Introduction to cryptography.

1. CRYPTO 101
SUBSPLASH TECH TALK 2017

2. 2
ATTACK AT DAWN
▸ How to hide a secret message?
THE PROBLEM
CRYPTO 101

3. 3
ATTACK AT DAWN
DW DFN DW GDZQW
CRYPTO 101

4. BITS & BYTES
▸ bits
▸ 0
▸ 1
▸ bytes
▸ 8 bits
▸ 0010 1111
▸ 1100 0001
4CRYPTO 101

5. ▸ Binary to decimal
▸ 0010 1111 -> 47
▸ 1100 0001 -> 193
▸ 0000 0000 -> 0
▸ 1111 1111 -> 255
▸ Decimal to binary
▸ 65 -> 0100 0001
5CRYPTO 101

6. CRYPTO 101
ASCII ENCODING
6
! " # $ % & ' ( ) * + , - . /
0 1 2 3 4 5 6 7 8 9 : ; < = > ?
@ 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 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 { | } ~

7. CRYPTO 101
ATTACK AT DAWN
7
A 65 0100 0001
T 84 0101 0100
T 84 0101 0100
A 65 0100 0001
C 67 0100 0011
K 75 0100 1011
32 0010 0000
A 65 0100 0001
T 84 0101 0100
32 0010 0000
D 68 0100 0100
A 65 0100 0001
W 87 0101 0111
N 78 0100 1110

8. CRYPTO 101
A 65 0100 0001
▸ Coin
▸ Head -> 0
▸ Tail -> 1
▸ 1 0 0 1 1 0 1 0
8

9. CRYPTO 101 9
“A” - 0100 0001
key - 1001 1010
??? - 1101 1011

10. CRYPTO 101
EXCLUSIVE OR (XOR)
10
A B A⊕B
0 0 0
1 0 1
0 1 1
1 1 0

11. CRYPTO 101 11
A 0100 0001 1001 1010 1101 1011
T 0101 0100 0101 1100 0000 1000
T 0101 0100 0111 0111 0010 0011
A 0100 0001 1110 1110 1010 1111
C 0100 0011 0010 0101 0110 0110
K 0100 1011 0010 1011 0110 0000
0010 0000 1100 1111 1110 1111
A 0100 0001 0010 0101 0110 0100
T 0101 0100 0111 1110 0010 1010
0010 0000 1111 0001 1101 0001
D 0100 0100 1001 1001 1101 1101
A 0100 0001 1101 1010 1001 1011
W 0101 0111 1110 0001 1011 0110
N 0100 1110 0011 1100 0111 0010
db
08
23
af
66
60
ef
64
2a
d1
dd
9b
b6
72
9a
5c
77
ee
25
2b
cf
25
7e
f1
99
da
e1
3c

12. CRYPTO 101 12
ATTACK AT DAWN
9a5c77ee252bcf257ef199dae13c
db0823af6660ef642ad1dd9bb672
⊕
||
??#?f`?d*????r

13. CRYPTO 101
ONE-TIME PAD
▸ Perfect secrecy
13
db0823af6660ef642ad1dd9bb672
1101 1011 0000 1000 0010 0011 1010 1111
0110 0110 0110 0000 1110 1111 0110 0100
0010 1010 1101 0001 1101 1101 1001 1011
1011 0110 0111 1100

14. CRYPTO 101
ONE-TIME PAD KEY
▸ As long as the plaintext
14
9a5c77ee252bcf257ef199dae13c
1001 1010 0101 1100 0111 0111 1110 1110
0010 0101 0010 1011 1100 1111 0010 0101
0111 1110 1111 0001 1001 1001 1101 1010
1110 0001 0011 1100

15. CRYPTO 101
IS ONE-TIME PAD GOOD ENOUGH?
▸ Not practical in real life
▸ “ATTACK AT DAWN” - 14 Bytes (B)
▸ A letter - 3 Kilo Bytes (KB)
▸ An audio - 4 Mega Bytes (MB)
▸ A video - 2 Giga Bytes (GB)
▸ Can we do better?
15

16. CRYPTO 101
BLOCK CIPHER
16
plaintext
↓
+--------------+
key -> | Block Cipher |
+--------------+
↓
ciphertext

17. CRYPTO 101
DATA ENCRYPTION STANDARD (DES)
▸ 1970s
▸ IBM
▸ Block size: 64 bits
▸ Key size: 56 bits
▸ password
▸ 3689207c138acf18
▸ 9adf272b96969988
17

18. CRYPTO 101
TRIPLE DES (3DES)
▸ 1998
▸ DES -> DES -> DES
▸ Key size: 56 * 3 = 168 bits
▸ Also 3 times slower than DES
18

19. CRYPTO 101
ADVANCED ENCRYPTION STANDARD (AES)
▸ 1997-2000
▸ NIST
▸ Finalists
▸ Rijndael/Serpent/Twofish/RC6/MARS
▸ Rijndael is the winner
19

20. CRYPTO 101
AES (RIJNDAEL)
▸ 2001
▸ Block size: 128 bits
▸ Key size: 128/192/256 bits
▸ e9e9fb64e075818aacb6d86c502d8b63
▸ 5ba36e701dd67693b4ada36cdc5b7a89624e94e0583
df09c
▸ cd8394660e5e4003d634cedbaa19b04d2ebaa8568ee
55197222f56ce1886ae17
20

21. CRYPTO 101 21
plaintext
↓
+--------------+
key -> | Block Cipher | (DES/3DES/AES)
+--------------+
↓
ciphertext

22. CRYPTO 101 22
ATTACK A T DAWN
-------- ------
64 bits 48 bits
ATTACK A
↓
+----------------+
password -> | DES Encryption |
+----------------+
↓
1a2dd25ec47c36fd
T DAWN00
--------
48+16 bits
T DAWN00
↓
+----------------+
password -> | DES Encryption |
+----------------+
↓
03fbff11a9a8e0f0
1a2dd25ec47c36fd03fbff11a9a8e0f0

23. CRYPTO 101 23
ATTACK A
↓
+----------------+
password -> | DES Encryption |
+----------------+
↓
1a2dd25ec47c36fd
T DAWN00
↓
+----------------+
password -> | DES Encryption |
+----------------+
↓
03fbff11a9a8e0f0
↓
+----------------+
password -> | DES Decryption |
+----------------+
↓
ATTACK A
↓
+----------------+
password -> | DES Decryption |
+----------------+
↓
T DAWN00
ATTACK AT DAWN00
ATTACK AT DAWN

24. CRYPTO 101
BLOCK CIPHER MODE
▸ ECB
▸ CBC
▸ CFB/OFB
▸ CTR
24

25. CRYPTO 101
ECB (ELECTRONIC CODEBOOK)
25
plaintext
↓
+-----------------+
key -> | Block Cipher Enc|
+-----------------+
↓
ciphertext
plaintext
↓
+-----------------+
key -> | Block Cipher Enc|
+-----------------+
↓
ciphertext
↓
+-----------------+
key -> | Block Cipher Dec|
+-----------------+
↓
plaintext
↓
+-----------------+
key -> | Block Cipher Dec|
+-----------------+
↓
plaintext

26. CRYPTO 101
ECB PENGUIN
26

27. CRYPTO 101
CBC (CIPHER BLOCK CHAINING)
27
plaintext
|
IV ----------> ⊕
↓
+-----------------+
key -> | Block Cipher Enc|
+-----------------+
↓
ciphertext
plaintext
|
+--------------> ⊕
| ↓
| +-----------------+
| key -> | Block Cipher Enc|
| +-----------------+
-----------+ ↓
ciphertext
↓
+-----------------+
key -> | Block Cipher Dec|
+-----------------+
|
IV ----------> ⊕
↓
plaintext
-----------+ ↓
| +-----------------+
| key -> | Block Cipher Dec|
| +-----------------+
| |
+--------------> ⊕
↓
plaintext

28. CRYPTO 101
WHICH ONE TO USE?
▸ ECB
▸ CBC
▸ CFB/OFB
▸ CTR
28

29. CRYPTO 101
STREAM CIPHER
▸ RC4
▸ Salsa20/ChaCha20
29

30. CRYPTO 101
THE PROBLEM
▸ How to hide a secret message?
▸ How to securely send my secret message to others?
30

31. CRYPTO 101
ALICE, BOB & EVE
31
secret
Alice -------------------------------------------> Bob
key key
secret secret'
Alice ------------------ Eve --------------------> Bob
key key

32. CRYPTO 101
CRYPTOGRAPHIC HASH FUNCTIONS
▸ Data integrity
32
secret
Alice -------------------------------------------> Bob
key key
secret
digest
Alice -------------------------------------------> Bob
key key

33. CRYPTO 101
IDEAL HASH FUNCTIONS
▸ digest = hash_function(input)
▸ Deterministic, same input always results in same digest
▸ Quick to compute for any input
▸ One-way function
▸ Avalanche effect, small change in input cause huge change in
digest
▸ Collision resistance, infeasible to find two different inputs with
same digest
33

34. CRYPTO 101
COMMON HASH FUNCTIONS
▸ MD5 (1992)
▸ SHA family
▸ SHA-1 (1995)
▸ SHA-2 (2001)
▸ SHA-224/SHA-256/SHA-384/SHA-512
▸ SHA-512/224, SHA512/256
▸ SHA-3 (2015)
34

35. CRYPTO 101
EXAMPLES
▸ hash(“ATTACK AT DAWN”)
35
MD5 042049321f10319f00d45dcc615f6bc1
SHA-1 19447f653ac5522b662bae641646bcce5102ade8
SHA-224 3db2c67781691fa0b7f82247fd8c7f5aa5977dc673d8c0979189919e
SHA-256 11e35eb1a2d18f4e2bfc43a1483c6b2b0f3ba4d6eb8b937278d3c9ab6897f4b8
SHA-384 51769418bf68409b6c41a4db245c7ff9d1e631f56c76661b1b84205d0419a179
6dd1ae3d474e204cd6c7ccf12907d46b
SHA-512 1df976845cfb9b2eddf77828c96e77a84ca17f864ab0513f70e8e50433f235be
9137ac500b6b181eb26d6db300304ba4a13b2c756c09cb073fb3926ea000a535

36. CRYPTO 101
IS DATA INTEGRITY ENOUGH?
36
secret
digest
Alice -------------------------------------------> Bob
key key
secret secret’
digest digest’
Alice ------------------ Eve --------------------> Bob
key key

37. CRYPTO 101
MAC (MESSAGE AUTHENTICATION CODE)
▸ Data integrity & authentication
▸ tag = mac_function(key, input)
▸ CMAC/CBC-MAC/VMAC/UMAC
▸ HMAC
▸ tag = hmac(hash_function, key, input)
▸ Poly1305
37

38. CRYPTO 101
HMAC
▸ hmac(hash, “mysecretkey”, “ATTACK AT DAWN”)
38
MD5 e9252bb2b93a94ac54af3fe1c6944e63
SHA-1 b22bf4d6ba5ca72a437def56f273025bc9480fa1
SHA-256 5a4590b9b30a2dc5899b0043ec407c561efd36eaa73abb3735752aa52f0c8c65
SHA-512 ad490070b7c792e4057d49d1b05d3515517505f9bb38413baab44326d0639f28
5791994496eeca1508471c8a97feb791dba129b4e3561cb1df73b4095d5d87d9

39. CRYPTO 101 39
secret
tag
Alice -------------------------------------------> Bob
(key, key2) (key, key2)
secret
digest
Alice -------------------------------------------> Bob
key key
secret secret’
tag ??????
Alice ------------------ Eve --------------------> Bob
(key, key2) (key, key2)

40. CRYPTO 101
SO FAR…
▸ Encryption
▸ DES/3DES/AES
▸ RC4/Chacha20
▸ Authentication
▸ HMAC/Poly1305
40

41. CRYPTO 101
HOW TO USE THEM?
▸ Encrypt then MAC
▸ IPsec
▸ MAC then Encrypt
▸ SSL/TLS
▸ Encrypt and MAC
▸ SSH
41

42. CRYPTO 101
AUTHENTICATED ENCRYPTION (AE)
▸ CCM/CWC/OCB
▸ GCM
42

43. CRYPTO 101
THE PROBLEM
▸ How to hide a secret message?
▸ How to securely send my secret message to others?
43

44. CRYPTO 101 44
secret secret’
tag ??????
Alice ------------------ Eve --------------------> Bob
(key, key2) (key, key2)
secret
tag
Alice -------------------------------------------> Bob
(key, key2) (key, key2)
(key, key2)
Alice ------------------ Eve --------------------> Bob
?????

45. CRYPTO 101
RESOURCES
▸ Cryptography I - Dan Boneh, Professor, Stanford U
▸ https://www.coursera.org/learn/crypto
▸ Enroll starts Aug 07, 2017
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