Also known as: chosen-plaintext attack, CPA

A chosen-plaintext attack (CPA) gives the attacker the power to choose the plaintexts and observe the matching ciphertexts.¹ Controlled inputs are far more informative than passively observed ones: by varying a single character, length, or bit at a time, the attacker drives the cipher’s internal state deliberately and reads its response — making it strictly stronger than a known-plaintext attack.

Choosing inputs that differ in one place isolates how that change propagates — the densest possible view of the cipher's state machine.

How it works

The attacker designs input sets that hold most of the state fixed and vary one factor: a length sweep exposes how the cipher is seeded; a single-character sweep exposes the per-position transform; single-bit differences feed directly into differential cryptanalysis. Because the inputs are chosen, the attacker can systematically cover the state×input combinations a passive corpus never reaches, often pinning an internal table outright.

Relevance to SDR

For an over-the-air obfuscation, a chosen-plaintext attack means programming a transmitter with selected text and capturing the result on an SDR — only on systems and spectrum you are licensed or authorized to key up. It is the decisive lever where passive known-plaintext data stalls: GopherTrunk’s analysis of the Motorola P25 talker-alias obfuscation (issue #773) found the per-character update is sparsely covered by real callsigns, so a short controlled sweep would supply exactly the dense coverage needed to finish it.

Sources

1. Chosen-plaintext attack — Wikipedia, for the attack model and its advantage over passively observed plaintext. ↩

See also

• Known-plaintext attack
• Ciphertext-only attack
• Differential cryptanalysis
• Brute-force attack (exhaustive key search)
• RC4 (ARC4) cipher