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 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.

3. 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.

4. 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.

5. 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.

6. 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
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 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 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

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

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

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.  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 - Cryptography

18. S/MIME - Message
MIME bodies + CMS.
Algorithm
Identifiers
Canonical
MIME
Certificates
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-mime
Pkcs7-signature
pkcs10-mime
Application

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

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

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
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
Registration request:
+
Subject’s name
Public-key in bit-
string representation
010111010011…
CertificationRequestInfo
User’s
private key
Public-key ID ?
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
 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
 Certificates 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 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 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.

37. 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

38. S/MIME- Attacks
Step 2- Registration

39. S/MIME- Attacks

40. S/MIME- Attacks

41. S/MIME- Attacks
shlomo@hotmail.com

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
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

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, 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.

51. 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.

52. 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

53. 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-Attacks

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.  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 - Summery

56. S/MIME
The End.