Adler-32 is a checksum algorithm which was invented by Mark Adler. Compared to a cyclic redundancy check of the same length, it trades reliability for speed. Adler-32 is more reliable than Fletcher-16, and slightly less reliable than Fletcher-32. 

## History

Adler-32 is a modification of the Fletcher checksum.

The Adler-32 checksum is part of the widely-used zlib compression library, as both were developed by Mark Adler. A "rolling checksum" version of Adler-32 is used in the rsync utility.

## The algorithm

An Adler-32 checksum is obtained by calculating two 16-bit checksums A and B and concatenating their bits into a 32-bit integer. A is the sum of all bytes in the string plus one, and B is the sum of the individual values of A from each step.

At the beginning of an Adler-32 run, A is initialized to 1, B to 0. The sums are done modulo 65521 (the largest prime number smaller than 216). The bytes are stored in network order (big endian), B occupying the two most significant bytes.

The function may be expressed as

``` A = 1 + D1 + D2 + ... + Dn (mod 65521)
B = (1 + D1) + (1 + D1 + D2) + ... + (1 + D1 + D2 + ... + Dn) (mod 65521)
= n×D1 + (n-1)×D2 + (n-2)×D3 + ... + Dn + n (mod 65521)

Adler-32(D) = B × 65536 + A
```

where D is the string of bytes for which the checksum is to be calculated, and n is the length of D.

## Example

The Adler-32 sum of the ASCII string "`Wikipedia`" would be calculated as follows:

```   ASCII code          A                   B
(shown as base 10)
W: 87           1 +  87 =  88        0 +  88 =   88
i: 105         88 + 105 = 193       88 + 193 =  281
k: 107        193 + 107 = 300      281 + 300 =  581
i: 105        300 + 105 = 405      581 + 405 =  986
p: 112        405 + 112 = 517      986 + 517 = 1503
e: 101        517 + 101 = 618     1503 + 618 = 2121
d: 100        618 + 100 = 718     2121 + 718 = 2839
i: 105        718 + 105 = 823     2839 + 823 = 3662
a: 97         823 +  97 = 920     3662 + 920 = 4582

A = 920  =  398 hex (base 16)
B = 4582 = 11E6 hex

Output = 300286872 = 11E60398 hex
```

(The modulo operation had no effect in this example, since none of the values reached 65521).

## Comparison with the Fletcher checksum

The first difference between the two algorithms is that Adler-32 sums are calculated modulo a prime number, whereas Fletcher sums are calculated modulo 24-1, 28-1, or 216-1 (depending on the number of bits used), which are all composite numbers. Using a prime number makes it possible for Adler-32 to catch differences in certain combinations of bytes that Fletcher is unable to detect.

The second difference, which has the largest effect on the speed of the algorithm, is that the Adler sums are computed over 8-bit bytes rather than 16-bit words, resulting in twice the number of loop iterations. This results in the Adler-32 checksum taking between one-and-a-half to two times as long as Fletcher's checksum for 16-bit word aligned data. For byte-aligned data, Adler-32 is faster than properly implemented (e.g., one found in the Hierarchical Data Format) Fletcher's checksum.

## Example implementation

In C, an inefficient but straightforward implementation is :

```#define MOD_ADLER 65521

uint32_t adler32(uint8_t *data, size_t len) /* data: Pointer to the data to be summed; len is in bytes */
{
uint32_t a = 1, b = 0;

while (len != 0)
{
a = (a + *data++) % MOD_ADLER;
b = (b + a) % MOD_ADLER;

len--;
}

return (b << 16) | a;
}
```

See the zlib source code for an efficient implementation.

• Like the standard CRC-32, the Adler-32 checksum can be forged easily and is therefore unsafe for protecting against intentional modification.
• It has the benefit over a CRC that it can be computed faster in software.
• Adler-32 has a weakness for short messages with few hundred bytes, because the checksums for these messages have a poor coverage of the 32 available bits.

## Weakness

Jonathan Stone discovered in 2001 that Adler-32 has a weakness for very short messages. He wrote "Briefly, the problem is that, for very short packets, Adler32 is guaranteed to give poor coverage of the available bits. Don't take my word for it, ask Mark Adler. :-)" The problem is that sum A does not wrap for short messages. The maximum value of A for a 128-byte message is 32640, which is below the value 65521 used by the modulo operation. An extended explanation can be found in RFC 3309, which mandates the use of CRC32 instead of Adler-32 for SCTP, the Stream Control Transmission Protocol.