Ilmari Karonen
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 Oct 26 comment Can a monoalphabetic substitution cipher attain perfect secrecy? (In any case, honestly, if you understand what "monoalphabetic substitution cipher" and "perfect secrecy" mean, what I posted above should essentially answer the question. All I really left out was the single word "yes/no" -- you'll have to spend a few seconds thinking for yourself to see whether my answer is a proof or a disproof of perfect secrecy.) Oct 26 comment Can a monoalphabetic substitution cipher attain perfect secrecy? @arsaKasra: There's no law against it, but many consider it poor form to spoon-feed complete answers to people asking homework questions. After all, the purpose of homework is to teach you how to figure out answers to such questions yourself. Now, if you honestly have the same problem, and need the answer for some purpose other than homework (although, for this particular question, I can't really imagine what it could be), let me know, and I'll be glad to amend my answer. Oct 23 comment Is it possible to combine cryptography with steganography Alternative "solution": Generate a bunch of watermarked versions Ms of the message M, encrypt each of them with the corresponding recipient's key (you can use standard symmetric or public-key crypto here) and combine all the encrypted messages into Me. Each recipient just extracts the piece of Me that is encrypted with their key, and decrypts it. Obviously, this produces a very long Me if there are lots of recipients, but otherwise it seems to meet your requirements. Oct 23 comment Is it possible to combine cryptography with steganography If each key only needs to be able to decrypt one message, why not put the watermarked message for each recipient in their "key", and just use a dummy "message"? (OK, one possible reason is if you need to be able to generate the keys before you know the message.) Oct 23 comment Is it possible to combine cryptography with steganography I assume there's an unstated requirement that Alice should be able to send the same encrypted message to multiple recipients, each having their own decryption key, and have them each obtain a distinct watermarked copy when they decrypt it? And presumably also that each decryption ḱey can be safely used to decrypt arbitrarily many messages, without allowing the watermark to be removed? Is it also possible for the recipients to collude to defeat the watermark? (In reality, it generally is, but it tends to make designing reliable watermarks much harder.) Oct 22 comment Is there an existing cryptography algorithm / method that both encrypts AND compresses text? @supercat: The closest thing I can think of, off the top of my head, is the use of straddling checkerboards in some WWII-era and later Soviet ciphers. But those ciphers had numerical ciphertext anyway, so they weren't particularly quick to transmit by Morse anyway. Oct 21 comment Is there an existing cryptography algorithm / method that both encrypts AND compresses text? @RickyDemer: You don't even need digital signatures for that. Just take any secure block cipher with a $k$-bit block size, and a (privacy-preserving) $k$-bit MAC, and define your encryption function to apply the block cipher if the input is exactly $k$ bits long, and the MAC otherwise. Then applying the inverse block cipher will, given any output, always yield a matching input. Oct 5 comment Generating unbiased numbers with a biased six sided die? I'm voting to close this question because it's about general mathematics without any specific cryptographic elements. Such questions are off-topic here, but may be asked on Mathematics. (Mind you, they don't tend to like "do my homework for me" questions there, either.) Oct 3 comment Is it possible to create “non overlapping” RNGs? +1. Also, if you need the outputs to lie in a range that doesn't conveniently match the block size of any standard block cipher, you can use the usual FPE tricks to construct a cipher with a suitable range. Sep 26 comment Signing and verifying message consisting of several parts @Mr.Vendetta: To verify message part $m_i$, given the master hash $h$, you first load the hash list $(h_0, h_1, \dots, h_n)$, check that $H(h_0 \| h_1 \| \dots \| h_n) = h$, and then check that $H(m_i) = h_i$. That's a lot faster than loading and hashing the entire message $m = (m_0, m_1, \dots, m_n)$. Sep 23 comment How can AES + cycle walking be used as an FPE? @erotavlas: No, not if you want to keep the system format-preserving. Sep 17 comment Does $(u_1=r_1\cdot x,\ \ w_1=r_1\cdot z, \ \ u_2=r_2\cdot x,\ \ w_2=r_2\cdot z)$ Leak Information? @fgrieu: That's a good suggestion. I've edited the question and my answer to do just that. Sep 17 comment Symmetric stream cipher, is it known? I can't really know, but I do have a hunch. We do, with some regularity, get "questions" here that basically go "Hey everybody! I've invented this awesome new cipher, isn't it great?" Usually, those get downvoted (and often closed, since we have a policy against asking for cryptanalysis of homebrew ciphers). I don't really think your question is one of those, or at least not a typical member of the bunch, but it does look similar at a glance. I can see how someone might have downvoted it on that basis. Anyway, voting (both up and down) is really up to each individual voter. Sep 17 comment Symmetric stream cipher, is it known? It's basically a combination of several (binary) Vigenère ciphers, applied consecutively. I don't know if it has a specific name, though. Sep 17 comment Does $(u_1=r_1\cdot x,\ \ w_1=r_1\cdot z, \ \ u_2=r_2\cdot x,\ \ w_2=r_2\cdot z)$ Leak Information? What are you trying to accomplish with this? In particular, which of the values $x$, $z$, $r_1$ and $r_2$ do you wish to keep secret, and why? Sep 17 comment Obtain 384 bits from a 256bit MasterSecret Yes, or derive the IV from something unique that you do send with the message, such as a message ID. (In fact, GCM accepts IVs of any size, so you can just use the message ID directly as the IV if you want.) Also, if two (or more) users are using the same key to encrypt messages, you'll need to ensure that they don't use the same IVs, e.g. by having one of them only use even IVs, and having the other use only odd IVs. Sep 17 comment Obtain 384 bits from a 256bit MasterSecret That's a good choice. That said, HKDF is pretty easy to implement yourself. Sep 16 comment Picking a nonce in the context of CCM (CTR with CBC-MAC) mode There is no need to encrypt the nonce. Sep 16 comment Picking a nonce in the context of CCM (CTR with CBC-MAC) mode Typically, you would either send the nonce along with the message, or derive it from other unique information associated with the message, such as a message ID. Sep 16 comment apprSVP in lattices Hi, preethi, and welcome to Crypto Stack Exchange. I've copyedited your question a bit to hopefully make it clearer and easier to understand. Could you please check that I didn't accidentally introduce any errors while doing so? If you do find anything that you think should be corrected, please feel free to edit your question further yourself. Thanks!