Encipherment Using
Modern Symmetric-Key
Ciphers
Objectives
❏ To show how modern standard ciphers, such as
DES or AES, can be used to encipher long
messages.
8.2
8-1 USE OF MODERN BLOCK CIPHERS
Symmetric-key encipherment can be done using
modern block ciphers. Modes of operation have been
devised to encipher text of any size employing either
DES or AES.
8.3
Continued
Modes of operation
8.4
Electronic Codebook (ECB) Mode
The simplest mode of operation is called the electronic
codebook (ECB) mode.
Electronic codebook (ECB) mode
8.5
 Pattern
at block level are preserved
 Block independence creates
opportunities for Eve to exchange some
cipher text block without knowing key.
8.6
Example
Assume that Eve works in a company a few hours per month (her
monthly payment is very low). She knows that the company uses
several blocks of information for each employee in which the
seventh block is the amount of money to be deposited in the
employee’s account. Eve can intercept the ciphertext sent to the
bank at the end of the month, replace the block with the
information about her payment with a copy of the block with the
information about the payment of a full-time colleague. Each
month Eve can receive more money than she deserves.
Error Propagation
A single bit error in transmission can create errors in the
corresponding block. However, the error does not have
any effect on the other blocks.
8.8
Cipher Block Chaining (CBC) Mode
In CBC mode, each plaintext block is exclusive-ored with
the previous ciphertext block before being encrypted.
Cipher block chaining (CBC) mode
8.9
Cipher block chaining (CBC) mode
8.10
Continued
It can be proved that each plaintext block at Alice’s site is
recovered exactly at Bob’s site. Because encryption and decryption
are inverses of each other,
Initialization Vector (IV)
The initialization vector (IV) should be known by the
sender and the receiver.
Error Propagation
In CBC mode, a single bit error in ciphertext block Cj
during transmission may create error in most bits in
plaintext block Pj during decryption.
8.12
Cipher Feedback (CFB) Mode
In some situations, we need to use DES or AES as secure
ciphers, but the plaintext or ciphertext block sizes are to
be smaller.
Encryption in cipher feedback (CFB) mode
8.13
Note
In CFB mode, encipherment and decipherment use
the encryption function of the underlying block
cipher.
The relation between plaintext and ciphertext blocks is
shown below:
8.14
CFB as a Stream Cipher
Cipher feedback (CFB) mode as a stream cipher
8.15
Output Feedback (OFB) Mode
In this mode each bit in the ciphertext is independent of
the previous bit or bits. This avoids error propagation.
Encryption in output feedback (OFB) mode
8.16
OFB as a Stream Cipher
Output feedback (OFB) mode as a stream cipher
8.17
Counter (CTR) Mode
In the counter (CTR) mode, there is no feedback. The
pseudorandomness in the key stream is achieved using a
counter.
Encryption in counter (CTR) mode
8.18
Counter (CTR) mode as a stream cipher
8.19
Comparison of Different Modes
8.20
USE OF STREAM CIPHERS
Although the five modes of operations enable the use
of block ciphers for encipherment of messages or files
in large units and small units, sometimes pure stream
are needed for enciphering small units of data such as
characters or bits.
RC4
A5/1
8.21
RC4
RC4 is a byte-oriented stream cipher in which a byte (8
bits) of a plaintext is exclusive-ored with a byte of key to
produce a byte of a ciphertext.
State
RC4 is based on the concept of a state.
The idea of RC4 stream cipher
8.23
Initialization
Initialization is done in two steps:
8.24
Key Stream Generation
The keys in the key stream are generated, one by one.
Algorithm
8.26
Algorithm Continued
8.27
A5/1
A5/1 (a member of the A5 family of ciphers) is used in the
Global System for Mobile Communication (GSM), a
network for mobile telephone communication..
General outline of A5/1
8.28
Key Generator
A5/1 uses three LFSRs with 19, 22, and 23 bits.
Three LFSR’s in A5/1
8.29
Initialization
1. set all bits in three LFSRs to 0.
2. Mix the 64 bit key with the value of register according
to following code
3. Repeat above procedure but use 22 bit frame buffer
8.30
4. For 100 cycles clock the whole generator but use
majority function to see which LFSR should be clocked.
8.31
A5/1 working example
0
18 17 16
01
1
0111
00
11
01
00
11
00
11
01
00
11
00
11
00
1 10
R1
C1
10
21 20
11
clock
control
0
10
0
1111
0 0 01 10 00 01 10 011001100111111000011
R2
C2
1
0
22 21 20
0
11
00
11
00
11
0 01 0 10 1 01 10 11 11 01 10 11 01 00 10 01 1001
C3
R3
0
 Prevent
(or at least detect) unauthorized
modification of data
 Encryption provides confidentiality
(prevents unauthorized disclosure)
 Encryption
alone does not assure integrity
8.34
 The
cryptography systems that we have studied
so far provide secrecy, or confidentiality, but not
integrity. However, there are occasions where we
may not even need secrecy but instead must have
integrity
 One way to preserve the integrity of a document
is through the use of a fingerprint. If Alice needs
to be sure that the contents of her document will
not be changed, she can put her fingerprint at the
bottom of the document.
8.35
Message and Message Digest
The electronic equivalent of the document and fingerprint
pair is the message and digest pair.
Message and digest
11.36
Difference
The two pairs (document / fingerprint) and (message /
message digest) are similar, with some differences. The
document and fingerprint are physically linked together.
The message and message digest can be unlinked
separately, and, most importantly, the message digest
needs to be safe from change.
Note
The message digest needs to be safe from change.
11.37
Checking Integrity
Checking integrity
11.38
Cryptographic Hash Function Criteria
A cryptographic hash function must satisfy three criteria:
preimage resistance, second preimage resistance, and
collision resistance.
Criteria of a cryptographic hash function
11.39
Preimage Resistance
Preimage
11.40
Second Preimage Resistance
Second preimage
11.41
Continued
Collision Resistance
Collision
11.42
 A message
digest does not authenticate the sender
of the message. To provide message
authentication, Alice needs to provide proof that it
is Alice sending the message and not an impostor.
The digest created by a cryptographic hash
function is normally called a modification
detection code (MDC). What we need for message
authentication is a message authentication code
(MAC).
8.43
 A modification
detection code (MDC) is a message
digest that can prove the integrity of the message:
that message has not been changed.
 If Alice needs to send a message to Bob and be sure
that the message will not change during
transmission, Alice can create a message digest,
MDC, and send both the message and the MDC to
Bob.
 Bob can create a new MDC from the message and
compare the received MDC and the new MDC. If
they are the same, the message has not been
changed.
8.44
8.45
Message Authentication Code (MAC)
Message authentication code
11.46
Note
The security of a MAC depends on the security of
the underlying hash algorithm.
11.47
Continued
Nested MAC
Nested MAC
11.48
Continued
HMAC
Details of HMAC
Ipad=(36)H
Opad=(5c)H
11.49
Continued
CMAC
11.50
8.51