The Damm algorithm is similar to the Verhoeff algorithm. It too will detect all occurrences of the two most frequently appearing types of transcription errors, namely altering one single digit, and transposing two adjacent digits. But the Damm algorithm has the benefit that it makes do without the dedicatedly constructed permutations and its position specificpowers being inherent in the Verhoeff scheme. Furthermore, a table of inverses can be dispensed with provided all main diagonal entries of the operation table are zero. The Damm algorithm does not suffer from exceeding the number of 10 possible values, resulting in the need for using a non-digit character. Prepending leading zeros does not affect the check digit. There are totally anti-symmetric quasigroups that detect all phonetic errors associated with the English language. The table used in the illustrating example is based on an instance of such kind.
Weaknesses
Despite its desirable properties in typical contexts where similar algorithms are used, the Damm algorithm is largely unknown and scarcely used in practice. Since prepending leading zeros does not affect the check digit, variable length codes should not be verified together since, e.g., 0, 01, and 001, etc. produce the same check digit. However, all checksum algorithms are vulnerable to this.
Design
Its essential part is a quasigroup of order 10 with the special feature of being weakly totally anti-symmetric. Damm revealed several methods to create totally anti-symmetric quasigroups of order 10 and gave some examples in his doctoral dissertation. With this, Damm also disproved an old conjecture that totally anti-symmetric quasigroups of order 10 do not exist. A quasigroup is called totally anti-symmetric if for all, the following implications hold:
,
and it is called weak totally anti-symmetric if only the first implication holds. Damm proved that the existence of a totally anti-symmetric quasigroup of order is equivalent to the existence of a weak totally anti-symmetric quasigroup of order. For the Damm algorithm with the check equation a weak totally anti-symmetric quasigroup with the property is needed. Such a quasigroup can be constructed from any totally anti-symmetric quasigroup by rearranging the columns in such a way that all zeros lay on the diagonal. And, on the other hand, from any weak totally anti-symmetric quasigroup a totally anti-symmetric quasigroup can be constructed by rearranging the columns in such a way that the first row is in natural order.
Algorithm
The validity of a digit sequence containing a check digit is defined over a quasigroup. A quasigroup table ready for use can be taken from Damm's dissertation. It is useful if each main diagonal entry is 0, because it simplifies the check digit calculation.
Validating a number against the included check digit
Process the number digit by digit: Use the number's digit as column index and the interim digit as row index, take the table entry and replace the interim digit with it.
The number is valid if and only if the resulting interim digit has the value of 0.
Calculating the check digit
Prerequisite: The main diagonal entries of the table are 0.
Set up an interim digit and initialize it to 0.
Process the number digit by digit: Use the number's digit as column index and the interim digit as row index, take the table entry and replace the interim digit with it.
The resulting interim digit gives the check digit and will be appended as trailing digit to the number.
Example
The following operation table will be used. It may be obtained from the totally anti-symmetric quasigroup in Damm's doctoral dissertation page 111 by rearranging the rows and changing the entries with the permutation and defining.
0
1
2
3
4
5
6
7
8
9
0
0
3
1
7
5
9
8
6
4
2
1
7
0
9
2
1
5
4
8
6
3
2
4
2
0
6
8
7
1
3
5
9
3
1
7
5
0
9
8
3
4
2
6
4
6
1
2
3
0
4
5
9
7
8
5
3
6
7
4
2
0
9
5
8
1
6
5
8
6
9
7
2
0
1
3
4
7
8
9
4
5
3
6
2
0
1
7
8
9
4
3
8
6
1
7
2
0
5
9
2
5
8
1
4
3
6
7
9
0
Suppose we choose the number 572.
Calculating the check digit
The resulting interim digit is 4. This is the calculated check digit. We append it to the number and obtain 5724.
Validating a number against the included check digit
The resulting interim digit is 0, hence the number is valid.
Graphical illustration
This is the above example showing the detail of the algorithm generating the check digit and verifying the number 572 with the check digit.
