ArcBlock's Technical Learning Series presents: Cryptography for everyone. Cryptography is a foundational piece in BlockChain technology. The reason BlockChain is secured is based on the promise of the underlying cryptographic primitives. In this talk, we will give an introduction to cryptography, from symmetric encryption to public key encryption. We will see how it makes blockchain technology possible. Also, we will talk about the crypto primitives behind cryptocurrency wallet. So you will know how important is your private key. 区块链是集多种高大上技术为大成的应用，密码学占其中最重要的一环。事实上，平日我们在互联网上做的任何事情都有密码学的身影在里面。区块链所用到的的sha256, secp256k1, ed25519都是些什么鬼？本次讲座将从一个最简单的问题开始——如何隐藏不能说的秘密——一直讲到现代密码学在区块链以及钱包中的应用，为您揭开密码学娇羞的面纱。 听完您会明白，为什么我不该在云上备份钱包里的私钥。 https://www.arcblock.io

1. 1/15/2019 Cryptography for Everyone
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Cryptography for Everyone
Brought to you by Boshan Sun
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How to hide a secret?
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ATTACK AT DAWN
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DWWDFN DW GDZQ
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Bits & Bytes
# bits
0
1
# bytes
0010 1111
1100 0001
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ASCII
------------US-ASCII-------------
0 1 2 3 4 5 6 7 8 9 a b c d e f
2 ! " # $ % & ' ( ) * + , - . /
3 0 1 2 3 4 5 6 7 8 9 : ; < = > ?
4 @ A B C D E F G H I J K L M N O
5 P Q R S T U V W X Y Z [ ] ^ _
6 ` a b c d e f g h i j k l m n o
7 p q r s t u v w x y z { | } ~
---------------------------------
A 41 0100 0001
T 54 0101 0100
T 54 0101 0100
A 41 0100 0001
C 43 0100 0011
K 4b 0100 1011
20 0010 0000
A 41 0100 0001
T 54 0101 0100
20 0010 0000
D 44 0100 0100
A 41 0100 0001
W 57 0101 0111
N 4e 0100 1110
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Coin toss
- head 0
- tail 1
0101
0011
1110
0100000101010100010101000100000101000011010010110010000001000001010101000010000001000100010000010101011101001110 ATTACK AT DAWN
0001000001001011001001111100001001011110001010110101011000010000010111110000010011110100011100101011010000110100 random coin toss
0101000100011111011100111000001100011101011000000111011001010001000010110010010010110000001100111110001101111010 ciphertext
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One-time Pad
• perfect secrecy
• as long as the plaintext
• not practical in real life
• can we do better?
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Block Cipher
.
plaintext
↓
+--------------+
key -> | Block Cipher |
+--------------+
↓
ciphertext
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DES
• Data Encryption Standard
• 1970s
• IBM
• block size: 8 bytes
• key size: 8 bytes
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3DES
• 1998
• DES -> DES -> DES
• key size: 8 * 3 = 24 bytes
• also 3 times slower than DES
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AES
• Advanced Encryption Standard
• 1997 - 2000
• NIST
• Finalists
• Rijndael/Serpent/Twofish/RC6/MARS
• Rijndael become AES
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AES
• 2001
• block size: 16 bytes
• key size: 16/24/32 bytes
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Block Cipher
.
plaintext
↓
+--------------+
key -> | Block Cipher | (DES/3DES/AES)
+--------------+
↓
ciphertext
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Block Cipher Encryption
ATTACK A T DAWN00
-------- --------
8 bytes 8 bytes
ATTACK A T DAWN00
↓ ↓
+--------------+ +--------------+
password -> | DES Encrypt | password -> | DES Encrypt |
+--------------+ +--------------+
↓ ↓
1a2dd25ec47c36fd 03fbff11a9a8e0f0
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Block Cipher Decryption
.
ATTACK A T DAWN00
↓ ↓
+--------------+ +--------------+
password -> | DES Encrypt | password -> | DES Encrypt |
+--------------+ +--------------+
↓ ↓
1a2dd25ec47c36fd 03fbff11a9a8e0f0
↓ ↓
+--------------+ +--------------+
password -> | DES Decrypt | password -> | DES Decrypt |
+--------------+ +--------------+
↓ ↓
ATTACK A T DAWN00
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Block Cipher Mode
• ECB
• CBC
• CFB/OFB
• CTR
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ECB Mode
.
plaintext plaintext
↓ ↓
+--------------+ +--------------+
key -> | BC Encrypt | key -> | BC Encrypt |
+--------------+ +--------------+
↓ ↓
ciphertext ciphertext
↓ ↓
+--------------+ +--------------+
key -> | BC Decrypt | key -> | BC Decrypt |
+--------------+ +--------------+
↓ ↓
plaintext plaintext
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ECB Penguin 19

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CBC Mode
.
plaintext plaintext
↓ ↓
iv -> ⊕ +-----------------> ⊕
↓ | ↓
+--------------+ | +--------------+
key -> | BC Encrypt | | key -> | BC Encrypt |
+--------------+ | +--------------+
↓ | ↓
ciphertext -------+ ciphertext
↓ | ↓
+--------------+ | +--------------+
key -> | BC Decrypt | | key -> | BC Decrypt |
+--------------+ | +--------------+
↓ | ↓
iv -> ⊕ +-----------------> ⊕
↓ ↓
plaintext plaintext
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Stream Cipher
• RC4
• Salsa20/ChaCha20
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How to securely send my message to others?
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Alice, Bob & Eve
.
secret
Alice ----------------------------------> Bob
key key
secret secret'
Alice ------------ Eve -------------> Bob
key key
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Cryptographic Hash Functions
• data integrity
.
secret
digest
Alice ----------------------------------> Bob
key key
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Ideal Hash Function
• deterministic, same input always generate same output
• quick to compute for any input
• one-way function
• avalanche effect, samell change in input cause huge change in output
• collision resistance, infeasible to find two different inputs generate same output
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Common hash functions
• MD5 (1992)
• SHA-1 (1995)
• SHA-224/256/384/512 (2001)
• SHA-3 (2015)
• RIPEMD-160 (1996)
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SHA3
• NIST, 2007
• Finalists, 2012
• BLAKE
• Grøstl
• JH
• Keccak <- this guy wins
• Skein
• SHA-3, 2015
• Ethereum uses original 2012 keccak256
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Is data integrity enough?
.
secret
digest
Alice ----------------------------------> Bob
key key
secret secret'
digest digest'
Alice ------------ Eve -------------> Bob
key key
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MAC
• Message Autehntication Code
• data integrity & authentication
• CMAC/CBC-MAC/VMAC/UMAC
• HMAC
• Poly1305
hmac
sha sha
msg + key -------> f023a7d109f1 + key -------> b15c701d5e63
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MAC
.
secret
digest
Alice ----------------------------------> Bob
key key
secret
tag
Alice ----------------------------------> Bob
{k1,k2} {k1,k2}
secret secret'
tag ??????
Alice ------------ Eve -------------> Bob
{k1,k2} {k1,k2}
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How do Alice and Bob swap keys at the rst place?
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Is key exchange possible?
.
secret secret'
tag ??????
Alice ------------ Eve -------------> Bob
{k1,k2} {k1,k2}
{k1,k2}
Alice ------------ Eve -------------> Bob
???
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Di e-Hellman-Merkle key exchange
• 1976
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RSA
• 1977
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Public Key Encryption
• private key (sk)
• public key (pk)
.
plaintext plaintext
↓ ↓
+--------------+ +--------------+
pk -> | RSA | sk -> | RSA |
+--------------+ +--------------+
↓ ↓
ciphertext ciphertext
↓ ↓
+--------------+ +--------------+
sk -> | RSA | pk -> | RSA |
+--------------+ +--------------+
↓ ↓
plaintext plaintext
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DSA
• Digital Signature Algorithm
• NIST, 1991
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ECDSA
• Elliptic Curve DSA
• secp256k1, Bitcoin, Ethereum
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EdDSA
• Edwards-curve DSA
• Curve25519
• Ed25519, HD key
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