# BSD checksum

The **BSD checksum algorithm** is a commonly used, legacy checksum algorithm. It has been implemented in BSD and is also available through the GNU sum command line utility.

## Computation of the BSD checksumEdit

Below is the relevant part of the GNU sum source code (GPL licensed). It computes a 16-bit checksum by adding up all bytes (8-bit words) of the input data stream. In order to avoid many of the weaknesses of simply adding the data, the checksum accumulator is circular rotated to the right by one bit at each step before the new char is added.

```
int bsdChecksumFromFile(FILE *fp) /* The file handle for input data */
{
int checksum = 0; /* The checksum mod 2^16. */
for (int ch = getc(fp); ch != EOF; ch = getc(fp)) {
checksum = (checksum >> 1) + ((checksum & 1) << 15);
checksum += ch;
checksum &= 0xffff; /* Keep it within bounds. */
}
return checksum;
}
```

Below is some sample java code that calculates an 8-bit checksum. It adds each byte from the input byte array after a circular rotation of the checksum.

```
byte checksum(byte[] input) {
byte checksum = 0;
for (byte cur_byte: input) {
checksum = (byte) (((checksum & 0xFF) >>> 1) + ((checksum & 0x1) << 7)); // Rotate the accumulator
checksum = (byte) ((checksum + cur_byte) & 0xFF); // Add the next chunk
}
return checksum;
}
```

## Description of the algorithmEdit

As mentioned above, this algorithm computes a checksum by segmenting the data and adding it to an accumulator that is circular right shifted between each summation. To keep the accumulator within return value bounds, bit-masking with 1's is done.

**Example:** Calculating a 4-bit checksum using 4-bit sized segments (big-endian)

Input: 101110001110 -> three segments: 1011, 1000, 1110.

**Iteration 1:**

segment: 1011 checksum: 0000 bitmask: 1111

a) Apply circular shift to the checksum:

0000 -> 0000

b) Add checksum and segment together, apply bitmask onto the obtained result:

0000 + 1011 = 1011 -> 1011 & 1111 = 1011

**Iteration 2:**

segment: 1000 checksum: 1011 bitmask: 1111

a) Apply circular shift to the checksum:

1011 -> 1101

b) Add checksum and segment together, apply bitmask onto the obtained result:

1101 + 1000 = 10101 -> 10101 & 1111 = 0101

**Iteration 3:**

segment: 1110 checksum: 0101 bitmask: 1111

a) Apply circular shift to the checksum:

0101 -> 1010

b) Add checksum and segment together, apply bitmask onto the obtained result:

1010 + 1110 = 11000 -> 11000 & 1111 = 1000

* Final checksum:* 1000

## SourcesEdit

- official FreeBSD sum source code
- official GNU sum manual page
- coreutils download page --- find and unpack the newest version of the coreutils package, read src/sum.c