DomainKeys Identified Mail, or DKIM, is a technical standard that helps protect email senders and recipients from spam, spoofing, and phishing.  It is a form of email authentication that allows an organization to claim responsibility for a message in a way that can be validated by the recipient.

Specifically, it uses an approach called “public key cryptography” to verify that an email message was sent from an authorized mail server, in order to detect forgery and to prevent delivery of harmful email like spam. It supplements SMTP, the basic protocol used to send email, because it does not itself include any authentication mechanisms.

How does it work?

It works by adding a digital signature a la headers of an email message. That signature can be validated against a public cryptographic key in the organization’s Domain Name System (DNS) records. In general terms, the process works like this:

El propietario de un dominio publica una clave pública criptográfica como un registro TXT con un formato especial en los registros DNS generales del dominio.

When a mail message is sent by an outbound mail server, the server generates and attaches a unique DKIM signature header a la message. This header includes two cryptographic hashes, one of specified headers, and one of the message body (or part of it). En header contains information about how the signature was generated.

Cuando un servidor de correo entrante recibe un mensaje, busca la clave pública DKIM del remitente en DNS. El servidor de entrada utiliza esta clave para descifrar la firma y compararla con una versión recién calculada. Si los dos valores coinciden, se puede demostrar que el mensaje es auténtico y no ha sido alterado en tránsito.

What is a DKIM signature?

A DKIM signature is a header added to email messages. En header contains values that allow a receiving mail server to validate the email message by looking up a sender’s DKIM key and using it to verify the encrypted signature. It looks something like this:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sparkpost.com; s=google; h=from:content-transfer-encoding:subject:message-id:date:to:mime-version; bh=ZkwViLQ8B7I9vFIen3+/FXErUuKv33PmCuZAwpemGco=; b=kF31DkXsbP5bMGzOwivNE4fmMKX5W2/Yq0YqXD4Og1fPT6ViqB35uLxLGGhHv2lqXBWwFhODPVPauUXxRYEpMsuisdU5TgYmbwSJYrFLFj5ZWTZ7VGgg6/nI1hoPWbzDaL9qh

Una cabecera de firma DKIM contiene mucha información, ya que está pensada para un procesamiento automatizado. Como puede ver en este ejemplo, la cabecera contiene una lista de partes de etiqueta=valor. Las etiquetas más destacadas son "d=" para el dominio de firma, "b=" para la firma digital real y "bh=" para un hash que puede verificarse recalculándolo con la clave pública del remitente.

Las firmas son, por definición, únicas de mensaje a mensaje, pero estos elementos básicos estarán presentes en todas las cabeceras de firma DKIM.

How is it related to FPS, DMARC, or other standards?

DKIM, SPF y DMARC son normas que permiten diferentes aspectos de la autenticación del correo electrónico. Abordan cuestiones complementarias.

• SPF allows senders to define which IP addresses are allowed to send mail for a particular domain.

• DKIM proporciona una clave de cifrado y una firma digital que verifica que un mensaje de correo electrónico no ha sido falsificado ni alterado.

• DMARC unifies the SPF and DKIM authentication mechanisms into a common framework and allows domain owners to declare how they would like email from that domain to be handled if it fails an authorization test.

¿Necesito DKIM?

If you are a business sending commercial or transactional email, you definitely need to implement one or more forms of email authentication to verify that an email is actually from you or your business. Properly configuring email authentication standards is one of the most important steps you can take to improve your deliverability. However, by itself it only goes so far; SparkPost and other email experts recommend also implementing SPF and DMARC to define a more complete email authentication policy.