Cryptographic Algorithm Implementation Questions

Skills and knowledge for correctly implementing cryptographic algorithms and primitives in code. Candidates should be able to translate algorithm specifications and mathematical definitions into correct implementations, handling binary data layouts, bit operations such as shifts and exclusive or, and large integer arithmetic required for modular operations. Expect to implement core operations and components including Advanced Encryption Standard encryption rounds, Rivest Shamir Adleman modular exponentiation, Secure Hash Algorithm 256 message scheduling, substitution boxes in block ciphers, mixing functions, substitution and permutation operations, and simplified cipher operations for demonstration. Understand appropriate data structure choices and their performance and security implications, including constant time considerations, endianness, padding rules, and proper randomness. Implement and verify against official test vectors and build verification procedures and test harnesses. Emphasize correctness and clear explanation over premature optimization and recognize when to prefer well vetted cryptographic libraries to custom implementations.

MediumTechnical

61 practiced

Implement a single AES-128 encryption round in Python: SubBytes, ShiftRows, MixColumns, and AddRoundKey. Input: 16-byte state and 16-byte round key. You do not need to implement key expansion or the final round special case. Provide the MixColumns implementation over GF(2^8) and test against a simple known vector.

MediumTechnical

83 practiced

Scenario: you find a legacy codebase that generates cryptographic keys using the C library rand() seeded with time(). Describe the steps you would take to assess impact (inventory affected keys), remediate (rotate keys and migrate to a CSPRNG), design a migration plan that minimizes downtime, and validate the remediation. Include tests and monitoring you would add.

MediumTechnical

68 practiced

Implement the SHA-256 message schedule (W[0..63]) for a single 512-bit block in Python. Input is a 64-byte block in big-endian byte order; output should be a list of 64 32-bit unsigned integers. Use right rotations and shifts as specified by the standard, and explain how you handle endian conversion and 32-bit wrap-around in Python.

EasyTechnical

73 practiced

Implement rotate-left and rotate-right operations for 32-bit and 64-bit unsigned integers in C with well-defined behavior for all rotate amounts (including 0). Provide portable implementations and mention how you would use compiler intrinsics or assembly for performance when available.

HardTechnical

62 practiced

Implement RSA decryption/signing using CRT optimization and RSA blinding to mitigate timing attacks. Given CRT parameters (p, q, dP, dQ, qInv) and public exponent e, create a function that takes ciphertext c and returns plaintext m using blinding before CRT exponentiation and proper unblinding. Explain blinding mechanics, why it helps, and what additional checks you would add to detect or mitigate fault-injection attacks.

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