Introduction to Cryptographic Engineering. Steven M. Bellovin
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1 Introduction to Cryptographic Engineering Steven M. Bellovin 1
2 Cryptographic Engineering? There are lots of introductions to encryption But using encryption in the real world requires more We have to engineer it If we get the engineering wrong, enemies can crack our systems 2
3 A Disclaimer I ll be talking about classical (and simple) encryption, because it s easier to see what s going on I don t have time to cover all of the issues even there Modern encryption systems also need engineering; many of the issues today are quite similar 3
4 Terminology Encryption is an algorithm It converts plaintext the message we want to protect and a key to ciphertext Decryption, of course, converts the ciphertext and the key to plaintext Design principle: the system should be secure even if you enemy knows the algorithm the security should rest entirely on protecting the key (Kerckhoff, 1883) 4
5 Codes and Ciphers Ciphers operate at the syntactic layer Replace a bit or a letter with a different bit or letter It doesn t matter what the language is Codes operate at the semantic layer Replace a word, phrase, or sentence with a codeword Language-dependent: you can t use an English language codebook to encode French 5
6 Caesar Cipher According to Suetonius (writing around 121 CE), Caesar used a cipher that shifted every letter by 3: A B C D W X Y Z A B C D E F G W X Y Z A B C We could say that the key is 3 the amount of the shift or we could say that it s D A becomes D (This cipher is very, very insecure, for lots of reasons, but it s a simple example for now. Many of Caesar s enemies were illiterate ) 6
7 Sample Encryption Winston Churchill: This is the kind of tedious nonsense up with which I will not put WKLV LV WKH NLQG RI WHGLRXV QRQVHQVH XS ZLWK ZKLFK L ZLOO QRW SXW What s wrong? 7
8 Patterns Show Through WKLV LV WKH NLQG RI WHGLRXV QRQVHQVH XS ZLWK ZKLFK L ZLOO QRW SXW Word lengths: L can only be A or I Repeated letter patterns can show through Solution: five-letter groups WKLVL VWKHN LQGRI WHGLR XVQRQ VHQVH XSZLW KZKLF KLZLO OQRWS XW 8
9 Patterns Show Through WKLV LV WKH NLQG RI WHGLRXV QRQVHQVH XS ZLWK ZKLFK L ZLOO QRW SXW Word lengths: L can only be A or I Repeated letter patterns can show through Solution: five-letter groups WKLVL VWKHN LQGRI WHGLR XVQRQ VHQVH XSZLW KZKLF KLZLO OQRWS XW 9
10 (How Many Words Have the Same Pattern as QRQVHQVH )? Look for letters 3-4 the same as such words, most rather uncommon, e.g., obtected Look for letters 1, 3, and 6 being the same 45 such words, most rather uncommon, e.g., anaplasm Look for both patterns: Only two, cachucha and nonsense Which do you think it is? 10
11 Multiple Keys Alice has to exchange secret messages with Bob, Carol, and David Bob and Carol are allowed read each other s messages Bob and Carol must not see David s messages; he must not see theirs 11
12 Multiple Keys Alice has to exchange secret messages with Bob, Carol, and David Bob and Carol are allowed read each other s messages Bob and Carol must not see David s messages; he must not see theirs Solution: Alice, Bob, and Carol share one key; Alice and David share a different one 12
13 Multiple Keys Alice has to exchange secret messages with Bob, Carol, and David Bob and Carol are allowed read each other s messages Bob and Carol must not see David s messages; he must not see theirs Solution: Alice, Bob, and Carol share one key; Alice and David share a different one Messages must show which key is being used 13
14 Indicators Messages must contain an indicator KIBYZ WKLVL VWKHN LQGRI WHGLR XVQRQ VHQVH XSZLW KZKLF KLZLO OQRWS XW versus ZSETK WKLVL VWKHN LQGRI WHGLR XVQRQ VHQVH XSZLW KZKLF KLZLO OQRWS XW To the enemy, the indicator looks just like another code group 14
15 Message Lengths Matter Knowledge of message lengths matters Why? Spot message importance, repeated messages, etc. We need to pad the real message with dummy stuff Also: recipient must be certain the entire message was received 15
16 Padding This is the kind of tedious nonsense up with which I will not put xxx blue red cat flower rock WKLVL VWKHN LQGRI WHGLR XVQRQ VHQVH XSZLW KZKLF KLZLO OQRWS XWAAA EOXHU HGFDW IORZH UURFN 16
17 The XXX is a Pattern This is the kind of tedious nonsense up with which I will not put the world wonders WKLVL VWKHN LQGRI WHGLR XVQRQ VHQVH XSZLW KZKLF KLZLO OQRWS XWWKH ZRUOG ZRQGH UV But now the recipient can be confused and besides, we still have to worry about receiving the whole thing 17
18 Lengths The original message is 11 groups long, plus an indicator KIBYZ 11 WKLVL VWKHN LQGRI WHGLR XVQRQ VHQVH XSZLW KZKLF KLZLO OQRWS XWWKH ZRUOG ZRQGH UV But that s no good the attacker can see the message length, so the padding is useless Encrypt the length KIBYZ ZNERL WKLVL VWKHN LQGRI WHGLR XVQRQ VHQVH XSZLW KZKLF KLZLO OQRWS XWWKH ZRUOG ZRQGH UV 18
19 ZNERL? Why does ZNERL mean 11? We re using a code for message lengths 19
20 A Commercial Codebook 20
21 Encoding Numbers 21
22 A World War II Military Codebook 22
23 Code Can Be Insecure The same codeword always means the same thing An enemy can recreate the codebook which was routinely done by military cryptanalysts Solution: superencipher the code by using a book of additives 23
24 Codebook Additives Page
25 Additives Users had a book of additives page upon page of random numbers Open the additive book to a random page; pick a random line Starting from there, use each number in turn and add it (without carrying!) to the code number from the codebook We now need an indicator for the additive: the page and line number , etc. 25
26 Additive Example You receive The additives for that line are Subtracting (but without borrowing!), we get Turning to our codebook, we get. 26
27 Arrived with captain under restraint, apparently insane Send fresh captain immediately (This codebook, the The A B C Universal Commercial Electric Telegraphic Code from 1901, is available at 27
28 The Enigma Machine Used by the Germans during World War II Initially cracked by the Poles, who gave their insights to the British The British made major improvements and were able to read Enigma traffic constantly (Photo: NSA) 28
29 Setting The Rotors The operator picked three random letters and encrypted them twice These encrypted letters were part of the indicators 29
30 Engineering and Usage Mistakes Encrypting one of the indicator fields twice was a fatal flaw Picking non-random letters for the indicator was a fatal flaw Sending the same, simple message every day was a fatal flaw Sending a message consisting of nothing but the letter L was a fatal flaw this is partly usage, and partly a design weakness in the Enigma The basic algorithm was decent but it wasn t engineered properly! 30
31 Questions? (these slides at 31
32 Vigenère Cipher Write each letter of the key above the message, repeating as necessary Encrypt each plaintext letter with the key letter above it Note: because the key changes constantly, a single plaintext value can have a different ciphertext (Invented circa 1585; general solution found in 1863 by Kasiski) Key: SECRE TSECR ETSEC RETSE CRETS ECRET Message: Thisi sthek indof tedio usnon sense Encrypted: LLKJM LLLGB MGVSH KIWAS WJRHF WGEWX 32
33 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A A B C D E F G H I J K L M N O P Q R S T U V W X Y Z B B C D E F G H I J K L M N O P Q R S T U V W X Y Z A C C D E F G H I J K L M N O P Q R S T U V W X Y Z A B D D E F G H I J K L M N O P Q R S T U V W X Y Z A B C E E F G H I J K L M N O P Q R S T U V W X Y Z A B C D F F G H I J K L M N O P Q R S T U V W X Y Z A B C D E G G H I J K L M N O P Q R S T U V W X Y Z A B C D E F H H I J K L M N O P Q R S T U V W X Y Z A B C D E F G I I J K L M N O P Q R S T U V W X Y Z A B C D E F G H J J K L M N O P Q R S T U V W X Y Z A B C D E F G H I K K L M N O P Q R S T U V W X Y Z A B C D E F G H I J L L M N O P Q R S T U V W X Y Z A B C D E F G H I J K M M N O P Q R S T U V W X Y Z A B C D E F G H I J K L N N O P Q R S T U V W X Y Z A B C D E F G H I J K L M O O P Q R S T U V W X Y Z A B C D E F G H I J K L M N P P Q R S T U V W X Y Z A B C D E F G H I J K L M N O 33
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