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E-mail Security. CS 470 Introduction to Applied Cryptography Instructor: Ali Aydin Selcuk. Security Services for E-mail. privacy authentication integrity non-repudiation anonymity proof of submission proof of delivery message flow confidentiality, etc. Key Management.

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e mail security

E-mail Security

CS 470

Introduction to Applied Cryptography

Instructor: Ali Aydin Selcuk

E-mail Security

security services for e mail
Security Services for E-mail
  • privacy
  • authentication
  • integrity
  • non-repudiation
  • anonymity
  • proof of submission
  • proof of delivery
  • message flow confidentiality, etc.

E-mail Security

key management
Key Management
  • A per-message symmetric key is used for message encryption,
  • which is conveyed in the mail, encrypted under a long-term key (typically a public key)
  • Long-term keys can be established,
    • offline
    • online, with help from a trusted third party
    • online, through a webpage (for public keys)

E-mail Security

multiple recipients
Multiple Recipients
  • Message key will be encrypted under each recipients long term key in the message header.
    • Bob’s ID, KBob{S}
    • Carol’s ID, KCarol{S}
    • Ted’s ID, KTed{S}
    • S{m}
  • E.g.:

To: Bob, Carol, Ted

From: Alice

Key-info: Bob-4276724736874376

Key-info: Carol-78657438676783457

Key-info: Ted-12873486743009

Msg-info: UHGuiy77t65fhj87oi.....

E-mail Security

text format issues
Text Format Issues
  • Mail gateways/forwarders may modify the format of the message (wrapping long lines, end-of-line character, high order bits, etc.), causing the integrity check to fail
  • Encode messages in a format supported by all mailers. 6-bit representation, no long lines, etc. (similar to uuencode)

E-mail Security

text format issues cont d
Text Format Issues (cont’d)
  • Problem: Non-supportive clients should be able to read authenticated (but not encrypted) messages, which they no longer can.
  • Two options:
    • MAC without encoding (subject to corruption by mail routers)
    • Encode & MAC/encrypt (may not be readable at the other end)

E-mail Security

providing different services
Providing Different Services
  • confidentiality: by encryption
  • auth./integrity: by signature or MAC
  • non-repudiation: by signature
  • some eccentric services,
    • anonymity
    • message flow confidentiality
    • non-repudiation with secret keys

can be provided by TTP support.

E-mail Security

pem s mime
PEM & S/MIME
  • Privacy Enhanced Mail (PEM)
    • Developed by IETF, to add encryption, source authentication & integrity protection to e-mail
    • Allows both public & secret long-term keysMessage key is always symmetric
    • Specifies a detailed certification hierarchy
  • Secure/MIME (S/MIME)
    • PEM never took off; CA hierarchy difficult to realize
    • S/MIME: PEM design incorporated into MIME

E-mail Security

pem key exchange encryption
PEM Key Exchange & Encryption
  • “Interchange keys”: Users’ long-term PEM keys
    • public (a detailed PKI is defined)
    • secret (pre-shared symmetric keys)
  • Encryption
    • A symmetric per-message key is sent encrypted under the interchange key.
    • The message is encrypted under the per-message key (typically with DES in CBC mode)
  • Authentication
    • Message is authenticated by a “MIC”(Q: Any authentication for the per-message key?)

E-mail Security

pem certificate hierarchy
PEM Certificate Hierarchy
  • The root CA: “Internet Policy Registration Authority” (IPRA)
  • “Policy Certification Authorities”: Second-level, CA-certifying CAs, each with a different policy:
    • High Assurance (HA): super-secure
      • implemented on secure platforms
      • regulates that the child CAs (also HACAs) enforce the same rules
    • Discretionary Assurance (DA): secure
      • requires that the child CAs own their names
    • No Assurance (NA): no constraints
      • can be used to certify Internet personas (pseudonyms)
  • Lower-level CAs, certifying individuals or other CAs

E-mail Security

s mime vs pem
S/MIME vs. PEM
  • Incorporated into MIME; no other encoding
  • Any sequence of sign & encrypt is supported (each as a recursive MIME encapsulation)
  • Has more options than PEM
  • ASN.1 header encoding
  • No prescribed certification hierarchy
  • Has a good prospect of deployment for commercial & organizational usage

E-mail Security

pretty good privacy pgp
Pretty Good Privacy (PGP)
  • Popular mail & file encryption tool
  • Developed by Phil Zimmermann, 1991
  • Based on RSA, IDEA, MD5 (later DSS, ElGamal (DH), 3DES, SHA1)
  • Many different versions have emerged (from PGP, from GNU (GPG), from IETF (Open PGP))

E-mail Security

pgp operation
PGP Operation
  • All long-term user keys are public
  • Signature:

Message & timestamp are hashed (MD5 or SHA1) and signed (RSA or DSS)

  • Compression (ZIP)
  • Encryption:
    • Message is encrypted with a per-message symmetric key (typically with IDEA in CFB mode)
    • which is encrypted with the recipient’s public key (RSA or DH (ElGamal))
  • Radix-64 (6-bit) encoding

E-mail Security

trust model key management
Trust Model & Key Management
  • Any user can certify any other (anarchy model)
  • Each user decides whom to trust and how much
  • “Key Ring”: Data structure to store public keys held by a user, with their levels of trust
  • Public keys can be obtained,
    • offline (in person, over the phone, etc.)
    • through personal webpages
    • through a trusted friend (“web of trust”)
    • through a trusted CA

E-mail Security

dkim domain keys identified mail
DKIM – Domain Keys Identified Mail
  • An effort to stop spam with forged domain addresses (e.g. phishing attacks).
  • Standardized by RFC 4871; supported by Yahoo, Gmail, FastMail etc.
  • Each domain has an email signature key.Public keys will be retrieved over DNS.
  • If signature verification fails, mail will be dropped.

E-mail Security

slide16
DKIM
  • Once deployed, it will significantly limit phishing attacks with forged domain addresses.
  • Deployment is increasing rapidly.
  • Example: Gmail’s collaboration with PayPal & eBay

E-mail Security

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