Messaging, Malware and Mobile Anti-Abuse Working Group recommends businesses replace 512- and 768-bit verification keys with 1024-bit or higher encryption to counter a current vulnerability that allows the shorter keys to be cracked within 72 hours using cheap cloud-based services. Florida mathematician earlier this year discovered well known companies were using weak encryption keys as part of their DomainKeys Indentified Mail (DKIM) implementation, thus making it easier for someone to spoof e-mail messages when they were signed using test or small-bit signing keys.
Source: https://threatpost.com/m3aawg-recommends-new-dkim-best-practices-110612/77192/