Rupali Bank Ltd. Assistant Network Engineer — Exam: 04.11.2023 (MIST)প্রশ্ন ও সমাধান

Aspect	Symmetric	Asymmetric
Keys	One shared secret key	Public key (encrypt) + private key (decrypt)
Speed	Fast — bulk data	Slower — key exchange/signatures
Key distribution	Hard	Easy (public key shareable openly)
Examples	AES, DES, 3DES, RC4	RSA, ECC, Diffie-Hellman

Cipher types:

graph TD
    C["Ciphers"] --> S["Symmetric-Key"]
    C --> A["Asymmetric-Key"]
    S --> S1["Substitution<br/>(Caesar, Playfair)"]
    S --> S2["Transposition<br/>(Rail Fence)"]
    S --> S3["Block Cipher<br/>(AES, DES)"]
    S --> S4["Stream Cipher<br/>(RC4)"]
    A --> A1["RSA"]
    A --> A2["ECC"]
    A --> A3["Diffie-Hellman"]

Factors affecting cryptographic strength:

1. Key length — longer keys exponentially widen the brute-force search space (AES-256 > AES-128).
2. Algorithm maturity — peer-reviewed with no known weaknesses (AES vs the broken DES).
3. Randomness of key generation — a weak/predictable PRNG undermines any algorithm.
4. Mode of operation — e.g., ECB leaks patterns; CBC/GCM are stronger.
5. Resistance to known attacks — differential/linear cryptanalysis, side-channel attacks.
6. Key management — secure storage, rotation, and distribution.

Exam takeaway: cryptographic strength is key length and algorithm soundness and correct implementation together — a strong algorithm with a weak key or bad mode of operation is still weak overall.

Subnetting divides one large network into smaller logical sub-networks by borrowing bits from the host portion of an IP address, improving routing efficiency, security isolation, and shrinking broadcast domains. A subnet mask is the 32-bit value that separates an IP address into its network and host parts (1-bits = network, 0-bits = host).

Worked example — 172.16.0.0/19:

• 172.16.0.0 is a Class B address, whose default (natural) mask is /16.
• Subnet bits borrowed = 19 − 16 = 3 bits → subnets = 2³ = 8 subnets.
• Remaining host bits = 32 − 19 = 13 bits → usable hosts = 2¹³ − 2 = 8,190 hosts per subnet.
• Subnet mask for /19 = 255.255.224.0.

Subnet #	Network Address	Broadcast Address
1	172.16.0.0	172.16.31.255
2	172.16.32.0	172.16.63.255
...	...	...
8	172.16.224.0	172.16.255.255

OSPF (Open Shortest Path First): a link-state interior gateway routing protocol. Each router builds a complete map of the network (link-state database) by exchanging Link-State Advertisements with neighbors, then independently runs Dijkstra's algorithm to compute the shortest-cost path to every destination. It supports fast convergence, classless (VLSM) addressing, and hierarchical design via Areas (Area 0 = backbone).

Exam takeaway: subnets = 2^(borrowed bits), hosts = 2^(host bits) − 2; OSPF is the classic link-state IGP using Dijkstra's shortest-path-first computation within an autonomous system.