21-02-2011, 12:22 PM
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Cryptography and Network Security
Message Authentication and Hash Functions
At cats' green on the Sunday he took the message from the inside of the pillar and added Peter Moran's name to the two names already printed there in the "Brontosaur" code. The message now read: “Leviathan to Dragon: Martin Hillman, Trevor Allan, Peter Moran: observe and tail.” What was the good of it John hardly knew. He felt better, he felt that at last he had made an attack on Peter Moran instead of waiting passively and effecting no retaliation. Besides, what was the use of being in possession of the key to the codes if he never took advantage of it?
—Talking to Strange Men, Ruth Rendell
Message Authentication
message authentication is concerned with:
protecting the integrity of a message
validating identity of originator
non-repudiation of origin (dispute resolution)
will consider the security requirements
then three alternative functions used:
message encryption
message authentication code (MAC)
hash function
Security Requirements
disclosure
traffic analysis
masquerade
content modification
sequence modification
timing modification
source repudiation
destination repudiation
Message Encryption
message encryption by itself also provides a measure of authentication
if symmetric encryption is used then:
receiver know sender must have created it
since only sender and receiver now key used
know content cannot of been altered
if message has suitable structure, redundancy or a checksum to detect any changes
if public-key encryption is used:
encryption provides no confidence of sender
since anyone potentially knows public-key
however if
sender signs message using their private-key
then encrypts with recipients public key
have both secrecy and authentication
again need to recognize corrupted messages
but at cost of two public-key uses on message
Message Authentication Code (MAC)
generated by an algorithm that creates a small fixed-sized block
depending on both message and some key
like encryption though need not be reversible
appended to message as a signature
receiver performs same computation on message and checks it matches the MAC
provides assurance that message is unaltered and comes from sender
MAC Properties
a MAC is a cryptographic checksum
MAC = CK(M)
condenses a variable-length message M
using a secret key K
to a fixed-sized authenticator
is a many-to-one function
potentially many messages have same MAC
but finding these needs to be very difficult
Requirements for MACs
taking into account the types of attacks
need the MAC to satisfy the following:
1. knowing a message and MAC, is infeasible to find another message with same MAC
2. MACs should be uniformly distributed
3. MAC should depend equally on all bits of the message
Using Symmetric Ciphers for MACs
can use any block cipher chaining mode and use final block as a MAC
Data Authentication Algorithm (DAA) is a widely used MAC based on DES-CBC
using IV=0 and zero-pad of final block
encrypt message using DES in CBC mode
and send just the final block as the MAC
• or the leftmost M bits (16≤M≤64) of final block
but final MAC is now too small for security
Cryptography and Network Security
Message Authentication and Hash Functions
At cats' green on the Sunday he took the message from the inside of the pillar and added Peter Moran's name to the two names already printed there in the "Brontosaur" code. The message now read: “Leviathan to Dragon: Martin Hillman, Trevor Allan, Peter Moran: observe and tail.” What was the good of it John hardly knew. He felt better, he felt that at last he had made an attack on Peter Moran instead of waiting passively and effecting no retaliation. Besides, what was the use of being in possession of the key to the codes if he never took advantage of it?
—Talking to Strange Men, Ruth Rendell
Message Authentication
message authentication is concerned with:
protecting the integrity of a message
validating identity of originator
non-repudiation of origin (dispute resolution)
will consider the security requirements
then three alternative functions used:
message encryption
message authentication code (MAC)
hash function
Security Requirements
disclosure
traffic analysis
masquerade
content modification
sequence modification
timing modification
source repudiation
destination repudiation
Message Encryption
message encryption by itself also provides a measure of authentication
if symmetric encryption is used then:
receiver know sender must have created it
since only sender and receiver now key used
know content cannot of been altered
if message has suitable structure, redundancy or a checksum to detect any changes
if public-key encryption is used:
encryption provides no confidence of sender
since anyone potentially knows public-key
however if
sender signs message using their private-key
then encrypts with recipients public key
have both secrecy and authentication
again need to recognize corrupted messages
but at cost of two public-key uses on message
Message Authentication Code (MAC)
generated by an algorithm that creates a small fixed-sized block
depending on both message and some key
like encryption though need not be reversible
appended to message as a signature
receiver performs same computation on message and checks it matches the MAC
provides assurance that message is unaltered and comes from sender
MAC Properties
a MAC is a cryptographic checksum
MAC = CK(M)
condenses a variable-length message M
using a secret key K
to a fixed-sized authenticator
is a many-to-one function
potentially many messages have same MAC
but finding these needs to be very difficult
Requirements for MACs
taking into account the types of attacks
need the MAC to satisfy the following:
1. knowing a message and MAC, is infeasible to find another message with same MAC
2. MACs should be uniformly distributed
3. MAC should depend equally on all bits of the message
Using Symmetric Ciphers for MACs
can use any block cipher chaining mode and use final block as a MAC
Data Authentication Algorithm (DAA) is a widely used MAC based on DES-CBC
using IV=0 and zero-pad of final block
encrypt message using DES in CBC mode
and send just the final block as the MAC
• or the leftmost M bits (16≤M≤64) of final block
but final MAC is now too small for security