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S/MIME S/MIME - Overview After 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 - Overview S/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 - Overview RFC 822 defines a format for text messages that are sent using electronic mail. SMTP/RFC822 scheme limitations: 1. SMTP cannot transmit executable files or other binary files. 2. 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. 3. SMTP servers may reject mail message over a certain size. 4. SMTP gateways that translate between ASCII to EBCDIC suffer translation problems. 5. Some SMTP implementations do not adhere completely to the SMTP standard defined in RFC 822. MIME (contd.) MIME specification includes the following elements: 1. Five new message header fields. These fields provide information about the body of the message. 2. A number of content formats are defined, thus standardizing representations that supports multimedia e-mail. 3. 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: Type Subtype Description Text Plain Enriched Unformatted text (ASCII or ISO 8859). Provides greater format flexibility. Multipart Mixed The different parts are independent but are to be transmitted together. Should be presented to the receiver in their original order. Parallel Differs from mixed only in that no order is defined. Alternative The different parts are alternative versions of the same information. Digest Similar to Mixed but the default type/subtype of each part is message/rfc822. Message rfc822 The body is itself an encapsulated message that conforms to RFC822. Partial Used to allow fragmentation in a transparent way to the recipient. External body Contains a pointer to an object exists else where. MIME (contd.) Type Subtype Description Image Jpeg gif The image is in JPEG format. The image is in GIF format. Video Mpeg MPEG format. Audio Basic Single-channel 8-bit ISDN mu-law encoding at a sample rate of 8kHz Application Postscript Octet-stream Adobe Postscirpt. General binary data consisting of 8-bit bytes. MIME (contd.) The other major component of MIME is a definition of transfer encodings for message contents: Encoding Description 7bit The data are all represented by short lines of ASCII chars. 8bit The lines are short, but there may be non-ASCII chars. Binary Not only may non-ASCII chars be presented but lines are not necessarily short enough for SMTP transport. Quoted-printable Encodes the data in such a way that if the data being encoded are mostly ASCII text, the encoded form of the data remains largely recognizable by humans. Base64 Encodes data by mapping 6-bit blocks to 8-bit printable ASCII characters blocks. x-token A nonstandard encoding. MIME (contd.) Canonical form is a format that is standardized for use between systems. Conclusions: MIME is a necessity in today’s 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 - Functions SignerInfo: allows the inclusion of unsigned and signed attributes to be included along with a signature. signingTime sMIMECapabilities sMIMEEncryptionKeyPreference S/MIME - Functions Clear 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 - Cryptography Be liberal in what you receive and conservative in what you send. Definitions: 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. S/MIME - Cryptography Used Algorithms: Function Requirement Creation of a MUST support SHA-1. SHOULD use sha-1. message digest. Receiving agents SHOULD support MD5 for the purpose of providing backward compatibility with S/MIME v2. A message digest Both sending and receiving agents MUST encryption to form a support DSS. digital signature. Receiving agents SHOULD support verification of RSA signatures with key sizes 512 bits to 1024 bits. Note that S/MIME v2 clients are only capable of verifying digital signatures using RSA. S/MIME - Cryptography Function Requirement A session key encryption Both sending and receiving agents MUST for transmission with the support Diffie-Hellman. message. Sending agents SHOULD support RSA encryption with key sizes 512 to 1024 bits. Receiving agents SHOULD support RSA decryption. A message Encryption Sending an receiving agent MUST support for transmission with Encryption/Decryption with 3DES. one-time session key. Receiving agents SHOULD support decryption with RC2/40. (S/MIME V 2. - Sending agents SHOULD support RSA encryption with 3DES and RC2/40. Receiving agents MUST support decryption with RC2/40.) S/MIME - Cryptography Algorithm 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 - Cryptography A 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 - Message Canonical MIME Algorithm Certificates Identifiers CMS MIME bodies + CMS. CMS object MIME Encoding + Canonical form S/MIME - Message S/MIME makes use of a number of new MIME content types: Type Subtype S/MIME Description parameter Multipart Signed A clear message in tow parts: One is the message and the other is the signature. Application pkcs7-mime signedData A signed S/MIE entity. pkcs7-mime envelopedData An encrypted S/MIME entity. multipart/signed message. Pkcs7-signature -- A certificate registration pkcs10-mime -- request message. S/MIME - Message Enveloped Data: Recipient’s public key Diffie-Hellman / RSA Encrypt the session key Pseudorandom session key (3DES or RC2/40)ׁׁ Certificate RecipientInfo M enveloped-data + S/MIME - Message SignedData: Sender’s private key Hash function Encryption M SHA-1 or MD5 Certificate SignerInfo Base64 encoding S/MIME - Message Clear 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 - Message This parameter indicates Content-Type: multipart/signed; that this is a two part clear- protocol=“application/pkcs7-signature”; signed entity. micalg=sha1; boundary=boundary42 --boundary42 This parameter indicates the type of message digest used. Unsigned Content-Type: text/plain Data This is a clear-signed message. --boundary42 SignerInfo Content-Type: application/pkcs7-signature; Header name=smime.p7s Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7s ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6 4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 --boundary42-- S/MIME - Message Certificate-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 - Message Creating 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 an application/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 - Message Registration 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 - Message Registration request: Subject’s name Public-key in bit- string representation 010111010011… + CertificationRequestInfo Public-key ID ? PKCS10 CA User’s private key S/MIME - Certificates S/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 - Certificates a 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 role Key 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 role Certificate 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 -Attacks Certificate 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 -Attacks Certificates classes (common practice by most CAs) – Class 1 – Class 2 Tighter identity Easier to – Class 3 validation issue 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? S-MIME -Attacks Attack 1: Class 1 Certificate Attack No 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 -Attacks Attack 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- Attacks Step 2- Registration S/MIME- Attacks S/MIME- Attacks S/MIME- Attacks email@example.com S/MIME- Attacks S/MIME-Attacks S/MIME-Attacks Step 3 – Setup local account S/MIME-Attacks Step 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-Attacks Attack 2: Use one’s 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- Attacks Step 1- setup another account S/MIME- Attacks Step 2- Send bogus signed msg. S/MIME-Attacks Consequences: During verification, e-mail client does not match the name in certificate with the name in e-mail. – Only e-mail addresses are matched (as mentioned in RFC 2632 (S/MIME Certificate Handling). Verifier’s 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-Attacks Attack 3: Forging the header The scope of a S/MIME signature does not include the 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-Attacks What 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 client’s. Try to identify people by their e-mail addresses. S/MIME-Attacks What 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 – Sender’s identity – Subject – Date Don’t let subject say all! S/MIME - Summery In 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 by all major vendors of UA products. It very likely that S/MIME will become the predominant technology for secure messaging on the Internet. S/MIME - Summery In 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.
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