diff --git a/misc/ecc-formulas.tex b/misc/ecc-formulas.tex
index e347dcd9f5c071a107b751259436e7eb48bed306..d696ca5035f51bbd49c18bb0998d565c83f5104f 100644
--- a/misc/ecc-formulas.tex
+++ b/misc/ecc-formulas.tex
@@ -231,8 +231,9 @@ Curve25519 is defined as the Montgomery curve
 \begin{equation*}
   y^2 = x^3 + b x^2 + x \pmod p
 \end{equation*}
-with $b = 486662$ and $p = 2^{255} -19$. It is equivalent to the
-Edwards curve
+with $b = 486662$ and $p = 2^{255} -19$. See
+\url{http://cr.yp.to/ecdh/curve25519-20060209.pdf}. It is equivalent
+to the Edwards curve
 \begin{equation*}
   u^2 + v^2 = 1 + d u^2 v^2 \pmod p
 \end{equation*}