Chapter 2
Cryptographic Tools

Symmetric Encryption
•
The universal technique for providing
confidentiality for transmitted or stored data
•
Also referred to as conventional encryption or
single-key encryption
•
Two requirements for secure use:
•
Need a strong encryption algorithm
•
Sender and receiver must have obtained copies
of the secret key in a secure fashion
and must
keep the key secure

Symmetric Encryption
Terminology
•
Plaintext:
The original message or data that is fed into the
algorithm as input.
•
Encryption algorithm:
The encryption algorithm performs
various substitutions and transformations on the plaintext.
•
Secret key:
The secret key is also input to the encryption
algorithm. The exact substitutions and transformations
performed by the algorithm depend on the key.
•
Ciphertext:
This is the scrambled message produced as
output. It depends on the plaintext and the secret key. For a
given message, two different keys will produce two different
ciphertexts.
•
Decryption algorithm:
This is essentially the encryption
algorithm run in reverse. It takes the ciphertext and the
secret key and produces the original plaintext.

Plaintext
input
Y
= E[
K
,
X
]
X
= D[
K
,
Y
]
X
K
K
Transmitted
ciphertext
Plaintext
output
Secret key shared by
sender and recipient
Secret key shared by
sender and recipient
Encryption algorithm
(e.g., DES)
Decryption algorithm
(reverse of encryption
algorithm)
Figure 2.1
Simplified Model of Symmetric Encryption

Attacking Symmetric
Encryption
Cryptanalytic Attacks
Brute-Force Attack
Rely on:
Nature of the algorithm
Some knowledge of the general
characteristics of the plaintext
Some sample plaintext-
ciphertext pairs
Exploits the characteristics of
the algorithm to attempt to
deduce a specific plaintext or
the key being used
If successful all future and past
messages encrypted with that
key are compromised
Try all possible keys on some
ciphertext until an intelligible
translation into plaintext is
obtained
On average half of all
possible keys must be tried to
achieve success

Attacking Symmetric
Encryption
•
Example ciphertext
ovd dpss dl iylhr aopz zljbyl jvkl
•
Frequency analysis
– the study of letters or
groups of letters in a cipher
o
‘E’ is the most common letter, occurring ~13% of the time
o
‘LL EE SS OO TT FF RR NN PP CC’ are the most common doubles
•
The above cipher is a Caesar Cipher, one of
the first encryption methods invented

Attacking Symmetric
Encryption
•
Brute-force example
+1 NUCCORRCKHXKGQZNOYYKIAXKIUJK
+2 MTBBNQQBJGWJFPYMNXXJHZWJHTIJ
+3 LSAAMPPAIFVIEOXLMWWIGYVIGSHI
+4 KRZZLOOZHEUHDNWKLVVHFXUHFRGH
+5 JQYYKNNYGDTGCMVJKUUGEWTGEQFG
+6 IPXXJMMXFCSFBLUIJTTFDVSFDPEF
+7 HOWWILLWEBREAKTHISSECURECODE
+8 GNVVHKKVDAQDZJSGHRRDBTQDBNCD
+9 FMUUGJJUCZPCYIRFGQQCASPCAMBC
+10
ELTTFIITBYOBXHQEFPPBZROBZLAB
+11
DKSSEHHSAXNAWGPDEOOAYQNAYKZA
+12
CJRRDGGRZWMZVFOCDNNZXPMZXJYZ
+13
BIQQCFFQYVLYUENBCMMYWOLYWIXY
+14
AHPPBEEPXUKXTDMABLLXVNKXVHWX
+15
ZGOOADDOWTJWSCLZAKKWUMJWUGVW

Symmetric Block
Encryption
•

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- Fall '16
- Randy Fortier
- Computer Science, Cryptography, Cryptographic hash function