# Tag Info

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 ...

7

It happens that a line usually (not always) cuts three points in a elliptic curve by the Bezout theorem. This is the case for the points and the curve you are asking for. So the sum of two points are defined like the inverse of the third point intercepted by the line that cut $P$ and $Q$ (let's name it $R$). So we need to find $-R$ because $P+Q=-R$ by ...

7

The tangent line at point $P(x_1,y_1)$ is: $\frac{\partial f}{\partial x}(x-x_1)+\frac{\partial f}{\partial y}(y-y_1)$ Solving differentials, we obtain: $\frac{\partial f}{\partial x}=3x^2+2x$ $\frac{\partial f}{\partial y}=-2y-1$ For $x_1=0$ and $y_1=0$ we have: $\frac{\partial f}{\partial x}=3x^2+2x = 0$ $\frac{\partial f}{\partial y}=-2y-1 = -1$ ...

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 =& ... 7 Actually, it is not possible to uniquely recover the public key from an ECDSA signature (r,s). This remains true even if we also assume you know the curve, the hash function used, and you also have the message that was signed. However, with the signature and the message that was signed, and the knowledge of the curve, it is possible to generate two ... 6 BouncyCastle has a really bad ECC implementation. It uses affine coordinates which incur a huge performance hit (factor 20 or so) since it computes a field inversion after every single step. Good implementations use Jacobi coordinates (or a similar approach) where denominators are kept and there is only one field inversion at the end. It's also potentially ... 6 Over large characteristic fields, I am not aware of any "point generation method" that can be computed faster than a base field exponentiation, and I would be very surprised if such a thing existed even if you do not require constant running time. So your best bet in general is probably Icart's function (I'd pick that one over Elligator if I didn't need ... 6 I don't know about computing things in parallel, so I will ignore that part of the question. First, please note that the encryption algorithm is rarely the the weak point of the security. It is far more likely that you will have problems with the implementation, some spyware installed on your computer, a weak password (If you use qwerty as your password, ... 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 mainly depends on how the algorithm was selected. If it was selected by a public competition like for AES, then it is likely to be secure. If it was forced in by the NSA such as Dual-EC random number generator, then you may have some doubts. Other questions you may want to ask yourself are: Is this an "original" algorithm or was the problem that it ... 5 Your second equation seems a bit off. In the curve of equation:$$ y^2 + cy = x^3 + ax + b $$in a binary field \mathbb{F}_{2^m}, to add point P_1 = (x_1,y_1) to point P_2 = (x_2,y_2), resulting in point P_3 = (x_3,y_3), then the two equations are: \begin{eqnarray*} x_3 &=& \lambda^2 + x_1 + x_2\\ y_3 &=& \lambda (x_1+x_3) + y_1 + c ... 5 The bad news is that projective coordinates do not work with Pollard's Rho like you want it to. Rho needs an unambiguous point representation to find meaningful collisions, and in projective coordinates each point can have up to p-1 valid distinct representations. The good news is that, sticking to affine coordinates, you can avoid most of the cost of the ... 5 SafeCurves lists some ways to compare the security of elliptic curves. Their security criteria are split to "ECDLP security" and "ECC security". Failing the former basically means "there is no way to use this curve securely in general" while the latter "it is difficult to implement this curve securely". None of the (few) BouncyCastle-supported curves that ... 5 Now to calculate Q this will take a lot of time since it means I will need to perform point addition an insane number of times unless I'm not understanding something about it. You're missing a point; elliptic curve point addition is associative; that is, for any three points A, B, C, we have:$$(A + B) + C = A + (B+C)$$Now, why is this a big deal? ... 5 Update: My previous answer, although technically true, didn't answer your question. The issue is that the strong-DDH is not hard when using pairing groups, so my answer was merely stating that your problem is at least as hard as an easy problem (duh!) After some thought, I realized your problem is at least as hard as the 2-weak Bilinear Diffie-Hellman ... 4 Given a EC public key, can a different, but plausible and functional private key be derived to match the public key? No, a public key will correspond to only one private key (with one minor exception, which I will explain below). With Elliptic Curve systems, the private key is an integer d between 1 and q (the order the generator point G), and ... 4 Trevor Perrin wrote a library doing exactly that. Explanation can be found on in the curves mailing list archives. To convert a Curve25519 public key x_C into an Ed25519 public key y_E, with a Ed25519 sign bit of 0:$$y_E = \frac{x_C - 1}{x_C + 1} \mod 2^{255}-19 The Ed25519 private key may need to be adjusted to match the sign bit of $0$: if ...

4

Yes. There are several different kinds of twist attacks. Curve25519 is safe against all the ones SafeCurves looks at. Specifically, the cost of a combined attack is $2^{124.3}$, which is considered safe.

4

Type-1 (symmetric pairings) are dead for curves over fields of small characteristic. Over prime fields of large prime characteristic they are not really dead, but as they only offer small embedding degrees ($k=2$), they are not really attractive from a performance point of view. You have to choose very large curves (which makes the curve arithmetic slow) ...

4

Designing such signature schemes from scratch without having strong experience is very likely to fail and very dangerous (see the tons of bad papers out there being accepted to "dubious" conferences and journals). Your proposed scheme Your verification relation is to check if: $sP - Q + R \stackrel{?}{=} zP + mP$ where $Q$ is the public key of the signer ...

4

Actually, strictly speaking, the $x$ and $y$ values on an elliptic curve point aren't integers; instead, they are field elements. That is, the elliptic curve is defined in a field, which is a group of elements with addition and multiplication operations defined on them (along with a group of identities); the $x$ and $y$ values are members from these ...

4

A curve with cofactor 1, like all Brainpool curves, cannot possibly satisfy the SafeCurves criteria, so the answer to your question is no. Whether that means that they are actually "unsafe" for use in practice is debatable. I think it would be fair to say that implementing such curves in a secure way is perfectly doable in practice, but it's trickier to do ...

4

The main difference is that secp256r1 is a prime field curve, while secp256k1 is a Koblitz curve. Koblitz curves are known to be a few bits weaker than prime field curves, but since we are talking about 256-bit curves, neither is broken in "5-10 years" unless there's a breakthrough. The other difference is how the parameters have been chosen. In secp256r1 ...

4

Both curves have similar form and primes close to powers of two ($2^{192}-2^{64}-1$ and $2^{224} - 2^{96} + 1$), so you wouldn't expect large differences in performance – all things equal, P-224 might be anywhere from 30% to 60% slower due to the computational scaling of curve operations. However, in practice different implementations will have different ...

4

What's difference between n & #E(FP)? The difference is that $n$ is the smallest positive integer where $nG = O$; while you correctly state that $\#E \cdot G = O$, that doesn't mean that $\#E$ is the smallest integer that makes this happen. There may be a smaller integer $n$; $n$ will always be a factor of $\#E$, however it can be smaller. As for ...

4

In the usual definition of security of Elliptic Curves, curve25519 security is in fact 126 bits. If look at safecurves's rho page you can see the rho complexity for curve25519 is $2^{125.8}$ in accordance to what you say. Curve25519 author basically doesn't accept that definition of security. In the Curve25519 paper he states in section 1: Every known ...

3

The paper itself has more details on this: ECDSA, like many other signature systems, asks users to generate not merely a random long-term secret key, but also a new random secret session key $r$ for each message to be signed. ... If the same value r is ever used for 2 diff erent messages the secret key can be computed as well, as ElGamal... It ...

3

ECDH or DH for that matter doesn't provide any authentication of a user. ECDSA as a public key scheme does provide authentication, but lacks validation. You need to certify that the exchanged public keys are indeed from Alice or Bob. So Alice and Bob must let an authority certify their own public keys such that Alice trusts the authority of Bob and Bob ...

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

As already mentioned in a previous comment, ECIES (a hybrid encryption scheme) is typically the way to go when implementing asymmetric encryption on elliptic curves, as it is standardized. It provides chosen ciphertext security (IND-CCA). But as you are looking for "pure" public key encryption schemes, here we go: ElGamal can not only be implemented in ...

Only top voted, non community-wiki answers of a minimum length are eligible