122 lines
4.3 KiB
Markdown
122 lines
4.3 KiB
Markdown
# Authentication
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- To allow some access to an asset we must ensure:
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- They are permitted to access that asset
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- They are who they say they are
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- We can attempt to verify identity using credentials
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- Something the user *is*
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- Something the user *has*
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- Something the user *knows*
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#### Usernames and Passwords
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- Identification - who are you
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- Authentication - verify that identity
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- Authentication should expire
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- *Remember my credentials* turns this into something you have
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- **T**ime **o**f **c**heck **t**o **t**ime **o**f **u**se - **TOCTTOU**
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- Repeated authentication
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- At the start and during a session
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##### Problems with passwords
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- People forget them
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- They can be guessed
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- Spoofing and phishing
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- Compromising password files
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- Key-logging
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- Many of these are made many times worse by weak passwords
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### Hash Functions
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- Another cryptographic primitive
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- Takes a message of any length, and returns a pseudorandom hash of fixed length
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$$
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h(M):\{0,1\}^n \rightarrow \{0,1\}^{128}
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$$
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- Hash functions are used everywhere. Message authentication, integrity, passwords etc
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#### Strong Hash Functions
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- The output must be indistinguishable from random noise
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- Bit changes must be diffused through the entire output
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For a hash function to be useful, we need it to have some important properties:
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1. Given a hash, we can’t reverse it
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2. It is impractical to find messages that produce the same hash - a *hash collision*
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##### Password Authentication (wrong way)
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If database is breached, passwords are stored in plaintext
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- Storing passwords in plaintext is a terrible idea
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- Administrators can read them
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- Storing encrypted passwords is better, but not perfect
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- Where are keys stored?
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- Administrators can read them
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Using a **one-way hash function** is a much better solution
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#### Password & Shadow Files
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- Operating systems have taken steps to stop people reading hashes for offline attacks
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- Linux stores hashes in a shadow file `/etc/shadow`
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- These files are now **read-protected**
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#### Cracking Passwords
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- Cracking a password isn’t always illegal
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- Password cracking falls into two basic types:
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- Offline: you have a copy of the password hash locally
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- This is trying possible passwords and seeing if we have a hash collision with the password list
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- Usually done via brute force however difficulty is $\{char\space count\}^{length}$
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- Online: You do not have the hash, and are instead attempting to gain access to an actual login terminal
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- Online is usually attempted via phishing
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##### Dictionary Attacks
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- Most password cracking is now achieved using **dictionary attacks** rather than brute force
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- Using a dictionary of common words and passwords
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- Apply small variations to this list, trying them all
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- Combine words from two different lists
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- `qwerty1234password1` is unbreakable using brute force, but won’t last against a dictionary attack
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##### Password Salting
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- We can improve security by pre-pending a random *salt* to a password before hashing
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- The salt is stored unencrypted with the hash
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- If a hacker has a list of hashed passwords, and three of them are the same, he can surmise they’re all a common password.
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- Salting adds non-secret randomness to passwords
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- If we use a different random salt for each user, we get the following security benefits:
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1. Cracking multiple passwords is slower - a hit is for a single user, not all users with that password
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2. Prevents **rainbow table** attacks - we can’t pre-compute that many password combinations
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- Salting has no effect on the speed of cracking a single password
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#### Hashing Speed
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- When password cracking, the most important factor is *hashing speed*
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- New algorithms take longer
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- Partly because they’re more complex
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- But some have been specifically designed to take a while
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- Iterate to increase complexity
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##### Social Cracking
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- Obtaining private details by offering some *pretext* as a reason for needing them
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- We continue to rely on email addresses, DOB and Mother’s maiden names as our *last line of defence* for security
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- How much information do we need to ring up a company as someone else
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