26 Email Security

Hiteishi Diwanji

Email Security

  • E-mail is vital for today’s commerce, as well a convenient medium for communications among ordinary users.
  • E-mail is public, exposed at every point from the sender’s workstation to the recipient’s screen.

Threats to E-Mail

  • message interception (confidentiality)
  • message interception (blocked delivery)
  • message interception and subsequent replay
  • message content modification
  • message origin modification
  • message content forgery by outsider
  • message origin forgery by outsider
  • message content forgery by recipient
  • message origin forgery by recipient
  • denial of message transmission

Encryption helps in limited case..

 

  • Encryption helps in a defense against replay, but each message must contain something unique that is encrypted.
  • Symmetric encryption cannot protect against forgery by a recipient, since both sender and recipient share a common key;
  • public key schemes can let a recipient decrypt but not encrypt.

Because of lack of control over the middle points of a network, senders or receivers generally cannot protect against blocked delivery

 

Requirements and Solutions

  • message confidentiality (the message is not exposed en route to the receiver)
  • message integrity (what the receiver sees is what was sent)
  • sender authenticity (the receiver is confident who the sender was)
  • nonrepudiation (the sender cannot deny having sent the message)

Designs

  • The standard for encrypted e-mail was developed by the Internet Society, through its architecture board (IAB) and research (IRTF) and engineering (IETF) task forces.
  • The encrypted e-mail protocols are documented as an Internet standard in documents 1421, 1422, 1423, and 1424 [LIN93, KEN93, BAL93, KAL93a].
  • This standard is actually the third refinement of the original specification.

Confidentiality

  • The sender chooses a (random) symmetric algorithm encryption key.
  • Then, the sender encrypts a copy of the entire message to be transmitted, including FROM:, TO:, SUBJECT:, and DATE: headers.
  • Next, the sender prepends plaintext headers.
  • The encrypted e-mail standard works with symmetric as well as asymmetric encryption.
  • The standard is also defined for symmetric encryption only:

To use symmetric encryption, the sender and receiver must have previously established a shared secret encryption key.

 

The processing type (“Proc-Type”) field identifies what privacy enhancement services have been applied. The data exchange key field (“DEK-Info”) identifies, the kind of key exchange (symmetric or asymmetric).The key exchange (“Key-Info”) field contains the message encryption key, encrypted under this shared encryption key. The field also identifies the originator (sender) so that the receiver can determine which shared symmetric key was used.

  • field would contain the message encryption field, encrypted under the recipient’s public key The sender’s certificate is also attached (for determining authenticity and for generating replies).
  • The encrypted e-mail standard is designed to support multiple encryption algorithms, using popular algorithms such as DES, triple DES, and AES for message confidentiality, and RSA and Diffie–Hellman for key exchange.
  • Encrypted e-mail messages always carry a digital signature, so the authenticity and nonrepudiability of the sender is assured.
  • The integrity is also assured because of a hash function (called a message integrity check, or MIC) in the digital signature.
  • If confidentiality is to be achieved, encrypted e-mail messages can be encrypted.

 

  • Header inside the message (in the encrypted portion) differs from that outside. A sender’s identity or the actual subject of a message can be concealed within the encrypted portion.
  • The encrypted e-mail processing can integrate with ordinary e-mail packages, so a person can send both enhanced and nonenhanced messages.
  • If the sender decides to add enhancements, an extra bit of encrypted e-mail processing is invoked on the sender’s end; the receiver must also remove the enhancements.
  • But without enhancements, messages flow through the mail handlers as usual.

 

Encryption for Secure E-Mail

 

  • The major problem with encrypted e-mail is key management.
  • Encrypted e-mail provides strong end-to-end security for electronic mail. Triple DES, AES, and RSA cryptography are quite strong, especially if RSA is used with a long bit key (1024 bits or more).
  • The vulnerabilities remaining with encrypted e-mail come from the points not covered: the endpoints.
  • An attacker with access could subvert a sender’s or receiver’s machine, modifying the code that does the privacy enhancements or arranging to leak a cryptographic key.

Example of Secure E-Mail Systems

  • PGP – PGP stands for Pretty Good Privacy. It was invented by Phil Zimmerman in 1991.
  • Originally a free package, it became a commercial product after being bought by Network Associates in 1996.
  • A freeware version is still available. PGP is widely available, both in commercial versions and freeware, and it is heavily used by individuals exchanging private e-mail.

PGP key management

  • PGP addresses the key distribution problem with what is called a “ring of trust” or a user’s “keyring.” One user directly gives a public key to another, or the second user fetches the first’s public key from a server. Some people include their PGP public keys at the bottom of e-mail messages. And one person can give a second person’s key to a third (and a fourth, and so on).

The PGP processing performs some or all of the following actions, depending on whether confidentiality, integrity, authenticity, or some combination of these is selected:

  • Create a random session key for a symmetric algorithm.
  • Encrypt the message, using the session key (for message confidentiality).
  • Encrypt the session key under the recipient’s public key.
  • Generate a message digest or hash of the message; sign the hash by encrypting it with the sender’s private key (for message integrity and authenticity).
  • Attach the encrypted session key to the encrypted message and digest.
  • Transmit the message to the recipient.

PGP consists of the following five services:

  1. Authentication
  2. Confidentiality
  3. Compression
  4. E-mail compatibility
  5. Segmentation

 

Terminology:

  • Ks – session key used in symmetric encryption key.
  • PRa – private key of user A used in public key encryption scheme
  • PUa – public key of user A used in public key encryption scheme
  • EP – public key encryption
  • DP – public key decryption
  • EC – symmetric encryption
  • DC – symmetric encryption
  • H – hash function
  • ||- concatenation
  • Z – compression using ZIP algorithm
  • R64 – conversion to radix 64 ASCII format S/MIME

 

  •   An Internet standard governs how e-mail is sent and received.
  • The general MIME specification defines the format and handling of e-mail attachments.
  • S/MIME (Secure Multipurpose Internet Mail Extensions) is the Internet standard for secure e-mail attachments.

Difference between S/MIME and PGP

  • The principal difference between S/MIME and PGP is the method of key exchange.
  • Basic PGP depends on each user’s exchanging keys with all potential recipients and establishing a ring of trusted recipients; it also requires establishing a degree of trust in the authenticity of the keys for those recipients.
  • S/MIME uses hierarchically validated certificates, usually represented in X.509 format, for key exchange.

 

 

  • S/MIME works with a variety of cryptographic algorithms, such as DES, AES, and RC2 for symmetric encryption.
  • S/MIME performs security transformations very similar to those for PGP. PGP was originally designed for plaintext messages, but S/MIME handles (secures) all sorts of attachments, such as data files (for example, spreadsheets, graphics, presentations, movies, and sound). Because it is integrated into many commercial e-mail packages, S/MIME is likely to dominate the secure e-mail market.

Suggested Reading:

  1. Cryptography and Network Security Principles and Practice by William Stallings, sixth Edition, PEARSON.
  2. Security in Computing by Charles Pfleeger & Shari Lawrence Pfleeger, fourth Edition, PEARSON.
  3. Network Security by Charlie Kaufman, Radia Perlman, Mike Speciner, second Edition, PHI.
  4. The Complete Reference – Network Security by Roberta Bragg, Mark Rhodes-Ousley & Keith Strassberg, Tata McGraw Hill
  5. Network Security Bible by Eric Cole, Ronald Krutz, James Conley, Wiley
  6. Hacking 6 Exposed by Stuart McClure, Joel Scambray & George Kurtz , Tata McGraw Hill .
  7. www.snort.org
  8. https://nmap.org