# Tag Info

133

RSA was there first. That's actually enough for explaining its preeminence. RSA was first published in 1978 and the PKCS#1 standard (which explains exactly how RSA should be used, with unambiguous specification of which byte goes where) has been publicly and freely available since 1993. The idea of using elliptic curves for cryptography came to be in 1985, ...

76

Edit: I have made some tests and I found something weird. See at the end. Initial answer: At least the Koblitz curves (K-163, K-233... in NIST terminology) cannot have been specially "cooked", since the whole process is quite transparent: Begin with a binary field $GF(2^m)$. For every m there is only one such field (you can have several representations, ...

50

I wouldn't try to explain the mathematics of the backdoor. Just explain that the NSA hid a secret backdoor in there. Instead, I would suggest focusing on the history and the context. For instance, you could explain about Crypto.AG, how they spiked their RNG to help the NSA spy on their customers. You could explain how random number generators are a ...

46

ECDSA is a digital signature algorithm ECIES is an Integrated Encryption scheme ECDH is a key secure key exchange algorithm First you should understand the purpose of these algorithms. Digital signature algorithms are used to authenticate a digital content. A valid digital signature gives a recipient reason to believe that the message was created by a ...

43

The main difference is that secp256k1 is a Koblitz curve, while secp256r1 is not. Koblitz curves are known to be a few bits weaker than other 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 they are supposedly ...

41

Curve25519 was designed to take advantage of the Montgomery ladder, which combined with Montgomery curves forgoes the $Y$ coordinates, is side-channel resistant, and enables public keys to be any 255-bit string. The ladder looks something like this (pseudocode): Q = P; Q = 2*P; for(int i = log2(exponent) - 2; i >= 0; --i) { Q[ bit(exponent, i)] =...

39

I've never heard that RSA becomes less secure when the modulus grows. Obviously the strength doesn't grow as fast as the number of bits, but that only means that it grows sub-exponentially. If it keeps growing (without the growth going near zero) then there is no "trap". Check for instance here where the conclusion is that there is no exponential growth but ...

36

This is mostly a supplement to @ThomasPornin's answer, not a complete answer on its own (but too long to fit in a comment). ECC uses a finite field, so even though elliptical curves themselves are relatively new, most of the math involved in taking a discrete logarithm over the field is much older. In fact, most of the algorithms used are relatively minor ...

32

There are some widely used cryptographic algorithms which need a finite, cyclic group (a finite set of element with a composition law which fulfils a few characteristics), e.g. DSA or Diffie-Hellman. The group must have the following characteristics: Group elements must be representable with relatively little memory. The group size must be known and be a ...

30

Ed25519 is a specific instance of the EdDSA family of signature schemes. Ed25519 is specified in RFC 8032 and widely used. The only other instance of EdDSA that anyone cares about is Ed448, which is slower, not widely used, and also specified in RFC 8032. Keys and signatures in one instance of EdDSA are not meaningful in another instance of EdDSA: Ed25519 ...

29

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 ...

28

Your answer is in the paper Elliptic curve cryptosystems from Neal Koblitz: Set up an elliptic curve $E$ over a field $\mathbb{F}_q$ and a point $P$ of order $N$ just the same as for EC-DDH as system parameters. You need a public known function $f : m \mapsto P_m$, which maps messages $m$ to points $P_m$ on $E$. It should be invertible, and one way is to ...

28

Your question is at least partially answered in FIPS 186-3 itself… Appendix A describes how to start with a seed and use an iterative process involving SHA-1 until a valid elliptic curve is found. Appendix D contains the NIST recommended curves and includes the seed used to generate each one according to the procedure in Appendix A. So to believe that NSA ...

28

Elliptic curves are not the only curves that have groups structure, or uses in cryptography. But they hit the sweet spot between security and efficiency better than pretty much all others. For example, conic sections (quadratic equations) do have a well-defined geometric addition law: given $P$ and $Q$, trace a line through them, and trace a parallel line ...

27

Most cryptosystems based on elliptic curves can be broken if you can solve the discrete logarithm problem, that is, given the point $P$ and $rP$, find the integer $r$. The MOV attack uses a bilinear pairing, which (roughly speaking) is a function $e$ that maps two points in an elliptic curve $E(\mathbb{F}_q)$ to a element in the finite field $\mathbb{F}_{q^... 27 Let's assume that everyone agreed on some elliptic curve and a public base point$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$times. She obtains some result$A=g^a=\underbrace{g\cdot ...

27

While it is true that Elliptic Curve Diffie Hellman, Elliptic Curve Signature Generation and Elliptic Curve Signature Verification rely on scalar multiplications, these are usually implemented as different types of scalar multiplication for both security and efficiency reasons. In fact there are three types of scalar multiplications used in practice for ...

26

Finite fields - which is a branch of algebra - is a must. It is, in some way, used in almost all types of cryptographic algorithms. Also, you need some sort of basic programming ability since you will need to calculate time and space complexity of cryptographic algorithms. An "Algorithms" course taken from a CS department would be very useful. My advice ...

25

All points on an elliptic curve verify, by definition, the curve equation, usually written as $Y^2 = X^3 + aX + b$, with two given $a$ and $b$ parameters (these two parameters actually define the curve). So, if you know $X$, you can use the curve equation to recompute $Y^2$. A square root extraction will yield $Y$ or $-Y$. The compressed point format ...

25

In the beginning SSL handshake, the client sends a list of supported ciphersuites (among other things). The server then picks one of the ciphersuites, based on a ranking, and tells the client which one they will be using. This step is the one that determines whether or not the future connection will have perfect forward secrecy. Note that, at this point, ...

24

There is a draft RFC which describes a way to implement deterministic (EC)DSA (with test vectors). In this draft, both $h(m)$ (the hash of the message) and $x$ are used as input to a deterministic PRNG which uses HMAC (that's HMAC-DRBG as specified by NIST); the PRNG output is used to yield $k$. I am not sure your simple multiplication with $x$ would be ...

24

Montgomery and twisted Edwards curves have even order, but the group law can be implemented using fewer multiplications than Weierstrass models. So that is why these curves are popular and we have to live with cofactors $> 1$. There are other reasons to prefer to use prime-order elliptic curves (e.g., small subgroup attacks). So you are right that in ...

24

I'd say that the whole argument hinges around a "secret attack" that possibly the NSA may know of, enabling them to break some instances of elliptic curves that the rest of the World considers as safe, because the secret attack is, well, secret. This yields to the only possible answer to your question: since secret attacks are secret, they are not known to ...

24

I don't understand at all what this claim is on the website. The claim that RSA becomes very expensive for large $N$ is true, but to say that the gap between encryption/decryption cost and factoring goes down makes no sense at all! The function describing the running time of the best factoring algorithms is clearly asymptotically larger than $n^3$ (the time ...

24

I feel that as it was my comment, I am obliged to answer this :-). First of all, birational equivalence is really a geometric notion. As far as I know, there is no analogue for groups, rings or fields and therefore the cryptographic relevance is limited. It becomes relevant when speaking of geometric objects: for example, elliptic curves. Given these ...

24

Summary: ECC+symmetric algorithms can do almost anything RSA+symmetric algorithms commonly do (plus forward secrecy where RSA struggles). But RSA is often preferred, sometime rightly so, in particular due to it's superior performance for the public-key side. At common security levels, the public-key RSA operation (used for signature verification, and on the ...

23

First of all, I'm no expert in this area. Generally $n$ bit ECC seems to have a security level of about $n/2$, but I found some claims that it's lower for certain types of curves. RFC4492 - Elliptic Curve Cryptography (ECC) Cipher Suites contains the following table: for Transport Layer Security (TLS) Symmetric | ECC | ...

23

Simply put, elliptic curves allow you to use smaller fields. Consider Diffie-Hellman: given $p$, $g$ and $x$, you compute $g^x \mod p$. To ensure security, you must use values of $x$ and $p$ which are large enough to defeat known attacks. In practice, $x$ will need to be at least 160-bit long, while $p$ will be a 1024-bit prime. Now, the elliptic curve ...

23

Here is a list of products and companies who have had their EC DRBG algorithm validated by NIST. http://csrc.nist.gov/groups/STM/cavp/documents/drbg/drbgval.html The validation lists all modes that have been validated, so you can see which ones have gone to the effort of having their implementation of Dual_EC_DRBG validated. Tim Dierks points out that, for ...

22

Elliptic curve cryptography is not presently vulnerable to quantum computing because there are no quantum computers big and reliable enough to matter. But it would be vulnerable to quantum computers big enough to run Shor's algorithm. All elliptic curve cryptography* is based on the difficulty of finding a secret integer $n$ given the scalar multiple \$Q = [...

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