# Tag Info

## Hot answers tagged diffie-hellman

12

Let's assume that everyone agreed on some elliptic curve and a public base point $g$ somewhere on the curve. When two parties Alice and Bob want to agree on a shared secret, they proceed as follows: Alice chooses some random number $a$ and applies the curve operation to $g$, the public base point, $a$ times. She obtains some result ...

10

$g^x \cdot g^y \;\;\; = \;\;\; (\hspace{.02 in}g\cdot g\cdot g\cdot \ldots$ [$\hspace{.02 in}x$ of them] $\ldots \cdot g\cdot g\cdot g) \: \cdot \: (\hspace{.02 in}g\cdot g\cdot g\cdot \ldots$ [$\hspace{.03 in}y$ of them] $\ldots \cdot g\cdot g\cdot g)$ $= \;\;\; g\cdot g\cdot g\cdot \ldots$ [$\hspace{.02 in}x\hspace{-0.05 in}+\hspace{-0.05 in}y$ of them] ...

7

If the DDH is hard in a group $G$ with generator $g$, then it is hard to decide given $(g,g^a,g^b,g^c)$ whether $ab\equiv c\pmod{ord(G)}$. If you take as $G$ the group $Z_p^*$ of order $p-1$ with $p$ being prime, then you will have $(p-1)/2$ elements being quadratic residues ($QR$) and the other half being non-quadratic residues ($QNR$). Now, we know that ...

7

A generator of a finite group is a value $g$ such that all elements of the group can be represented as $g^k$ for some integer $k$. Another key of looking at it is that if we consider the sequence $g,\ \ g \cdot g,\ \ g \cdot g \cdot g, ...$, saying $g$ is a generator means that all values in the group will appear somewhere in the sequence. Now, when it ...

7

Yes, you are correct. The simplest way without stepping outside NaCl would be to have both create an ephemeral, random crypto_box_keypair, then exchange public keys using their long term keys. Further communication would use that new keypair for crypto_box during that session. After they are done with the session, delete those ephemeral keys from memory. ...

7

There are actually only 5 unique $x$-coordinates one needs to be concerned about: $(0, \ldots)$ $(1, \ldots)$ $(-1, \ldots)$ $(x_1, \ldots)$ $(x_2, \ldots)$, where $$\begin{eqnarray} x_1 =& 393823572354896145817230607815530211125 \\ & 29911719440698176882885853963445705823 \end{eqnarray}$$ and \begin{eqnarray} x_2 =& ...

6

This expands CodesInChaos's comment into an answer. Forward Secrecy (that is, maintaining confidentiality of messages enciphered before compromise of the long term key) can be achieved in a protocol using a public-key signature scheme with a long-term public key, and a public-key encryption scheme with a per-session key; but in the case of RSA signature and ...

6

It is equivalent to the computational Diffie-Hellman problem; if you can one of the two problems, you can solve the other (with a polynomial number of queries to the oracle which solves the other). If you can solve the Diffie-Hellman problem, you can solve your problem: this can be seen by first noting that, with a Diffie-Hellman solver, given $g^b$, you ...

5

The problem doesn't lie with curves in Weierstrass form necessarily, but with naive implementations of elliptic curve arithmetic on such curves. Basically, if you implement an ECC scheme (ECDH, ECDSA or whatever) on a smart card using a curve in Weierstrass form in the most straightforward way possible (by writing a simple double-and-add loop for ...

5

It's an element of the field of all integers modulo $p$, and these are represented by the numbers $0,\ldots, p-1$. And $g$ will be one of them.

5

$\mathbb{Z}^*_{13}$ is a group with 12 elements, not 13. A group is defined by a set of elements, and a "law". The law combines two elements and yields a third one within the set. You get a group if the law fulfils some properties (the law is associative, there is a neutral element, each element has an opposite in the group). $\mathbb{Z}_{13}$ is a group ...

5

Update: My previous answer, although technically true, didn't answer your question. The issue is that the strong-DDH is not hard when using pairing groups, so my answer was merely stating that your problem is at least as hard as an easy problem (duh!) After some thought, I realized your problem is at least as hard as the 2-weak Bilinear Diffie-Hellman ...

5

It does not; the equation holds for any element $g$. The fact that $g$ is a generator means only that every element of the group can be obtained a key. This is not at all necessary for the protocol.

4

Given a EC public key, can a different, but plausible and functional private key be derived to match the public key? No, a public key will correspond to only one private key (with one minor exception, which I will explain below). With Elliptic Curve systems, the private key is an integer $d$ between 1 and $q$ (the order the generator point $G$), and ...

4

When using a Discrete Logarithm based scheme, such as SRP, the rule of thumb is to always use private exponents with a bit length twice the desired security strength. Hence, a 128 bit exponent $a$ will at most give you 64 bits of security. If you want 128 bit security, you need (at least) a 256 bit exponent. This is because the algebraic structure of the ...

4

What we have to show for random self reducibility is that we can reduce an efficient algorithm for solving an arbitrary (worst-case) instance to an algorithm that solves a random instance efficiently. Consequently, an efficient algorithm for the average case implies an efficient algorithm for the worst case. You already have outlined how this is ...

4

No, DHE is secure and allows to share a common secret between two parties over an insecure channel. But you cannot know, if the one you share the secret with is the one you want (DHE is vulnerable to man in the middle attacks). So DHE-RSA uses DHE to share a common secret and signs the communication with RSA to make sure, that both persons communicate with ...

4

The encryption of the signatures $\;$ keeps the identity of the initiator (Alice) confidential, even against active attackers $\;\;\;\;$ and $\;$ keeps the identity of the responder (Bob) confidential against passive eavesdroppers $\;\;\;\;$ and $\;$ provides some protection against identity misbinding attacks, $\;$ although not as much as a good protocol ...

4

What you are envisioning has basically been standardized as the integrated encryption scheme being a hybrid encryption scheme providing message authenticity (IND-CCA security).

3

To use the proper terminology: in TLS, cipher suites which include "some Diffie-Hellman" are: Anonymous Diffie-Hellman: DH_anon Static Diffie-Hellman: DH-RSA, DH-DSS... Ephemeral Diffie-Hellman: DHE-RSA, DHE-DSS... There is no "plain DHE" cipher suite in TLS; it is called "DH_anon". As the name indicates, with DH_anon, the server is "anonymous": you ...

3

How does Diffie-Hellman prevent a man-in-the-middle attack? Answer: Diffie-Hellman does not prevent a man-in-the-middle attack. If you're using Diffie-Hellman without any sort of authentication, then Oscar can certainly change the keys. When he does that, what's effectively happen is that Alice and Bob aren't actually negotiating keys; Alice is ...

3

Yes, it meets the formal definition of a Schnorr group; however it was constructed somewhat differently. Normally, when we generate a Schnorr group, we pick a prime $q$, and then search for an $r$ so that $qr+1$ is also prime; with the RFC3526 group, they picked $p$ and $q$ simultaneously. In addition, when it came to selecting the generator, they did not ...

3

Unless you are absolutely sure that you don't need to and that the cost is going to be significant then I would absolutely say you should use authenticated encryption. One reason is bit-flipping attacks - flipping a few bits at the 'right' point in your encrypted message might lead well to a message that is legal (the classic example is if someone learns ...

3

In DH if you want to compute $g^a$ from $K$ you have to know $b$ (which the legitimate receiver of $g^a$ clearly knowns, so this does not really make sence). This party can compute the inverse of $b$, namely $b^{-1}$, and then compute $g^a=K^{b^{-1}}$. Note that this is not the same as $(K^b)^{-1}=(g^{ab})^{-1}$ (as I will discuss below). But that is not ...

3

That depends on the groups you are working in. Using a $\Sigma$-protocol If you have a group $G$ of prime order $q$ where the DDH is hard and you have a DH tuple $(g,g^u,g^v,g^w)$ with $w\equiv uv \pmod q$, then if your prover knows one of these values, say $u$, then we can write the DH tuple as $(g,g^u,h,h^u)$ and he is able to convince a verifier that ...

3

This algorithm is vulnerable to a Man in the middle. From Wikipedia: In the original description, the Diffie–Hellman exchange by itself does not provide authentication of the communicating parties and is thus vulnerable to a man-in-the-middle attack. Mallory may establish two distinct key exchanges, one with Alice and the other with Bob, effectively ...

3

Are there any advantages to “1.”, especially when users must communicate the password/key through a separate channel in both cases? As the comments (1, 2) already indicated: the first option “1.” will be easier to communicate. When you talk about a “high-entropy key”, I assume you are generating that high-entropy with a cryptographically secure random ...

3

That depends entirely on the size of $p$ and $q$. Given a factorization of $N = pq$, an attacker can compute $g^u \bmod p$ and $g^v \bmod p$, and then attempt to solve the CDH problem modulo $p$, giving him $g^{uv} \bmod p$. Then, he can then compute $g^u \bmod q$ and $g^v \bmod q$, and then attempt to solve the CDH problem modulo $q$, giving him \$g^{uv} ...

3

In TLS, the key exchange step results in a key called the master secret which is then derived into as much key material as needed with a custom key derivation function, called in TLS terminology the PRF. It is not slow -- contrary to PBKDF2, the "PRF" of TLS is not for handling password and thus has no need to be slow.

3

I'm guessing whoever made the program was confused about key lengths. A 256-bit DH modulus will not give 256-bits of security. It will provide far less. If you're reading http://www.keylength.com, the value here corresponds to the "discrete logarithm group". These days you really want a minimum of 2048 bits.

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