The prefix $2a$ identifies thebcrypthashing algorithm, which is based on the Blowfish symmetric encryption cipher (developed by Bruce Schneier). Bcrypt is purpose-built for password hashing, incorporating:
Salt:A random string (e.g., 22 Base64 characters) to thwart rainbow table attacks.
Work Factor:A cost parameter (e.g., $2a$10$ means 2^10 iterations), making it computationally expensive to brute-force.
Format:$2a$[cost]$[salt][hash]
Example: $2a$10$N9qo8uLOickgx2ZMRZoMyeIjZAgcfl7p92ldGxad68LJZdL17lhWy
$2a$: Bcrypt variant (original is $2$; $2a$ fixes a minor bug).
$10$: 1024 iterations.
Next 22 characters: Salt.
Remaining: Hashed password.
Used in /etc/shadow on Linux, bcrypt’s adaptive nature ensures it remains secure as hardware improves. CNSP likely includes it in cryptography modules for its strength over older algorithms like MD5.
Why other options are incorrect:
B. SHA256:Part of the SHA-2 family, outputs a 64-character hexadecimal string (e.g., e3b0c442...), no $ prefix. It’s faster, less suited for passwords.
C. MD5:Produces a 32-character hex string (e.g., d41d8cd9...), no prefix. It’s cryptographically broken (collisions found).
D. SHA512:SHA-2 variant, 128-character hex (e.g., cf83e135...), no $ prefix, not salted by default.
Real-World Context:Bcrypt protects SSH keys and web app passwords (e.g., in PHP’s password_hash()).References:CNSP Official Documentation (Cryptography); Bcrypt Specification, RFC 1321 (MD5).
