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12

What happens if the sender is at another point in the sequence? ... the key is pressed while out of range to the car. In a rolling code (code hopping) system, the keyfob transmitter maintains a synchronization counter C, incremented every time a button is pushed. The car receiver stores the most recent validated synchronization counter it has received ...


7

As @D.W. guessed, the branching program for a circuit essentially reveals the original circuit. It's not clear what you mean by "apply the whole obfuscation process to the circuit-revealing branching program," but the prospects for that do not seem good: evaluating the branching program is highly sequential (polynomial depth), and you would need to ...


6

SSL was designed long ago when encrypt-then-MAC wasn't that popular yet. Even TLS 1.2, published in 2008, is pretty old by now, and while encrypt-then-MAC was preferred by then, the practical risks were underestimated for a long time. Padding oracles attacks became well known after several high profile attacks in 2010. With stream ciphers, MAC-then-encrypt ...


5

When they write “well-typed”, they’re simply stating that the process $P$ is well-typed in context, or type environment. (Where the type environment contains a set of type assumptions occurring in $P$.) Keeping it simple: you can think of the term as a kind of classification. The term origins in Type Theory and is (more-or-less frequently) used in relation ...


5

For the moment assume $g$ is a secret (uniformly random) generator, but that $p$ may be known to the adversary. Then given only $g^a, g^b$, the Diffie-Hellman key $g^{ab}$ is information-theoretically uniform (up to small statistical error), i.e., it cannot even be found by brute force because the adversary does not have enough information to determine it. ...


5

No, this protocol does not provide perfect forward secrecy. Record the initial key transport message (shared via RSA-OAEP). If the attacker later gets access to the corresponding RSA private key, and decrypts the original key transport message, the entire symmetric key evolution sequence for that session will trivially unfold.


5

Designing your own crypto protocol (using existing primitives) is dangerous if you're not sufficiently familiar with cryptographic protocol design and the ways such protocols might be attacked. If you wish to gain such familiarity, I'd recommend taking a few introductory crypto courses that focus on protocol design and analysis.* This won't turn you ...


4

I would say there are three general areas of necessary expertise for most crypto-related jobs: Knowledge of primitives and their use cases. Knowledge of protocols and understanding how to reason about their security. Deep and abiding understanding of how incredibly stupid people are, including oneself. The most that knowing the math is going to do for ...


4

I am wondering if using Skein or the Keccak hash algorithm in this construction (as a stream cipher) is secure: In the case of Skein and Keccak it should be secure. However, both of those have defined their own cipher modes which you should IMO prefer. (For speed and compatibility, if not security.) The Skein one is defined in section 4.10 of the ...


4

I would generate this key, then encrypt it in such a way that it would take years (but not decades!) to crack, then release it publicly. Yes, you are in effect putting the master key in a time capsule. The problems of time capsules in general apply: the release time will not be exact and a breakthrough in e.g. CPU design could hasten it. If no one's ...


4

Canetti ("Towards realizing random oracles," Crypto 1997) gave a reasonably efficient (very efficient, by the standards of most obfuscation work) "virtual black-box" obfuscator for "point functions," i.e., functions of the form $I_x(y) = 1$ if $x=y$, $0$ otherwise. Such functions can be used, e.g., for password checking. Virtual black-box obfuscation ...


4

“Well-typed” relates to a type system. This is a general concept in computer science, the usage here is an example of the general concept and is not specific to cryptography. “Well-typed” does not refer to a cryptographic protocol, but to a theory (model) in which a protocol is described. A type system is a way to assign properties (called types) to ...


3

Pairings in cryptography is a very important tool, the introduction of which has developed a new field, that is pairing-based cryptography. After the independent pioneering work by Joux and by Sakai et al.("Cryptosystems based on pairing"), many pairing-based crypto-systems emerged. In cryptography, pairings are often treated as "black-box", and then we ...


3

The answer depends on how you would layer the encryption on top of the existing protocol. If you implemented your own Skype client, you could deal with compression issues yourself. That might allow you to use format preserving encryption, perhaps on the compressed data stream and not the audio itself. However, you would need to be careful – speech ...


3

If you want $N$ serial numbers, your serial numbers will have to use $n$ bits for uniqueness, where $n = \log_2 N$. So if you have 100 bits to use for the serial, you could use 20 to get about a million serials and have 80 bits to use for a cryptographic MAC or signature. Now there are two approaches, the symmetric and the asymmetric. In the symmetric ...


3

In a rolling code both the sender and the receiver always move forward in the sequence. If the sender has sent the $n$th code, then it will send the $(n+1)$th next. Contrarily, if the receiver has seen the $n$th code it will only accept the $(n+1)$th code or some later code. What happens if the sender is at another point in the sequence? Think of that ...


3

If you aren't worried about collusion or dynamic group membership, then a very simple solution is to simply have one key for encrypting the messages and another for signing them. The encryption key gives someone read access and the signing key gives them write access. Only nodes with the encryption key will be able to successfully decrypt the messages and ...


3

Asmuth and Blakley provided a proof that, assuming the keys for each cryptosystem are chosen independently, breaking their composite cryptosystem is at least as hard as breaking the hardest part of either. [1] Building on their work, cascade ciphers have been shown to in fact be harder to break than the hardest part of either. Admittedly, what you're ...


3

In your example, $Encryption_1$ is $\textsf{AES}_{CTR}$ and $Encryption_2$ is $\textsf{Salsa20}$. Then, the encryption method you are proposing is $Encryption_1(Encryption_2(plaintext))$, which is in fact a cascade of stream ciphers. Note that, because you simply XOR the streams, this cascade cipher commutes, that is, you will have the same result if you use ...


3

I may be interpreting your question incorrectly, but it sounds to me like you are asking if Caroline can prove (in court or whatever) that she can only gain access to some secret $S$ if both Alice and Bob collaborate in revealing it to her. Unfortunately as you have currently set up the question, I don't think that is possible, because your question ...


2

Yes, that is possible -- that's exactly the problem that secure multiparty computation solves. You should start by reading standard references on secure multiparty computation. You might enjoy the following paper, and follow-on work: Secure Multiparty Computations on Bitcoin, Marcin Andrychowicz and Stefan Dziembowski and Daniel Malinowski and Łukasz ...


2

Choose a public key encryption scheme, where $(s(x||r), p(x||r))$ is a key-pair derived from $x||r$ using a cryptographic hash function so that $s(x||r)$ is invertible. To make a commitment $C(x, r)$, Alice derives the key-pair and publishes the public key $c = p(x||r)$. If she reveals $x$ and $r$ then anyone can see if the same key-pair can be derived: ...


2

If there is _______________ obfuscation scheme for such $\hspace{.03 in}f$s then there is a scheme that does what you describe, where _______________ is either "a differing-inputs" or "an extractability", depending on how you define "doesn't know Alice's secret". Note that if there is no a-priori bound on the length of $x$, then the $\hspace{.03 in}f$s ...


2

In my experience the persons doing the standardization may not know about formal methods in the first place. And even if a formal method was used, they would not know how to assess it. Note that whatever mathematical method is applied, the security of a protocol is still dependent on how the domain was modelled. If the model is even slightly incorrect, a ...


2

Well, hope that it's not late for this answer. Because it was yesterday that I encountered this problem and I'm new to this wonderful website. According to your description, and as far as I know, this protocol meets your demands very well. First, it works with RSA as you have mentioned in the second paragraph. The original version of this protocol is ...


2

The answer to the first question is both. TLS uses a custom PRF based on HMAC to generate symmetric and MAC keys from a shared secret. The shared secret is created during the asymmetric key exchange between client and server as part of the handshake. The PRF generates key material of a required length. That length is determined by the key sizes and the key ...


2

One algorithm that is especially suited to one-use key pars is lamport signatures. Like many (all?) other signature functions, lamport signatures first hash the message to get it down to a size that is more reasonable to sign. For this use case, if you are willing to have $n^{2}$-bit signatures and $2n^{2}$-bit keys (public and private), you can sign a ...


2

The purpose is to prevent a two-for-one guessing attack, where an active adversary, impersonating the server, can test two password guesses per attempt. The attack and why the multiplier prevents it is described in Section 2 of the SRP-6 paper (ps). (According to MacKenzie, it was discovered by Bleichenbacher.) In brief, the attack goes like this: Instead ...


2

At one point, most music legally sold digitally was protected by DRM (all iTunes music, for instance). Eventually the labels backed down and started allowing the music to be published DRM-free. So yes the music industry has attempted this, but it encountered all of the fundamental problems with DRM and was abandoned. Crypto fundamentally can't protect you ...


2

I don't think there's an exact "correct" behaviour in this case. It would be up to the implementation to decide, since the spec is only concerned about the DER encoded portion. If your implementation parses the input as it moves along only, and doesn't concern itself with the overall size, then it would work fine. Having said that, I believe the best ...



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