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E-mail Security

E-mail Security. CS 519 Cryptography and Network Security 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

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  1. E-mail Security CS 519 Cryptography and Network Security Instructor: Ali Aydin Selcuk E-mail Security

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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. Providing Services (cont’d) • Authentication with plausible deniability: • with MAC, if shared key exists • with public-key signature: • Alice, to send message m to Bob, • chooses a random symmetric key S • computes [{S}Bob]Alice • computes MACS(m) • sends m, MACS(m), [{S}Bob]Alice Bob can prove that Alice sent him S, but not that she sent him m. E-mail Security

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