[midas007]

Pretty hilariously wrong, and you know it.

Supposed OTP constructions are defined as

e(i) == E(...) ^ m(i)

m(i) == D(...) ^ e(i)

where E(...) = D(...)

and where ... doesnt contain any of the following

e(j) for any j

m(k) for any k

j and k in same domain as i

Then, take a look at CTR...

CTR is E(i) = blockcipher(key, nonce . i) and D(i) = E(i)

e(i) == blockcipher(key, nonce . i) ^ m(i)

m(i) == blockcipher(key, nonce . i) ^ e(i)

(i == counter, since it's the same in this example where counter and blocks start at the same number)

Therefore CTR is an OTP.

[schoen]

CTR isn't an OTP in the classic sense of OTP, because you rely on the security of blockcipher. For example, if you used blockcipher=single DES, the attacker can break the cipher by breaking single DES by brute force.

Indeed, even if blockcipher=AES256, the attacker can still break CTR by merely guessing key in 2²⁵⁶ operations. (Likely only one such value of key will yield meaningful plaintext throughout the entire multi-block message.) That is contrary to the information-theoretic security property of OTP, where the attacker can't tell whether they've correctly guessed the key.

More to tptacek's point, if you're using the block offset as i, then if you write the same block 30 times, you used the same value blockcipher(key, nonce . i) each time. That isn't a one-time use of that part of the pad, it's a 30-time use of that part of the pad. It's extremely possible that an attacker who has observed all 30 ciphertexts can actually decrypt many of them in combination. In Boneh's Coursera class, we did it successfully with like 4 or 5 ciphertexts, and I've seen a paper that describes doing it automatically for the majority of the text with only two ciphertexts, assuming the plaintext is English written in ASCII.

[midas007]

That's trivial to add on, outside of CTR.

You have a system of keys derived from a master key. Too many bytes encrypted with one key? Use a new key for subsequent writes.

(And for god's sake use a PBKDF to derive a master key from a password, don't memcpy() it directly.)

[lvh]

That looks like the definition of a symmetric stream cipher, not OTP. You're missing the part where the OTP keystream has to be truly random. The output of a block cipher in CTR mode is not truly random.

[midas007]

Indistinguishable from a PRF A good block cipher satisfied this property, otherwise it's not a PRF and insecure.

Hair-splitting, really. Actual OTP is an imaginary construction that requires an endless supply of truly random bits that have to be securely stored or somehow recreated during decryption. It shifts the hard part to that fn, and just XORs the result with the pt or ct block.

[tptacek]

No.