I am aware that when the equation $\#E(\mathbb{Z}_p) = p$ holds for prime $p$, the elliptic curve is called "anomalous" and is insecure do to "Smart's attack".

Consider the similar case that $E(\mathbb{Z}_p)$ has a proper subgroup of (prime) order $p$, so $p \neq \#E$. In this case $p$ divides $\#E$ and $\#E > p$. This relationship of the (sub)group order matching the field size (thus forcing primality) is desirable for some applications.

Is such a curve secure for the discrete log performed in the $p$-order subgroup, or is this vulnerable to a similar attack? What would be the process to generate such a curve that is arguably safe for cryptographic use?

From what I read, curves over $GF(2^n)$ for some $n$ are less stable. But analogously, for $E(GF(2^n))$ there could be a subgroup of order $2^n$ and as long as $2^n$ has a large prime factor it should be safe for the discrete log. Is this also a possibility?

  • 1
    $\begingroup$ "As long as $2^n$ has a large prime factor" - $2^n$ never has a large prime factor... $\endgroup$
    – poncho
    Aug 13, 2019 at 14:38

1 Answer 1


It seems that curves like that do not exist.

Hasse's bound states that $|N-(p+1)| \le 2\sqrt p$, where $N=\#E$. Writing the order of the group as $N = ph$, you get:

$|ph-(p+1)| \le 2\sqrt p \rightarrow$

$|p(h-1)-1)| \le 2\sqrt p \rightarrow$

$|\sqrt p(h-1)-\frac{1}{\sqrt p}| \le 2$

The only way this inequality is true for $p > 5$ is if $h = 1$, i.e., $\#E=p$.

  • 4
    $\begingroup$ The curve $y^2=x^3+3x$ over $\mathbb F_5$ has $10$ rational points. (This is the only counterexample.) $\endgroup$
    – yyyyyyy
    Aug 13, 2019 at 13:03

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