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EECC: Symmetric Key Encryption

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Symmetric Key Encryption

Symmetric Key Encryption

The exam tests whether you can pick the right symmetric algorithm and mode for a scenario, state realistic key strengths, and explain why the shared-key model creates a distribution problem.

Algorithms you must recognize

  • DES - 64-bit block, 56-bit key, 16-round Feistel network. Exhaustible by brute force; the 1998 EFF "Deep Crack" machine settled it. Never approved for new work.
  • 3DES (TDEA) - Encrypt-Decrypt-Encrypt with two or three keys. Because of meet-in-the-middle, 3-key 3DES gives roughly 112 bits of security, not 168. Its 64-bit block invites Sweet32 birthday collisions on long-lived connections.
  • AES - 128-bit block, 128/192/256-bit keys, 10/12/14 rounds, substitution-permutation network. Selected by NIST in 2000 from the Rijndael proposal.
  • Legacy and alternatives - Blowfish (64-bit block, up to 448-bit key, no longer advised for large volumes), Twofish, IDEA, and RC4, a stream cipher whose keystream biases killed it in TLS.

Modes of operation

  • ECB - deterministic, no IV. Identical plaintext blocks produce identical ciphertext blocks, exposing structure. Only defensible for single-block key wrapping.
  • CBC - random, unpredictable IV; padding required; vulnerable to padding-oracle attacks when errors leak.
  • CTR - turns the block cipher into a stream cipher; the nonce/counter must never repeat under one key.
  • GCM - AEAD: confidentiality plus an authentication tag in one pass.

Worked example

A developer encrypts a payroll file containing 40 identical 16-byte records with AES-256-ECB and sees 40 identical ciphertext blocks; the record count and repetition leak. Re-encrypting with AES-256-CBC and a fresh random IV produces distinct ciphertext per record and hides the repetition. ECB was the fault, not the 256-bit key.

Traps

  • Believing 3-key 3DES delivers 168 bits.
  • Treating the CBC IV as secret - it must be unpredictable, not confidential.
  • Choosing a longer key to fix an ECB design flaw.
  • Forgetting that symmetric key distribution needs n(n−1)/2 keys for n parties.