Part_III.pdf

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Symmetric Key Cryptography
Model of symmetric key cryptography
Symmetric Encryption
¯
Stream ciphers
¯
Block ciphers
Symmetric Authentication
¯
Manipulation Detection Codes (MDC)
¯
Message Authentication Codes (MAC)
Nigel Smart - Frederik Vercauteren
1
COMS 30124
March 3, 2002
Model of Symmetric Key Cryptography
Alice
m
¹
Eve
e
k
secure channel
Bob
¹
c
d
k
¹
m
KG
m
=
plaintext,
c
=
ciphertext,
k
=
key, KG
=
Key Generator.
Nigel Smart - Frederik Vercauteren
2
COMS 30124
March 3, 2002
Model of Symmetric Key Cryptography
We write
c
=
e
k
(m),
where
¯
m
is the plaintext,
¯
e
is the encryption function,
¯
k
is the secret key,
¯
c
is the ciphertext.
Decryption is given by
m
=
d
k
(c).
Both sides
need to know the
key
k,
but
k
needs to be kept
secret.
µ
secret-key, single-key or one-key algorithms.
Kerckhoffs’ principle:
¯
Cryptanalyst has
complete knowledge
of
e
¡
(
¡
)
and
d
¡
(
¡
).
¯
Secrecy of message must reside entirely in the secrecy of the key.
Nigel Smart - Frederik Vercauteren
3
COMS 30124
March 3, 2002
Attacks on Symmetric Ciphers
Worst case
assumptions: adversary has
¯
Full knowledge
of the encryption algorithm
e
¡
(
¡
)
and decryption
algorithm
d
¡
(
¡
).
¯
Number of
plaintext/ciphertext pairs
associated to the target key
k.
Exhaustive keysearch:
¯
Given a few plaintext/ciphertext pairs, search through all possible
keys until the correct key is found.
µ
The
number of keys
must be
large enough.
In practice ciphers are used which are
believed to be strong,
which
means that the best attempts of experienced cryptanalysts cannot
break them.
Nigel Smart - Frederik Vercauteren
4
COMS 30124
March 3, 2002
Attacks on Symmetric Ciphers
Ciphertext-only attack:
¯
Adversary
only
has
ciphertext
of several messages.
¯
Recover the plaintext or deduce the keys used during encryption.
Known-plaintext attack:
¯
Adversary has several
ciphertexts
and
corresponding plaintexts.
¯
Deduce keys used to encrypt messages or decrypt new
messages.
Chosen-plaintext attack:
¯
Adversary can
choose the plaintexts
that get encrypted.
¯
More powerful than known-plaintext attack, because specific
plaintexts can be chosen.
Nigel Smart - Frederik Vercauteren
5
COMS 30124
March 3, 2002

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