![]() ![]() The signer (the email sending software or service that creates the DKIM signature) adds the selector into the DKIM header in the email. Examples of selectors can be: google010118, mailchimp2017 or mailserver1 It is recommended to use the name of the service and some date indicator, so it's easy to remember where this key is used. You can choose any value as the selector, as long as it is permitted to use as a DNS hostname (that is: all lowercase letters, numbers and hyphens). The DKIM public key is expected to be found in a DNS record at address. The selector is an identifier for the DKIM key, it tells the receiver which DNS address to query to find the public key. That is why a DKIM signature in an email contains an identification field known as the selector. Since a domain can use multiple DKIM keys (usually one per service), there must be a way for the validator (the receiver) to know which key to use to validate the email. The private key must never be shared with anyone else. It should only be installed on the server which is responsible for DKIM signing (for example: your SMTP server). You can read more about selectors later in this article.Ĭare must be taken when handling this file, a private key must always stay private. The selector is an address where a validator (a receiver of the email) can find the public key. The DKIM signing software requires at least two pieces of information: the private key, and the public key selector. Please refer to the documentation of your software or library for instructions on how set up DKIM signing. The resulting file dkim_private.pem contains the private key that will be used by your email software or library to create the DKIM signature. So although it may be tempting to create a stronger key here, there is no guarantee that a key larger than 2048 bit will be accepted. Please note that the DKIM specification only requires DKIM validators to support RSA keys up to 2048 bit. openssl genrsa -out dkim_private.pem 2048 The key we are generating here is a 2048-bit RSA key. To start, use openssl to generate a new RSA private key. Update: Besides RSA, it is now also possible to use Ed25519 elliptic curve signatures with DKIM, we published a new guide: how to use DKIM with Ed25519. ![]() If you are new to DKIM, we recommend reading our main article about DKIM first. The email service provider will create the keys for you, and give you the value of the DKIM DNS record that you should place in your DNS zone. then you do not need to create DKIM keys. Note that the key pair is to be created by the sender, so if you use a cloud service such as Gmail, Microsoft 365, Mailchimp, Mailgun, etc. In this guide we will explain how to use OpenSSL to create an RSA key pair suitable for DKIM signing. The private key stays at the server or service that sends the email, the public key is published using a DNS TXT record. To create and validate DKIM signatures a pair of keys known as the public and private key must be created by the signer (the sender). With DKIM, digital signatures are added to email messages, for authorization of the sender and authentication of the email itself. For instance, Windows systems use DPAPI for storing user's private keys, and DPAPI makes some extra efforts at not letting stored keys leak (whether these efforts are successful remains to be proven).Knowledge base > How to create a DKIM record with OpenSSL How to create a DKIM record with OpenSSL Details depend a lot on what system is actually used for private key storage. Of course, if a private key has ever been stored on some physical medium (say, a hard disk) without any extra protection, then it may have left exploitable traces there. Password protection is really an orthogonal issue. Correspondingly, there is nothing special in a RSA key pair which would make it suitable or unsuitable for password protection. to sign something), then it is first decrypted in the RAM of some computer, which then proceeds to use the non-encrypted private key. ![]() The important point here is that the password is all about storage: when the private key is to be used (e.g. A private key is readily encodable as a sequence of bytes, and can be copied, encrypted and decrypted just like any file. When a private is "protected by a password", it merely means that the key bytes, as stored somewhere, are encrypted with a password-derived symmetric key. ![]()
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