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a3fb911e
Commit
a3fb911e
authored
Oct 02, 2014
by
Niels Möller
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Notes on EdDSA decompression.
parent
1281c778
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misc/eccformulas.tex
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misc/eccformulas.tex
View file @
a3fb911e
...
...
@@ 181,7 +181,8 @@ suggests using the twisted Edwards curve,
\begin{equation*}
x
^
2 + y
^
2 = 1 + d' x
^
2 y
^
2
\pmod
{
p
}
\end{equation*}
(For this we use the same
$
d'
=

d
=
(
121665
/
121666
)
\bmod
p
$
).
(For this we use
$
d'
=

d
$
, with
$
d
=
(
121665
/
121666
)
\bmod
p
$
, where
$
d
$
is the same as in the curve25519 equivalence described below).
Assuming 1 has a square root modulo
$
p
$
, a point
$
(
x, y
)
$
lies on
this curve if and only if
$
(
\sqrt
{

1
}
x, p
)
$
lies of the nontwisted
Edwards curve. The point addition formulas for the twisted Edwards
...
...
@@ 225,6 +226,18 @@ because they are complete. See
In our notation
$
a
=

1
$
, and the
$
d'
$
above is
$

d
$
.
\subsection
{
Decompression
}
For EdDSA, points are represented by the
$
y
$
coordinate and only the
low bit, or ``sign'' bit, of the
$
x
$
coordinate. Then
$
x
^
2
$
can be
computed as
\begin{align*}
x
^
2
&
= (1y
^
2) (d y
^
2  1)
^{
1
}
\\
&
= 121666 (1y
^
2) (121665 y
^
2  121666)
^{
1
}
\end{align*}
We then get
$
x
$
from a square root, and we can use a trick of djb's to
avoid the inversion.
\section
{
Curve25519
}
Curve25519 is defined as the Montgomery curve
...
...
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