Integrity Methods (Cambridge (CIE) A Level Computer Science) : Revision Note
Validation & verification
What is validation?
Validation is an automated process where a computer checks if a user input is sensible and meets the program's requirements
There are seven categories of validation which can be carried out on fields and data types, these are
Range check
Format check
Length check
Presence check
Existence check
Limit check
Check digit
There can be occasions where more than one type of validation will be used on a field
An example of this could be a password field which could have a length, presence and type check on it
What is verification?
Verification is the act of checking data is accurate when being transferred or entered into a system
Verification methods include:
Parity check (transfer)
Checksum (transfer)
Double entry checking (entry)
Visual checks (entry)
Validation methods
Validation type | Definition | Example |
|---|---|---|
Range check | Ensures a number falls within a set range | Validating that a percentage is between 0 and 100 |
Limit check | Checks that a value does not exceed a maximum limit | Ensuring no more than 5 items can be bought in a special offer |
Length check | Checks the length of a string | Confirming a PIN is exactly 4 digits long |
Type check | Checks that the input is of the correct data type | Ensuring age is entered as a whole number |
Presence check | Checks that data has been entered and the field is not blank | Making sure a registration form is not submitted with blank fields |
Existence check | Checks that a referenced value exists in a database or list | Confirming a student ID entered exists in the school records |
Format check | Ensures data is in the correct format (e.g. patterns) | Making sure an email address contains '@' and a domain like '.com' |
What is a check digit?
A check digit is the last digit included in a code or sequence, used to detect errors in numeric data entry
Examples of errors that a check digit can help to identify are:
Incorrect digits entered
Omitted or extra digits
Phonetic errors
Added to the end of a numerical sequence they ensure validity of the data
Calculated using standardised algorithms to ensure widespread compatibility
Examples of where check digits can be used include:
ISBN book numbers
Barcodes
ISBN book numbers
Each book has a unique ISBN number that identifies the book
A standard ISBN number may be ten digits, for example, 965-448-765-9
The check digit value is the final digit (9 in this example).
This number is chosen specifically so that when the algorithm is completed the result is a whole number (an integer) with no remainder parts
A check digit algorithm is performed on the ISBN number and if the result is a whole number, then the ISBN is valid
Barcodes
Barcodes consist of black and white lines which can be scanned using barcode scanners
Barcode scanners shine a laser on the black and white lines which reflect light into the scanner
The scanner reads the distance between these lines as numbers and can identify the item
Barcodes also use a set of digits to uniquely identify each item
The final digit on a barcode is usually the check digit, this can be used to validate and authenticate an item
Verification methods
What is a parity check?
A parity check determines whether bits in a transmission have been corrupted
Every byte transmitted has one of its bits allocated as a parity bit
The sender and receiver must agree before transmission whether they are using odd or even parity
If odd parity is used then there must be an odd number of 1’s in the byte, including the parity bit
If even parity is used then there must be an even number of 1’s in the byte, including the parity bit
The value of the parity bit is determined by counting the number of 1’s in the byte, including the parity bit
If the number of 1’s does not match the agreed parity then an error has occurred
Parity checks only check that an error has occurred, they do not reveal where the error(s) occurred
Even parity
Below is an arbitrary binary string
EVEN | Byte | ||||||
|---|---|---|---|---|---|---|---|
0 | 1 | 0 | 1 | 1 | 0 | 1 | 0 |
If an even parity bit is used then all bits in the byte, including the parity bit, must add up to an even number
There are four 1’s in the byte
This means the parity bit must be 0 otherwise the whole byte, including the parity bit, would add up to five which is an odd number
Odd parity
Below is an arbitrary binary string
ODD | Byte | ||||||
|---|---|---|---|---|---|---|---|
1 | 1 | 0 | 1 | 1 | 0 | 1 | 0 |
If an odd parity bit is used then all bits in the byte, including the parity bit, must add up to an odd number
There are four 1’s in the byte. This means the parity bit must be a 1 otherwise the whole byte, including the parity bit, would add up to four which is an even number
The table below shows a number of examples of the agreed parity between a sender and receiver and the parity bit used for each byte
Example # | Agreed parity | Parity bit | Main bit string | Total number of 1’s | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
#1 | ODD | 0 | 1 | 1 | 0 | 1 | 0 | 1 | 1 | 5 |
#2 | EVEN | 1 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 2 |
#3 | EVEN | 1 | 0 | 1 | 0 | 1 | 1 | 1 | 1 | 6 |
#4 | ODD | 1 | 0 | 1 | 1 | 1 | 0 | 0 | 1 | 5 |
#5 | ODD | 1 | 1 | 0 | 1 | 0 | 1 | 0 | 1 | 5 |
#6 | EVEN | 0 | 1 | 0 | 0 | 1 | 1 | 1 | 0 | 4 |
Example #1: The agreed parity is odd. All of the 1’s in the main bit string are added (5). As this number is odd already the parity bit is set to 0 so the whole byte stays odd
Example #2: The agreed parity is even. All of the 1’s in the main bit string are added (1). As this number is odd the parity bit is set to 1 to make the total number of 1’s even (2)
Example #6: The agreed parity is even. All of the 1’s in the main bit string are added (4). As this number is even already the parity bit is set to 0 so the whole byte stays even
How do errors occur?
When using parity bits, an error occurs when the number of total bits does not match the agreed parity
Bits can be flipped or changed due to interference on a wire or wirelessly due to weather or other signals
Example # | Agreed parity | Parity bit | Main bit string | Total number of 1’s | Error | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
#1 | ODD | 1 | 1 | 1 | 0 | 1 | 0 | 1 | 1 | 6 | Error |
#2 | EVEN | 1 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 2 | No error |
#3 | EVEN | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 7 | Error |
#4 | ODD | 1 | 0 | 1 | 1 | 1 | 0 | 0 | 1 | 5 | No error |
#5 | ODD | 1 | 1 | 0 | 1 | 0 | 1 | 1 | 1 | 6 | Error |
#6 | EVEN | 0 | 1 | 0 | 0 | 0 | 1 | 1 | 0 | 3 | Error |
Parity checks are quick and easy to implement but fail to detect bit swaps that cause the parity to remain the same
What are parity byte & block checks?
Parity blocks and parity bytes can be used to check an error has occurred and where the error is located
Parity checks on their own do not pinpoint where errors in data exist, only that an error has occurred
A parity block consists of a block of data with the number of 1’s totalled horizontally and vertically
A parity byte is also sent with the data which contains the parity bits from the vertical parity calculation
Below is a parity block with a parity byte at the bottom and a parity bit column in the second column
ODD | Parity bit | Bit 2 | Bit 3 | Bit 4 | Bit 5 | Bit 6 | Bit 7 | Bit 8 |
|---|---|---|---|---|---|---|---|---|
Byte 1 | 0 | 1 | 1 | 0 | 1 | 0 | 1 | 1 |
Byte 2 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 |
Byte 3 | 1 | 0 | 1 | 0 | 1 | 1 | 1 | 1 |
Byte 4 | 1 | 0 | 1 | 1 | 1 | 0 | 0 | 1 |
Byte 5 | 1 | 1 | 0 | 1 | 0 | 1 | 0 | 1 |
Byte 6 | 1 | 1 | 0 | 0 | 1 | 1 | 1 | 0 |
Byte 7 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 0 |
Byte 8 | 0 | 1 | 0 | 1 | 1 | 0 | 0 | 0 |
Parity byte | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
The above table uses odd parity
Each byte row calculates the horizontal parity as a parity bit as normal
Each bit column calculates the vertical parity for each row, the parity byte
It is calculated before transmission and sent with the parity block
Each parity bit tracks if a flip error occurred in a byte while the parity byte calculates if an error occurred in a bit column
By cross referencing both horizontal and vertical parity values the error can be pinpointed
In the above example the byte 3 / bit 5 cell is the error and should be a 0 instead
The error could be fixed automatically or a retransmission request could be sent to the sender
What is a checksum?
A checksum is a value that can be used to determine if data has been corrupted or altered
It indicates whether data differs from its original form but does not specify where
Checksums are calculated using an algorithm and the value is added to the transmission
The receiving device re-calculates the checksum and compares to the original
If the checksums do not match, it is assumed an error has occurred
What is double entry checking?
Double entry checking involves entering the data twice in separate input boxes and then comparing the data to ensure they both match
If they do not, an error message is shown
What is a visual check?
A visual check involves the user visually checking the data on the screen
A popup or message then asks if the data is correct before proceeding
If it isn’t the user then enters the data again
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