Anatomy of a DKIM Signature: Every Field Explained

Mon, 23 Mar 2026 00:00:00 +0000

You know that DKIM signs your emails. But what exactly is inside that DKIM-Signature header? Let’s break down every field and follow the verification process from start to finish.

A Real DKIM Header, Dissected

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
 d=example.com; s=selector2024;
 h=from:to:subject:date:mime-version:content-type;
 bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
 t=1714123456;
 b=dHJ5aW5nIHRvIHJlYWQgdGhpcyBzaWduYXR1cmU/IE5pY2Ug
 dHJ5IDspIFRoaXMgaXMganVzdCBhIGR1bW15IGV4YW1wbGU=

Every Field Explained

`v=1`: Version

The DKIM protocol version. Only one version exists (1), defined by RFC 6376. This field is mandatory.

`a=rsa-sha256`: Algorithm

Two elements combined:

Cryptosystem: rsa (most common) or ed25519 (emerging, RFC 8463)
Hash function: sha256 (standard) or sha1 (obsolete and vulnerable, do not use)

Combination	Status
`rsa-sha256`	Standard, recommended
`rsa-sha1`	Obsolete, some providers reject it
`ed25519-sha256`	Future standard, partial support

`c=relaxed/relaxed`: Canonicalization

Defines how content is normalized before computing the signature. The format is headers/body:

DKIM RSA Key Size: 1024 vs 2048 Bits and the Future with Ed25519

Sat, 14 Mar 2026 00:00:00 +0000

Your DKIM setup works, signatures pass. But have you checked the size of your RSA key? A key that’s too short is a ticking time bomb: it could be cracked, allowing an attacker to sign emails on your behalf.

The History of DKIM Keys: From 512 to 2048 Bits

2012: The End of 512-Bit Keys

In 2012, researchers demonstrated that a 512-bit RSA key could be cracked in under 72 hours using cheap cloud computing power. The result: anyone could impersonate a domain using a 512-bit key and send perfectly DKIM-signed emails.

Cryptography on Sender Audit Blog