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Hash Functions 2
- Introduction
- One way hash
functions are easy
to compute, but
hard to reverse
- Brute force can be
used against simple
one-way hash
functions
- Brute force can be
used against simple
one-way hash
functions
- Sometimes they
have no inverse at all
- One way hash
functions are usually
a lot smaller than the
origional input
- Examples
- MD5
- MD4
- SHA
- RIPE-MD-60
- MD5
- One way hash
functions are easy
to compute, but
hard to reverse
- Hashing
- Hashing is used for hash
tables data so that there is
a direct relation to the data
content and storage
location
- Instead of storing each data item in the
next free memory location, it's data
location is determined by a an algorithem
that uses a key part of the data
- We can then
access that bit of
data using it's key
value alone
- Hashing is used for hash
tables data so that there is
a direct relation to the data
content and storage
location
- Simple hash
function methods
- Truncation
- Take a few of the first or
last characters of the key
as the hash code. Works
well if the characters are
well distributed
- Take a few of the first or
last characters of the key
as the hash code. Works
well if the characters are
well distributed
- Mid-square
- The key is squared
and the middle digits
of the re sult are
used as the hashed
value
- The key is squared
and the middle digits
of the re sult are
used as the hashed
value
- Folding
- the key is partitioned
into several parts and
the su m of the parts is
used to produce the
hash code
- the key is partitioned
into several parts and
the su m of the parts is
used to produce the
hash code
- Truncation
- One-way hash functions
- Simple hash
functions cause
collisions - where
there is more than
one hash keys
resulting in the
same index.
- For security, we need
unique keys to be
generated. This is known
as collision-free hashing
- Needs to be easy
to compute, but
hard to inverse
- Simple hash
functions cause
collisions - where
there is more than
one hash keys
resulting in the
same index.
- Passwords
- It is possible to extract
authentication information
from the target system
- Plain text
passwords should
NEVER be stored
- The same hashing function is
used when the user logs in,
and it is compared with the
value in the database
- If the files containing the hashed is
stolen, the thief has not got the
passwords, just the irreversable hashes
- If the thief knows the length of
password, then it is possible to
crack. This can be made
harder by adding a salt.
- It is possible to extract
authentication information
from the target system
- Hash Salt
- Hash salt is additional
data that is used as an
additional input to a
one-way hash function
- Helps defend against
dictionary attacks
- A new salt is
randomly
generated for
each password
- Usually the the salt
and the password
are concatenated
and processed with
a cryptographic
hash function
- The resulting
output is then
stored with
the salt in the
database
- These are used for
storing nearly all
user credentials
- The resulting
output is then
stored with
the salt in the
database
- Hash salt is additional
data that is used as an
additional input to a
one-way hash function
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