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Sep
7
comment Is this “layered” XOR cipher secure?
@paulo Ebermann - Exactly. What I really want to do is use the key as a seed to a PRNG but no PRNG is available so instead I built a really lame LFSR with really lame tap positions. Something better could be built by running a LFSR on the key with good tap positions but then I'm just reinventing a bad linear-stream cipher.
Sep
1
comment How can we reason about the cryptographic capabilities of code-breaking agencies like the NSA or GCHQ?
@D.W. Yes that is what I generally intended, with the idea that we can place attacks on some sort of quantifiable continuum (2^60, etc ...). Feel free to edit the question to clarify it.
Sep
1
comment How can we reason about the cryptographic capabilities of code-breaking agencies like the NSA or GCHQ?
@D.W. I meant what sort of lower bounds can we assume in regards to the ease of breaking the cipher, but I agree the wording is confusing, I have edited the question to clarify the issue.
Sep
1
comment How can we reason about the cryptographic capabilities of code-breaking agencies like the NSA or GCHQ?
@gokoon - are my edits in line with what you wished to ask with this question?
Aug
11
comment The use of cribs
@mixedmath in your defense, codes and code books are still used by militaries in all parts of the world. Breaking such code books are not particular interesting to academic cryptographers as the methods and techniques have already been researched exhaustively, but intelligence agencies still have to break code books.
Aug
11
comment Is it possible to derive the encryption method from encrypted text?
@Nayuki Minase - The functions themselves may by pseudorandom, but what about the increments, the other bits of information that no one likes to talk about. Can a block cipher running in 'ciphertext stealing' mode produce an odd number of bits, can it operate on the bit level rather than the byte level? What does it do if the input size is less than one block? How does it fail?
Aug
11
comment Is it possible to derive the encryption method from encrypted text?
@Nayuki Minase - Can you distinguish a stream cipher from a block cipher running in 'ciphertext stealing' mode?
Aug
5
comment What is the general justification for the hardness of finding preimages for cryptographic hash functions?
+1 Interesting. Are there any research papers that you would recommend that use proof of resistance to 3SAT to argue the strength of various hash functions/ciphers?
Jul
27
comment About Cryptography in a Character Language
Frequency analysis would still work as some words are more common than others (even in chinese).
Jul
20
comment Are any of the major asymmetric ciphers distinguishable (EG, RSA, ECC)?
If a given public_key can generate n possible ciphertexts for any message (this is what you mean by plausible ciphertexts right?), and the first bit of that plaintexts reduces the space of possible (plausible) ciphertexts, given that public_key, to n-1 doesn't that imply a bias in the possible (plausible) ciphertexts. If the first bit of the message does not reduce the number of possible ciphertexts I'm not sure how the process can possibly be reversible.
Jul
20
comment Are any of the major asymmetric ciphers distinguishable (EG, RSA, ECC)?
It's good to know that I'm not duplicating someone elses work. I'll probably write up my design as a blog post and not sully crypto.so with a hobby cipher. Thanks for your help and comments.
Jul
20
comment Are any of the major asymmetric ciphers distinguishable (EG, RSA, ECC)?
As a first glance designing a secure RSA block cipher with a CBC-like mode of operation does not seem to present any difficulties. Use a random secret value (a secret IV) as the input to the first block, then xor each input with the output of the last block (CBC mode). Sure the ciphertext is one block longer than the plaintext but you have your randomness and you only need to worry about your asymmetric cipher being hard to break (no symmetric weakness). Of course if the cipher has bias guessing the secret IV becomes becomes easier.
Jul
20
comment Are any of the major asymmetric ciphers distinguishable (EG, RSA, ECC)?
Is it unimportant in the sense that given two asymmetric ciphers where one cipher has bias and the other cipher does not you would not prefer one without bias over the other? AES is used because it is significantly faster than RSA, but it introduces more surface area since a break in either RSA or AES results in decryption of your message. Is the idea that RSA + AES is good enough born out of the fact that we don't have a fast unbiased asymmetric cipher?
Jul
20
comment Are any of the major asymmetric ciphers distinguishable (EG, RSA, ECC)?
Is bias required by any asymmetric algorithm? Is there a proof for this?
Jul
19
comment Is it possible to derive the encryption method from encrypted text?
I disagree. What about all the detail one can deduce from block sizes, encryption modes, relative key size to block size, padding method, ciphertext length, non-random plaintext, key reuse (a problem for OTPs), related keys, fixed points.
Jul
15
comment What is the general justification for the hardness of finding preimages for cryptographic hash functions?
@Paulo Ebermann - Consider a less efficient function LP, that is essentially a mapping of all $2^n$ possible outputs to inputs. Such a function can find preimages in O(1) time, but requires 2^n space. Now compress this mapping into less than 2^n. Thus, for every hash function H there must exist an inverter function that finds preimages cheaper than brute force. All hash functions are "broken" the trick is discovering the function that breaks the hash function. If such a preimage finding function is too big, discovering it by brute force is hard.
Jul
15
comment What is the general justification for the hardness of finding preimages for cryptographic hash functions?
@Paulo Ebermann - For most hash functions there is no claim that such an efficient preimage finding function (or collision finding function) can't exist, but rather that the cryptography has not discovered (hence contests like SHA3). My question is what makes finding such a function hard?
Jul
15
comment What is the general justification for the hardness of finding preimages for cryptographic hash functions?
@Paulo Ebermann - The minimum number of bits that is required to describe a function. For example machine code can be used to describe a particular mapping from one set of integers to another. Given a hash function H, that is encoded using language L, can you find another function P such that P inverts H in the same time/memory complexity as H and that P encoded in L is smaller than the output size of H (the encoded description of P is smaller than the output size of H).
Jul
15
comment What is the general justification for the hardness of finding preimages for cryptographic hash functions?
@Paulo Ebermann - I mean that if an efficient preimage finding function is smaller in number of bits than the output size of the hash function then one could guess the preimage finding function faster than one could guess the preimage thereby breaking the hash function. By efficient, I mean a that the preimage function finds the preimage in roughly the same time as the hash function generates an output (that is the preimage function is an inversion of the hash function and not just a program that guesses at preimages).
Jul
13
comment What is the general justification for the hardness of finding preimages for cryptographic hash functions?
What are the philosophical underpinnings of 'fuzzy' or 'cowboy' cryptography that attempt to explain why it is hard to break. While there is probably no known hard-science theory that explains why SHA2 hasn't been broken yet, there may be explanations in philosophy or other softer fields. What are the stories cryptographers tell themselves about why SHA2 provides resistance to attempts to break it?