ÓBUDA UNIVERSITY, BUDAPEST SSL CERTIFICATES
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1 ÓBUDA UNIVERSITY, BUDAPEST SSL CERTIFICATES
2 INTRODUCTION KESZTHELYI András PhD Óbuda University, Keleti Károly Faculty of Economics
3 BASIC RULE Ask questions whenever you need! (Or whenever you think you need:)
4 Question What is the title of this lesson?
5 STRUCTURE Want to hide your data? One-key encryption Two-key encryption Certificates Known problems
6 Want to hide your data? close your computer close the door use strong passwords encryption
7 STRUCTURE Want to hide your data? One-key encryption Two-key encryption Certificates Known problems
8 One-key encryption examples security rules key is real random series must be kept in secret big question need a secure channel for key exchange remark: more sophisticated methods exist
9 One-key encryption, example 1. This is the key: A = B = C = etc.
10 One-key encryption, example 1. Which is the most frequent sign? Letter frequency in the language
11 One-key encryption, example 2. Key: 3 (shift right) A=C B=D C=E etc. Which is the most frequent sign? Letter frequency in the language
12 One-key encryption, example 3. Key: 34 (shift right) A=C or D B=D or E C=E or F etc. This is the plain text ??????????????????
13 One-key encryption, example 3. Key: 34 (shift right) A=C or D B=D or E C=E or F etc. This is the plain text W
14 One-key encryption, example 3. Key: 34 (shift right) A=C or D B=D or E C=E or F etc. This is the plain text W If you have enough captured text?
15 One-key encryption, example 3. key: the longer the better key: the longest the best
16 Security rules key: real random series kept in secret => 100% secure X + Y = Z
17 Question Have you ever read a book 800 miles on the Amazon? Have you ever read a book written by Jules Verne?
18 Big question Alice & Bob have their own keys real random series each kept in absolute secret (if exists;) Alice encrypts Bob also encrypts Alice decrypts Bob decrypts and reads?
19 Big question Alice & Bob have their own keys KeyA & KeyB text + KeyA text + KeyA + KeyB text + KeyA + KeyB KeyA = = text + KeyB text + KeyB KeyB = text?
20 Big question Alice & Bob have their own keys KeyA & KeyB msg1: text + KeyA msg2: text + KeyA + KeyB postman: msg2 msg1 = text So?
21 Secure channel for key exchange if the key must be kept in secret......you need a secure channel practically: personal meeting In the bottom of the copper mine? If the other guy lives in New-Zealand?
22 Remark Less or more sophisticated methods do exist
23 STRUCTURE Want to hide your data? One-key encryption Two-key encryption Certificates Known problems
24 Tow-key encryption secure channel theoretical background example: dictionary breakable Hard enough, so who cares? prime factorization how it works digital signature security rules secret (private) key must be kept in secret collected public keys must be checked MITM
25 Secure channel for key exchange if the other guy lives in New-Zealand expensive so we'd like to get rid of
26 Example no need for a secure channel for key exchg pair of dictionaries
27 Example no need for a secure channel for key exchg pair of dictionaries Serbian-English: public key place it at Yellow Gulliver everyone can use it English-Serbian: secret key the only copy is at home your dog stands guard replace Serbian words of your message
28 Breakable capture an encrypted message go to Yellow Gulliver search it => You can find the decryption!
29 Breakable ~1.200 pages you must carefully read 600 pages approx. to decrypt one word 100 word long message: pages to read So what if it is breakable?
30 Breakable So what if it is breakable? Hard enough, so who cares?
31 Prime factorization real method based on prime factorization much-much more secure
32 Prime factorization Try! Multiply two 100 digit prime numbers Find the factors of the result!
33 How it works for the exact math background see: Wikipedia, e.g. pair of keys are generated public (P) and secret (S) one encrypts, other decrypts (and vice versa) (secret OR private key)
34 How it works coding [ coding(text,p), S ] = text OR coding [ coding(text,s), P ] = text
35 How it works coding [ coding(text,p), S ] = text OR coding [ coding(text,s), P ] = text SO: public key can be distributed secret key must be kept in secret
36 How it works Alice wants to send an ecrypted msg to Bob which key will she use? A: her own secret B: her own public C: Bob's public D: Bob's secret
37 How it works Bob received an encrypted msg from Ann which key will he use? A: his own secret B: his own public C: Ann's public D: Ann's secret
38 Digital signature Ann wants Bob to be sure the message is really from her She can encrypt the message with her own secret key, too
39 Digital signature Better solution: instead of whole docu you encrypt only a cheksum of the docu
40 Digital signature
41 Security rule 1. Secret key must be kept in total secret! if not, others may... read our messages digitally sign docus instead of us
42 Security rule 2. Collected public keys must be checked! Why?
43 Security rule 2. Collected public keys must be checked! Why? For this:
44 MITM Man In The Middle Monkey In The Middle when the other guy is not the one you think he is
45 MITM Alice (P A ;S A ) & Bob (P B ;S B ) Alice wants to send an encrypted msg to Bob What does she need for that?
46 MITM Alice (P A ;S A ) & Bob (P B ;S B ) Alice wants to send an encrypted msg to Bob What does she need for that? Bob's public key, P B.
47 MITM Bob's public key, P B, can be sent via Ann P B Bob...can be?
48 MITM What if Cecil is is in the middle?
49 MITM What if Cecil is is in the middle? Ann P B P C Bob Cecil (P C ;S C ) P B
50 MITM You must carefully check whether the collected public keys really belong to the person you think they belong to!
51 MITM check public keys Get personally from them Get via many different channels Build the web of trust:
52 Web of trust Alice has the authentic public key of Bob.
53 Web of trust Alice has the authentic public key of Bob. Cecil is close to Bob.
54 Web of trust Alice has the authentic public key of Bob. Cecil is close to Bob. Cecil can put his personal data and his public key into a document.
55 Web of trust Alice has the authentic public key of Bob. Cecil is close to Bob. Cecil can put his personal data and his public key into a document. Bob can sign this document means: described person and public key belong to each other.
56 Web of trust Alice has the authentic public key of Bob. Cecil is close to Bob. Cecil can put his personal data and his public key into a document. Bob can sign this document means: described person and public key belong to each other. Alice can check the signature...
57 Web of trust Chain of trust web of trust No need for central organization
58 STRUCTURE Want to hide your data? One-key encryption Two-key encryption Certificates Known problems
59 CERTIFICATES standard docu format for automatic key exchange
60 CERTIFICATES Certificate Authorities, CA
61 CERTIFICATES See in Firefox, e.g. Preferences / Advanced / Certificates contains a lot of authentic root CERTs
62 in case of an https connection: CERT of other side is acquired if it is among the stored ones, OK. if not, its signature is checked (etc.) CERTIFICATES
63 CERTIFICATES if there is an error message, it is YOU, who must check the situation (and the certificate). You must be sure that the other side is the one you think it should be, e.g. your bank!
64 STRUCTURE Want to hide your data? One-key encryption Two-key encryption Certificates Known problems
65 Known problems human factor social engineering browser CERTs are tampered with a few organizations can probably break it
66 ADVICES Learn what you can knowledge is power! Believe in God AND keep gunpowder dry! in other words: 100% security level does not exist. You want to be as close to it as possible.
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