Lecture Eleven - Data Security & Recovery
(Reference: C. J. Date, An introduction to Database Systems)
We can protect our data from unauthorised access by some security control such
as system password or so, however, some infiltrators may physically removing
part of the database, or by tapping into a communication line to get the data they
want.
It’s hardly to prevent infiltrators to get the data, how can we protect our data?
One of the most effective measure is data encryption: the infiltrators may get the
data, but they cannot interpret the data. We store and transmit data in an
encrypted form.
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Terminology in Data Encryption:
Plaintext
Encryption
Encryption
Key
Ciphertext
The original data.
An algorithm which converts the
plaintext into another form.
An input value for use in the encryption
algorithm.
The encrypted form of plaintext.
An example for data encryption.
Plain text : AS KINGFISHERS CATCH FIRE
Encryption key: ELIOT
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Steps:
1. Divide the plaintext into blocks of length equal to the encryption key:
AS_KI NGFIS HERS_ CATCH _FIRE
(Note: ‘_’ means blank space)
2. Replace each character of the plaintext by an integer in the range 00-26, using
blank space as 00, A as 01, ..Z=26
3. Repeat Step 2 for the encryption key:
ELIOT = 0512091520
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4. For each block of the plaintext, replace each character by the sum modulo 27 of
its integer encoding and the integer encoding and the integer encoding of the
corresponding character of the encryption key:
0119001109
0512091520
0604092602
1407060919
0512091520
1919152412
0805181900 0301200308 0006091805
0512091520 0512091520 0512091520
1317000720 0813021801 0518180625
5. Replace each integer encoding in the result of Step 4 by its character equivalent:
FDIZB SSOXL MQ_GT HMBRA ERRFY
Q:
Is it really secure?
Ans.:
Well, obviously it is not difficult to break this security.
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Public-Key Encryption
In this method, both the encryption algorithm and the encryption key are made
freely available, therefore everyone is able to make use of this method to convert
plaintext into Ciphertext. Whereas the corresponding decryption key is kept
secret.
Note that in this method, there are two keys, one is the encryption key and the
other is decryption key. Decryption key cannot be deduced from the encryption
key, thus even the person performing the original encryption cannot perform the
corresponding decryption if not authorised to do so.
The original idea of public-key encryption is from Diffie and Hellman, in which a
specific approach is from Rivest, Shamir and Adleman(RSA scheme).
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The RSA scheme is based on the facts that:
1. There is a known fast algorithm for determining whether a given number is
prime or not
2. There is no known algorithm for finding the prime factors of a given
composite number.
Algorithm for RSA scheme:
1. Randomly choose two distinct large prime numbers p and q, then compute the
product r = p * q.
2. Randomly choose a large integer e that is relatively prime to the product (p 1) * (q - 1). The integer e is the encryption key.
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3. Take the decryption key d to be unique “multiplicative inverse” of e modulo
(p - 1) * (q - 1);
d = e modulo (p-1) * (q-1)
4. Publish the integers r and e but not d.
5. To encrypt a piece of plaintext P, replace it by the Ciphertext C, computed as
follows:
C= Pe modulo r
6. To decrypt a piece of Ciphertext C, replace it by the plaintext P, compute as
follows:
P = Cd modulo r
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Example:
Let p = 3, q = 5, r = 15.
(p -1) * (q - 1) = 8
Let e = 11 (a prime greater than both p and q).
d = 11 modulo 8
Now let the plaintext P consist of the integer 13, then the Ciphertext C is:
C = Pe modulo r
= 1311 modulo 15
= 1,792,160,394,037 modulo 15
=7
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Now the original plaintext P is given by
P = Cd modulo r
= 73 modulo 15
= 13
Q: If A sends a Ciphertext to B, how does B know that the Ciphertext did indeed
come from A?
Suppose ECA and ECB are for encrypting messages to be sent to A and B, DCA
and DCB are for decrypting messages message for A and B.
Now, A wishes to send a plaintext P to B, A can first DCA the message and then
ECB the result and transmits that as Ciphertext C:
C = ECB ( DCA (P))
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When B receives the C, the DCB will be applied to it followed by the ECA, thus
producing the final result P:
ECA ( DCB (C))
= ECA (DCB ( ECB ( DCA (P))))
= ECA (DCA (P))
----- DCB and ECB cancel
=P
----- ECA and DCA cancel
Now, because ECA will produce P only if algorithm DCA was used in the
encryption process, and that algorithm is known only to A, so no one can forge
A’s message.
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Recovery
In database, recovery means restoring the database to a state that is know to be
correct after some failure has caused the current state to be incorrect.
We must make sure that the database is recoverable that any piece of information
it contains can be reconstructed from some other information stored, somewhere
in the system.
Transaction Recovery
We can define any database update into a logical unit of work, i.e. a transaction.
A transaction begins with the successful execution of BEGIN TRANSACTION
and a COMMIT or ROLLBACK command.
Begin Transaction
Select * from table;
Update table set ........
Commit;
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Begin Transaction
Update table set .....
Rollback;
Note carefully that COMMIT and ROLLBACK terminate a transaction, not the
program. Generally speaking, a single program consists of a sequence of
transactions.
Money Transfer: ($1000 from John’s account to May’s account)
John’s account = John’s account– 1000
May’s account = May’s account + 1000
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There are four important properties for each transaction:
Atomicity:
Consistency:
Transactions are atomic (all or nothing)
Transformations preserve database consistency. That is, a
transaction transforms a consistent state into another consistent
state.
Isolation:
No matter how many transactions are running concurrently, any
given transaction’s updates are concealed from all others until
that transaction ends.
Durability:
Q: What’s the implication of that?
Once a transaction committed, its updates will be valid, even any
subsequent operations fail, including system crash.
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System Recovery
System
Such as power failure, OS crash which affect all transactions
Failures:
currently in progress but do not physically damage the database.
A system failure is also called a soft crash.
Media
Failures:
Such as disk crash or cartridge failure which cause damage to the
database. It affects at least those transactions currently using the
damaged data.
A media failure is also called a hard crash.
Q: What mechanism can be used to recover a crashed system?
Checkpoint
At certain prescribed intervals, the system automatically takes a checkpoint which
involves physically writing the contents of the database buffers out to the physical
database and writing a special checkpoint record to the physical log.
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The checkpoint record gives a list of all transactions that were in progress at the
time the checkpoint was taken.
Checkpoint example
Time f
Time c
Time
T1
T2
T3
T4
T5
Checkpoint
System failure
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When the system restarts after Time f, transaction T3 and T5 will be undone, T2
and T4 will be redone, T1 has finishing physically writing the database and thus
will not be included in the restart process.
Media Recovery
A media failure is a failure such as a disk head crash, or a disk controller failure.
Recovery form such cases basically involves reloading the database from a
backup copy and then using the log to redo all transactions that completed since
that backup copy was taken.
Q: For non-stop applications, such as banking systems, which does not tolerate
system down time, what configuration can be used to tackle the problem?
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DATABASE-W
Disk A
Disk
Controller-1
CPU-X
DATABASE-W
Disk B
Disk
Controller-2
CPU-Y
Media-Fault-Tolerant System Configuration
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