large amount of digests. A better solution in terms of storage space and security is using a secret sharing scheme.
- "What is partial password verification?". The Co-operative Bank. Retrieved 2011-03-03.
- ""Give Me Letters 2, 3 and 6!": Partial Password Implementations & Attacks" (PDF). Retrieved 2015-10-14.
- "Partial Passwords and Keystroke Loggers". Retrieved 2011-03-03.
- "Partial Passwords - How?". Retrieved 2011-03-03.
- Update to Partial Passwords