CS 513 - System Security
Lecture 15
Lecturer: Professor Fred B. Schneider
Notes by: Vicky Weissman
Lecture Date: 3/16/00
Today's Topic - Human Authentication
- What you know
- What you are
- What you have
Approaches to Human Authentication
What you know (from last lecture)
- Example: equate identity with knowledge of a password
- Requirements:
- a trusted path between keyboard and security kernel
- passwords that are difficult for an attacker to learn
- a simple way to change passwords
- Advantages:
- passwords can be changed to limit damage of a successful attack
- approach does not require additional hardware
- Disadvantages:
- user must remember a frequently changed, hard to guess password
- Protocol:
- user opens a trusted path and types in password
- system verifies password by comparing it with stored password data (this may require inputted password to be combined with salt and/or hashed)
What you are
- Examples: equate identity with particular voice, typing style, retina, fingerprint, or signature characteristics.
- Requirements:
- reliability
- non-invasive way to get user's characteristics
- may require additional hardware
- Advantages:
- 'something you are' cannot be lost or forgotten
- Disadvantages:
- 'something you are' cannot be changed (difficult to recover from attack)
- Protocol:
- user's personal data is scanned into the machine
- personal data is translated into a bit stream
- bit stream is compared with stored bit stream of user data (this may require the bit stream to be combined with salt and/or hashed)
In other words, the personal data is acquired and converted to a bit stream. Then, a password-based authentication scheme is used in which the bit stream is the password
What you have
- Definitions:
- A smart card is a small card containing memory that can be accessed by special hardware. Some smart cards also have a processor and/or a keyboard.
- A cryptographic calculator has memory, a processor, and a screen. It may also have a keyboard.
- Examples: equate identity with procession of a physical key, credit card, smart card or cryptographic calculator
- Requirements:
- Advantages:
- can recover from attack ('something' is invalidated and replaced)
- Disadvantage:
- 'something you have' can be lost or stolen
- user has to carry something
- Some Protocols/Capabilities:
- If authentication is done with a smart card, then the user can sign documents by storing his/her private key on the smart card.
- If authentication is done with a smart card that has a processor, then the user can do encryption and decryption.
- If authentication is done with a cryptographic calculator, then the calculator can display the user's password (ex. the current time in encrypted form) and the user types this information into the system.
- If authentication is done with a cryptographic calculator that has a keyboard, then the system can issue a challenge, the user types the challenge into his/her calculator, the calculator displays the reply on it's screen, and the user types the reply into the system.
- Lamport's System:
- Protocol:
- user maintains userid and password
- for each user, system maintains 2 numbers (n and m) and the user's password hashed m times. If below procedure is successful, then the user's m is set to n and the hashed password is updated accordingly.
- Attacks and Defenses
- Attack 1:
Now that the attacker knows hq(password), the attacker can impersonate the user as long as n is not less than q.
- Defense 1: user stores n and does not answer a challenge if n is less than expected
- Attack 2: intercept and discard the system's message. User will no longer accept challenges from system, therefore, user can't authenticate self.
Note: The keyboard on the smart card and the cryptographic calculator can be used to do password-based authentication between the user and the device. This provides a second line of defense against attack.