Internet E-mail Security

Issues and standards for securing Internet e-mail

Laurence Lundblade QUALCOMM Inc., Eudora Division

The following are slides from a presentation on e-mail security, May 1997

Common Services

Message body security:

End-to-end encryption
Authentication of sender
End-to-end message integrity

Download security (SASL/SSL/TLS for POP or IMAP)

Prevents unauthorized download of your e-mail
Useful to retrieve e-mail across a firewall

Message submission (SASL/SSL/TLS for SMTP)

Helps with authentication of sender
Can be used to prevent SPAM injection

Transmission of a Message

Message Standards Reviewed

Classic PGP - a defacto standard
PGP/MIME - on the Internet standards track
S/MIME - an industry consortium standard

Four Components

Trust Management - How certificates are created, distributed and used
Certificates - Data structure used to convey identity, algorithm keys and trust
Algorithms - Cryptography used to secure messages
Message Formats - Format of messages and data sent and received

Diagram of Four Components

shows client, message, certificate

Trust Management

An open-ended category: client or server software network protocols certificate authorities systems for distributing and validating certificates
Kinds of trust

public trust
enterprise trust
web of trust
direct trust

Trust Management: The Problem

Failure to validate credentials/certificate results in:

Encrypting for the wrong person (possibly the attacker)
Not knowing that a message is authentic

Trust Management: Public Trust

People & organizations are issued certificates by a public certificate authority.
Can be verified by the public easily Likely to be either for fee or by a government
Best solution for short interactions with unknown people, e.g., electronic commerce
Usually part of a hierarchy of trust with a trusted “root”
Examples:

Passports, issued by governments, trusted everywhere “
VISA, it’s everywhere you want to be”

Trust Management: Enterprise

Certificates are issued by the organization
Policy and trust controlled by organization
Usually part of a hierarchy with a trusted “root”
Breaks-down outside of organization
Can be used across organizations with cross-certification
Examples: QUALCOMM badges Conference badges Theater ticket

Trust Management: Web

Individuals and organizations certify each other
No central authority required
No guarantee that trust can be established
Requires understanding by users
Examples:

A person introduces one of his friends to another

Trust Management: Direct Trust

The two parties authenticate each other directly
Can be accomplished by:

One time out of band verification of certificates
Recognizing each others voice, e.g., a telephone call
“What color socks where you wearing last time I saw you?”

Simple, requires no central server or authority
Requires understanding by users
Can scale poorly
Solves a significant part of the problem

Trust Management: PGP

Today, PGP and PGP/MIME are primarily used to implement web and direct trust
Not restricted to web and direct trust models
Simple to set up and operate because no central infrastructure is required

Trust Management: S/MIME

The S/MIME standard does not specify any trust model
S/MIME can be used to implement nearly any trust model
Certificates cannot be changed or signed multiple times
Can’t be used for web of trust

Trust Management: Examples

PGP: Trust mainly friends or parties you have personally established trust with
Entrust, Netscape: Trust your organization, enterprise or your IS department, and who they say you should trust
VeriSign, US Postal Service: Trust us! We’ll make sure everyone is who they say they are

Trust Management: Issues

Trust can be established between two parties without a third party, without extensive infrastructure, and with fairly simple client implementations
Important to understand how trust is established, conveyed and maintained for a particular implementation
The kind of trust is determined by the standard and an implementation

Certificates

Certificates are a data structure (in a file, or protocol message) used to convey identity and trust
They usually include: Owner’s name, and/or e-mail address Owner's public key
A signature which guarantees the above information

Certificates: PGP

The PGP equivalent of a certificate is a “PGP key”.
Its format is used exclusively by PGP Carries only basic data needed for a certificate
Can be signed by many people, each indicating a different degree of trust
PGP will soon support X.509 certificates

Certificates: X.509 for S/MIME

S/MIME uses X.509 format certificates
X.509 is used widely for other than secure e-mail
X.509 certificates can carry a large variety of data fields:

Validity dates
Issuer name, issuer contact information, serial number
“Policy” or nature of the trust that is conveyed

Certificates: SPKI certs

SPKI: “Simple Public Key Infrastructure”
A recent alternative to X.509
Each certificate contains only essential information
Examples:

Name and public key for identity
Bank reference and public key for anonymous payments
Plain public key for authorization

Certificates: Issues

What kind of trust information is carried? (e.g., the policy)
Certificate interoperable depends on the algorithm used to generate the public key
Security depends on the user picking the correct certificate, not a forged one.
This can be aided by:

Having the e-mail address and other info in the certificate
Good user interface and implementation of secure e-mail client
Smart cards and certificate “wallets”

Algorithms

Core cryptographic functions used to secure messages
Three kinds of algorithms are used

Public key algorithms, e.g., RSA, El Gamal, DSS, Elliptic curves
Secret key algorithms, e.g., DES, IDEA, RC-2, RC-5
Message Digest, e.g., MD-5, SHA-1

Algorithms: PGP & PGP/MIME

PGP allows use of many different algorithms, but in practice only one set is commonly used:

Public key: RSA, 512 to 2048 bit keys, moving to El Gamal/DSS
Secret key: IDEA, 128 bit keys
Message Digest: MD-5 now, moving to SHA-1

PGP is strong crypto, and not exportable

Algorithms: S/MIME

S/MIME also allows the use of nearly any set of algorithms.
An implementation guide that is part of the standard makes specific recommendations to insure interoperability

Public Key: RSA
Secret Key: DES, Triple DES, RC-2, RC-5
Message Digest: MD-5, SHA-1

S/MIME has two profiles:

Weak and exportable
Strong and non-exportable

Algorithms: Issues

Public Review - It takes a few years of public review to know a new algorithms is secure - be cautious about new ones, especially ones that aren’t published
Interoperability - What is encrypted using one algorithm can’t be decrypted by another. Thus, for true global interoperability we must standardize on a very small set of algorithms. The algorithm choice affect certificates as well as messages

Message Formats: General

Some message formats are oriented around text and files, others secure the full MIME message
Every standard (PGP, S/MIME, MOSS, MSP...) uses a different format, and they don’t interoperate
Format concern in two places:

The “inside” format of the message before it is secured
The “outside” format of the secured message as it is sent

Message Formats:

MIME “Multipurpose Internet Mail Extension”
It provides:

Type tagging mechanism (e.g. image, HTML, encrypted)
Transfer encoding for binary data
Structures e-mail message into parts and sub-parts

Is very stable and an Internet standard
Is widely deployed in e-mail clients today

Message Formats: MIME Uses

Adding attachments including their type and file name
Including inline images International character sets
Integrating voice mail with Internet e-mail
Bundling components of an HTML page in e-mail
Representing secured messages

Message Formats: Picture

Shows MIME and security encapsulation

Message Formats: Clear-signing

Message are not encrypted
Signed to guarantee identity of sender and integrity of message
Solves the Internet mail forgery problem
Recipients without secure e-mail software should be able to read the message as if it were a normal message

Message Formats: PGP

Secures plain text messages or files which then may be attached
Does not secure multimedia, HTML, inline audio and images
Is easy to integrate with e-mail software
Uses a text format that works very well for clear signing.

Message Formats: PGP/MIME

Secures full MIME structure of message including

Attachments
Order of attachments
HTML, including MIME/HTML
Voice, images, multimedia
Apple-double format for MIME transmission of Macintosh files

Uses RFC-1847 format for clear-signing full MIME structure of message

Message Formats: S/MIME

Secures full MIME structure just as PGP/MIME
Uses two formats for clear-signing:

The RFC-1847 format is useful when all recipients must be able to read the message regardless of whether or not they can verify it
The “SignedData” format is useful when it is more critical that the message be verifiable in all circumstances, than that it can be read in all circumstances

Message Formats: Issues

Plain text-oriented formats are easy to integrate with the mail program
MIME formats are harder to integrate Consider which clear-signing formats an implementation supports
Consider how a particular implementation integrates with the mail program. You should be able to reply, forward, file, search and open attachments for a secured message in your mail program as if it were any other message