Key management. Pretty Good Privacy

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1 ECE Lecture 4 Key management Pretty Good Privacy Using the same key for multiple messages M 1 M 2 M 3 M 4 M 5 time E K time C 1 C 2 C 3 C 4 C 5 1

2 Using Session Keys & Key Encryption Keys K 1 K 2 K 3 time E KEK time E KEK (K 1 ) E KEK (K 2 ) E KEK (K 3 ) M 1 M 2 M 3 M 4 M 5 time E K1 E K2 E K3 time C 1 C 2 C 3 C 4 C 5 Key Distribution Center (KDC) B K B-KDC K A-KDC A C K C-KDC K A-KDC K B-KDC K C-KDC K D-KDC KDC E K E-KDC D K D-KDC 2

3 Simple key establishment protocol based on KDC KDC K A-KDC K B-KDC K C-KDC K D-KDC... (1) let me talk with (2b) K B-KDC (, K AB ) (2a) K A-KDC (, K AB ) A K A-KDC K B-KDC B Key establishment protocol based on KDC KDC K A-KDC K B-KDC K C-KDC K D-KDC... (1) let me talk with (2) K A-KDC (, K AB, ticket ) (3) ticket = K B-KDC (, K AB ) A B K A-KDC K B-KDC 3

4 A s private key Key agreement B s private key A s public key B s public key Secret derivation Secret derivation Key derivation Key of A and B Key derivation Key of A and B x A Diffie-Hellman key agreement scheme α, q - global public elements x B y A = α x A mod q y B = α x B mod q x A S AB = y B mod q x B S AB = y A mod q Key derivation Key derivation Key K AB Key K AB 4

5 Man-in-the-middle attack A s private key B s private key A s public key B s public key Charlie Secret derivation C s public key C s public key Secret derivation Key derivation Key derivation Key of A and C Key of B and C Does public key cryptography have an Achilles heel?, send me your public key, s public key, message encrypted using s public key Charlie 5

6 Does public key cryptography have an Achilles heel?, send me your public key, s public key, Charlie s public key message encrypted using s public key Charlie Charlie s public key Does public key cryptography have an Achilles heel?, send me your public key, s public key, Charlie s public key message encrypted using Charlies s public key Charlie message reencrypted using s public key 6

7 Directory of public keys (1) On-line database, s public key, s public key, s public key Charlie, Charlie s public key Dave, Dave s public key Eve, Eve s public key. message encrypted using s public key Charlie Directory of public keys (2) On-line database, s public key Charlie s public key, s public key, s public key Charlie, Charlie s public key Dave, Dave s public key Eve, Eve s public key. message encrypted using s public key Charlie s public key Charlie 7

8 Directory of public keys (3) On-line database, s public key Charlie s public key, s public key, s public key Charlie, Charlie s public key Dave, Dave s public key Eve, Eve s public key. message encrypted using Charlie s public key Charlie message reencrypted using s public key PGP: Flow of trust Manual exchange of public keys: Las Vegas David Edinburgh David Betty (Washington) David (New York) Betty (London) David, send me Betty s public key Betty s public key signed by David message encrypted using Betty s public key 8

9 Certification Authority Loren Kohnfelder, Towards a Practical Public-Key Cryptosystem, Bachelor s Thesis, MIT, May Proof of identity Public key of Certification Authority Certificate Public key of Certification Authority Certificate Subject s distinguished name Subject s public key Subject s Credentials Serial number Issuer (CA) name Period of validity Signature algorithm identifier CA s signature 9

10 Distinguished Name (DN) according to X.500 Example: Common name (CN) = Kris Gaj Country name (C) = US State or province name (ST) = VA Locality name (L) = Fairfax Organization name (O) = George Mason University Organizational unit name (OU) = ECE Other fields permitted: Street address (SA) Post office box (PO Box) Postal code (PC) Title (T) Description (D) Telephone number (TN) Serial number (SN) The exact X.509 Certificate Format 10

11 Non-repudiation only M, SGN A (M), Cert CA (A, KU A ) s private key - KR A CA s public key - KU CA Notation: KU X - public key of X KR X - private key of X SGN X (M) - signature of X for the message M Cert Y (X, KU X ) - certificate issued by Y for the user X Cert CA (B, KU B ) Confidentiality only Cert CA (A, KU A ) Cert CA (B, KU B ) Cert CA (C, KU C ) Cert CA (D, KU D ). On-line database K AB (M), KU B (K AB ) CA s public key - KU CA s private key - KR B 11

12 Confidentiality and Non-repudiation Cert CA (B, KU B ) Cert CA (A, KU A ) Cert CA (B, KU B ) Cert CA (C, KU C ) Cert CA (D, KU D ). On-line database SGN A (M), Cert CA (A, KU A ), K AB (M), KU B (K AB ) s private key - KR A CA s public key - KU CA s private key - KR B CA s public key - KU CA Public Key Infrastructure with Strict Hierarchy US VA MA CA Fairfax Herndon Worcester Boston Santa Clara San Jose GMU MIT A M, SGN A (M), All users know KU US Cert GMU (A, KU A ), Cert Fairfax (GMU, KU GMU ), Cert VA (Fairfax, KU Fairfax ), Cert US (VA, KU VA ), B 12

13 Public Key Infrastructure with Reverse Certificates US VA MA CA Fairfax Herndon Worcester Boston Santa Clara San Jose GMU MIT A knows KU GMU B B knows KU MIT A M, SGN A (M), Cert GMU (A, KU A ), Cert Fairfax (GMU, KU GMU ), Cert VA (Fairfax, KU Fairfax ), Cert US (VA, KU VA ), Cert MA (US, KU US ), Cert Boston (MA, KU MA ), Cert MIT (Boston, KU Boston ) Public Key Infrastructure with Cross-Certificates Cert GMU (MIT, KU MIT ) Cert MIT (GMU, KU GMU ) GMU MIT A A knows KU GMU B B knows KU MIT M, SGN A (M), Cert GMU (A, KU A ), Cert MIT (GMU, KU GMU ) 13

14 VeriSign Public-Key Certificate Classes Certificate Revocation Lists (CRLs) This update date Next update date Issuer (CA) name List of revoked certificates (serial number + revocation date) Signature algorithm CA s signature Certificate is valid if it has a valid signature of CA did not expire is not listed in the CA s most recent CRL 14

15 The exact X.509 CRL Format Advantages of Certification Authorities over Key Distribution Centers CA does not need to be on-line CA is relatively easy to implement CA crash = no new users in the network but all old users operate normally certificates are not security sensitive, they can be stored in a public database, and transmitted over a public network compromised CA cannot decrypt messages (without first impersonating one of the users) only active attacks can be mounted using CAs private key 15

16 A s static private key A s ephemeral private key Authenticated key agreement A s ephemeral public key A s static public key certificates B s static public key B s ephemeral public key B s static private key B s ephemeral private key Secret derivation Key derivation key Secret derivation Key derivation key Pretty Good Privacy PGP 16

17 Security is one of the most widely used and regarded network services currently message contents are not secure may be inspected either in transit or by suitably privileged users on destination system Pretty Good Privacy (PGP) widely used de facto secure developed by Phil Zimmermann selected best available crypto algs to use integrated into a single program available on Windows, Unix, Macintosh, and other systems originally free, now have commercial versions available also 17

18 PGP Authentication Only Notation: M - message H hash function EP public key encryption - concatenation Z - compression using ZIP algorithm KR a private key of user A KU a public key of user A Non-repudiation Message Signature Message Signature Hash function Hash function Hash value 1 Hash value yes no Public key cipher Hash value 2 Public key cipher s private key s public key 18

19 PGP Confidentiality Only Notation: M - message Z - compression using ZIP algorithm EC / DC classical (secret-key) encryption / decryption EP / DP public key encryption / decryption - concatenation K s - session key KR b private key of user B KU b public key of user B Hybrid Systems - Sender s Side (2) 1 session key random message Secret key cipher Public key cipher 3 s public key 2 Session key encrypted using s public key Message encrypted using session key 19

20 Hybrid Systems - Receiver s Side (2) session key random 1 Public key cipher s private key message 2 Secret key cipher Session key encrypted using s public key Message encrypted using session key PGP Confidentiality and Authentication Notation: M - message H hash function Z - compression using ZIP algorithm EP / DP public key encryption / decryption - concatenation EC / DC classical (secret-key) encryption / decryption K s - session key KR a / KR b private key of user A / B KU a / KU b public key of user A / B 20

21 Transmission and Reception of PGP Messages PGP Operation Compression by default PGP compresses message after signing but before encrypting so can store uncompressed message & signature for later verification & because compression is non deterministic uses ZIP compression algorithm 21

22 Major idea behind ZIP compression Radix-64 Conversion 22

23 Radix-64 Encoding General Format of PGP Message 23

24 Summary of PGP functions Private Key Ring 24

25 Public Key Ring PGP Message Generation (without compression or radix-64 conversion) 25

26 PGP Message Reception (without compression or radix-64 conversion) Manual exchange of public keys: PGP: Flow of trust Las Vegas David Edinburgh David Betty (Washington) David (New York) Betty (London) David, send me Betty s public key Betty s public key signed by David message encrypted using Betty s public key 26

27 PGP Trust Model Contents of Trust Flag Byte 27

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