<s>
In	O
cryptography	O
,	O
Curve25519	O
is	O
an	O
elliptic	O
curve	O
used	O
in	O
elliptic-curve	O
cryptography	O
(	O
ECC	O
)	O
offering	O
128	O
bits	O
of	O
security	O
(	O
256-bit	O
key	O
size	O
)	O
and	O
designed	O
for	O
use	O
with	O
the	O
elliptic	B-Protocol
curve	I-Protocol
Diffie	I-Protocol
–	I-Protocol
Hellman	I-Protocol
(	O
ECDH	B-Protocol
)	O
key	O
agreement	O
scheme	O
.	O
</s>
<s>
The	O
reference	O
implementation	O
is	O
public	B-Application
domain	I-Application
software	I-Application
.	O
</s>
<s>
Daniel	O
J	O
.	O
Bernstein	O
has	O
since	O
proposed	O
that	O
the	O
name	O
Curve25519	O
be	O
used	O
for	O
the	O
underlying	O
curve	O
,	O
and	O
the	O
name	O
X25519	B-Application
for	O
the	O
DH	O
function	O
.	O
</s>
<s>
The	O
protocol	O
uses	O
compressed	O
elliptic	O
point	O
(	O
only	O
X	O
coordinates	O
)	O
,	O
so	O
it	O
allows	O
efficient	O
use	O
of	O
the	O
Montgomery	O
ladder	O
for	O
ECDH	B-Protocol
,	O
using	O
only	O
XZ	O
coordinates	O
.	O
</s>
<s>
Starting	O
in	O
2014	O
,	O
OpenSSH	B-Language
defaults	O
to	O
Curve25519-based	O
ECDH	B-Protocol
and	O
GnuPG	B-Application
adds	O
support	O
for	O
Ed25519	O
keys	O
for	O
signing	O
and	O
encryption	O
.	O
</s>
<s>
Behavior	O
for	O
general	O
SSH	B-Protocol
protocol	O
is	O
still	O
being	O
standardized	O
as	O
of	O
2018	O
.	O
</s>
<s>
In	O
2018	O
,	O
DKIM	B-Protocol
specification	O
was	O
amended	O
so	O
as	O
to	O
allow	O
signatures	O
with	O
this	O
algorithm	O
.	O
</s>
<s>
Also	O
in	O
2018	O
,	O
RFC	O
8446	O
was	O
published	O
as	O
the	O
new	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
v1.3	O
standard	O
.	O
</s>
<s>
It	O
recommends	O
support	O
for	O
X25519	B-Application
,	O
Ed25519	O
,	O
X448	O
,	O
and	O
Ed448	O
algorithms	O
.	O
</s>
<s>
NaCl	B-Application
for	O
Tcl	O
—	O
a	O
port	O
to	O
the	O
Tcl	O
language	O
.	O
</s>
