Adler-32 Checksum Generator

What is Adler-32?

Adler-32 is the checksum used by the zlib data format described in RFC 1950. It keeps two running sums modulo 65,521, the largest prime below 65,536, and combines them into a single 32-bit value.

Check typed text or uploaded files. The result includes the unsigned integer, hexadecimal text, and the raw 4-byte digest.

Why addition instead of polynomial division

Speed and simplicity. A CRC needs table lookups or dedicated instructions; Adler-32 needs two additions per byte and a modulo, which made it attractive when zlib was designed and keeps it cheap today.

The two sums do different jobs. The first accumulates the bytes, catching changes in their values. The second accumulates the first sum, which makes the result depend on where each byte sits, so reordering data changes the checksum. One sum alone would not notice a swap.

Are there alternatives?

CRC-32 is stronger for the same output width and is what most formats specify. CRC-64 widens the result for large data sets. FNV-1 targets hash tables rather than integrity checking.

For any guarantee against a deliberate change, use BLAKE3, SHA256, or SHA3 instead.

Fun facts and more info

Adler-32 is noticeably weak on short inputs. The sums simply do not have room to spread out over a few bytes, so the checksum covers only part of its range and misses more errors than a CRC of the same width. Protocol designers have moved away from it for short messages for exactly this reason, and it remains a good fit for zlib streams, which are rarely tiny.

It detects accidental change only. It is not cryptographic and offers no defence against deliberate modification or forgery.