Cryptographic data integrity algorithms Flashcards
what are Hash functions?
Functions that accepts a variable length block of data and produces a fixed size hash value
Are hash values public or private?
public
What do hash functions detect?
changes to message
Give to cryptographic hash function properties and define each.
1- One-Way property : computationally infeasible to find data mapping to a specific hash value
2- Collision-free property : computationally infeasible to find two data to same hash
What are the applications of Cryptographic hash functions?
- Message authentication
- digital signatures
- one way password file
- intrusion detection and virus detection
- PRF, PRNG
What is message authentication>
verify the integrity of the message
What is digital signature?
the hash value of a message is encrypted using the user’s private key
What is one way password file?
keeping the hash value of the password not the actual password
what is intrusion and virus detection
Keep and check hash files on the system
h=H(x), we say that x is a ….. of h, and x is a data block whose hash function is —. the function is —
preimage,h,H
Collision is:
a. one to one hash mapping
b. one to many hash mapping
c. many to one hash mapping
d. many to many hash mapping
c
State the hash function requirements.
- variable input size
- fixed output size
- efficiency: H(x) is easy to compute for any given ‘x’ making SW and HW implementations practical.
- preimage resistant(one way property): for given ‘h’ it is infeasible to find y in H(y) = h
- second preimage resistant(weak collision resistant) : for given block x, it is infeasible to find y!x in H(x) = H(y)
- Collision resistant(strong): it is infeasible to find a pair(x,y) such that H(y) = H(x)
pseudo-randomness : Output of H meets standard tests for pseudo-randomness
State attacks on Hash Functions
- Brute force
- cryptanalysis
- preimage and secon preimage: find y in H(y) such that H(y) = h
- collision resistance : determine two values with the same hash value(x and y) such the H(x)=H(y)
what determines the length of the hash code against brute force?
2^m/2
True or False:
128 bits is inadequate , 160 bits suspect
True
MD4 produces a ….. bit hash value
160
SHA-1 is considered :
a. secure
b. not secure
b
versions of SHA (also known as SHA-2) are:
SHA-256
SHA-384
SHA-512
What are message security requirements?
-Disclosure
-Source repudiation
-destination repudiation
-Traffic analysis
-Masquerade
-content modification
- sequence modification
-timing modification
what is the type that is based on a hash function
keyed hash functions as MAC
True or False:
Hash functions are slower than MACs
False, Hash function are faster than MACs
In key hashed functions , what does a hash include?
A key
Give the Original Proposal of key-hashed functions as Mac.
KeyHashed = Hash(Key|Message)
The security of MACs can be an advantage in which type of attack?
Brute Force, and MACs work as MAC pairs
What does MAC use encryption for?and does it use the same keys
Secrecy, No, it uses separate keys for each.
What are the requirements for MAC?
MAC needs to satisfy the following:
- Knowing a message and MAC is infeasible to find another message with the same MAC
- MAC should be distributed uniformly
- MAC should depend equally on all bits in a message
State MAC properties.
- MAC is a cryptographic checksum
- it has an input data of variable length
- it has a fixed sized authenticator(Output)
- it uses a secret key K
- MAC is many-to-one function meaning that many messages can have the same MAC, but finding these needs to be very difficult.
MAC is a cryptographic …..
Checksum
MAC = CK(M)
Why do we use MAC?
For Authentication
True or False:
MAC is a Digital Signature
False
What is the fixed sized block of data from MAC generated from?
a message and a secret key
MAC = C(K, M)
What does a MAC create?
A fixed sized block
How does the receiver make sure that the message is not altered?
By performing a certain computation on the message and checks if it matches the MAC
True or False:
In MAC, Encryption need not to be reversible.
True
When can public Key Encryption be efficient?
When the sender signs(signature) their message using their private key, and encrypts it using the public key of the receiver. this assures secrecy and authentication
MAC is appended to a message as a…..
Signature
Allows for easy replicability of embedded Hash functions
HMAC
What is Symmetric Message Encryption?
- Receiver knows that the sender must have created the message
- Only the sender and receiver know the key used
- know content cannot be altered
- if a message has a suitable structure, redundancy or a checksum to detect and changes.
What does HMAC preserve?
the original performance of the hash function without significant degradation .
HMAC handles keys in a very difficult way
This statement is False, HMAC handles keys in a simple way
Internet Standard of HMAC
RFC2104
Write the whole function of HMAC
HMACK(M) = Hash[(K+ XOR opad )|| Hash[(K+ XOR ipad || M)]]
In HMAC what is K+?
the key padded to size opad
how much is the overhead of HMAC
1
what does the security of a HMAC depend on?
the hash algorithm
Attacking HMAC requires:
- brute force attack on key used
- birthday attack
Digital Signatures provide the ability to:
- Verify the author, date and time
- Authenticate message contents
- Be checked by others to settle disagreements
In DSA , how many bits is the signature ? And how many bits is the security?
320 bits
512-1024
Respectively
DSA is smaller and faster than RSA
This statement is true
In direct digital signature, who is involved?
Sender and Receiver only.
Security depends on the sender/receiver’s private/public key.
Sender’s , private key.
