1. S/MIME

2. 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 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 security for many.

3. S/MIME - Overview There are three versions of S/MIME:
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 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.

4. MIME - Overview
RFC 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.

5. 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 .
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.

6. MIME (contd.) Here is a summary of the different MIME content types:
Subtype
Description
Text
Plain
Enriched
Unformatted text (ASCII or ISO 8859).
Provides greater format flexibility.
Multipart
Mixed
Parallel
Alternative
Digest
The different parts are independent but are to be transmitted together. Should be presented to the receiver in their original order.
Differs from mixed only in that no order is defined.
The different parts are alternative versions of the same information.
Similar to Mixed but the default type/subtype of each part is message/rfc822.
Message
rfc822
Partial
External body
The body is itself an encapsulated message that conforms to RFC822.
Used to allow fragmentation in a transparent way to the recipient.
Contains a pointer to an object exists else where.

7. MIME (contd.) Type Subtype Description Image Video Audio Application
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.

8. 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.

9. MIME (contd.)
Canonical form is a format that is standardized for use between systems.
Conclusions:
MIME is a necessity in today’s Internet and 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

10. 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.

11. S/MIME - Functions
SignerInfo: allows the inclusion of unsigned and signed attributes to be included along with a signature.
signingTime
sMIMECapabilities
sMIMEEncryptionKeyPreference

12. 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.

13. 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.

14. S/MIME - Cryptography Used Algorithms: Function Requirement
Creation of a message digest.
MUST support SHA-1. SHOULD use sha-1.
Receiving agents SHOULD support MD5 for the purpose of providing backward compatibility with S/MIME v2.
A message digest encryption to form a digital signature.
Both sending and receiving agents MUST support DSS.
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.

15. S/MIME - Cryptography Function Requirement
A session key encryption for transmission with the message.
Both sending and receiving agents MUST support Diffie-Hellman.
Sending agents SHOULD support RSA encryption with key sizes 512 to 1024 bits.
Receiving agents SHOULD support RSA decryption.
A message Encryption for transmission with one-time session key.
Sending an receiving agent MUST support Encryption/Decryption with 3DES.
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.)

16. 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.

17. 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.

18. S/MIME - Message Canonical MIME Algorithm Identifiers Certificates CMS
MIME bodies + CMS.
CMS object
MIME
Encoding + Canonical form

19. S/MIME - Message
S/MIME makes use of a number of new MIME content types:
Description
S/MIME parameter
Subtype
Type
A clear message in tow
parts:
One is the message and
the other is the signature.
Signed
Multipart
A signed S/MIE entity.
An encrypted S/MIME entity.
multipart/signed message.
A certificate registration
request message.
signedData
envelopedData --
pkcs7-mime
Pkcs7-signature
pkcs10-mime
Application

20. S/MIME - Message M + Enveloped Data: Recipient’s public key
Encrypt the session key
Diffie-Hellman / RSA
Recipient’s public key
Pseudorandom
session key
(3DES or RC2/40)ׁׁ
Certificate
RecipientInfo M
enveloped-data +

21. S/MIME - Message M SignedData: Sender’s private key Hash function
Encryption
Sender’s private key
Certificate M
Hash function
SHA-1 or MD5
SignerInfo
Base64 encoding

22. 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.

23. S/MIME - Message Unsigned Data
This parameter indicates that this is a two part clear-signed entity.
Content-Type: multipart/signed;
protocol=“application/pkcs7-signature”;
micalg=sha1; boundary=boundary42
--boundary42
Content-Type: text/plain
This is a clear-signed message.
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 the type of message digest used.
Unsigned Data
SignerInfo
Header

24. 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.

25. S/MIME - Message The CMS signedData object is enclosed in an
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".

26. 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.

27. S/MIME - Message ? CA + Registration request: Subject’s name
Public-key in bit-string representation …
CertificationRequestInfo
Public-key ID
User’s private key ?
PKCS10 CA

28. 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

29. S/MIME - Certificates to protect the authenticity and
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

30. 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.

31. S/MIME - Certificates

32. 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.

33. 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.

34. 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.

35. S-MIME -Attacks How to determine user
Certificates classes (common practice by most CAs)
– Class 1
– Class 2
– Class 3
CA certification policies
– ID-control practices
• Class 1: only 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 address?
Tighter identity
validation
Easier to
issue

36. S-MIME -Attacks Attack 1: Class 1 Certificate Attack
No identity check during registration.
Binding between public key and address.
It is possible to enroll under a different name.
Name spoofing is possible in signed messages
clients do not make this fact explicit to average users.

37. S-MIME -Attacks Attack 1: Class 1 Certificate Attack
• Step 1: Get an address that implies the
person you want to imitate.
• Step 2: Register for a certificate with that
bogus name and address.
• Step 3: Step up an outgoing account
at your favorite client software
with that bogus name.
• Step 4: Send bogus signed messages

38. S/MIME- Attacks Step 2- Registration

39. S/MIME- Attacks

40. S/MIME- Attacks

41. S/MIME- Attacks

42. S/MIME- Attacks

43. S/MIME-Attacks

44. S/MIME-Attacks Step 3 – Setup local account

45. S/MIME-Attacks Step 4 – Send signed but bogus msgs.

46. S/MIME-Attacks Consequences: Loose control for Class 1 certificates.
The system becomes less secure for the name of security.

47. S/MIME-Attacks
Attack 2: Use one’s certificate to send s under another name.
• Step 1: Set up another account at local client.
– Same address
– But a different name
• Step 2: Send bogus signed messages

48. S/MIME- Attacks Step 1- setup another account

49. S/MIME- Attacks Step 2- Send bogus signed msg.

50. S/MIME-Attacks Consequences:
During verification, client does not
match the name in certificate with the name
in .
– Only 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.
- clients are not concerned with certificate
classes.

51. S/MIME-Attacks Attack 3: Forging the header
The scope of a S/MIME signature does not include
the header.
– from, to, cc, subject, date
Indeed, the mail header is modified without changing the verification status.
Problem of all classes of certificates.

52. S/MIME-Attacks What should be done?
Class 1 certificates should be discontinued.
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 client’s.
Try to identify people by their addresses.

53. 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!

54. 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.

55. 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.

56. S/MIME The End.