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3 months ago in Quantum Computing By Shashank

Can quantum computers break the one-time pad (OTP) cipher?

I'm analyzing encryption methods for sensitive, long-term data archiving. The one-time pad is theoretically proven secure, but all proofs assume a classical adversary. With the advent of quantum computing, I need to be absolutely certain: do quantum algorithms like Shor's or Grover's introduce any vulnerability to a correctly implemented OTP, or does its information-theoretic security hold firm even against a quantum adversary? I'm specifically concerned about the core cryptographic principle, not implementation flaws

All Answers (2 Answers In All)

By Maninder Answered 1 month ago

No, a correctly implemented one-time pad (OTP) remains provably unbreakable by any computer, classical or quantum. This is not a matter of computational difficulty but of information-theoretic security.

The core principle of the OTP—that the key is truly random, at least as long as the plaintext, and used only once—ensures that the ciphertext reveals zero information about the original message. An attacker with infinite computational power, including a quantum computer running Shor's or Grover's algorithm, still faces an infinite number of equally probable plaintexts for any given ciphertext. There is no mathematical relationship for a quantum algorithm to exploit.

Therefore, the security of the OTP is not threatened by quantum computing. The practical vulnerabilities of the OTP (key distribution, true randomness, and perfect secrecy) remain the same as in the classical world.

Replied 1 month ago

By Shashank

Thanks, that makes a lot of sense! So even the perfect cipher can fail if keys aren’t handled correctly. definitely something to be careful about.

By Binita Sinha Answered 1 month ago

No. The one-time pad is information-theoretically secure, meaning its security does not depend on computational difficulty. Even a perfect quantum computer cannot break an OTP as long as the key is truly random, kept secret, at least as long as the message, and never reused. Under these conditions, the ciphertext contains no exploitable information about the plaintext, making the scheme secure against all adversaries—classical or quantum.

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