DKIM key rotation is an important security measure that ensures your DKIM records and email ecosystem aren’t exploited for long if keys are compromised. While the frequency of rotation depends on the nature of your organization, the complexity level of your email infrastructure, the availability of resources, etc., it’s suggested that you rotate them at least once every six months.
Using the same set of keys for an extended period is a vulnerability, risking your communication and safety at various levels.
DKIM Keys
DKIM keys use asymmetric cryptography to secure a message and come in pairs: private and public keys. The sender keeps the private key secure and uses it to generate the signature. The public key is published in the sender’s DNS records so that recipients’ servers can retrieve it for signature verification.
Longer DKIM keys increase complexity, providing many possible combinations and making it difficult for threat actors to guess or brute-force them. Keys that are 2048 bits or longer future-proof cryptographic systems by facilitating a greater margin of security against advances in computing power and cryptographic techniques.
Ideally, the length of DKIM keys depends on the cryptographic algorithm on which they are based. Commonly used algorithms are RSA and ECDSA–
RSA
RSA is short for Rivest-Shamir-Adleman, named after its inventors, Ron Rivest, Adi Shamir, and Leonard Adleman. For RSA, 1024-bit long keys were once considered ideal; however, due to advances in computing power, they are now seen as insufficient for strong security.
So, the recommended minimum key length for DKIM keys based on the RSA algorithm is 2048 bits, and several domain owners prefer using 3072 bits or even higher as it’s challenging for hackers to break longer keys.
ECDSA Keys
Elliptic Curve Digital Signature Algorithm (ECDSA) keys offer similar security to RSA but with shorter key lengths. For ECDSA, key lengths of 256 bits (equivalent to 3072-bit RSA keys) are considered sufficient for most purposes.
Many security standards and best practices recommend using longer keys to ensure adequate security. Adhering to these recommendations can help organizations maintain compliance and uphold security standards.
DKIM Selector
A DKIM selector is a subdomain prefix that identifies which DKIM key should be used to verify the authenticity of the email sender. When DKIM is deployed, a sender or domain owner publishes their DKIM public key in their DNS records under a specific selector domain.
For example, if a domain example.com implements DKIM, the domain owner might publish their DKIM public key under a subdomain like “selector1._domainkey.example.com” or “selector2._domainkey.example.com”. In this case, “selector1” or “selector2” would be the DKIM selectors.
DKIM selectors let domains have multiple keys, each linked to a different sending source. This helps organizations send messages from various departments using different email servers or services. Specifying selectors also helps manage and rotate DKIM keys independently for different sending sources while maintaining overall email authentication.
When a receiving mail server receives an email, it looks up the DKIM public key based on the selector specified in the email’s DKIM-Signature header. This ensures that the appropriate DKIM key is used to verify the email’s signature, enhancing security and flexibility in DKIM implementation.
How Often Should You Rotate DKIM Keys?
To be honest, there is no one-size-fits-all answer to this question, as the frequency of DKIM key rotation depends on your security practices and expectations, industry standards, and risk tolerance capabilities. But it’s suggested that you rotate them at least once every six months, and if your resources allow, four times a year is an even safer choice.
Here are a few considerations to help you determine an appropriate rotation schedule-
Risk Assessment
Consider and understand the sensitivity and criticality of the data that is shared via email. If you have a high-risk environment, then dealing with sensitive information requires more frequent key rotations to maintain security.
Industry Standards and Compliances
Industries and regulatory frameworks specify key rotation frequency requirements. Certain compliance standards, such as the Payment Card Industry Data Security Standard (PCI DSS), or healthcare regulations, such as the Health Insurance Portability and Accountability Act (HIPAA), may mandate specific key rotation intervals.
Key Usage Patterns
When you use DKIM selectors to handle multiple keys for different sending sources or departments within your company, you need to establish schedules for using different keys based on the risk associated with each source.
Cryptographic Strength
As cryptographic algorithms age or vulnerabilities are discovered, rotating keys more frequently may become necessary to maintain adequate security levels. Monitor developments in cryptographic research and standards to stay informed about recommended key lengths and rotation practices.
Manual DKIM Key Rotation
After creating a DKIM record and publishing the public key on your DNS, replace the old ones and share the private key with your email service provider or upload it to your email server if an internal team handles email safety.
Subdomain Delegation
This refers to using external services for periodic DKIM key rotation so that you can handle other responsibilities.
CNAME Delegation
CNAME delegation lets domain administrators route DKIM record details through a third-party vendor. It’s almost like subdomain delegation, with just one change: You publish specific CNAME records in your DNS, and then your vendor handles key rotation.
Automated DKIM Key Rotation
After generating and distributing DKIM keys to appropriate email servers or domains, set time-based rotation schedules or key expiration dates to ensure that rotation occurs without manual intervention. In the event of key compromise or security incidents, automatic DKIM key rotation systems may include mechanisms for revoking compromised keys and replacing them with new ones.
Automatic key rotation is better than manually doing it, as the latter has greater chances of oversights or delays, potentially leaving email communication vulnerable to attacks. Additionally, automatic rotation facilitates scalability, particularly for organizations with complex email environments or multiple sending sources. It streamlines key management processes, allowing for seamless integration with existing email systems and minimizing administrative overhead. Visit us at Autospf.com for more information.
