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S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for public key encryption and signing of MIME data. S/MIME is on an IETF standards track and defined in a number of documents, most importantly RFC 3369, 3370, 3850 and 3851. It was originally developed by RSA Data Security Inc. and the original specification used the IETF MIME specification with the de facto industry standard PKCS#7 secure message format. Change control to S/MIME has since been vested in the IETF and the specification is now layered on Cryptographic Message Syntax, an IETF specification that is identical in most respects with PKCS #7. S/MIME functionality is built into the majority of modern email software and interoperates between them.
S/MIME provides the following cryptographic security services for electronic messaging applications:
S/MIME specifies the MIME type
application/pkcs7-mime (smime-type "enveloped-data") for data enveloping (encrypting) where the whole (prepared) MIME entity to be enveloped is encrypted and packed into an object which subsequently is inserted into an application/pkcs7-mime MIME entity.
Before S/MIME can be used in any of the above applications, one must obtain and install an individual key/certificate either from one's in-house certificate authority (CA) or from a public CA. The accepted best practice is to use separate private keys (and associated certificates) for signature and for encryption, as this permits escrow of the encryption key without compromise to the non-repudiation property of the signature key. Encryption requires having the destination party's certificate on store (which is typically automatic upon receiving a message from the party with a valid signing certificate). While it is technically possible to send a message encrypted (using the destination party certificate) without having one's own certificate to digitally sign, in practice, the S/MIME clients will require the user to install their own certificate before they allow encrypting to others.
A typical basic ("class 1") personal certificate verifies the owner's "identity" only insofar as it declares that the sender is the owner of the "From:" email address in the sense that the sender can receive email sent to that address, and so merely proves that an email received really did come from the "From:" address given. It does not verify the person's name or business name. If a sender wishes to enable email recipients to verify the sender's identity in the sense that a received certificate name carries the sender's name or an organization's name, the sender needs to obtain a certificate ("class 2") from a CA who carries out a more in-depth identity verification process, and this involves making inquiries about the would-be certificate holder. For more detail on authentication, see digital signature.
Depending on the policy of the CA, the certificate and all its contents may be posted publicly for reference and verification. This makes the name and email address available for all to see and possibly search for. Other CAs only post serial numbers and revocation status, which does not include any of the personal information. The latter, at a minimum, is mandatory to uphold the integrity of the public key infrastructure.
Any message that an S/MIME email client stores encrypted cannot be decrypted if the applicable key pair's private key is unavailable or otherwise unusable (e.g., the certificate has been deleted or lost or the private key's password has been forgotten). However, an expired, revoked, or untrusted certificate will remain usable for cryptographic purposes. Indexing of encrypted messages' clear text may not be possible with all email clients. Neither of these potential dilemmas is specific to S/MIME but rather cipher text in general and do not apply to S/MIME messages that are only signed and not encrypted.
S/MIME signatures are usually "detached signatures": the signature information is separate from the text being signed. The MIME type for this is multipart/signed with the second part having a MIME subtype of application/(x-)pkcs7-signature. Mailing list software is notorious for changing the textual part of a message and thereby invalidating the signature; however, this problem is not specific to S/MIME, and a digital signature only reveals that the signed content has been changed.
On May 13th 2018, the Electronic Frontier Foundation (EFF) announced critical vulnerabilities in S/MIME, together with an obsolete form of PGP that is still used in many email clients. Dubbed EFAIL, this is a particularly critical hit to S/MIME that will require significant coordinated effort by many vendors to fix.
S/MIME adds new MIME content types that provide data confidentiality, integrity protection, nonrepudiation, and authentication services: application/pkcs7-mime, multipart/signed, and application/pkcs7-signature
Only for personal use (non-commercial)