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PKI: Public Key Infrastructure – tell me in plain English AND THEN deep technical how PKI works

PKI: Public Key Infrastructure – tell me in plain English AND THEN deep technical how PKI works. Mostly borrowed & updated from Steve Lamb in Microsoft Land…. Scott Rea, PKI Architect, Dartmouth College + HEBCA. Objectives. Demystify commonly used terminology Explain how PKI works

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PKI: Public Key Infrastructure – tell me in plain English AND THEN deep technical how PKI works

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  1. PKI: Public Key Infrastructure– tell me in plain English AND THEN deep technical how PKI works Mostly borrowed & updated from Steve Lamb in Microsoft Land…. Scott Rea, PKI Architect, Dartmouth College + HEBCA

  2. Objectives • Demystify commonly used terminology • Explain how PKI works • Get you playing with PKI in the lab • Make some simple recommendations

  3. Agenda • Foundational Concept • PKI and Signatures • Recommendations • Reference material • Common Algorithms

  4. What can PKI enable? Secure Email – sign and/or encrypt messages Secure browsing – SSL – authentication and encryption Secure code – authenticode Secure wireless – PEAP & EAP-TLS Secure documents – Rights Management Secure networks – segmentation via IPsec Secure files – Encrypted File System(EFS)

  5. Foundational Concepts

  6. Encryption vs. Authentication • Encrypted information cannot be automatically trusted • You still need authentication • Which we can implement using encryption, of course

  7. Assets • What we are securing? • Data • Services (i.e. business etc. applications or their individually accessible parts) • This session is not about securing: • People (sorry), cables, carpets, typewriters and computers (!?) • Some assets are key assets • Passwords, private keys etc…

  8. Strong PhysicalSecurity of KA Strong PhysicalSecurity of KA Weak PhysicalSecurity of KA Weak DigitalSecurity Strong DigitalSecurity Strong DigitalSecurity InsecureEnvironment Good SecurityEverywhere InsecureEnvironment Digital Security as Extension of Physical Security of Key Assets

  9. Remember CP and CPS! • “The Certification Practice & Certification Practice Statement (CP/CPS) is a formal statement that describes who may have certificates, how certificates are generated and what they may be used for.” • http://www.ietf.org/rfc/rfc3647.txt

  10. Symmetric Key Cryptography Plain-text input Plain-text output Cipher-text “The quick brown fox jumps over the lazy dog” “The quick brown fox jumps over the lazy dog” “AxCv;5bmEseTfid3)fGsmWe#4^,sdgfMwir3:dkJeTsY8R\s@!q3%” Encryption Decryption Same key(shared secret)

  11. Symmetric Pros and Cons • Strength: • Simple and really very fast (order of 1000 to 10000 faster than asymmetric mechanisms) • Super-fast (and somewhat more secure) if done in hardware (DES, Rijndael) • Weakness: • Must agree the key beforehand • Securely pass the key to the other party

  12. Public Key Cryptography • Knowledge of the encryption key doesn’t give you knowledge of the decryption key • Receiver of information generates a pair of keys • Publish the public key in a directory • Then anyone can send him messages that only she can read

  13. private public Public Key Encryption Clear-text Input Clear-text Output Cipher-text “The quick brown fox jumps over the lazy dog” “The quick brown fox jumps over the lazy dog” “Py75c%bn&*)9|fDe^bDFaq#xzjFr@g5=&nmdFg$5knvMd’rkvegMs” Encryption Decryption Different keys Recipient’s private key Recipient’s public key

  14. Public Key Pros and Cons • Weakness: • Extremely slow • Susceptible to “known ciphertext” attack • Problem of trusting public key (see later on PKI) • Strength • Solves problem of passing the key • Allows establishment of trust context between parties

  15. Symmetric encryption (e.g. DES) *#$fjda^j u539!3t t389E *&\@ 5e%32\^kd Symmetric key encrypted asymmetrically (e.g., RSA) Digital Envelope User’s public key (in certificate) As above, repeated for other recipientsor recovery agents DigitalEnvelope Randomly- Generated symmetric“session” key Other recipient’s or agent’s public key (in certificate) in recovery policy RNG Hybrid Encryption (Real World) Launch key for nuclear missile “RedHeat” is...

  16. Symmetricdecryption (e.g. DES) Launch key for nuclear missile “RedHeat” is... Symmetric “session” key Recipient’s privatekey Asymmetric decryption of “session” key (e.g. RSA) Session key must be decrypted using the recipient’s private key Digital envelope contains “session” key encrypted using recipient’s public key Digital Envelope Hybrid Decryption *#$fjda^j u539!3t t389E *&\@ 5e%32\^kd

  17. PKI and Signatures

  18. Public Key Distribution Problem • We just solved the problem of symmetric key distribution by using public/private keys • But… • Scott creates a keypair (private/public) and quickly tells the world that the public key he published belongs to Bill • People send confidential stuff to Bill • Bill does not have the private key to read them… • Scott reads Bill’s messages 

  19. Eureka! • We need PKI to solve that problem • And a few others…

  20. private Creating a Digital Signature Message or File Digital Signature 128 bits Message Digest This is a really long message about something… Jrf843kjfgf*£$&Hdif*7oUsd*&@:<CHDFHSD(** Py75c%bn&*)9|fDe^bDFaq#xzjFr@g5=&nmdFg$5knvMd’rkvegMs” Hash Function (SHA, MD5) AsymmetricEncryption Calculate a short message digest from even a long input using a one-way message digest function (hash) Signatory’s private key

  21. Digital Signature Jrf843kjfgf*£$&Hdif*7oUsd*&@:<CHDFHSD(** Asymmetricdecryption (e.g. RSA) Py75c%bn&*)9|fDe^bDFaq#xzjFr@g5=&nmdFg$5knvMd’rkvegMs” ? == ? Are They Same? Signatory’s publickey Everyone has access to trusted public key of the signatory Verifying a Digital Signature Py75c%bn&*)9|fDe^bDFaq#xzjFr@g5=&nmdFg$5knvMd’rkvegMs” Same hash function(e.g. MD5, SHA…) This is a really long message about something… Original Message

  22. Word About Smartcards • Some smartcards are “dumb”, i.e. they are only a memory chip • Not recommended for storing a private key used in a challenge test (verifying identity) • Anyway, they are still better than leaving keys on a floppy disk or on the hard drive • Cryptographically-enabled smartcards are more expensive but they give much more security • Private key is secure and used as needed • Additional protection (password, biometrics) is possible • Hardware implements some algorithms • Self-destruct is possible

  23. Recommendations • Don’t be scared of PKI! • Set up a test environment to enable you to “play” • Minimise the scope of your first implementation • Read up on CP & CPS • Document the purpose and operating procedures of your PKI

  24. Summary • Cryptography is a rich and amazingly mature field • We all rely on it, everyday, with our lives • Know the basics and make good choices avoiding common pitfalls • Plan your PKI early • Avoid very new and unknown solutions • Certificate Policy • Certification Practises statement

  25. References • Visit http://www.pki-page.org/ • Read sci.crypt (incl. archives) • For more detail, read: • Cryptography: An Introduction, N. Smart, McGraw-Hill, ISBN 0-07-709987-7 • Practical Cryptography, N. Ferguson & B. Schneier, Wiley, ISBN 0-471-22357-3 • Contemporary Cryptography, R. Oppliger, Artech House, ISBN 1-58053-642-5 (to be published May 2005, see http://www.esecurity.ch/Books/cryptography.html) • Applied Cryptography, B. Schneier, John Wiley & Sons, ISBN 0-471-11709-9 • Handbook of Applied Cryptography, A.J. Menezes, CRC Press, ISBN 0-8493-8523-7, www.cacr.math.uwaterloo.ca/hac (free PDF) • PKI, A. Nash et al., RSA Press, ISBN 0-07-213123-3 • Foundations of Cryptography, O. Goldereich, www.eccc.uni-trier.de/eccc-local/ECCC-Books/oded_book_readme.html • Cryptography in C and C++, M. Welschenbach, Apress, ISBN 1-893115-95-X (includes code samples CD)

  26. Thanks to Rafal Lukawiecki and Steve Lamb for providing some of the content for this presentation deck – their contact details are as follows… • rafal@projectbotticelli.co.uk • stephlam@microsoft.com

  27. Common Algorithms

  28. DES, IDEA, RC2, RC5, Twofish S/MIME, SSL, Kerberos • Symmetric • DES (Data Encryption Standard) is still the most popular • Keys very short: 56 bits • Brute-force attack took 3.5 hours on a machine costing US$1m in 1993. Today it is done real-time • Triple DES (3DES) more secure, but better options about • Just say no, unless value of data is minimal • IDEA (International Data Encryption Standard) • Deceptively similar to DES, and “not” from NSA • 128 bit keys • RC2 & RC5 (by R. Rivest) • RC2 is older and RC5 newer (1994) - similar to DES and IDEA • Blowfish, Twofish • B. Schneier’s replacement for DES, followed by Twofish, one of the NIST competition finalists PGP .NET Fx S/MIME, SSL Java

  29. Rijndael (AES) • Standard replacement for DES for US government, and, probably for all of us as a result… • Winner of the AES (Advanced Encryption Standard) competition run by NIST (National Institute of Standards and Technology in US) in 1997-2000 • Comes from Europe (Belgium) by Joan Daemen and Vincent Rijmen. “X-files” stories less likely (unlike DES). • Symmetric block-cipher (128, 192 or 256 bits) with variable keys (128, 192 or 256 bits, too) • Fast and a lot of good properties, such as good immunity from timing and power (electric) analysis • Construction, again, deceptively similar to DES (S-boxes, XORs etc.) but really different

  30. CAST and GOST • CAST • Canadians Carlisle Adams & Stafford Tavares • 64 bit key and 64 bit of data • Chose your S-boxes • Seems resistant to differential & linear cryptanalysis and only way to break is brute force (but key is a bit short!) • GOST • Soviet Union’s “version” of DES but with a clearer design and many more repetitions of the process • 256 bit key but really 610 bits of secret, so pretty much “tank quality” • Backdoor? Who knows…

  31. Careful with Streams! • Do NOT use a block cipher in a loop • Use a crypto-correct technique for treating streams of data, such as CBC (Cipher Block Chaining) • For developers: • .NET Framework implements it as ICryptoTransform on a crypto stream with any supported algorithm

  32. RC4 PPTP • Symmetric • Fast, streaming encryption • R. Rivest in 1994 • Originally secret, but “published” on sci.crypt • Related to “one-time pad”, theoretically most secure • But! • It relies on a really good random number generator • And that is the problem • Nowadays, we tend to use block ciphers in modes of operation that work for streams

  33. RSA, DSA, ElGamal, ECC • Asymmetric • Very slow and computationally expensive – need a computer • Very secure • Rivest, Shamir, Adleman – 1978 • Popular and well researched • Strength in today’s inefficiency to factorise into prime numbers • Some worries about key generation process in some implementations • DSA (Digital Signature Algorithm) – NSA/NIST thing • Only for digital signing, not for encryption • Variant of Schnorr and ElGamal sig algorithm • ElGamal • Relies on complexity of discrete logarithms • ECC (Elliptic Curve Cryptography) • Really hard maths and topology • Improves RSA (and others) SSL, PGP .NET Fx

  34. Quantum Cryptography • Method for generating and passing a secret key or a random stream • Not for passing the actual data, but that’s irrelevant • Polarisation of light (photons) can be detected only in a way that destroys the “direction” (basis) • So if someone other than you observes it, you receive nothing useful and you know you were bugged • Perfectly doable over up-to-120km dedicated long fibre-optic link • Seems pretty perfect, if a bit tedious and slow • Practical implementations still use AES/DES etc. for actual encryption • Don’t confuse it with quantum computing, which won’t be with us for at least another 50 years or so, or maybe longer…

  35. MD5, SHA • Hash functions – not encryption at all! • Goals: • Not reversible: can’t obtain the message from its hash • Hash much shorter than original • Two messages won’t have the same hash • MD5 (R. Rivest) • 512 bits hashed into 128 • Mathematical model still unknown • But it resisted major attacks • SHA (Secure Hash Algorithm) • US standard based on MD5 S/MIME, SSL, PGP, Digital Sigs .NET Fx

  36. Diffie-Hellman, “SSL”, Certs PGP • Methods for key generation and exchange • DH is very clever since you always generate a new “key-pair” for each asymmetric session • STS, MTI, and certs make it even safer • Certs (certificates) are the most common way to exchange public keys • Foundation of Public Key Infrastructure (PKI) • SSL uses a protocol to exchange keys safely • See later Everyone

  37. Cryptanalysis • Brute force • Good for guessing passwords, and some 40-bit symmetric keys (in some cases needed only 27 attempts) • Frequency analysis • For very simple methods only (US mobiles) • Linear cryptanalysis • For stronger DES-like, needs 243 plain-cipher pairs • Differential cryptanalysis • Weaker DES-like, needs from 214 pairs • Power and timing analysis • Fluctuations in response times or power usage by CPU

  38. Strong Systems • It is always a mixture! Changes all the time… • Symmetric: • AES, min. 128 bits for RC2 & RC5, 3DES, IDEA, carefully analysed RC4, 256 bit better • Asymmetric: • RSA, ElGamal, Diffie-Hellman (for keys) with minimum 1024 bits (go for the maximum, typically 4096, if you can afford it) • Hash: • Either MD5 or SHA but with at least 128 bit results, 256 better

  39. Weak Systems • Anything with 40-bits (including 128 and 56 bit versions with the remainder “fixed”) • Most consider DES as fairly weak algorithm • CLIPPER • A5 (GSM mobile phones outside US) • Vigenère (US mobile phones) • Dates from 1585! • Unverified certs with no trust • Weak certs (as in many “class 1” personal certs)

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