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100-413: 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 . DKIM allows the receiver to check that an email that claimed to have come from a specific domain was indeed authorized by the owner of that domain. It achieves this by affixing a digital signature , linked to

200-544: A Mail from: at the beginning of each message. Both of them can contain a domain name. The SPF verifier queries the Domain Name System (DNS) for a matching SPF record, which if it exists will specify the IP addresses authorized by that domain's administrator. The result can be "pass", "fail", or some intermediate result - and systems will generally take this into account in their anti-spam filtering. DKIM checks

300-470: A Received: field by the same domain from which the message was relayed. Additional Received: fields may appear between that and the top of the header, as the message got transferred internally between servers belonging to that same, trusted ADMD. The Internet Assigned Numbers Authority maintains a registry of Email Authentication Parameters . Not all parameters need to be registered, though. For example, there can be local "policy" values designed for

400-469: A symmetric key , which is then used by symmetric-key cryptography to transmit data using the now-shared symmetric key for a symmetric key encryption algorithm. PGP , SSH , and the SSL/TLS family of schemes use this procedure; they are thus called hybrid cryptosystems . The initial asymmetric cryptography-based key exchange to share a server-generated symmetric key from the server to client has

500-639: A " brute-force key search attack ". However, such an attack is impractical if the amount of computation needed to succeed – termed the "work factor" by Claude Shannon – is out of reach of all potential attackers. In many cases, the work factor can be increased by simply choosing a longer key. But other algorithms may inherently have much lower work factors, making resistance to a brute-force attack (e.g., from longer keys) irrelevant. Some special and specific algorithms have been developed to aid in attacking some public key encryption algorithms; both RSA and ElGamal encryption have known attacks that are much faster than

600-411: A " man-in-the-middle attack " is possible, making any subordinate certificate wholly insecure. Most of the available public-key encryption software does not conceal metadata in the message header, which might include the identities of the sender and recipient, the sending date, subject field, and the software they use etc. Rather, only the body of the message is concealed and can only be decrypted with

700-629: A British cryptographer at the UK Government Communications Headquarters (GCHQ), conceived of the possibility of "non-secret encryption", (now called public key cryptography), but could see no way to implement it. In 1973, his colleague Clifford Cocks implemented what has become known as the RSA encryption algorithm , giving a practical method of "non-secret encryption", and in 1974 another GCHQ mathematician and cryptographer, Malcolm J. Williamson , developed what

800-1056: A DKIM signature. Discussions about DKIM signatures passing through indirect mail flows, formally in the DMARC working group, took place right after the first adoptions of the new protocol wreaked havoc on regular mailing list use. However, none of the proposed DKIM changes passed. Instead, mailing list software was changed. Email authentication The original base of Internet email, Simple Mail Transfer Protocol (SMTP), has no such feature, so forged sender addresses in emails (a practice known as email spoofing ) have been widely used in phishing , email spam , and various types of frauds. To combat this, many competing email authentication proposals have been developed. By 2018 three had been widely adopted – SPF , DKIM and DMARC . The results of such validation can be used in automated email filtering , or can assist recipients when selecting an appropriate action. This article does not cover user authentication of email submission and retrieval. In

900-407: A command line: nslookup -q=TXT brisbane._domainkey.example.net ) as in this example: The available tags are: A CNAME record can also be used to point at a different TXT record, for example when one organization sends email on behalf of another. The receiver can use the public key (value of the p tag) to then validate the signature on the hash value in the header field, and check it against

1000-419: A correct source domain, other filtering techniques can work more effectively. In particular, the source domain can feed into a reputation system to better identify spam. Conversely, DKIM can make it easier to identify mail that is known not to be spam and need not be filtered. If a receiving system has a whitelist of known good sending domains, either locally maintained or from third party certifiers, it can skip

1100-473: A data collection involving 21 mail servers and millions of messages. 92.3% of observed signatures were successfully verified, a success rate that drops slightly (90.5%) when only mailing list traffic is considered. The problems might be exacerbated when filtering or relaying software makes changes to a message. Without specific precaution implemented by the sender, the footer addition operated by most mailing lists and many central antivirus solutions will break

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1200-525: A different character set, and often document this with X-MIME-Autoconverted: header fields. In addition, servers in certain circumstances have to rewrite the MIME structure, thereby altering the preamble , the epilogue , and entity boundaries, any of which breaks DKIM signatures. Only plain text messages written in us-ascii , provided that MIME header fields are not signed, enjoy the robustness that end-to-end integrity requires. The OpenDKIM Project organized

1300-472: A direct handler of the message, such as the author, mail submission agent , site, or further intermediary along the transit path, or an indirect handler such as an independent service that is providing assistance to a direct handler. Signing modules insert one or more DKIM-Signature: header fields, possibly on behalf of the author organization or the originating service provider. The specification allows signers to choose which header fields they sign, but

1400-407: A document or communication. Further applications built on this foundation include: digital cash , password-authenticated key agreement , time-stamping services and non-repudiation protocols. Because asymmetric key algorithms are nearly always much more computationally intensive than symmetric ones, it is common to use a public/private asymmetric key-exchange algorithm to encrypt and exchange

1500-548: A domain administrator will authorize the IP addresses used by their own outbound MTAs, including any proxy or smarthost. The IP address of the sending MTA is guaranteed to be valid by the Transmission Control Protocol , as it establishes the connection by checking that the remote host is reachable. The receiving mail server receives the HELO SMTP command soon after the connection is set up, and

1600-466: A domain name, to each outgoing email message. The recipient system can verify this by looking up the sender's public key published in the DNS . A valid signature also guarantees that some parts of the email (possibly including attachments ) have not been modified since the signature was affixed. Usually, DKIM signatures are not visible to end-users, and are affixed or verified by the infrastructure rather than

1700-440: A field with that name will break the signature. The DKIM-Signature: field of the signature being created, with bh equal to the computed body hash and b equal to the empty string, is implicitly added to the second hash, albeit its name must not appear in h — if it does, it refers to another, preexisting signature. For both hashes, text is canonicalized according to the relevant c algorithms. The result, after encryption with

1800-503: A file, so as to obtain a signed copy of the message. Use of the l tag in signatures makes doctoring such messages even easier. The signed copy can then be forwarded to a million recipients, for example through a botnet , without control. The email provider who signed the message can block the offending user, but cannot stop the diffusion of already-signed messages. The validity of signatures in such messages can be limited by always including an expiration time tag in signatures, or by revoking

1900-414: A key length, the chief security risk is that the private key of a pair becomes known. All security of messages, authentication, etc., will then be lost. Additionally, with the advent of quantum computing , many asymmetric key algorithms are considered vulnerable to attacks, and new quantum-resistant schemes are being developed to overcome the problem. All public key schemes are in theory susceptible to

2000-691: A long list of "self-signed identity certificates" from PKI providers – these are used to check the bona fides of the certificate authority and then, in a second step, the certificates of potential communicators. An attacker who could subvert one of those certificate authorities into issuing a certificate for a bogus public key could then mount a "man-in-the-middle" attack as easily as if the certificate scheme were not used at all. An attacker who penetrates an authority's servers and obtains its store of certificates and keys (public and private) would be able to spoof, masquerade, decrypt, and forge transactions without limit, assuming that they were able to place themselves in

2100-604: A major advantage over your opponent. Only at the end of the evolution from Berners-Lee designing an open internet architecture for CERN , its adaptation and adoption for the Arpanet ... did public key cryptography realise its full potential. — Ralph Benjamin These discoveries were not publicly acknowledged for 27 years, until the research was declassified by the British government in 1997. In 1976, an asymmetric key cryptosystem

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2200-550: A man-in-the-middle attack relatively straightforward. Capturing the public key would only require searching for the key as it gets sent through the ISP's communications hardware; in properly implemented asymmetric key schemes, this is not a significant risk. In some advanced man-in-the-middle attacks, one side of the communication will see the original data while the other will receive a malicious variant. Asymmetric man-in-the-middle attacks can prevent users from realizing their connection

2300-418: A non-wanted feature of DKIM, forced by behaviors such as those just described. Indeed, DKIM protocol provides for expiration. There is an optional x tag on each signature, which establishes a formal expiration time; however, verifiers can ignore it. In addition, domain owners can revoke a public key by removing its cryptographic data from the record, thereby preventing signature verification unless someone saved

2400-556: A policy that specifies which mechanism (DKIM, SPF, or both) is employed when sending email from that domain; how to check the From: field presented to end users; how the receiver should deal with failures—and a reporting mechanism for actions performed under those policies. The primary advantage of this system for e-mail recipients is in allowing the signing domain to reliably identify a stream of legitimate email, thereby allowing domain-based blacklists and whitelists to be more effective. This

2500-597: A prior shared secret. Merkle's "public key-agreement technique" became known as Merkle's Puzzles , and was invented in 1974 and only published in 1978. This makes asymmetric encryption a rather new field in cryptography although cryptography itself dates back more than 2,000 years. In 1977, a generalization of Cocks's scheme was independently invented by Ron Rivest , Adi Shamir and Leonard Adleman , all then at MIT . The latter authors published their work in 1978 in Martin Gardner 's Scientific American column, and

2600-578: A process for verifying a signed message. Verifying modules typically act on behalf of the receiver organization, possibly at each hop . All 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. Hence, DKIM signatures survive basic relaying across multiple message transfer agents . The signing organization can be

2700-493: A public key encryption system is for encrypting communication to provide confidentiality – a message that a sender encrypts using the recipient's public key, which can be decrypted only by the recipient's paired private key. Another application in public key cryptography is the digital signature . Digital signature schemes can be used for sender authentication . Non-repudiation systems use digital signatures to ensure that one party cannot successfully dispute its authorship of

2800-400: A public key periodically or upon a notification of an incident. Effectiveness of the scenario can hardly be limited by filtering outgoing mail, as that implies the ability to detect if a message might potentially be useful to spammers. DKIM currently features two canonicalization algorithms, simple and relaxed , neither of which is MIME -aware. Mail servers can legitimately convert to

2900-484: A purpose-built program running on a server computer – vouches for the identities assigned to specific private keys by producing a digital certificate. Public key digital certificates are typically valid for several years at a time, so the associated private keys must be held securely over that time. When a private key used for certificate creation higher in the PKI server hierarchy is compromised, or accidentally disclosed, then

3000-512: A site's internal use only, which correspond to local configuration and need no registration. Public-key cryptography Public-key cryptography , or asymmetric cryptography , is the field of cryptographic systems that use pairs of related keys. Each key pair consists of a public key and a corresponding private key . Key pairs are generated with cryptographic algorithms based on mathematical problems termed one-way functions . Security of public-key cryptography depends on keeping

3100-416: A trace header field Authentication-Results: where a receiver can record the results of email authentication checks that it carried out. Multiple results for multiple methods can be reported in the same field, separated by semicolons and wrapped as appropriate. For example, the following field is purportedly written by receiver.example.org and reports SPF and DKIM results: The first token after

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3200-486: A trusted courier. This key, which both parties must then keep absolutely secret, could then be used to exchange encrypted messages. A number of significant practical difficulties arise with this approach to distributing keys . In his 1874 book The Principles of Science , William Stanley Jevons wrote: Can the reader say what two numbers multiplied together will produce the number 8616460799 ? I think it unlikely that anyone but myself will ever know. Here he described

3300-485: A wired route inside the sender's own building. In summation, public keys are easier to alter when the communications hardware used by a sender is controlled by an attacker. One approach to prevent such attacks involves the use of a public key infrastructure (PKI); a set of roles, policies, and procedures needed to create, manage, distribute, use, store and revoke digital certificates and manage public-key encryption. However, this has potential weaknesses. For example,

3400-517: Is a fixed part of the specification. This gives the TXT resource record to be looked up as: brisbane._domainkey.example.net Note that the selector and the domain name can be UTF-8 in internationalized email. In that case the label must be encoded according to IDNA before lookup. The data returned from the query of this record is also a list of tag-value pairs. It includes the domain's public key , along with other key usage tokens and flags (e.g. from

3500-612: Is a hallmark of digital postmarks, making sending bulk spam more (computationally) expensive. This facet of DKIM may look similar to hashcash , except that the receiver side verification is a negligible amount of work, while a typical hashcash algorithm would require far more work. DKIM's non-repudiation feature prevents senders (such as spammers) from credibly denying having sent an email. It has proven useful to news media sources such as WikiLeaks , which has been able to leverage DKIM body signatures to prove that leaked emails were genuine and not tampered with. Many consider non-repudiation

3600-573: Is a necessary first step towards identifying the origin of messages, and thereby making policies and laws more enforceable. Hinging on domain ownership is a stance that emerged in the early 2000. It implies a coarse-grained authentication, given that domains appear on the right part of email addresses, after the at sign . Fine-grain authentication, at user level, can be achieved by other means, such as Pretty Good Privacy and S/MIME . At present, digital identity needs to be managed by each individual. An outstanding rationale for email authentication

3700-567: Is added to the message header itself, as a trace field. Any number of relays can receive and forward the message and at every hop, the signature can be verified by retrieving the public key from the DNS. As long as intermediate relays do not modify signed parts of a message, its DKIM-signatures remain valid. DMARC allows the specification of a policy for authenticated messages. It is built on top of two existing mechanisms, Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM). It allows

3800-411: Is also likely to make certain kinds of phishing attacks easier to detect. There are some incentives for mail senders to sign outgoing e-mail: DKIM is a method of labeling a message, and it does not itself filter or identify spam. However, widespread use of DKIM can prevent spammers from forging the source address of their messages, a technique they commonly employ today. If spammers are forced to show

3900-610: Is an email authentication system designed to allow an intermediate mail server like a mailing list or forwarding service to sign an email's original authentication results. This allows a receiving service to validate an email when the email's SPF and DKIM records are rendered invalid by an intermediate server's processing. ARC is defined in RFC 8617, published in July 2019, as "Experimental". In October 2012, Wired reported that mathematician Zach Harris detected and demonstrated an email source spoofing vulnerability with short DKIM keys for

4000-413: Is compromised. This remains so even when one user's data is known to be compromised because the data appears fine to the other user. This can lead to confusing disagreements between users such as "it must be on your end!" when neither user is at fault. Hence, man-in-the-middle attacks are only fully preventable when the communications infrastructure is physically controlled by one or both parties; such as via

4100-401: Is genuine by verifying the signature using the public key. As long as the software publisher keeps the private key secret, even if a forger can distribute malicious updates to computers, they cannot convince the computers that any malicious updates are genuine. For example, a journalist can publish the public key of an encryption key pair on a web site so that sources can send secret messages to

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4200-454: Is never trivial and very rapidly becomes unmanageable as the number of participants increases, or when secure channels are not available, or when, (as is sensible cryptographic practice), keys are frequently changed. In particular, if messages are meant to be secure from other users, a separate key is required for each possible pair of users. By contrast, in a public-key cryptosystem, the public keys can be disseminated widely and openly, and only

4300-462: Is now known as Diffie–Hellman key exchange . The scheme was also passed to the US's National Security Agency . Both organisations had a military focus and only limited computing power was available in any case; the potential of public key cryptography remained unrealised by either organization: I judged it most important for military use ... if you can share your key rapidly and electronically, you have

4400-465: Is slightly harder to learn what identities it can trust. Since users can receive email from multiple domains—e.g., if they have multiple email addresses -— any of those domains could let Authentication-Results: fields pass through because they looked neutral. That way, a malicious sender can forge an authserv-id that the user would trust if the message arrived from a different domain. A legitimate Authentication-Results: typically appears just above

4500-475: Is the ability to automate email filtering at receiving servers. That way, spoofed messages can be rejected before they arrive to a user's Inbox. While protocols strive to devise ways to reliably block distrusted mail, security indicators can tag unauthenticated messages that still reach the Inbox. A 2018 study shows that security indicators can lower the click-through ratio by more than ten points, 48.9% to 37.2% of

4600-594: Is transparent to existing e-mail systems that lack DKIM support. This design approach also is compatible with other, related services, such as the S/MIME and OpenPGP content-protection standards. DKIM is compatible with the DNSSEC standard and with SPF . DKIM requires cryptographic checksums to be generated for each message sent through a mail server, which results in computational overhead not otherwise required for e-mail delivery. This additional computational overhead

4700-406: The From: field must always be signed. The resulting header field consists of a list of tag=value parts as in the example below: where the tags used are: 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

4800-724: The google.com corporate domain, as well as several other high-profile domains. He stated that authentication with 384-bit keys can be factored in as little as 24 hours "on my laptop," and 512-bit keys, in about 72 hours with cloud computing resources. Harris found that many organizations sign email with such short keys; he factored them all and notified the organizations of the vulnerability. He states that 768-bit keys could be factored with access to very large amounts of computing power, so he suggests that DKIM signing should use key lengths greater than 1,024. Wired stated that Harris reported, and Google confirmed, that they began using new longer keys soon after his disclosure. According to RFC 6376

4900-579: The Internet Message Format . SMTP defines the trace information of a message, which is saved in the header using the following two fields: A mail user agent (MUA) knows the outgoing mail SMTP server from its configuration. An MTA (or a relay server) typically determines which server to connect to by looking up the MX (Mail eXchange) DNS resource record for each recipient's domain name . The path depicted below can be reconstructed on

5000-489: The message content , deploying digital signatures . Rather than using digital certificates, the keys for signature-verification are distributed via the DNS. That way, a message gets associated to a domain name. A DKIM-compliant domain administrator generates one or more pairs of asymmetric keys , then hands private keys to the signing MTA, and publishes public keys on the DNS. The DNS labels are structured as selector ._domainkey.example.com , where selector identifies

5100-535: The DKIM signature. A possible mitigation is to sign only designated number of bytes of the message body. It is indicated by l tag in DKIM-Signature header. Anything added beyond the specified length of the message body is not taken into account while calculating DKIM signature. This won't work for MIME messages. Another workaround is to whitelist known forwarders; e.g., by SPF . For yet another workaround, it

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5200-502: The DNS queries for those keys and filtering out the high number of queries due to e-mail being sent to large mailing lists or malicious queries by bad actors. For a comparison of different methods also addressing this problem see e-mail authentication . As mentioned above, authentication is not the same as abuse prevention. A malicious email user of a reputable domain can compose a bad message and have it DKIM-signed and sent from that domain to any mailbox from where they can retrieve it as

5300-459: The IP was set up properly in the DNS. The reverse resolution of a range of IP addresses can be delegated to the ADMD that uses them, or can remain managed by the network provider. In the latter case, no useful identity related to the message can be obtained. Looking up a DNSWL (DNS-based whitelist) may provide an assessment of the sender, possibly including its identification. RFC 8601 defines

5400-421: The PKI system (software, hardware, and management) is trust-able by all involved. A " web of trust " decentralizes authentication by using individual endorsements of links between a user and the public key belonging to that user. PGP uses this approach, in addition to lookup in the domain name system (DNS). The DKIM system for digitally signing emails also uses this approach. The most obvious application of

5500-577: The administrative owner of a domain to publish a policy in their DNS records to specify which mechanism (DKIM, SPF or both) is employed when sending email from that domain; how to check the From: field presented to end users; how the receiver should deal with failures - and a reporting mechanism for actions performed under those policies. A range of other methods have been proposed, but are now either deprecated or have not yet gained widespread support. These have included Sender ID , Certified Server Validation , DomainKeys and those below: ADSP allowed

5600-517: The advantage of not requiring that a symmetric key be pre-shared manually, such as on printed paper or discs transported by a courier, while providing the higher data throughput of symmetric key cryptography over asymmetric key cryptography for the remainder of the shared connection. As with all security-related systems, there are various potential weaknesses in public-key cryptography. Aside from poor choice of an asymmetric key algorithm (there are few that are widely regarded as satisfactory) or too short

5700-411: The algorithm came to be known as RSA , from their initials. RSA uses exponentiation modulo a product of two very large primes , to encrypt and decrypt, performing both public key encryption and public key digital signatures. Its security is connected to the extreme difficulty of factoring large integers , a problem for which there is no known efficient general technique. A description of the algorithm

5800-413: The attacker using the correct public keys for the different communication segments so as to avoid suspicion. A communication is said to be insecure where data is transmitted in a manner that allows for interception (also called " sniffing "). These terms refer to reading the sender's private data in its entirety. A communication is particularly unsafe when interceptions can not be prevented or monitored by

5900-434: The available metadata to a third party. The concept is based around an open repository containing separately encrypted metadata blocks and encrypted messages. Only the intended recipient is able to decrypt the metadata block, and having done so they can identify and download their messages and decrypt them. Such a messaging system is at present in an experimental phase and not yet deployed. Scaling this method would reveal to

6000-683: The basis for a series of IETF standards-track specifications and support documents which eventually resulted in STD 76 , currently RFC 6376. "Identified Internet Mail" was proposed by Cisco as a signature-based mail authentication standard, while DomainKeys was designed by Yahoo to verify the DNS domain of an e-mail sender and the message integrity . Aspects of DomainKeys, along with parts of Identified Internet Mail, were combined to create DomainKeys Identified Mail (DKIM). Trendsetting providers implementing DKIM include Yahoo , Gmail , AOL and FastMail . Any mail from these organizations should carry

6100-447: The brute-force approach. None of these are sufficiently improved to be actually practical, however. Major weaknesses have been found for several formerly promising asymmetric key algorithms. The "knapsack packing" algorithm was found to be insecure after the development of a new attack. As with all cryptographic functions, public-key implementations may be vulnerable to side-channel attacks that exploit information leakage to simplify

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6200-441: The certificate authority issuing the certificate must be trusted by all participating parties to have properly checked the identity of the key-holder, to have ensured the correctness of the public key when it issues a certificate, to be secure from computer piracy, and to have made arrangements with all participants to check all their certificates before protected communications can begin. Web browsers , for instance, are supplied with

6300-495: The communication stream. Despite its theoretical and potential problems, Public key infrastructure is widely used. Examples include TLS and its predecessor SSL , which are commonly used to provide security for web browser transactions (for example, most websites utilize TLS for HTTPS ). Aside from the resistance to attack of a particular key pair, the security of the certification hierarchy must be considered when deploying public key systems. Some certificate authority – usually

6400-415: The confidentiality and authenticity of electronic communications and data storage. They underpin numerous Internet standards, such as Transport Layer Security (TLS) , SSH , S/MIME , and PGP . Compared to symmetric cryptography , public-key cryptography can be too slow for many purposes, so these protocols often combine symmetric cryptography with public-key cryptography in hybrid cryptosystems . Before

6500-436: The corresponding private keys need be kept secret. The two best-known types of public key cryptography are digital signature and public-key encryption : For example, a software publisher can create a signature key pair and include the public key in software installed on computers. Later, the publisher can distribute an update to the software signed using the private key, and any computer receiving an update can confirm it

6600-538: The destination's MX (that is B → D in the figures). The sender's ADMD can add authentication tokens only if the message goes through its boxes. The most common cases can be schematized as follows: Access Providers MUST NOT block users from accessing the external Internet using the SUBMISSION port 587. SPF allows the receiver to check that an email claimed to have come from a specific domain comes from an IP address authorized by that domain's administrators. Usually,

6700-466: The early 1980s, when Simple Mail Transfer Protocol (SMTP) was designed, it provided for no real verification of sending user or system. This was not a problem while email systems were run by trusted corporations and universities, but since the commercialization of the Internet in the early 1990s, spam , phishing , and other crimes have been found to increasingly involve email. Email authentication

6800-474: The field name, receiver.example.org , is the ID of the authentication server, a token known as an authserv-id . A receiver supporting RFC 8601 is responsible to remove (or rename) any false header claiming to belong to its domain so that downstream filters cannot get confused. However, those filters still need to be configured, as they have to know which identities the domain may use. For a Mail User Agent (MUA), it

6900-403: The filtering on signed mail from those domains, and perhaps filter the remaining mail more aggressively. DKIM can be useful as an anti- phishing technology. Mailers in heavily phished domains can sign their mail to show that it is genuine. Recipients can take the absence of a valid signature on mail from those domains to be an indication that the mail is probably forged. The best way to determine

7000-515: 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. System administrators also have to deal with complaints about malicious email that appears to have originated from their systems, but did not. DKIM provides the ability to sign a message, and allows the signer ( author organization) to communicate which email it considers legitimate. It does not directly prevent or disclose abusive behavior. DKIM also provides

7100-417: The ground of the trace header fields that each host adds to the top of the header when it receives the message: The first few lines at the top of the header are usually trusted by the recipient. Those lines are written by machines in the recipient's Administrative Management Domain ( ADMD ), which act upon their explicit mandate. By contrast, the lines that prove the involvement of A and B , as well as of

7200-563: The hash value for the mail message (headers and body) that was received. If the two values match, this cryptographically proves that the mail was signed by the indicated domain and has not been tampered with in transit. Signature verification failure does not force rejection of the message. Instead, the precise reasons why the authenticity of the message could not be proven should be made available to downstream and upstream processes. Methods for doing so may include sending back an FBL message , or adding an Authentication-Results: header field to

7300-469: The key pair, and _domainkey is a fixed keyword, followed by the signing domain's name so that publication occurs under the authority of that domain's ADMD. Just before injecting a message into the SMTP transport system, the signing MTA creates a digital signature that covers selected fields of the header and the body (or just its beginning). The signature should cover substantive header fields such as From: , To: , Date: , and Subject: , and then

7400-515: The message as described in RFC 7001. DomainKeys was covered by U.S. patent 6,986,049 , now expired. Yahoo! licensed its patent claims under a dual license scheme: the DomainKeys Patent License Agreement v1.2 , or GNU General Public License v2.0 (and no other version) . In essence, both DKIM and SPF provide different measures of email authenticity. DMARC provides the ability for an organisation to publish

7500-492: The message envelope, which holds the return-path and message recipients. Since DKIM does not attempt to protect against mis-addressing, this does not affect its utility. A number of concerns were raised and refuted in 2013 at the time of the standardization. A concern for any cryptographic solution would be message replay abuse, which bypasses techniques that currently limit the level of abuse from larger domains. Replay can be inferred by using per-message public keys, tracking

7600-436: The message's authors and recipients. DKIM is an Internet Standard . It is defined in RFC 6376, dated September 2011, with updates in RFC 8301 and RFC 8463. 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. For example, a fraudster may send a message claiming to be from sender@example.com , with

7700-414: The mid-1970s, all cipher systems used symmetric key algorithms , in which the same cryptographic key is used with the underlying algorithm by both the sender and the recipient, who must both keep it secret. Of necessity, the key in every such system had to be exchanged between the communicating parties in some secure way prior to any use of the system – for instance, via a secure channel . This requirement

7800-461: The news organization in ciphertext. Only the journalist who knows the corresponding private key can decrypt the ciphertexts to obtain the sources' messages—an eavesdropper reading email on its way to the journalist cannot decrypt the ciphertexts. However, public-key encryption does not conceal metadata like what computer a source used to send a message, when they sent it, or how long it is. Public-key encryption on its own also does not tell

7900-411: The private key of the intended recipient. This means that a third party could construct quite a detailed model of participants in a communication network, along with the subjects being discussed, even if the message body itself is hidden. However, there has been a recent demonstration of messaging with encrypted headers, which obscures the identities of the sender and recipient, and significantly reduces

8000-454: The private key secret; the public key can be openly distributed without compromising security. There are many kinds of public-key cryptosystems, with different security goals, including digital signature , Diffie-Hellman key exchange , public-key key encapsulation , and public-key encryption . Public key algorithms are fundamental security primitives in modern cryptosystems , including applications and protocols that offer assurance of

8100-417: The public key data beforehand. DKIM key rotation is often recommended just to minimize the impact of compromised keys. However, in order to definitely disable non-repudiation, expired secret keys can be published, thereby allowing everyone to produce fake signatures, thus voiding the significance of original ones. The RFC itself identifies a number of potential attack vectors. DKIM signatures do not encompass

8200-410: The purported author's MUA could be a counterfeit created by C . The Received: field shown above is an epoch-making piece of the header. The Return-Path: is written by E , the mail delivery agent (MDA), based on the message envelope . Additional trace fields, designed for email authentication, can populate the top of the header. Normally, messages sent out by an author's ADMD go directly to

8300-439: The receiver to make an informed decision whether a certain mail is spam or not. For example, using DMARC, eBay and PayPal both publish policies that all of their mail is authenticated, and requesting that any receiving system, such as Gmail , should reject any that is not. Because it is implemented using DNS records and an added RFC 5322 header field, DKIM is compatible with the existing e-mail infrastructure. In particular, it

8400-497: The receiving party must be able to validate signatures with keys ranging from 512 bits to 2048 bits, thus usage of keys shorter than 512 bits might be incompatible and shall be avoided. RFC 6376 also states that signers must use keys of at least 1024 bits for long-lived keys, though long-livingness is not specified there. DKIM resulted in 2004 from merging two similar efforts, "enhanced DomainKeys" from Yahoo and "Identified Internet Mail" from Cisco . This merged specification has been

8500-576: The recipient anything about who sent a message —it just conceals the content of the message. One important issue is confidence/proof that a particular public key is authentic, i.e. that it is correct and belongs to the person or entity claimed, and has not been tampered with or replaced by some (perhaps malicious) third party. There are several possible approaches, including: A public key infrastructure (PKI), in which one or more third parties – known as certificate authorities – certify ownership of key pairs. TLS relies upon this. This implies that

8600-467: The relationship of one-way functions to cryptography, and went on to discuss specifically the factorization problem used to create a trapdoor function . In July 1996, mathematician Solomon W. Golomb said: "Jevons anticipated a key feature of the RSA Algorithm for public key cryptography, although he certainly did not invent the concept of public key cryptography." In 1970, James H. Ellis ,

8700-415: The reputation of domains. A sender can apply for a reference at a vouching authority. The reference, if accepted, is published on the DNS branch managed by that authority. A vouched sender should add a VBR-Info: header field to the messages it sends. It should also add a DKIM signature, or use some other authentication method, such as SPF. A receiver, after validating the sender's identity, can verify

8800-547: The search for a secret key. These are often independent of the algorithm being used. Research is underway to both discover, and to protect against, new attacks. Another potential security vulnerability in using asymmetric keys is the possibility of a "man-in-the-middle" attack , in which the communication of public keys is intercepted by a third party (the "man in the middle") and then modified to provide different public keys instead. Encrypted messages and responses must, in all instances, be intercepted, decrypted, and re-encrypted by

8900-451: The selector. An Agent or User Identifier (AUID) can optionally be included. The format is an email address with an optional local-part. The domain must be equal to, or a subdomain of, the signing domain. 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. Both header and body contribute to

9000-514: The sender. A man-in-the-middle attack can be difficult to implement due to the complexities of modern security protocols. However, the task becomes simpler when a sender is using insecure media such as public networks, the Internet , or wireless communication. In these cases an attacker can compromise the communications infrastructure rather than the data itself. A hypothetical malicious staff member at an Internet service provider (ISP) might find

9100-469: The set of domains that merit this degree of scrutiny remains an open question. DKIM used to have an optional feature called ADSP that lets authors that sign all their mail self-identify, but it was demoted to historic status in November 2013. Instead, DMARC can be used for the same purpose and allows domains to self-publish which techniques (including SPF and DKIM) they employ, which makes it easier for

9200-418: The signature. First, the message body is hashed, always from the beginning, possibly truncated to a given length l (which may be zero). Second, selected header fields are hashed, in the order given by h . Repeated field names are matched from the bottom of the header upward, which is the order in which Received: fields are inserted in the header. A non-existing field matches the empty string, so that adding

9300-444: The signer's private key and encoding using Base64, is b . In addition to the list of header fields listed in h , a list of header fields (including both field name and value) present at the time of signing may be provided in z . This list need not match the list of headers in h . Algorithms, fields, and body length are meant to be chosen so as to assure unambiguous message identification while still allowing signatures to survive

9400-444: The specification of a policy for messages signed by the author's domain. A message had to go through DKIM authentication first, then ADSP could demand a punishing treatment if the message was not signed by the author domain(s) —as per the From: header field. ADSP was demoted to historic in November 2013. VBR adds a vouch to an already authenticated identity. This method requires some globally recognized authorities that certify

9500-407: The third party only the inbox server being used by the recipient and the timestamp of sending and receiving. The server could be shared by thousands of users, making social network modelling much more challenging. During the early history of cryptography , two parties would rely upon a key that they would exchange by means of a secure, but non-cryptographic, method such as a face-to-face meeting, or

9600-403: The unavoidable changes which are going to occur in transit. No end-to-end data integrity is implied. A receiving SMTP server wanting to verify uses the domain name and the selector to perform a DNS lookup. For example, given the example signature above: the d tag gives the author domain to be verified against, example.net  ; the s tag the selector, brisbane . The string _domainkey

9700-430: The users who open spoofed messages. SMTP defines message transport , not the message content . Thus, it defines the mail envelope and its parameters, such as the envelope sender , but not the header (except trace information ) nor the body of the message itself. STD 10 and RFC   5321 define SMTP (the envelope), while STD 11 and RFC   5322 define the message (header and body), formally referred to as

9800-573: The vouch claimed in VBR-Info: by looking up the reference. Applications should avoid using this method as a means of authentication. Nevertheless, it is often carried out and its results, if any, written in the Received: header field besides the TCP information required by the SMTP specification. The IP reverse, confirmed by looking up the IP address of the name just found, is just an indication that

9900-399: Was proposed that forwarders verify the signature, modify the email, and then re-sign the message with a Sender: header. However, this solution has its risk with forwarded third party signed messages received at SMTP receivers supporting the RFC 5617 ADSP protocol. Thus, in practice, the receiving server still has to whitelist known message streams . The Authenticated Received Chain (ARC)

10000-517: Was published by Whitfield Diffie and Martin Hellman who, influenced by Ralph Merkle 's work on public key distribution, disclosed a method of public key agreement. This method of key exchange, which uses exponentiation in a finite field , came to be known as Diffie–Hellman key exchange . This was the first published practical method for establishing a shared secret-key over an authenticated (but not confidential) communications channel without using

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