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microsoft exchange server and dkim failures with non-ascii characters: a deep dive

microsoft exchange servers, while powerful and widely used, have historically presented challenges when dealing with dkim (domainkeys identified mail) signatures and non-ascii characters within email content or headers. this tutorial will explore the intricacies of this issue, explaining the underlying mechanisms and providing practical solutions.

*understanding dkim and its dependence on ascii*

dkim uses public-key cryptography to verify the authenticity of emails. a sender's domain publishes a public key, and an email client uses this key to verify a digital signature embedded in the email's headers. the signature is generated using a private key held by the sender. crucially, the standard dkim implementation relies heavily on ascii encoding. problems arise when non-ascii characters (those outside the standard 7-bit ascii range) are present in the email's canonicalization process.

*canonicalization: the root of the problem*

dkim signatures are calculated based on a canonicalized version of the email headers. canonicalization is a process that standardizes the header fields, removing whitespace inconsistencies and potentially modifying line endings. there are two main canonicalization algorithms defined in rfc 6376:

*relaxed/simple:* this method is more lenient, allowing for some whitespace variations.
*relaxed/relaxed:* this method is even more lenient, handling whitespace variations more aggressively.

the issue often arises because exchange servers, depending on their version and configuration, might handle non-ascii characters differently during canonicalization than the receiving mail servers. this mismatch leads to a signature verification failure, even though the signature is technically valid. the receiving server sees a different canonicalized version than the one signed.

*how non-ascii characters break dkim*

here's a breakdown of how non-ascii c ...

#Microsoft #EmailSecurity #windows
Microsoft
email servers
DKIM
non-ASCII characters
email authentication
email security
email integrity
character encoding
domain keys
digital signatures
email delivery
spam prevention
email configuration
email standards
Microsoft Exchange