janagiram-kumar/cns-new

Purpose and Execution Model

This is a collection of standalone Java cryptography exercises, organized by assignment (EX1–EX7). There is no shared application framework, build system, README guidance, or central launcher. Each class is expected to be compiled and run independently, likely through its own main method and console input/output.

Number-Theory Foundations

EX1 contains the arithmetic primitives and primality testing used conceptually by later public-key exercises:

• Euclidean GCD variants: EX1/GCD.java, EX1/GCDE.java

• Extended Euclidean algorithm: EX1/ExtendedEuclid.java

• Euler totient computation: EX1/EulerTotient.java

• Miller–Rabin probabilistic primality testing: EX1/MillerRabin.java

These programs establish modular arithmetic, coprimality, inverses, and prime validation boundaries needed for RSA, Diffie–Hellman, and ElGamal.

Classical Cipher Implementations

EX2 implements substitution and polyalphabetic cipher workflows, generally transforming plaintext to ciphertext and reversing the process from user-provided keys:

• EX2/CaesarCipher.java

• EX2/VigenereCipher.java

• EX2/PlayfairCipher.java

• EX2/HillCipher.java

• EX2/OneTimePad.java

The main boundary is text normalization and key handling: these algorithms operate on character sequences rather than shared domain models or persisted data.

Transposition Cipher Stage

EX3 isolates positional rearrangement ciphers:

• EX3/RailFenceCipher.java

• EX3/RowColumnTranspositionCipher.java

Unlike substitution ciphers, these preserve character identities while changing ordering. They are independent console-oriented implementations, with no indicated integration into the EX2 cipher classes.

Symmetric-Key Demonstration

EX4/SDES.java implements Simplified DES, representing the repository’s block-cipher / symmetric-encryption exercise. Its architecture is expected to center on bit-level permutation, key scheduling, round functions, and encrypt/decrypt control paths, rather than production DES interoperability.

Public-Key Key Agreement and Encryption

The public-key exercises separate major cryptographic roles:

• EX5/RSA.java covers RSA key construction and modular-exponentiation encryption/decryption.

• EX6/DiffeHellman.java models Diffie–Hellman key agreement between parties.

These components depend on externally supplied or generated numeric parameters; no certificate authority, keystore, network transport, or secure randomness infrastructure is represented in the tree.

Digital Signature Workflow

EX7/ElGamalSignature.java implements ElGamal signing and verification. It forms the repository’s authentication/integrity stage: private-key-based signature generation and public-parameter-based verification. Its trust boundary is the supplied message, key material, and domain parameters; there is no external identity service or persistence layer.

Runtime and Infrastructure Boundaries

The material runtime is Java source compiled directly with a JDK. The repository has no declared package hierarchy, dependency manager, test suite, service runtime, database, network API, or external cloud/service integration. Each exercise is therefore an isolated educational executable rather than a deployable cryptographic system.