<s>
Application	B-Application
Layer	I-Application
Transport	O
Security	O
(	O
ALTS	B-Protocol
)	O
is	O
a	O
Google-developed	O
authentication	B-Protocol
and	O
transport	O
encryption	O
system	O
used	O
for	O
securing	O
Remote	O
Procedure	O
Call	O
(	O
RPC	O
)	O
within	O
Google	B-Application
machines	O
.	O
</s>
<s>
Google	B-Application
started	O
its	O
development	O
in	O
2007	O
,	O
as	O
a	O
tailored	O
modification	O
of	O
TLS	B-Protocol
.	O
</s>
<s>
ALTS	B-Protocol
,	O
similar	O
to	O
TLS	B-Protocol
,	O
was	O
designed	O
specifically	O
for	O
Google	B-Application
’s	O
data	O
centers	O
and	O
relies	O
on	O
two	O
protocols	O
,	O
Handshake	B-Protocol
and	O
Record	O
.	O
</s>
<s>
Google	B-Application
began	O
developing	O
ATLS	O
in	O
2007	O
in	O
order	O
to	O
create	O
a	O
security	O
system	O
solution	O
for	O
the	O
company	O
’s	O
infrastructure	O
.	O
</s>
<s>
The	O
ALTS	B-Protocol
whitepaper	O
was	O
published	O
in	O
December	O
2017	O
.	O
</s>
<s>
At	O
that	O
time	O
the	O
dominant	O
Application	B-Application
layer	I-Application
protocols	O
were	O
SSL	O
and	O
TLS	B-Protocol
1.1	O
(	O
TLS	B-Protocol
1.2	O
was	O
only	O
published	O
as	O
an	O
RFC	O
in	O
2008	O
)	O
,	O
those	O
supported	O
many	O
legacy	O
algorithms	O
and	O
had	O
poor	O
security	O
standards	O
.	O
</s>
<s>
As	O
Google	B-Application
was	O
in	O
full	O
control	O
over	O
the	O
machines	O
that	O
needed	O
secure	O
transport	O
of	O
RPCs	O
,	O
deployment	O
of	O
systems	O
was	O
relatively	O
easy	O
,	O
and	O
so	O
Google	B-Application
developers	O
could	O
afford	O
designing	O
their	O
own	O
system	O
from	O
scratch	O
.	O
</s>
<s>
in	O
TLS	B-Protocol
,	O
the	O
server	O
side	O
is	O
committed	O
to	O
its	O
own	O
domain	O
name	O
(	O
and	O
corresponding	O
naming	O
scheme	O
)	O
,	O
while	O
Google	B-Application
needed	O
the	O
same	O
identity	O
(	O
i.e.	O
</s>
<s>
The	O
ALTS	B-Protocol
handshake	B-Protocol
protocol	O
is	O
based	O
on	O
authenticated	O
Diffie-Hellman	B-Protocol
key	I-Protocol
exchange	I-Protocol
scheme	O
,	O
and	O
supports	O
both	O
perfect	O
forward	O
secrecy	O
(	O
access	O
to	O
current	O
keys	O
does	O
not	O
compromise	O
future	O
security	O
)	O
and	O
session	O
resumption	O
(	O
noticeable	O
speedups	O
in	O
the	O
protocol	O
after	O
the	O
first	O
session	O
between	O
the	O
parties	O
)	O
.	O
</s>
<s>
Unlike	O
TLS	B-Protocol
,	O
in	O
ALTS	B-Protocol
both	O
parties	O
—	O
server	O
and	O
client	O
—	O
have	O
a	O
certificate	O
proving	O
their	O
respective	O
identities	O
.	O
</s>
<s>
The	O
certificate	O
chains	O
to	O
a	O
trusted	O
signing	O
service	O
verification	O
key	O
,	O
with	O
the	O
leaf	O
being	O
an	O
Elliptic	B-Protocol
curve	I-Protocol
Diffie-Hellman	I-Protocol
key	O
,	O
that	O
is	O
eventually	O
used	O
for	O
key	O
exchange	O
.	O
</s>
<s>
The	O
handshake	B-Protocol
protocol	O
consists	O
of	O
four	O
messages	O
,	O
sent	O
in	O
plaintext	O
:	O
</s>
<s>
ClientInit	O
,	O
initiated	O
by	O
the	O
client	O
,	O
and	O
contains	O
the	O
client	O
's	O
certificate	O
,	O
list	O
of	O
available	O
cipher	B-Protocol
suites	I-Protocol
,	O
and	O
a	O
session	O
resumption	O
attempt	O
;	O
</s>
<s>
ServerInit	O
,	O
sent	O
by	O
the	O
server	O
as	O
a	O
response	O
,	O
and	O
contains	O
its	O
own	O
certificate	O
,	O
chosen	O
cipher	B-Protocol
suite	I-Protocol
,	O
and	O
optionally	O
an	O
encrypted	O
resumption	O
ticket	O
;	O
</s>
<s>
ServerFinished	O
,	O
sent	O
by	O
the	O
server	O
(	O
concatenated	O
to	O
the	O
previous	O
message	O
in	O
ALTS	B-Protocol
default	O
implementation	O
)	O
,	O
and	O
contains	O
a	O
handshake	B-Protocol
authenticator	O
,	O
i.e.	O
</s>
<s>
HMAC	B-Algorithm
over	O
a	O
known	O
bitstring	O
using	O
the	O
calculated	O
session	O
key	O
;	O
</s>
<s>
ClientFinished	O
,	O
sent	O
by	O
the	O
client	O
,	O
and	O
contains	O
a	O
handshake	B-Protocol
authenticator	O
,	O
similarly	O
to	O
the	O
one	O
in	O
ServerFinished	O
.	O
</s>
<s>
Once	O
both	O
parties	O
computed	O
the	O
session	O
key	O
(	O
record	O
protocol	O
in	O
the	O
whitepaper	O
)	O
,	O
they	O
can	O
start	O
encrypting	O
traffic	O
with	O
the	O
symmetric	O
encryption	O
algorithm	O
128-bit	O
AES	B-Algorithm
,	O
using	O
mostly	O
GCM	B-Algorithm
as	O
its	O
mode	B-Algorithm
of	I-Algorithm
operation	I-Algorithm
.	O
</s>
<s>
On	O
older	O
machines	O
,	O
a	O
Google	B-Application
developed	O
VCM	O
was	O
used	O
.	O
</s>
<s>
The	O
handshake	B-Protocol
protocol	O
was	O
verified	O
using	O
the	O
ProVerif	B-Application
formal	O
verification	O
tool	O
.	O
</s>
<s>
In	O
order	O
to	O
avoid	O
repeating	O
computationally	O
expensive	O
operations	O
,	O
ALTS	B-Protocol
supports	O
session	O
resumption	O
.	O
</s>
<s>
The	O
resumption	O
tickets	O
are	O
created	O
by	O
either	O
the	O
server	O
or	O
the	O
client	O
,	O
and	O
may	O
be	O
used	O
in	O
the	O
handshake	B-Protocol
protocol	O
,	O
if	O
both	O
parties	O
hold	O
the	O
same	O
resumption	O
ticket	O
,	O
indexed	O
by	O
a	O
resumption	O
identifier	O
.	O
</s>
<s>
Perfect	O
forward	O
secrecy	O
(	O
PFS	O
)	O
is	O
not	O
enabled	O
by	O
default	O
in	O
ALTS	B-Protocol
;	O
however	O
,	O
it	O
is	O
supported	O
.	O
</s>
<s>
Instead	O
of	O
using	O
an	O
inherent	O
PFS	O
algorithm	O
,	O
ALTS	B-Protocol
achieves	O
PFS	O
by	O
frequently	O
rotating	O
the	O
certificates	O
,	O
which	O
have	O
a	O
short	O
lifespan	O
(	O
6	O
,	O
20	O
,	O
or	O
48	O
hours	O
;	O
see	O
)	O
.	O
</s>
