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Information about smime

Published on October 7, 2007

Author: Belly


Enhancing Email Security with S/MIME:  Enhancing Email Security with S/MIME Chuck Connell,, Introduction:  Introduction Worked at Lotus from 90 to 95 Managed Notes C API team, architect in (short-lived) “enterprise applications” group, business partner technical liaison Began my own business in 1995 Notes/Domino consulting, writing, teaching CS at Boston University Security expert at Outline:  Outline What is S/MIME? Why do we care about it? Secrecy, authentication, and integrity Cryptography primer, including public key techniques and certificates How S/MIME works Where S/MIME is used in Notes/Domino How to use S/MIME Audience:  Audience Experienced with Notes, Domino, general email topics Used some encryption/privacy tools Not a security expert or mathematician (will skip gory details) My goal is to explain a fairly complex topic to a generally knowledgeable computer audience What is S/MIME?:  What is S/MIME? When email was first developed, people could only send plain text messages MIME was developed in early 90s to allow people to send pictures, sound, programs and general attachments -- “Multipurpose Internet Mail Extension” MIME has no security features, can be read along its route or forged (easily) S/MIME is a secure version of MIME What does S/MIME give us?:  What does S/MIME give us? Secrecy – Only intended recipient can read the message. (A thick envelope and trustworthy couriers.) Authentication – Recipient knows the message came from the apparent sender. (An ink signature that you recognize.) Integrity – Recipient knows the message was not changed en route. (Un-erasable ink in a letter.) Cryptography primer:  Cryptography primer Secret key (a.k.a symmetric cipher) Public key (a.k.a. asymmetric cipher) Secrecy Authentication Secrecy and authentication Hashing (a.k.a. message digest) Public key certificate (X.509) Symmetric cipher:  Symmetric cipher Dates back thousands of years A “key” is scrambled into the message in a way that makes the message unreadable Scrambling method can be pencil and paper, mechanical, or mathematical Key can be numbers, letters, text from a book Only way to read the message (easily) is to unscramble it with the same key Sender and receiver must exchange key somehow Symmetric cipher:  Symmetric cipher Public key cryptography (PKC):  Public key cryptography (PKC) Invented in 1970s There are two keys; one public for all to see, the other kept secret to one person Keys are pairs of large numbers, related to prime number theory Message is scrambled with one key; only unscrambled easily with the other key Can be used for secrecy, authentication, or both Public key cryptography:  Public key cryptography PKC for secrecy only:  PKC for secrecy only Chuck wants to send message that only Katie can read Ciphertext = PKC(plaintext, katie’s public key) Plaintext = PKC(ciphertext, katie’s private key) Only Katie can decrypt the message, and Chuck does not have to send her a key PKC for authentication only:  PKC for authentication only Chuck wants to send message to Katie and prove it is from him Ciphertext = PKC(plaintext1, chuck’s private key) Chuck sends ciphertext and plaintext1 Plaintext2 = PKC(ciphertext, chuck’s public key) Katie compares plaintext1 (sent) with plaintext2 (decrypted) If they match, only Chuck could have sent the message. PKC for secrecy and authentication:  PKC for secrecy and authentication Chuck wants to send secret message to Katie and prove it is from him Cipher1 = PKC(plaintext1, chuck’s private key) Cipher2 = PKC(Cipher1 and plaintext1, katie’s public key) Chuck sends Cipher2 Cipher1 and Plaintext1 = PKC(Cipher2, katie’s private key) Plaintext2 = PKC(Cipher1, chuck’s public key) Katie compares plaintext1 (sent) with plaintext2 (decrypted) Hashing:  Hashing A one-way operation that is hard to undo Often results in a shorter message, which is called a message digest Example: “Let’s have breakfast at Dunkin Donuts”  “h7tfd8Fr” Public key certificate:  Public key certificate But, there is a problem with PKC… How does Katie know it is really Chuck sending her the message. Someone could pretend to be Chuck. Public key certificates solve this problem (mostly) A public key certificate contains A person’s name That person’s public key Name of a trusted certifying authority (CA) Digital signature of the CA, using their private key Certificate can be verified with CA’s public key X.509 is most common format Questions ?:  Questions ? So what is S/MIME?:  So what is S/MIME? S/MIME puts all these techniques together to create a practical, efficient, reasonably secure email protocol Standard (symmetric) cipher – RC2 or TripleDES Public key (asymmetric) cipher – RSA Hashing – SHA-1 or MD5 (Mathematical details found in references) S/MIME for secrecy only:  S/MIME for secrecy only Chuck’s email program creates a random key (session key) to be used in a symmetric cipher. Chuck’s email program encrypts the message with the symmetric cipher and session key. Chuck’s email program encrypts the session key with PKC and Katie's public key. Chuck’s email program creates a package of: encrypted message, encrypted session key, his X.509 certificate, names of encryption algorithms. S/MIME for secrecy, continued:  S/MIME for secrecy, continued Chuck’s email program sends package to Katie. This is an S/MIME email message. Katie’s email program receives package. Katie's email program uses her private key (and named PKC method) to decrypt the session key. Katie’s email program uses session key (and named symmetric cipher) to decrypt the message. S/MIME for authentication only:  S/MIME for authentication only Chuck’s email program uses hash function to create message digest Chuck’s email program encrypts message digest with PKC and his private key Chuck’s email program creates a package of: original message, encrypted message digest, his X.509 certificate, names of encryption algorithms Chuck’s email program sends package to Katie. Katie's email program receives package S/MIME for authentication, continued:  S/MIME for authentication, continued Katie’s email program verifies Chuck’s X.509 certificate by testing signature of CA Katie’s email program gets Chuck’s public key from his certificate Katie's email program uses Chuck’s public key to decrypt the message digest Katie's email program independently computes the message digest, using the same hash function Katie's email program compares the two message digests to verify sender and message integrity S/MIME for secrecy and authentication:  S/MIME for secrecy and authentication Message is authenticated just as shown above Authenticated package is made secret, just as shown above Secret package is sent to recipient Receiver uses his/her private key to decrypt session key Receiver uses session key to decrypt rest of secret package, yielding authenticated message Receiver authenticates message, just as shown above Questions ?:  Questions ? So S/MIME is used for Notes mail?:  So S/MIME is used for Notes mail? No! For pure Notes email (Notes and Domino) S/MIME is not needed. Notes has its own, similar, methods. S/MIME is used whenever pure Notes email is not available From Notes, through Domino, to other email From Notes, through standard server, to any email From other email, through Domino, to any email Using S/MIME:  Using S/MIME Get a digital identification Set up Domino server for S/MIME Use S/MIME with general email clients Use S/MIME with Notes Getting a digital identification:  Getting a digital identification A digital ID is Your name Public/private key pair Public key certificate for this ID Most popular vendors are and Thawte is free, but VeriSign is only $15/year and simpler to use Setting up Domino for S/MIME:  Setting up Domino for S/MIME Do nothing! (other than standard Internet mail set up) (If anyone is aware of special settings that are required, please let me know.) S/MIME with standard email clients (e.g. Outlook Express):  S/MIME with standard email clients (e.g. Outlook Express) If you got your digital ID on this computer, it is already installed (Can see the ID with Start / Settings / Control Panel / Internet Options / Content / Certificates) For secrecy, just press Encrypt For authentication, just press Sign When receiving a message, you will see security symbols near the attachment paperclip Using S/MIME with Notes:  Using S/MIME with Notes (Assuming digital ID already on Windows computer) Export digital ID from Windows Import digital ID to Notes ID file Make sure this certificate will be used for Internet mail from Notes Use digital ID as you send and receive email Demonstration… For further reading:  For further reading Excellent online overview of cryptography: Cryptography and Network Security by William Stallings – Good general security textbook.  S/MIME Internet task force: Relationship between S/MIME and PGP/MIME:

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