1
.
A
"
cryptosystem
"
is
the
ordered
list
of
elements
of
finite
possible
plaintexts
,
finite
possible
,
finite
possible
keys
,
and
the
encryption
and
decryption
algorithms
which
correspond
to
each
key
.
Keys
are
important
,
as
ciphers
without
variable
keys
can
be
trivially
broken
with
only
the
knowledge
of
the
cipher
used
and
are
therefore
useless
(
or
even
counter
-
productive
)
for
most
purposes
.
Historically
,
ciphers
were
often
used
directly
for
encryption
or
decryption
without
additional
procedures
such
as
authentication
or
checks
.
2
.
Symmetric
key
ciphers
are
implemented
as
either
block
ciphers
or
ciphers
.
A
block
cipher
enciphers
input
in
blocks
of
plaintext
as
opposed
to
individual
characters
,
the
input
form
used
by
a
stream
cipher
.
3
.
Stream
ciphers
,
in
contrast
to
the
'block'
type
,
create
an
arbitrarily
stream
of
key
material
,
which
is
combined
with
the
plaintext
bit
-
by
-
bit
or
character
-
by
-
character
,
somewhat
like
the
one
-
time
pad
.
In
a
stream
cipher
,
the
output
stream
is
created
based
on
a
hidden
internal
state
which
changes
as
the
cipher
operates
.
4
.
A
significant
disadvantage
of
symmetric
ciphers
is
the
key
management
to
use
them
securely
.
Each
distinct
pair
of
communicating
parties
must
,
ideally
,
share
a
different
key
,
and
perhaps
each
ciphertext
exchanged
as
well
.
5
.
Diffie
and
Hellman's
publication
sparked
widespread
academic
efforts
in
finding
a
practical
-
key
encryption
system
.
This
race
was
finally
won
in
1978
by
Ronald
Rivest
,
Adi
Shamir
,
and
Len
Adleman
,
whose
solution
has
since
become
known
as
the
RSA
algorithm
.
6
.
The
goal
of
is
to
find
some
weakness
or
insecurity
in
a
cryptographic
scheme
,
thus
permitting
its
subversion
or
evasion
.
7
.
Some
widely
known
cryptosystems
include
RSA
encryption
,
Schnorr
signature
,
El
-
Gamal
encryption
,
PGP
,
etc
.
More
complex
cryptosystems
include
electronic
cash
systems
,
signcryption
systems
,
etc
.
Some
more
'theoretical'
cryptosystems
include
proof
systems
,
(
like
zero
-
knowledge
proofs
)
,
systems
for
secret
sharing
,
etc
.
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