Reid

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I am an undergraduate university student double majoring in Computer Science and Mathematics hoping to pursue a research career in cryptography. Other interests include software engineering and Linux-based system administration.

	716 reputation	bio	website		visits	member for	11 months
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32 Comments

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Jun 14	comment	OpenSSL AES 256-bit Key Management @Gilles: I said "somewhat" questionable, not "somewhat questionable". :)
Jun 13	comment	Encrypting and MACing different data with same key @B-Con: The way I interpret this question, he isn't asking whether or not you should encrypt and MAC the same data with the same key. Rather, he's asking is it okay if you MAC some data and then encrypt some other data with the same key.
Jun 13	comment	Is it possible to cryptographically prove when was the last time a ciphertext was decrypted/encrypted? Can you control access to the ciphertext? (That is, can you log when people downloaded it, for example?) If so, then you could use write-once memory to log who downloaded it and when, and if your authentication includes knowledge of the key, then you can assume it was decrypted when downloaded. That seems a fairly safe assumption. After all, once the ciphertext leaves your system, your options become much more limited.
Jun 13	comment	Why is it impractical to generate a semiprime dictionary? @PaŭloEbermann: I was envisioning storing each 2048-bit semiprime plus the two factors, each around 1024 bits.
Jun 12	comment	Why is it impractical to generate a semiprime dictionary? @CodesInChaos: Oh, come on, that's only $2.743 \times 10^{279}$ yottabytes! (Just to store the semiprimes, that is. You'd need approximately double the space to store the factors too.)
Jun 11	comment	Can any one explain Circuit Privacy using fully homomorphic encryption from Gentry's thesis? To be brutally honest, it is a Ph.D. dissertation. That document earned someone a doctoral degree at a top university. I would not expect it to be easy to read. That being said, what is it exactly that you are having trouble with? Are you having problems with the definition of circuit privacy, or is it how Gentry transforms his scheme into a circuit-private one? Or both?
Jun 5	comment	Distributing blocks with validation and non-dependant list generation Ha! Don't worry about it. We all have our moments.
Jun 5	comment	Encrypt-then-MAC paradigm @MartinLundberg: If you're going to use HMAC for authentication, I'd probably go with AES-CTR or AES-CBC. People seem to love CBC, but I prefer CTR. Alternatively, if you'd like to skip out on the HMAC bit, you can use one of the AEAD modes of operation (like GCM). These provide authentication along with confidentiality, which is really quite nice.
Jun 5	comment	Distributing blocks with validation and non-dependant list generation If you're concerned about nodes maliciously modifying the data, are you also wanting confidentiality (e.g. encryption)? Something to think about, anyway.
Jun 5	comment	Distributing blocks with validation and non-dependant list generation The scheme I would use is $$b_n = n \|\| c \|\| \operatorname{MAC}_k(n\|\|c)$$ where $b_n$ is the block in question, $n$ is the index of the block, $c$ is the contents of the block, and $k$ is a symmetric key for the MAC. Embedding $n$ lets you quickly order messages, which is nice, since missing blocks don't screw everything up (necessarily). I can't think of any real advantage in using the MAC for a different block in the current block. But I may be missing something.
Jun 4	comment	HMAC and assumptions on the cryptographic hash @Gilles: The structure of the above construction is entirely different from usual hash schemes. How would you define the compression function for $H'$? And note that in order for NMAC's security proof to be relevant, the scheme needs to be iterated, so you somehow have to define the compression function in such a way that the final state will include the first $n$ bytes of the message. Of course, the compression function needs to be a PRF, but leaking even the first byte of the message sounds like a death knell for that idea.
Jun 4	comment	HMAC and assumptions on the cryptographic hash The last part of this answer is the crucial bit: HMAC's security proof relies iterated hash functions, specifically MD constructions. The construction in the question fails this criterion.
Jun 4	comment	Is entropy affected by time? @Jaay: you might say security against brute-force attacks is a function of entropy and time, but that doesn't mean entropy is a function of time. Entropy is associated with the process of selection. That's why it only makes sense to talk about the entropy of a random variable.
Jun 3	comment	Can i modify data “protected” by a CRC16? @fgrieu: Interesting! I wasn't aware of that. Fortunately, as you said, it doesn't seem to affect the basic gist of the above.
Jun 2	comment	Can i modify data “protected” by a CRC16? I hope my answer is demonstrative of the fact that CRC cannot be used to protect against an intelligent adversary. It's fantastic for transmission errors, but if you need this sort of protection, use a MAC instead (like HMAC).
Jun 2	comment	Pen-and-paper one-way function for externally-anonymous survey That's a really interesting technique. +1
Jun 2	comment	Best way to create password manager @ttouch: You would be mistaken. scrypt and bcrypt are key derivation functions and aren't used (directly) for encrypting files. No offense, but I think you would be better off sticking with a well-known password manager.
May 31	comment	Reversing SHA1 (don't know the correct term) @SmitJohnth: I did too, but it reared its head when I tried to implement the attack I described in the earlier revision of the post.
May 31	comment	Encrypt a file once with 50 characters password or twice with 25 characters? @user3321: 50 char password
May 31	comment	Reversing SHA1 (don't know the correct term) @SmitJohnth: Agreed! I didn't realize just how long it had gotten. I've now summarized the key points of my previous post, and I think the result is much better. It actually fits on one screen now.