14 Passwords and Shadow Hashes

Changing User Passwords
- passwd allows users to change their own password.
- Running sudo passwd <username> allows changing another user's password.
- Some systems enforce password complexity, rejecting weak passwords.
Understanding the /etc/passwd File
- /etc/passwd contains user account details but not passwords.
- Lists usernames, home directories, and shell programs.
- Each entry includes colon-separated fields for user metadata.
Storing Password Hashes in /etc/shadow
- /etc/shadow holds hashed passwords, not plain text.
- Only root users can view the shadow file (sudo cat /etc/shadow).
- Passwords are stored as hashed values with added "salt" for security.
How Password Hashing Works
- Linux hashes passwords before storing them in /etc/shadow.
- Hashing ensures passwords aren’t stored as plain text.
- Login attempts rehash input and compare it to stored values.
Using Python to Generate Hashes
- Python’s crypt module can simulate password hashing.
- Running crypt.crypt("password", "$6$salt") generates a hashed password.
- Changing one character in input completely alters the hash output.
Security Implications of Stolen Hashes
- Hackers attempt brute force or dictionary attacks to crack hashes.
- Adding salt makes it harder to use precomputed attack tables.
- sudo chage -l <username> checks password expiration policies.