Also known as: codebreaking
Cryptanalysis is the study of analyzing and breaking cryptosystems — recovering the plaintext, the key, or the internal structure of an algorithm without being handed the key.1 It is the analytic counterpart to cryptography: one side builds, the other probes.
How it works
Cryptanalysts almost always assume Kerckhoffs’s premise — that the algorithm is known and only the key is secret — so the work is to exploit any structure the cipher leaks. Classic attacks are named by what the analyst can obtain:
- Ciphertext-only — only intercepted ciphertext is available, as in classic frequency analysis of a substitution cipher.
- Known-plaintext — some plaintext/ciphertext pairs are known.
- Chosen-plaintext — the analyst can encrypt inputs of their choosing.
- Brute force — exhaustively trying keys, bounded by the key space.
Deeper techniques attack a cipher’s mathematics directly — differential and linear analysis, algebraic modeling, and solver-driven approaches that treat recovery as a constraint problem. A scheme that relies on the method staying secret is not really analyzed but merely reverse-engineered; that is obfuscation, not encryption.
Relevance to SDR
Cryptanalysis in the radio world is mostly about understanding formats, not breaking strong ciphers. Recovering an undocumented on-air transformation — framing, scrambling, or an obfuscation layer — is a cryptanalytic exercise carried out entirely from public observation. GopherTrunk’s clean-room work on the Motorola P25 talker-alias scheme (issue #773) is a worked example: candidate models such as shift-register and round-function update rules were hypothesized and tested against captured data until the actual substitution table emerged, all without reference to any third-party source code. Properly encrypted voice — P25 AES-256 or DMR RC4 — is not in scope: the math is sound and the key is absent.
Sources
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Cryptanalysis — Wikipedia, for attack models and the assumption that the algorithm is public. ↩