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Gmail signature settings3/31/2023 ![]() First, the message body is hashed, always from the beginning, possibly truncated at a given length (which may be zero). The semantics of the AUID are intentionally left undefined, and may be used by the signing domain to establish a more fine-grained sphere of responsibility.īoth header and body contribute to the signature. The domain must be equal to, or a subdomain of, the signing domain. The format is an email address with an optional local-part. The most relevant ones are b for the actual digital signature of the contents (headers and body) of the mail message, bh for the body hash (optionally limited to the first l octets of the body), d for the signing domain, and s for the selector.Īn Agent or User Identifier (AUID) can optionally be included. b (required), signature of headers and body.z (optional), header fields - copy of selected header fields and values.h (required), header fields - list of those that have been signed.i (optional), Agent or User Identifier (AUID).c (optional), canonicalization algorithm(s) for header and body.d (required), Signing Domain Identifier (SDID).The resulting header field consists of a list of tag=value parts as in the example below: The specification allows signers to choose which header fields they sign, but the From: field must always be signed. ![]() Signing modules insert one or more DKIM-Signature: header fields, possibly on behalf of the author organization or the originating service provider. The signing organization can be a direct handler of the message, such as the author, the submission site or a further intermediary along the transit path, or an indirect handler such as an independent service that is providing assistance to a direct handler. ![]() Hence, DKIM signatures survive basic relaying across multiple MTAs. Verifying modules typically act on behalf of the receiver organization, possibly at each hop.Īll of this is independent of Simple Mail Transfer Protocol (SMTP) routing aspects, in that it operates on the RFC 5322 message-the transported mail's header and body-not the SMTP "envelope" defined in RFC 5321. It does not directly prevent or disclose abusive behavior.ĭKIM also provides a process for verifying a signed message. ĭKIM provides the ability to sign a message, and allows the signer ( author organization) to communicate which email it considers legitimate. System administrators also have to deal with complaints about malicious email that appears to have originated from their systems, but did not. For example, a fraudster may send a message claiming to be from with the goal of convincing the recipient to accept and to read the email-and it is difficult for recipients to establish whether to trust this message. The need for email validated identification arises because forged addresses and content are otherwise easily created-and widely used in spam, phishing and other email-based fraud. It is defined in RFC 6376, dated September 2011 with updates in RFC 8301 and RFC 8463. Usually, DKIM signatures are not visible to end-users, and are affixed or verified by the infrastructure rather than the message's authors and recipients.ĭKIM is an Internet Standard. A valid signature also guarantees that some parts of the email (possibly including attachments) have not been modified since the signature was affixed. The recipient system can verify this by looking up the sender's public key published in the DNS. It achieves this by affixing a digital signature, linked to a domain name, to each outgoing email message. DomainKeys Identified Mail ( DKIM) is an email authentication method designed to detect forged sender addresses in email ( email spoofing), a technique often used in phishing and email spam.ĭKIM allows the receiver to check that an email claimed to have come from a specific domain was indeed authorized by the owner of that domain.
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