<s>
Symmetric-key	B-Algorithm
algorithms	I-Algorithm
are	O
algorithms	O
for	O
cryptography	O
that	O
use	O
the	O
same	O
cryptographic	O
keys	O
for	O
both	O
the	O
encryption	O
of	O
plaintext	O
and	O
the	O
decryption	O
of	O
ciphertext	O
.	O
</s>
<s>
The	O
requirement	O
that	O
both	O
parties	O
have	O
access	O
to	O
the	O
secret	O
key	O
is	O
one	O
of	O
the	O
main	O
drawbacks	O
of	O
symmetric-key	O
encryption	O
,	O
in	O
comparison	O
to	O
public-key	B-Application
encryption	I-Application
(	O
also	O
known	O
as	O
asymmetric-key	O
encryption	O
)	O
.	O
</s>
<s>
However	O
,	O
symmetric-key	B-Algorithm
encryption	I-Algorithm
algorithms	I-Algorithm
are	O
usually	O
better	O
for	O
bulk	O
encryption	O
.	O
</s>
<s>
With	O
exception	O
of	O
the	O
one-time	B-Algorithm
pad	I-Algorithm
they	O
have	O
a	O
smaller	O
key	O
size	O
,	O
which	O
means	O
less	O
storage	O
space	O
and	O
faster	O
transmission	O
.	O
</s>
<s>
Symmetric-key	O
encryption	O
can	O
use	O
either	O
stream	B-Algorithm
ciphers	I-Algorithm
or	O
block	O
ciphers	O
.	O
</s>
<s>
Stream	B-Algorithm
ciphers	I-Algorithm
encrypt	O
the	O
digits	O
(	O
typically	O
bytes	B-Application
)	O
,	O
or	O
letters	O
(	O
in	O
substitution	O
ciphers	O
)	O
of	O
a	O
message	O
one	O
at	O
a	O
time	O
.	O
</s>
<s>
The	O
Advanced	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
(	O
AES	B-Algorithm
)	O
algorithm	O
,	O
approved	O
by	O
NIST	O
in	O
December	O
2001	O
,	O
uses	O
128-bit	O
blocks	O
.	O
</s>
<s>
Examples	O
of	O
popular	O
symmetric-key	B-Algorithm
algorithms	I-Algorithm
include	O
Twofish	B-Algorithm
,	O
Serpent	B-Algorithm
,	O
AES	B-Algorithm
(	O
Rijndael	B-Algorithm
)	O
,	O
Camellia	B-Algorithm
,	O
Salsa20	B-Algorithm
,	O
ChaCha20	O
,	O
Blowfish	B-Algorithm
,	O
CAST5	B-Algorithm
,	O
Kuznyechik	B-Algorithm
,	O
RC4	B-Algorithm
,	O
DES	B-Algorithm
,	O
3DES	B-Algorithm
,	O
Skipjack	B-Algorithm
,	O
Safer	B-Algorithm
,	O
and	O
IDEA	B-Algorithm
.	O
</s>
<s>
Symmetric	B-Algorithm
ciphers	I-Algorithm
are	O
commonly	O
used	O
to	O
achieve	O
other	O
cryptographic	O
primitives	O
than	O
just	O
encryption	O
.	O
</s>
<s>
Hence	O
,	O
often	O
a	O
message	B-Algorithm
authentication	I-Algorithm
code	I-Algorithm
is	O
added	O
to	O
a	O
ciphertext	O
to	O
ensure	O
that	O
changes	O
to	O
the	O
ciphertext	O
will	O
be	O
noted	O
by	O
the	O
receiver	O
.	O
</s>
<s>
Message	B-Algorithm
authentication	I-Algorithm
codes	I-Algorithm
can	O
be	O
constructed	O
from	O
an	O
AEAD	O
cipher	O
(	O
e.g.	O
</s>
<s>
AES-GCM	B-Algorithm
)	O
.	O
</s>
<s>
However	O
,	O
symmetric	B-Algorithm
ciphers	I-Algorithm
cannot	O
be	O
used	O
for	O
non-repudiation	O
purposes	O
except	O
by	O
involving	O
additional	O
parties	O
.	O
</s>
<s>
Another	O
application	O
is	O
to	O
build	O
hash	B-Error_Name
functions	I-Error_Name
from	O
block	O
ciphers	O
.	O
</s>
<s>
See	O
one-way	B-Algorithm
compression	I-Algorithm
function	I-Algorithm
for	O
descriptions	O
of	O
several	O
such	O
methods	O
.	O
</s>
<s>
Symmetric	B-Algorithm
ciphers	I-Algorithm
have	O
historically	O
been	O
susceptible	O
to	O
known-plaintext	O
attacks	O
,	O
chosen-plaintext	O
attacks	O
,	O
differential	O
cryptanalysis	O
and	O
linear	O
cryptanalysis	O
.	O
</s>
<s>
Most	O
modern	O
symmetric-key	B-Algorithm
algorithms	I-Algorithm
appear	O
to	O
be	O
resistant	O
to	O
the	O
threat	O
of	O
post-quantum	O
cryptography	O
.	O
</s>
<s>
Quantum	B-Architecture
computers	I-Architecture
would	O
exponentially	O
increase	O
the	O
speed	O
at	O
which	O
these	O
ciphers	O
can	O
be	O
decoded	O
;	O
notably	O
,	O
Grover	B-Algorithm
's	I-Algorithm
algorithm	I-Algorithm
would	O
take	O
the	O
square-root	O
of	O
the	O
time	O
traditionally	O
required	O
for	O
a	O
brute-force	O
attack	O
,	O
although	O
these	O
vulnerabilities	O
can	O
be	O
compensated	O
for	O
by	O
doubling	O
key	O
length	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
128	O
bit	O
AES	B-Algorithm
cipher	I-Algorithm
would	O
not	O
be	O
secure	O
against	O
such	O
an	O
attack	O
as	O
it	O
would	O
reduce	O
the	O
time	O
required	O
to	O
test	O
all	O
possible	O
iterations	O
from	O
over	O
10	O
quintillion	O
years	O
to	O
about	O
six	O
months	O
.	O
</s>
<s>
By	O
contrast	O
,	O
it	O
would	O
still	O
take	O
a	O
quantum	B-Architecture
computer	I-Architecture
the	O
same	O
amount	O
of	O
time	O
to	O
decode	O
a	O
256	O
bit	O
AES	B-Algorithm
cipher	I-Algorithm
as	O
it	O
would	O
a	O
conventional	O
computer	O
to	O
decode	O
a	O
128	O
bit	O
AES	B-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
AES-256	B-Algorithm
is	O
believed	O
to	O
be	O
"	O
quantum	O
resistant	O
"	O
.	O
</s>
<s>
Symmetric-key	B-Algorithm
algorithms	I-Algorithm
require	O
both	O
the	O
sender	O
and	O
the	O
recipient	O
of	O
a	O
message	O
to	O
have	O
the	O
same	O
secret	O
key	O
.	O
</s>
<s>
Nearly	O
all	O
modern	O
cryptographic	O
systems	O
still	O
use	O
symmetric-key	B-Algorithm
algorithms	I-Algorithm
internally	O
to	O
encrypt	O
the	O
bulk	O
of	O
the	O
messages	O
,	O
but	O
they	O
eliminate	O
the	O
need	O
for	O
a	O
physically	O
secure	O
channel	O
by	O
using	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
key	I-Protocol
exchange	I-Protocol
or	O
some	O
other	O
public-key	B-Application
protocol	I-Application
to	O
securely	O
come	O
to	O
agreement	O
on	O
a	O
fresh	O
new	O
secret	O
key	O
for	O
each	O
session/conversation	O
(	O
forward	O
secrecy	O
)	O
.	O
</s>
<s>
When	O
used	O
with	O
asymmetric	B-Application
ciphers	I-Application
for	O
key	O
transfer	O
,	O
pseudorandom	O
key	O
generators	O
are	O
nearly	O
always	O
used	O
to	O
generate	O
the	O
symmetric	B-Algorithm
cipher	I-Algorithm
session	O
keys	O
.	O
</s>
<s>
However	O
,	O
lack	O
of	O
randomness	O
in	O
those	O
generators	O
or	O
in	O
their	O
initialization	B-Algorithm
vectors	I-Algorithm
is	O
disastrous	O
and	O
has	O
led	O
to	O
cryptanalytic	O
breaks	O
in	O
the	O
past	O
.	O
</s>
<s>
Practically	O
all	O
mechanical	O
cipher	O
machines	O
implement	O
a	O
reciprocal	O
cipher	O
,	O
a	O
mathematical	B-Algorithm
involution	I-Algorithm
on	O
each	O
typed-in	O
letter	O
.	O
</s>
<s>
The	O
majority	O
of	O
all	O
modern	O
ciphers	O
can	O
be	O
classified	O
as	O
either	O
a	O
stream	B-Algorithm
cipher	I-Algorithm
,	O
most	O
of	O
which	O
use	O
a	O
reciprocal	O
XOR	B-Algorithm
cipher	I-Algorithm
combiner	O
,	O
or	O
a	O
block	O
cipher	O
,	O
most	O
of	O
which	O
use	O
a	O
Feistel	B-Algorithm
cipher	I-Algorithm
or	O
Lai	B-Algorithm
–	I-Algorithm
Massey	I-Algorithm
scheme	I-Algorithm
with	O
a	O
reciprocal	O
transformation	O
in	O
each	O
round	O
.	O
</s>
