<s>
In	O
public-key	B-Application
cryptography	I-Application
,	O
a	O
public	B-Algorithm
key	I-Algorithm
fingerprint	I-Algorithm
is	O
a	O
short	O
sequence	O
of	O
bytes	B-Application
used	O
to	O
identify	O
a	O
longer	O
public	B-Application
key	I-Application
.	O
</s>
<s>
Fingerprints	B-Algorithm
are	O
created	O
by	O
applying	O
a	O
cryptographic	B-Algorithm
hash	I-Algorithm
function	I-Algorithm
to	O
a	O
public	B-Application
key	I-Application
.	O
</s>
<s>
Since	O
fingerprints	B-Algorithm
are	O
shorter	O
than	O
the	O
keys	O
they	O
refer	O
to	O
,	O
they	O
can	O
be	O
used	O
to	O
simplify	O
certain	O
key	O
management	O
tasks	O
.	O
</s>
<s>
In	O
Microsoft	O
software	O
,	O
"	O
thumbprint	O
"	O
is	O
used	O
instead	O
of	O
"	O
fingerprint.	O
"	O
</s>
<s>
A	O
public	B-Algorithm
key	I-Algorithm
fingerprint	I-Algorithm
is	O
typically	O
created	O
through	O
the	O
following	O
steps	O
:	O
</s>
<s>
A	O
public	B-Application
key	I-Application
(	O
and	O
optionally	O
some	O
additional	O
data	O
)	O
is	O
encoded	O
into	O
a	O
sequence	O
of	O
bytes	B-Application
.	O
</s>
<s>
To	O
ensure	O
that	O
the	O
same	O
fingerprint	B-Algorithm
can	O
be	O
recreated	O
later	O
,	O
the	O
encoding	O
must	O
be	O
deterministic	O
,	O
and	O
any	O
additional	O
data	O
must	O
be	O
exchanged	O
and	O
stored	O
alongside	O
the	O
public	B-Application
key	I-Application
.	O
</s>
<s>
The	O
additional	O
data	O
is	O
typically	O
information	O
which	O
anyone	O
using	O
the	O
public	B-Application
key	I-Application
should	O
be	O
aware	O
of	O
.	O
</s>
<s>
Examples	O
of	O
additional	O
data	O
include	O
:	O
which	O
protocol	O
versions	O
the	O
key	O
should	O
be	O
used	O
with	O
(	O
in	O
the	O
case	O
of	O
PGP	B-Application
fingerprints	B-Algorithm
)	O
;	O
and	O
the	O
name	O
of	O
the	O
key	O
holder	O
(	O
in	O
the	O
case	O
of	O
X.509	O
trust	O
anchor	O
fingerprints	B-Algorithm
,	O
where	O
the	O
additional	O
data	O
consists	O
of	O
an	O
X.509	O
self-signed	B-Protocol
certificate	I-Protocol
)	O
.	O
</s>
<s>
The	O
data	O
produced	O
in	O
the	O
previous	O
step	O
is	O
hashed	O
with	O
a	O
cryptographic	B-Algorithm
hash	I-Algorithm
function	I-Algorithm
such	O
as	O
SHA-1	B-Algorithm
or	O
SHA-2	B-Algorithm
.	O
</s>
<s>
If	O
desired	O
,	O
the	O
hash	B-Error_Name
function	I-Error_Name
output	O
can	O
be	O
truncated	O
to	O
provide	O
a	O
shorter	O
,	O
more	O
convenient	O
fingerprint	B-Algorithm
.	O
</s>
<s>
This	O
process	O
produces	O
a	O
short	O
fingerprint	B-Algorithm
which	O
can	O
be	O
used	O
to	O
authenticate	O
a	O
much	O
larger	O
public	B-Application
key	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
whereas	O
a	O
typical	O
RSA	B-Architecture
public	B-Application
key	I-Application
will	O
be	O
2048	O
bits	O
in	O
length	O
or	O
longer	O
,	O
typical	O
MD5	B-Algorithm
or	O
SHA-1	B-Algorithm
fingerprints	B-Algorithm
are	O
only	O
128	O
or	O
160	O
bits	O
in	O
length	O
.	O
</s>
<s>
When	O
displayed	O
for	O
human	O
inspection	O
,	O
fingerprints	B-Algorithm
are	O
usually	O
encoded	O
into	O
hexadecimal	O
strings	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
128-bit	O
MD5	B-Algorithm
fingerprint	B-Algorithm
for	O
SSH	B-Protocol
would	O
be	O
displayed	O
as	O
follows	O
:	O
</s>
<s>
When	O
a	O
public	B-Application
key	I-Application
is	O
received	O
over	O
an	O
untrusted	O
channel	O
,	O
such	O
as	O
the	O
Internet	O
,	O
the	O
recipient	O
often	O
wishes	O
to	O
authenticate	O
the	O
public	B-Application
key	I-Application
.	O
</s>
<s>
Fingerprints	B-Algorithm
can	O
help	O
accomplish	O
this	O
,	O
since	O
their	O
small	O
size	O
allows	O
them	O
to	O
be	O
passed	O
over	O
trusted	O
channels	O
where	O
public	B-Application
keys	I-Application
wo	O
n't	O
easily	O
fit	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
Alice	O
wishes	O
to	O
authenticate	O
a	O
public	B-Application
key	I-Application
as	O
belonging	O
to	O
Bob	O
,	O
she	O
can	O
contact	O
Bob	O
over	O
the	O
phone	O
or	O
in	O
person	O
and	O
ask	O
him	O
to	O
read	O
his	O
fingerprint	B-Algorithm
to	O
her	O
,	O
or	O
give	O
her	O
a	O
scrap	O
of	O
paper	O
with	O
the	O
fingerprint	B-Algorithm
written	O
down	O
.	O
</s>
<s>
Alice	O
can	O
then	O
check	O
that	O
this	O
trusted	O
fingerprint	B-Algorithm
matches	O
the	O
fingerprint	B-Algorithm
of	O
the	O
public	B-Application
key	I-Application
.	O
</s>
<s>
Exchanging	O
and	O
comparing	O
values	O
like	O
this	O
is	O
much	O
easier	O
if	O
the	O
values	O
are	O
short	O
fingerprints	B-Algorithm
instead	O
of	O
long	O
public	B-Application
keys	I-Application
.	O
</s>
<s>
Fingerprints	B-Algorithm
can	O
also	O
be	O
useful	O
when	O
automating	O
the	O
exchange	O
or	O
storage	O
of	O
key	O
authentication	O
data	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
key	O
authentication	O
data	O
needs	O
to	O
be	O
transmitted	O
through	O
a	O
protocol	O
or	O
stored	O
in	O
a	O
database	O
where	O
the	O
size	O
of	O
a	O
full	O
public	B-Application
key	I-Application
is	O
a	O
problem	O
,	O
then	O
exchanging	O
or	O
storing	O
fingerprints	B-Algorithm
may	O
be	O
a	O
more	O
viable	O
solution	O
.	O
</s>
<s>
In	O
addition	O
,	O
fingerprints	B-Algorithm
can	O
be	O
queried	O
with	O
search	O
engines	O
in	O
order	O
to	O
ensure	O
that	O
the	O
public	B-Application
key	I-Application
that	O
a	O
user	O
just	O
downloaded	O
can	O
be	O
seen	O
by	O
third	O
party	O
search	O
engines	O
.	O
</s>
<s>
If	O
the	O
search	O
engine	O
returns	O
hits	O
referencing	O
the	O
fingerprint	B-Algorithm
linked	O
to	O
the	O
proper	O
site(s )	O
,	O
one	O
can	O
feel	O
more	O
confident	O
that	O
the	O
key	O
is	O
not	O
being	O
injected	O
by	O
an	O
attacker	O
,	O
such	O
as	O
a	O
Man-in-the-middle	O
attack	O
.	O
</s>
<s>
PGP	B-Application
developed	O
the	O
PGP	B-Application
word	O
list	O
to	O
facilitate	O
the	O
exchange	O
of	O
public	B-Algorithm
key	I-Algorithm
fingerprints	I-Algorithm
over	O
voice	O
channels	O
.	O
</s>
<s>
In	O
systems	O
such	O
as	O
SSH	B-Protocol
,	O
users	O
can	O
exchange	O
and	O
check	O
fingerprints	B-Algorithm
manually	O
to	O
perform	O
key	O
authentication	O
.	O
</s>
<s>
Once	O
a	O
user	O
has	O
accepted	O
another	O
user	O
's	O
fingerprint	B-Algorithm
,	O
that	O
fingerprint	B-Algorithm
(	O
or	O
the	O
key	O
it	O
refers	O
to	O
)	O
will	O
be	O
stored	O
locally	O
along	O
with	O
a	O
record	O
of	O
the	O
other	O
user	O
's	O
name	O
or	O
address	O
,	O
so	O
that	O
future	O
communications	O
with	O
that	O
user	O
can	O
be	O
automatically	O
authenticated	O
.	O
</s>
<s>
In	O
systems	O
such	O
as	O
X.509-based	O
PKI	B-Architecture
,	O
fingerprints	B-Algorithm
are	O
primarily	O
used	O
to	O
authenticate	O
root	O
keys	O
.	O
</s>
<s>
This	O
use	O
of	O
certificates	O
eliminates	O
the	O
need	O
for	O
manual	O
fingerprint	B-Algorithm
verification	O
between	O
users	O
.	O
</s>
<s>
In	O
systems	O
such	O
as	O
PGP	B-Application
or	O
Groove	B-Protocol
,	O
fingerprints	B-Algorithm
can	O
be	O
used	O
for	O
either	O
of	O
the	O
above	O
approaches	O
:	O
they	O
can	O
be	O
used	O
to	O
authenticate	O
keys	O
belonging	O
to	O
other	O
users	O
,	O
or	O
keys	O
belonging	O
to	O
certificate-issuing	O
authorities	O
.	O
</s>
<s>
In	O
PGP	B-Application
,	O
normal	O
users	O
can	O
issue	O
certificates	O
to	O
each	O
other	O
,	O
forming	O
a	O
web	O
of	O
trust	O
,	O
and	O
fingerprints	B-Algorithm
are	O
often	O
used	O
to	O
assist	O
in	O
this	O
process	O
(	O
e.g.	O
,	O
at	O
key-signing	O
parties	O
)	O
.	O
</s>
<s>
In	O
systems	O
such	O
as	O
CGA	B-Protocol
or	O
SFS	B-Application
and	O
most	O
cryptographic	O
peer-to-peer	O
networks	O
,	O
fingerprints	B-Algorithm
are	O
embedded	O
into	O
pre-existing	O
address	O
and	O
name	O
formats	O
(	O
such	O
as	O
IPv6	B-Protocol
addresses	O
,	O
file	O
names	O
or	O
other	O
identification	O
strings	O
)	O
.	O
</s>
<s>
If	O
addresses	O
and	O
names	O
are	O
already	O
being	O
exchanged	O
through	O
trusted	O
channels	O
,	O
this	O
approach	O
allows	O
fingerprints	B-Algorithm
to	O
piggyback	O
on	O
them	O
.	O
</s>
<s>
In	O
PGP	B-Application
,	O
most	O
keys	O
are	O
created	O
in	O
such	O
a	O
way	O
so	O
that	O
what	O
is	O
called	O
the	O
"	O
key	O
ID	O
"	O
is	O
equal	O
to	O
the	O
lower	O
32	O
or	O
64	O
bits	O
respectively	O
of	O
a	O
key	B-Algorithm
fingerprint	I-Algorithm
.	O
</s>
<s>
PGP	B-Application
uses	O
key	O
IDs	O
to	O
refer	O
to	O
public	B-Application
keys	I-Application
for	O
a	O
variety	O
of	O
purposes	O
.	O
</s>
<s>
These	O
are	O
not	O
,	O
properly	O
speaking	O
,	O
fingerprints	B-Algorithm
,	O
since	O
their	O
short	O
length	O
prevents	O
them	O
from	O
being	O
able	O
to	O
securely	O
authenticate	O
a	O
public	B-Application
key	I-Application
.	O
</s>
<s>
The	O
primary	O
threat	O
to	O
the	O
security	O
of	O
a	O
fingerprint	B-Algorithm
is	O
a	O
second-preimage	O
attack	O
,	O
where	O
an	O
attacker	O
constructs	O
a	O
key	B-Application
pair	I-Application
whose	O
public	B-Application
key	I-Application
hashes	B-Error_Name
to	O
a	O
fingerprint	B-Algorithm
that	O
matches	O
the	O
victim	O
's	O
fingerprint	B-Algorithm
.	O
</s>
<s>
The	O
attacker	O
could	O
then	O
present	O
his	O
public	B-Application
key	I-Application
in	O
place	O
of	O
the	O
victim	O
's	O
public	B-Application
key	I-Application
to	O
masquerade	O
as	O
the	O
victim	O
.	O
</s>
<s>
A	O
secondary	O
threat	O
to	O
some	O
systems	O
is	O
a	O
collision	B-Algorithm
attack	I-Algorithm
,	O
where	O
an	O
attacker	O
constructs	O
multiple	O
key	B-Application
pairs	I-Application
which	O
hash	B-Error_Name
to	O
his	O
own	O
fingerprint	B-Algorithm
.	O
</s>
<s>
To	O
prevent	O
preimage	O
attacks	O
,	O
the	O
cryptographic	B-Algorithm
hash	I-Algorithm
function	I-Algorithm
used	O
for	O
a	O
fingerprint	B-Algorithm
should	O
possess	O
the	O
property	O
of	O
second	O
preimage	O
resistance	O
.	O
</s>
<s>
If	O
collision	B-Algorithm
attacks	I-Algorithm
are	O
a	O
threat	O
,	O
the	O
hash	B-Error_Name
function	I-Error_Name
should	O
also	O
possess	O
the	O
property	O
of	O
collision-resistance	O
.	O
</s>
<s>
While	O
it	O
is	O
acceptable	O
to	O
truncate	O
hash	B-Error_Name
function	I-Error_Name
output	O
for	O
the	O
sake	O
of	O
shorter	O
,	O
more	O
usable	O
fingerprints	B-Algorithm
,	O
the	O
truncated	O
fingerprints	B-Algorithm
must	O
be	O
long	O
enough	O
to	O
preserve	O
the	O
relevant	O
properties	O
of	O
the	O
hash	B-Error_Name
function	I-Error_Name
against	O
brute-force	B-Algorithm
search	I-Algorithm
attacks	O
.	O
</s>
<s>
In	O
practice	O
,	O
most	O
fingerprints	B-Algorithm
commonly	O
used	O
today	O
are	O
based	O
on	O
non-truncated	O
MD5	B-Algorithm
or	O
SHA-1	B-Algorithm
hashes	I-Algorithm
.	O
</s>
<s>
As	O
of	O
2017	O
,	O
collisions	O
but	O
not	O
preimages	O
can	O
be	O
found	O
in	O
MD5	B-Algorithm
and	O
SHA-1	B-Algorithm
.	O
</s>
<s>
The	O
future	O
is	O
therefore	O
likely	O
to	O
bring	O
increasing	O
use	O
of	O
newer	O
hash	B-Error_Name
functions	I-Error_Name
such	O
as	O
SHA-256	B-Algorithm
.	O
</s>
<s>
However	O
,	O
fingerprints	B-Algorithm
based	O
on	O
SHA-256	B-Algorithm
and	O
other	O
hash	B-Error_Name
functions	I-Error_Name
with	O
long	O
output	O
lengths	O
are	O
more	O
likely	O
to	O
be	O
truncated	O
than	O
(	O
relatively	O
short	O
)	O
MD5	B-Algorithm
or	O
SHA-1	B-Algorithm
fingerprints	B-Algorithm
.	O
</s>
<s>
In	O
situations	O
where	O
fingerprint	B-Algorithm
length	O
must	O
be	O
minimized	O
at	O
all	O
costs	O
,	O
the	O
fingerprint	B-Algorithm
security	O
can	O
be	O
boosted	O
by	O
increasing	O
the	O
cost	O
of	O
calculating	O
the	O
fingerprint	B-Algorithm
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
context	O
of	O
Cryptographically	B-Protocol
Generated	I-Protocol
Addresses	I-Protocol
,	O
this	O
is	O
called	O
"	O
Hash	B-Error_Name
Extension	O
"	O
and	O
requires	O
anyone	O
calculating	O
a	O
fingerprint	B-Algorithm
to	O
search	O
for	O
a	O
hashsum	B-Error_Name
starting	O
with	O
a	O
fixed	O
number	O
of	O
zeroes	O
,	O
which	O
is	O
assumed	O
to	O
be	O
an	O
expensive	O
operation	O
.	O
</s>
