S/MIME

S/MIME - OverviewAfter the development of PEM industry working group led by RSA Security, Inc. started to develop another specification for conveying digitally signed and/or encrypted and digitally enveloped data in accordance to the MIME message formats.

S/MIME (Secure/Multipurpose Internet Mail Extension) is a security enhancement to the MIME Internet e-mail format standard.

S/MIME is not restricted to mail; it can be used with any transport mechanism that transports MIME data, such as HTTP.

S/MIME is likely to emerge as the industry standard for commercial and organizational use, while PGP will remain the choice for personal e-mail security for many.

S/MIME - OverviewS/MIME provides the following cryptography security services:Authentication.Message Integrity. By using digital signing Non-repudiation of origin.Privacy and data security. By using encryption

There are three versions of S/MIME: S/MIME version 1 (1995)- was specified and officially published in 1995 by RSA Security, Inc.

S/MIME version 2 (1998)- was specified in a pair of informational RFC documents - RFC 2311 and RFC 2312 - in March1998.

The work was continued in the IETF S/MIME Mail Security (SMIME) WG and resulted in S/MIME version 3 (1999) specified in RFCs 2630 to 2634 in June 1999.

MIME - OverviewRFC 822 defines a format for text messages that are sent using electronic mail.

SMTP/RFC822 scheme limitations:SMTP cannot transmit executable files or other binary files.SMTP cannot transmit text data that includes national language characters because these are represented by 8-bit codes with values of 128 decimal or higher, and SMTP is limited to 7-bit ASCII.SMTP servers may reject mail message over a certain size.SMTP gateways that translate between ASCII to EBCDIC suffer translation problems.Some SMTP implementations do not adhere completely to the SMTP standard defined in RFC 822.

MIME (contd.)MIME specification includes the following elements:Five new message header fields. These fields provide information about the body of the message.A number of content formats are defined, thus standardizing representations that supports multimedia e-mail.Transfer encodings are defined that enable that protect any content format to be altered by the mail system.

MIME provides a standardized way of dealing with a wide variety of information representations in a multimedia environment.

MIME (contd.)Here is a summary of the different MIME content types:

MIME (contd.)

MIME (contd.)The other major component of MIME is a definition of transfer encodings for message contents:

MIME (contd.)Canonical form is a format that is standardized for use between systems.

Conclusions:MIME is a necessity in todays Internet and e-mail traffic requirements.

The Object Oriented structure of the MIME message enhances its capability to serve as multipurpose standard.

The MIME is capable of transferring data between two distinct systems which uses different formats

S/MIME - Functions

S/MIME is based on the Cryptographic Message Syntax (CMS) specified in RFC 2630.Enveloped data: This consists of encrypted content of any type and encrypted content encryption keys for one or more users. This functions provides privacy and data security. Signed data: A digital signature is formed by signing the message digest and then encrypting that with the signer private key. The content and the signature are then encoded using base64 encoding.This function provides authenticity, message integrity and non-repudiation of origin.

S/MIME - FunctionsSignerInfo: allows the inclusion of unsigned and signed attributes to be included along with a signature. signingTimesMIMECapabilities sMIMEEncryptionKeyPreference

S/MIME - FunctionsClear signed data: In this case a digital signature of the content is formed, However only the signature is encoded with base64.

Signed and enveloped data: Because of S/MIME encapsulating capability (multipart type), signed only and encrypted only entities may be nested, so that encrypted data may be signed and signed data may be encrypted.

S/MIME - CryptographyDefinitions:MUST: The definition is an absolute requirement of the specification.SHOULD: There may exist valid reasons in particular circumstances to ignore this feature or function, but it is recommended that an implementation include the feature or function.

Be liberal in what you receive and conservative in what you send.

S/MIME - Cryptography Used Algorithms:

S/MIME - Cryptography

S/MIME - CryptographyAlgorithm use decision procedure:

Preferred decrypting capabilities: SHOULD choose the first (highest preference) capability on the list.

No list of capabilities but has received message/s: SHOULD use the same encryption algorithm as was used on the last signed and encrypted message.

No knowledge & Willing to risk: willing to risk that the recipient may not be able to decrypt the message, then the sending agent SHOULD use 3DES.

No knowledge & Not willing to risk:sending agent MUST use RC2/40.

S/MIME - CryptographyA possible problem: A multiple recipients message: Performance Security

The Solution: This problem is solved using an Enhanced Security service called S/MIME Mail List Agent (MLA). An MLA perform the recipient-specific encryption for each recipient, and forward the message.

S/MIME - MessageMIME bodies + CMS.

Canonical MIMECertificatesCMSCMS objectMIMEEncoding + Canonical form

S/MIME - MessageS/MIME makes use of a number of new MIME content types:

S/MIME - MessageEnveloped Data:

S/MIME - MessageSignedData:

S/MIME - MessageClear signing:Clear signing is achieved using the multipart content type with a signed sub-type .

Two parts:Clear text (or any MIME type) encoded in base64.SignedData.

S/MIME - MessageContent-Type: multipart/signed; protocol=application/pkcs7-signature; micalg=sha1; boundary=boundary42

--boundary42

Content-Type: text/plain This is a clear-signed message. --boundary42

Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7s

ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6 4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4

--boundary42-- This parameter indicates that this is a two part clear-signed entity.This parameter indicates the type of message digest used.SignerInfoHeaderUnsigned Data

S/MIME - MessageCertificate-only message:Used to transport certificates.

contains only certificates or a certificate revocation list (CRL).

Sent in response to a registration request.

The message is an application/pkcs7-mime type/subtype.

S/MIME - MessageCreating a Certificates-only Message:Step 1: The certificates are made available to the CMS generating process which creates a CMS object of type signedData. Step 2: The CMS signedData object is enclosed in anapplication/pkcs7-mime MIME entity.

The smime-type parameter for a certs-only message is "certs-only".

The file extension for this type of message is ".p7c".

S/MIME - MessageRegistration request:A message signer MUST have a certificate for the signature so that the receiving agent can verify the signature.

Exchange with CA, hardware token, diskette etc.

S/MIME v3- does not specify how to request a certification.

S/MIME v2- request by applying to a CA using the application/pkcs10 S/MIME entity.

A typical app. Only needs to send certification request.

S/MIME - MessageRegistration request:

S/MIME - CertificatesS/MIME uses public-key certificates that conform to version 3 of X.509.

A hybrid between a strict X.509 certification hierarchy and PGP's web of trust.

A receiving agent MUST provide some certificate retrieval mechanism.

Receiving and sending agents SHOULD also provide a mechanism to allow a user to "store and protect" certificates

S/MIME - Certificates Public key certificates are required to protect the authenticity and integrity of public keys, thus protecting against man-in-the-middle attack.

A certificate chain must be verified until a root CA is reached

S/MIME - Certificatesa certificate can only be trusted if: every certificate in the chain is successfully verified.every CA in the certificate chain is trusted.

In practice, certificate chains are short and seldom verified for trustworthiness.

Also, the concept of cross-certification is of low practical value and seldom used between certification service providers.

S/MIME - Certificates

S/MIME- User roleKey generation: MUST be capable of generating separate Diffie-Hellman and DSS key pairs. SHOULD be capable of generating RSA key pairs. good source of non-deterministic random. protected in a secure fashion.

Registration: A user's public key must be registered with a certification authority in order to receive an X.509 public-key certificate.

S/MIME- User roleCertificate storage and retrieval: access to a local list of certificates in order to verify incoming signatures and encrypt outgoing messages.

maintained by the user local administrative entity on behalf of number of users.

S-MIME -AttacksCertificate Management in S/MIME:CA-centered.

CA certificates come with the client software.

An ordinary user is not aware of the CAs that he/she trusts.

Certificates are sent along with the signed messages.

S-MIME -AttacksCertificates classes (common practice by most CAs) Class 1 Class 2 Class 3

CA certification policies ID-control practices Class 1: only email address Class 2: against third party database Class 3: apply in person and submit picture IDs and/or hard documentation

How to determine user by name? by e-mail address?

Tighter identityvalidationEasier toissue

S-MIME -AttacksAttack 1: Class 1 Certificate AttackNo identity check during registration.

Binding between public key and e-mail address.

It is possible to enroll under a different name.Name spoofing is possible in signed messages

E-mail clients do not make this fact explicit to average users.

S-MIME -AttacksAttack 1: Class 1 Certificate Attack

Step 1: Get an e-mail address that implies the person you want to imitate.

Step 2: Register for a certificate with that bogus name and e-mail address.

Step 3: Step up an outgoing e-mail account at your favorite e-mail client software with that bogus name.

Step 4: Send bogus signed messages

S/MIME- AttacksStep 2- Registration

S/MIME- Attacks

S/MIME- Attacks

S/MIME- Attacksshlomo@hotmail.com

S/MIME- Attacks

S/MIME-Attacks

S/MIME-AttacksStep 3 Setup local account

S/MIME-AttacksStep 4 Send signed but bogus msgs.

S/MIME-Attacks

Consequences:Loose control for Class 1 certificates.

The system becomes less secure for the name of security.

S/MIME-AttacksAttack 2: Use ones certificate to send emails under another name. Step 1: Set up another e-mail account at local client. Same e-mail address But a different name

Step 2: Send bogus signed messages

S/MIME- AttacksStep 1- setup another account

S/MIME- AttacksStep 2- Send bogus signed msg.

S/MIME-AttacksConsequences:During verification, e-mail client does notmatch the name in certificate with the namein e-mail. Only e-mail addresses are matched (as mentioned in RFC 2632 (S/MIME Certificate Handling).

Verifiers manual check is needed.

Not a specific problem of class-1 certificates-Same attack is possible using class-2 and class-3 certificates.-E-mail clients are not concerned with certificate classes.

S/MIME-AttacksAttack 3: Forging the headerThe scope of a S/MIME signature does not includethe e-mail header. from, to, cc, subject, date

Indeed, the mail header is modified without changing the verification status.

Problem of all classes of certificates.

S/MIME-AttacksWhat should be done?

Class 1 certificates should be discontinued.

E-mail clients must be aware of certificate classes and issue appropriate warnings to the verifiers.

It is up to you whether to believe a digital signature is valid or not Use your reasoning, not your e-mail clients.

Try to identify people by their e-mail addresses.

S/MIME-AttacksWhat should be done (2)?

Examine the details of certificate of the other party.

Do not trust Class 1 certificates.

Ask the sender to put all sensitive information within the message Senders identity Subject Date

Dont let subject say all!

S/MIME - SummeryIn summary, S/MIME provides a thoroughly designed and widely deployed technological approach to provide basic message protection services for the Internet.

S/MIME makes use of a hierarchical trust model based on ITU-T X.509.

Most importantly, S/MIME is strongly supported byall major vendors of UA products.

It very likely that S/MIME will become the predominant technology for secure messaging on the Internet.

S/MIME - SummeryIn contrast to PGP S/MIME cannot be used by user agent which don't support MIME.

There are problems in the stiches (certificate handling).

With the release of S/MIME v3, standardization activities have slowed down.

S/MIME

The End.