CMACs and MACS based on block ciphers, Digital signature

MACs based on Block Ciphers
Digital Signature
Property, Requirements and Security of Digital Signature
Various digital signature schemes ( Elgamal and Schnorr )
2I N F O R M A T I O N A N D N E T W O R K S E C U R I T Y

MACs based on Block Ciphers

 We look at two MACs that are based on the use of a block cipher
mode of operation.
• Data Authentication Algorithm ( DAA )
• Cipher Based Message Authentication Codes ( CMAC )

 Data Authentication Algorithm (DAA) is a widely used MAC based
on DES-CBC.
•Send final block as the MAC or the leftmost M bits (16≤M≤64) of final block.
•If necessary, the final block is padded on the right with zeroes to form a full
64-bit block.
O1 = E(K, D)
O2 = E(K, [D2 ⊕ O1])
O3 = E(K, [D3 ⊕ O2])
:
ON = E(K, [DN ⊕ ON-1])

Fig. 1 Data Authentication Algorithm Process [1]

 CMAC (Cipher-based Message Authentication Code) is a block
cipher based MAC algorithm. It may be used to provide assurance of
the authenticity and, hence, the integrity of binary data. This mode
of operation fixes security deficiencies of CBC-MAC .
C1 = E(K, M1)
C2 = E(K, [M2 ⊕ C1])
C3 = E(K, [M3 ⊕ C2])
:
Cn = E(K, [Mn ⊕ Cn-1 ⊕ K1])
T = MSBTlen(Cn)
Where,
T = message authentication code,
also referred to as the tag
Tlen = bit length of T
MSBs(X) = the s leftmost bits of the
bit string X

Fig. 2 CMAC Process [1]

 Advantages
•Can use existing encryption functions.
•Encryption functions have properties that resist pre image and collision
attacks
 Disadvantage
•Encryption algorithms (particularly when chained) can be much slower than
hash algorithms

Macs based on Block Ciphers

 The authenticity of many legal, financial, and other documents is
done by the presence or absence of an authorized handwritten
signature.
 “Digital Signature” is the best solution for authenticity in various
fields.
 A digital signature is nothing but an attachment to any piece of
electronic information, which represents the content of the
document and the identity of the owner of that document
uniquely[2].

 A digital code (generated and authenticated by public key
encryption) which is attached to an electronically transmitted
document to verify its contents and the sender's identity.
 Digital Signature of a person therefore varies from document to
document thus ensuring authenticity of each word of that
document.
 As the public key of the signer is known, anybody can verify the
message and the digital signature.

 The digital signature must have the following properties[2]
•It must verify the author and the date and time of the signature.
•It must authenticate the contents at the time of the signature.
•It must be verifiable by third parties, to resolve disputes.

 Private Key
•The private key is one which is accessible only to the signer. It is used to
generate the digital signature which is then attached to the message.[2]
 Public Key
•The public key is made available to all those who receive the signed messages
from the sender. It is used for verification of the received message. [2]

 Digital Signature Certificate
•A subscriber of the private key and public key pair makes the public key
available to all those who are intended to receive the signed messages from
the subscriber.[3]
•But in case of any dispute between the two sides, there must be some entity
with the receiver which will allow the receiver of the message to prove that
the message was sent by the subscriber of the key pair. This can be done with
the Digital Signature Certificate.[3]

Fig. 3 Working of Digital Signature

Fig. 4 Digital Signature generation and verification [4]

Parameter Paper Electronic
Authenticity May be forged Can not be copied
Integrity Signature
independent of the
document
Signature depends on the contents of the document
Non-
repudiation
a. Handwriting
expert needed
b. Error prone
a. Any computer user
b. Error free

 The following are the widely used Schemes for Digital Signatures.
•RSA Scheme
•Elgamal Scheme
•Schnorr Scheme
•Digital Signature Standard (DSS)

 Elgamal encryption scheme is designed to enable encryption by a
user’s public key with decryption by the user’s private key. The
Elgamal signature scheme involves the use of the private key for
encryption and the public key for decryption.[2]
 In the signing process , two functions create two signatures. In
verifying process the output of two functions are compared for
verification .

Fig. 5 Verifying and Signing process in Elgamal [5]

 The Schnorr scheme minimizes the message-dependent amount of
computation required to generate a signature. The main work for
signature generation does not depend on the message and can be
done during the idle time of the processor.[2]
 In the signing process , two functions create two signatures. In
verifying process , the output of one function is compared to the
first signature for verification.
 The important point is that the scheme uses two prime modulus
p and q.

Fig. 5 Verifying and Signing process in Schnorr [5]

 Signing
1. Alice chooses a random number r.
2. Alice calculates S1 = h(M|e1^r mod p).
3. Alice calculates S2 = r + d × S1 mod q.
4. Alice sends M, S1, and S2.
 Verifying message
1. Bob calculates V = h (M | e1^S2 e2^−S1 mod p).
2. if S1 is congruent to V modulo p, the message is accepted; otherwise
rejected.

1. http://flylib.com/books/en/3.190.1.99/1/
2. Cryptography & Network Security : Principal & Practices, 5th edition
- William Stallings
3. https://en.m.wikipedia.org/wiki/Digital_Signature
4. http://ksn5-simonmuerzl.blogspot.in/
5. http://www.slideshare.net/sonalbisla/elgamal-amp-schnorr-digital-
signature-scheme-copy

CMACs and MACS based on block ciphers, Digital signature

CMACs and MACS based on block ciphers, Digital signature

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