Messaging security experts have published best practice guidance on how to avoid flawed implementation of the DKIM anti-phishing standard.
Google was among companies caught out in October using DomainKeys Identified Mail (DKIM) keys that were too short, allowing a researcher to spoof emails to Sergey Brin and Larry Page, purportedly from each other.
The Messaging, Malware and Mobile Anti-Abuse Working Group (M3AAWG), which counts Google as one of its sponsors, published guidance (PDF) calling on business enterprises to replace previously secure 512- and 768-bit verification keys with 1024-bit and higher encryption.
"Technology is advancing, and to keep pace with hackers, the industry needs to revisit its practices in light of their expanding capabilities," M3AAWG co-chairman Chris Roosenraad said in a statement.
The group recommended that keys be rotated quarterly, and that signatures should have an expiration period greater than the current key rotation period. Email services using DKIM should avoid sending messages in testing mode apart from during a testing period, and should monitor receiver performance using Domain-based Message Authentication, Reporting and Conformance (DMARC).