Part I: Introduction

Chapter 8
Network Security
第８章
ネットワークセキュリティ
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Thanks and enjoy! JFK/KWR
Computer Networking:
A Top Down Approach
Featuring the Internet,
3rd edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2004.
All material copyright 1996-2004
J.F Kurose and K.W. Ross, All Rights Reserved
8: Network Security
8-1
邦訳版
インターネット技術のすべて：ト
ップダウンアプローチによる実
践ネットワーク技法第２版
ジェームズ・F・クロセ (著), キ
ース・W・ロス (著), 岡田博美
(翻訳)
出版社: ピアソン・エデュケーシ
ョン (2003/12/25)
ASIN: 4894714949
8: Network Security
8-2
Chapter 8: Network Security
第８章：ネットワークセキュリティ
Chapter goals: この章の目標
 understand principles of network security:
ネットワークセキュリティの原理を理解すること：
 cryptography and its many uses beyond
“confidentiality”
 authentication
 message integrity
 key distribution
 security in practice:
実践でのセキュリティ
 firewalls
 security in application, transport, network, link
layers
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8-3
Chapter 8 roadmap
8.1 What is network security?
ネットワークセキュリティとは？
8.2 Principles of cryptography
8.3 Authentication
8.4 Integrity
8.5 Key Distribution and certification
8.6 Access control: firewalls
8.7 Attacks and counter measures
8.8 Security in many layers
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8-4
What is network security?
ネットワークセキュリティとは？
Confidentiality: 機密性： only sender, intended
receiver should “understand” message contents
 sender encrypts message
 receiver decrypts message
Authentication: 認証： sender, receiver want to
confirm identity of each other
Message Integrity: メッセージ保全： sender, receiver
want to ensure message not altered (in transit, or
afterwards) without detection
Access and Availability: アクセスと有効性： services
must be accessible and available to users
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Friends and enemies: Alice, Bob, Trudy
友達と敵： Alice, Bob, Trudy
 well-known in network security world
 Bob, Alice (lovers!) want to communicate “securely”
 Trudy (intruder) may intercept, delete, add messages
Alice
data
channel
secure
sender
Bob
data, control
messages
secure
receiver
data
Trudy
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Who might Bob, Alice be?
誰がBob,Aliceであってよいか？
 … well,
real-life Bobs and Alices!
 Web browser/server for electronic
transactions (e.g., on-line purchases)
 on-line banking client/server
 DNS servers
 routers exchanging routing table updates
 other examples?
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There are bad guys (and girls) out there!
そこには悪い連中もいる！
Q: What can a “bad guy” do?
“悪い奴”ができることは何か？
A: a lot! たくさん！
eavesdrop: 盗聴： intercept messages
 actively insert messages into connection
 impersonation: 偽装： can fake (spoof) source

address in packet (or any field in packet)
 hijacking: ハイジャック： “take over” ongoing
connection by removing sender or receiver,
inserting himself in place
 denial of service: サービス不能： prevent service
from being used by others (e.g., by overloading
resources)
more on this later ……
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8-8
Chapter 8 roadmap
8.1 What is network security?
8.2 Principles of cryptography
暗号の原理
8.3 Authentication
8.4 Integrity
8.5 Key Distribution and certification
8.6 Access control: firewalls
8.7 Attacks and counter measures
8.8 Security in many layers
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8-9
The language of cryptography
暗号の言語
Alice’s
K encryption
A
key
plaintext
encryption
algorithm
ciphertext
Bob’s
K decryption
B key
decryption plaintext
algorithm
symmetric key crypto: sender, receiver keys identical
対称鍵暗号：送信者、受信者の鍵は同一
public-key crypto: encryption key public, decryption key
secret (private)
公開鍵暗号：暗号鍵は公開、復号鍵は秘密（プライベート）
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Symmetric key cryptography
対称鍵暗号方式
substitution cipher: substituting one thing for another
置換式暗号：あるものを他のもので置換

monoalphabetic cipher: substitute one letter for another
plaintext:
abcdefghijklmnopqrstuvwxyz
ciphertext:
mnbvcxzasdfghjklpoiuytrewq
E.g.:
Plaintext: bob. i love you. alice
ciphertext: nkn. s gktc wky. mgsbc
Q: How hard to break this simple cipher?:
 brute force (how hard?)
 other?
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Symmetric key cryptography
対称鍵暗号方式
KA-B
KA-B
plaintext
message, m
encryption ciphertext
algorithm
K (m)
A-B
decryption plaintext
algorithm
m = K ( KA-B(m) )
A-B
symmetric key crypto: Bob and Alice share know same
(symmetric) key: KA-B
対称鍵暗号：ボブとアリスは同じ（対称の）鍵を共有
 e.g., key is knowing substitution pattern in mono
alphabetic substitution cipher
 Q: how do Bob and Alice agree on key value?
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Symmetric key crypto: DES
対称鍵暗号： DES
DES: Data Encryption Standard
 US encryption standard [NIST 1993]
 56-bit symmetric key, 64-bit plaintext input
 How secure is DES?
DES Challenge: 56-bit-key-encrypted phrase
(“Strong cryptography makes the world a safer
place”) decrypted (brute force) in 4 months
 no known “backdoor” decryption approach
 making DES more secure:
 use three keys sequentially (3-DES) on each datum
 use cipher-block chaining

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Symmetric key
crypto: DES
対称鍵暗号: DES
DES operation
initial permutation
16 identical “rounds” of
function application,
each using different
48 bits of key
final permutation
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AES: Advanced Encryption Standard
 new (Nov. 2001) symmetric-key NIST
standard, replacing DES
 processes data in 128 bit blocks
 128, 192, or 256 bit keys
 brute force decryption (try each key)
taking 1 sec on DES, takes 149 trillion
years for AES
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Public Key Cryptography
公開鍵暗号方式
symmetric key crypto
public key cryptography
対称鍵暗号
 requires sender,
receiver know shared
secret key
 Q: how to agree on key
in first place
(particularly if never
“met”)?
公開鍵暗号方式
 radically different
approach [DiffieHellman76, RSA78]
 sender, receiver do not
share secret key
 public encryption key
known to all
 private decryption key
known only to receiver
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Public key cryptography
公開鍵暗号方式
+ Bob’s public
B key
K
K
plaintext
message, m
encryption ciphertext
algorithm
+
K (m)
B
- Bob’s private
B key
decryption plaintext
algorithm message
+
m = K B(K (m))
B
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Public key encryption algorithms
公開鍵暗号アルゴリズム
Requirements:
1
2
+
need K ( ) and K - ( ) such that
B
B
- +
K (K (m)) = m
B B
.
.
+
given public key KB , it should be
impossible to compute
private key KB
RSA: Rivest, Shamir, Adelson algorithm
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RSA: Choosing keys
RSA：鍵の選択
1. Choose two large prime numbers p, q.
(e.g., 1024 bits each)
2. Compute n = pq, z = (p-1)(q-1)
3. Choose e (with e<n) that has no common factors
with z. (e, z are “relatively prime”).
4. Choose d such that ed-1 is exactly divisible by z.
(in other words: ed mod z = 1 ).
5. Public key is (n,e). Private key is (n,d).
+
KB
-
KB
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RSA: Encryption, decryption
RSA: 暗号化、復号化
0. Given (n,e) and (n,d) as computed above
1. To encrypt bit pattern, m, compute
e
e
c = m mod n (i.e., remainder when m is divided by n)
2. To decrypt received bit pattern, c, compute
d
m = c d mod n (i.e., remainder when c is divided by n)
Magic
m = (m e mod n) d mod n
happens!
c
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RSA example: RSA例：
Bob chooses p=5, q=7. Then n=35, z=24.
e=5 (so e, z relatively prime).
d=29 (so ed-1 exactly divisible by z.
encrypt:
decrypt:
letter
m
me
l
12
1524832
c
17
d
c
481968572106750915091411825223071697
c = me mod n
17
m = cd mod n letter
12
l
8: Network Security
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なぜそうなるのか
RSA: Why is that
m = (m e mod n) d mod n
Useful number theory result: If p,q prime and
n = pq, then:
y
y mod (p-1)(q-1)
x mod n = x
mod n
e
(m mod n) d mod n = medmod n
= m
ed mod (p-1)(q-1)
mod n
(using number theory result above)
1
= m mod n
(since we chose ed to be divisible by
(p-1)(q-1) with remainder 1 )
= m
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RSA: another important property
RSA: もう１つの重要な特性
The following property will be very useful later:
以下の特性は後々非常に奴に立つ：
- +
+ K (K (m)) = m = K (K (m))
B B
B B
use public key
first, followed
by private key
use private key
first, followed
by public key
Result is the same!
8: Network Security
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Chapter 8 roadmap
8.1 What is network security?
8.2 Principles of cryptography
8.3 Authentication
認証
8.4 Integrity
8.5 Key Distribution and certification
8.6 Access control: firewalls
8.7 Attacks and counter measures
8.8 Security in many layers
8: Network Security
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Authentication 認証
Goal: Bob wants Alice to “prove” her identity to him
目標：ボブはアリスに彼女がアリスであることを“証明”して欲しい
Protocol ap1.0: Alice says “I am Alice”
アリスが“私はアリスです”と言う
“I am Alice”
Failure scenario??
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Authentication 認証
Goal: Bob wants Alice to “prove” her identity to him
目標：ボブはアリスに彼女がアリスであることを“証明”して欲しい
Protocol ap1.0: Alice says “I am Alice”
アリスが“私はアリスです”と言う
“I am Alice”
in a network,
Bob can not “see”
Alice, so Trudy simply
declares
herself to be Alice
8: Network Security
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Authentication: another try
認証：もう１つの試み
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address
アリスが彼女のホストのIPアドレスを含む
IPパケットの中で“私はアリスです”と言う
Alice’s
IP address “I am Alice”
Failure scenario??
8: Network Security
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Authentication: another try
認証：もう１つの試み
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address
アリスが彼女のホストのIPアドレスを含む
IPパケットの中で“私はアリスです”と言う
Alice’s
IP address
Trudy can create
a packet
“spoofing”
“I am Alice”
Alice’s address
8: Network Security
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Authentication: another try
認証：他の試み
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
アリスが“私はアリスです”と言い、さらに
それを証明する秘密のパスワードを送る
Alice’s
Alice’s
“I’m Alice”
IP addr password
Alice’s
IP addr
OK
Failure scenario??
8: Network Security
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Authentication: another try
認証：他の試み
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
アリスが“私はアリスです”と言い、さらに
それを証明する秘密のパスワードを送る
Alice’s
Alice’s
“I’m Alice”
IP addr password
Alice’s
IP addr
OK
playback attack: Trudy
records Alice’s packet
and later
plays it back to Bob
Alice’s
Alice’s
“I’m Alice”
IP addr password
8: Network Security
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Authentication: yet another try
認証：さらに他の試み
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
アリスが“私はアリスです”と言い、さらにそれを証
明する暗号化された秘密のパスワードを送る
Alice’s encrypted
“I’m Alice”
IP addr password
Alice’s
IP addr
OK
Failure scenario??
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Authentication: another try
認証：さらに他の試み
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
アリスが“私はアリスです”と言い、さらにそれを証
明する暗号化された秘密のパスワードを送る
Alice’s encrypted
“I’m Alice”
IP addr password
Alice’s
IP addr
OK
record
and
playback
still works!
Alice’s encrypted
“I’m Alice”
IP addr password
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Authentication: yet another try
認証：さらに他の試み
Goal: avoid playback attack
Nonce: number (R) used only once –in-a-lifetime
ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice
must return R, encrypted with shared secret key
アリスが本物であることを証明するため、ボブはアリスにナンス（１回
限りの乱数）Rを送り、アリスはRを共有秘密鍵で暗号化して返信する
“I am Alice”
R
KA-B(R)
Failures, drawbacks?
Alice is live, and
only Alice knows
key to encrypt
nonce, so it must
be Alice!
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Authentication: ap5.0
認証: ap5.0
ap4.0 requires shared symmetric key
 can we authenticate using public key techniques?
ap5.0: use nonce, public key cryptography
ナンスと公開鍵暗号方式を使用
“I am Alice”
R
Bob computes
+ -
-
K A (R)
“send me your public key”
+
KA
KA(KA (R)) = R
and knows only Alice
could have the private
key, that encrypted R
such that
+ K (K (R)) = R
A A
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ap5.0: security hole セキュリティホール
Man (woman) in the middle attack: 中間で攻撃するもの：
Trudy poses as Alice (to Bob) and as Bob (to Alice)
I am Alice
R
I am Alice
R
K (R)
T
K (R)
A
Send me your public key
+
K
T
Send me your public key
+
K
A
- +
m = K (K (m))
A A
+
K (m)
A
Trudy gets
- +
m = K (K (m))
T Alice
sends T
m to
+
K (m)
T
encrypted with
Alice’s public key
8: Network Security
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ap5.0: security hole セキュリティホール
Man (woman) in the middle attack:中間で攻撃するもの：
Trudy poses as Alice (to Bob) and as Bob (to Alice)
Difficult to detect: 見つけにくい：
 Bob receives everything that Alice sends, and vice
versa. (e.g., so Bob, Alice can meet one week later and
recall conversation)
 problem is that Trudy receives all messages as well!
8: Network Security
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Chapter 8 roadmap
8.1 What is network security?
8.2 Principles of cryptography
8.3 Authentication
8.4 Message integrity
メッセージ保全
8.5 Key Distribution and certification
8.6 Access control: firewalls
8.7 Attacks and counter measures
8.8 Security in many layers
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Digital Signatures デジタル署名
Cryptographic technique analogous to handwritten signatures.
手で書かれた署名（サイン）と類似した暗号手法
 sender (Bob) digitally signs document,
establishing he is document owner/creator.
 verifiable, nonforgeable: recipient (Alice) can
prove to someone that Bob, and no one else
(including Alice), must have signed document
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Digital Signatures
デジタル署名
Simple digital signature for message m:
メッセージmのための単純なデジタル署名:
- private key
- signs m by encrypting with his
 Bob
KB, creating “signed” message, KB(m)
Bob’s message, m
Dear Alice
Oh, how I have missed
you. I think of you all the
time! …(blah blah blah)
Bob
K B Bob’s private
key
Public key
encryption
algorithm
-
K B(m)
Bob’s message,
m, signed
(encrypted) with
his private key
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Digital Signatures (more)
デジタル署名（さらに）
-
 Suppose Alice receives msg m, digital signature KB(m)
 Alice verifies m signed by Bob by applying Bob’s
+
-
+
-
public key KB to KB(m) then checks KB(KB(m) ) = m.
+
-
 If KB(KB(m) ) = m, whoever signed m must have used
Bob’s private key.
Alice thus verifies that:
 Bob signed m.
 No one else signed m.
 Bob signed m and not m’.
Non-repudiation:
 Alice can take m, and signature KB(m) to
court and prove that Bob signed m.
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Message Digests
メッセージ・ダイジェスト
Computationally expensive
to public-key-encrypt
long messages
長いメッセージの公開鍵暗号化は
計算機的コストが高くつく
Goal: fixed-length, easyto-compute digital
“fingerprint”
目標：固定長で計算が容易なデジ
タル“指紋”
 apply hash function H
to m, get fixed size
message digest, H(m).
large
message
m
H: Hash
Function
H(m)
Hash function properties:
ハッシュ関数特性:
 many-to-1
 produces fixed-size msg
digest (fingerprint)
 given message digest x,
computationally
infeasible to find m such
that x = H(m)
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Internet checksum: poor crypto hash function
インターネット・チェックサム:貧弱な暗号ハッシュ関数
Internet checksum has some properties of hash function:
 produces fixed length digest (16-bit sum) of message
 is many-to-one
But given message with given hash value, it is easy to find
another message with same hash value:
message
I O U 1
0 0 . 9
9 B O B
ASCII format
49 4F 55 31
30 30 2E 39
39 42 D2 42
B2 C1 D2 AC
message
I O U 9
0 0 . 1
9 B O B
ASCII format
49 4F 55 39
30 30 2E 31
39 42 D2 42
B2 C1 D2 AC
different messages
but identical checksums!
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Digital signature = signed message digest
デジタル署名＝署名されたメッセージダイジェスト
Alice verifies signature and
integrity of digitally signed
message:
Bob sends digitally signed
message:
large
message
m
H: Hash
function
Bob’s
private
key
+
-
KB
encrypted
msg digest
H(m)
digital
signature
(encrypt)
encrypted
msg digest
KB(H(m))
large
message
m
H: Hash
function
KB(H(m))
Bob’s
public
key
+
KB
digital
signature
(decrypt)
H(m)
H(m)
equal
?
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Hash Function Algorithms
ハッシュ関数アルゴリズム
 MD5 hash function widely used (RFC 1321)
computes 128-bit message digest in 4-step
process.
 arbitrary 128-bit string x, appears difficult to
construct msg m whose MD5 hash is equal to x.
 SHA-1 is also used.
 US standard [NIST, FIPS PUB 180-1]
 160-bit message digest

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Chapter 8 roadmap
8.1 What is network security?
8.2 Principles of cryptography
8.3 Authentication
8.4 Integrity
8.5 Key distribution and certification
鍵配送と認証
8.6 Access control: firewalls
8.7 Attacks and counter measures
8.8 Security in many layers
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Trusted Intermediaries
信頼された仲介者
Symmetric key problem:
対称鍵の問題：
Public key problem:
公開鍵の問題：
 How do two entities
 When Alice obtains
establish shared secret
key over network?
Solution:
 trusted key distribution
center (KDC) acting as
intermediary between
entities
Bob’s public key (from
web site, e-mail,
diskette), how does she
know it is Bob’s public
key, not Trudy’s?
Solution:
 trusted certification
authority (CA)
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Key Distribution Center (KDC)
鍵配送センター（KDC)
 Alice, Bob need shared symmetric key.
 KDC: server shares different secret key with
each
registered user (many users)
 Alice, Bob know own symmetric keys, KA-KDC KB-KDC , for
communicating with KDC.
KDC
KA-KDC KP-KDC
KP-KDC
KB-KDC
KA-KDC
KX-KDC
KY-KDC
KB-KDC
KZ-KDC
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Key Distribution Center (KDC)
鍵配送センター（KDC）
Q: How does KDC allow Bob, Alice to determine shared
symmetric secret key to communicate with each other?
Q: KDCはどのようにボブとアリスに相互に通信するための共有された対
称鍵の決定を許可するか？
KDC
generates
KA-KDC(A,B)
R1
Alice
knows
R1
KA-KDC(R1, KB-KDC(A,R1) )
KB-KDC(A,R1)
Bob knows to
use R1 to
communicate
with Alice
Alice and Bob communicate: using R1 as
session key for shared symmetric encryption
8: Network Security
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Certification Authorities
認証局
 Certification authority (CA): 公開鍵認証（CA）：
binds public key to particular entity, E.
 E (person, router) registers its public key with CA.



E provides “proof of identity” to CA.
CA creates certificate binding E to its public key.
certificate containing E’s public key digitally signed by CA
– CA says “this is E’s public key”
Bob’s
public
key
Bob’s
identifying
information
+
KB
digital
signature
(encrypt)
CA
private
key
K-
CA
+
KB
certificate for
Bob’s public key,
signed by CA
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Certification Authorities
認証局
 When Alice wants Bob’s public key:
アリスがボブの公開鍵を必要としたとき：
 gets Bob’s certificate (Bob or elsewhere).
ボブの証明書を取得
 apply CA’s public key to Bob’s certificate, get Bob’s
public key
認証局の公開鍵をボブの証明書に適用、ボブの公開鍵を取得
+
KB
digital
signature
(decrypt)
CA
public
key
Bob’s
public
+
key
KB
+
K CA
8: Network Security
8-50
A certificate contains:
証明書は以下を含む：
 Serial number (unique to issuer)
 info about certificate owner, including algorithm
and key value itself (not shown)
 info about
certificate
issuer
 valid dates
 digital
signature by
issuer
8: Network Security
8-51
Chapter 8 roadmap
8.1 What is network security?
8.2 Principles of cryptography
8.3 Authentication
8.4 Integrity
8.5 Key Distribution and certification
8.6 Access control: firewalls
アクセス制御：ファイアウォール
8.7 Attacks and counter measures
8.8 Security in many layers
8: Network Security
8-52
Firewalls ファイアウォール
firewall
isolates organization’s internal net from larger
Internet, allowing some packets to pass,
blocking others.
あるパケットには通過を許可し、あるパケットはブロックして、より
大きなインターネットから組織の内部ネットを分離
public
Internet
administered
network
firewall
8: Network Security
8-53
Firewalls: Why ファイアウォール：なぜ必要
prevent denial of service attacks:
サービス強制停止攻撃を防ぐ：
 SYN flooding: attacker establishes many bogus TCP
connections, no resources left for “real” connections.
prevent illegal modification/access of internal data.
内部データの不法な修正/アクセスを防ぐ
 e.g., attacker replaces CIA’s homepage with something else
allow only authorized access to inside network (set of
authenticated users/hosts)
内部のネットワーク(確証されたユーザ/ホストのセット)への認可されたア
クセスのみを許可
two types of firewalls:
 application-level
 packet-filtering
8: Network Security
8-54
Packet Filtering
パケットフィルタリング
Should arriving
packet be allowed
in? Departing packet
let out?
 internal network connected to Internet via router firewall
内部ネットワークはルータのファイアウォールを通してインターネットに繋
がれる
 router filters packet-by-packet（パケットごとにフィルタをかける）
, decision to forward/drop packet based on:




source IP address, destination IP address
TCP/UDP source and destination port numbers
ICMP message type
TCP SYN and ACK bits
8: Network Security
8-55
Packet Filtering パケットフィルタリング
 Example 1: block incoming and outgoing
datagrams with IP protocol field = 17 and with
either source or dest port = 23.
 All incoming and outgoing UDP flows and telnet
connections are blocked.
 Example 2: Block inbound TCP segments with
ACK=0.
 Prevents external clients from making TCP
connections with internal clients, but allows
internal clients to connect to outside.
8: Network Security
8-56
Application gateways
アプリケーションゲートウェイ
 Filters packets on
application data as well
as on IP/TCP/UDP fields.
host-to-gateway
telnet session
application
gateway
gateway-to-remote
host telnet session
router and filter
IP/TCP/UDPフィールドと同様に
アプリケーションデータ上でのパ
ケットのフィルタリング
 Example: allow select
internal users to telnet
outside.
1. Require all telnet users to telnet through gateway.
2. For authorized users, gateway sets up telnet connection to
dest host. Gateway relays data between 2 connections
3. Router filter blocks all telnet connections not originating
from gateway.
8: Network Security
8-57
Limitations of firewalls and gateways
ファイアウォールとゲートウェイの制限
 IP spoofing（IP偽装）:
router can’t know if
data “really” comes
from claimed source
 if multiple app’s. need
special treatment, each
has own app. gateway.
 client software must
know how to contact
gateway.

e.g., must set IP address
of proxy in Web
browser
 filters often use all or
nothing policy for UDP.
 tradeoff: degree of
communication with
outside world, level of
security
トレードオフ：外部との通信程
度とセキュリティのレベル
 many highly protected
sites still suffer from
attacks.
8: Network Security
8-58
Chapter 8 roadmap
8.1 What is network security?
8.2 Principles of cryptography
8.3 Authentication
8.4 Integrity
8.5 Key Distribution and certification
8.6 Access control: firewalls
8.7 Attacks and counter measures
攻撃と対策
8.8 Security in many layers
8: Network Security
8-59
Internet security threats
インタネットセキュリティの脅威
Mapping: マッピング（情報収集）
 before
attacking: “case the joint” – find out
what services are implemented on network
 Use ping to determine what hosts have
addresses on network
 Port-scanning: try to establish TCP connection
to each port in sequence (see what happens)
 nmap (http://www.insecure.org/nmap/) mapper:
“network exploration and security auditing”
Countermeasures?
8: Network Security
8-60
Internet security threats
インタネットセキュリティの脅威
Mapping: countermeasures
マッピング：対策
record traffic entering network
 look for suspicious activity (IP addresses, pots
being scanned sequentially)

8: Network Security
8-61
Internet security threats
インタネットセキュリティの脅威
Packet sniffing: パケットスニフィング（読み取り）
broadcast media
 promiscuous NIC reads all packets passing by
 can read all unencrypted data (e.g. passwords)
 e.g.: C sniffs B’s packets

C
A
src:B dest:A
payload
B
Countermeasures?
8: Network Security
8-62
Internet security threats
インタネットセキュリティの脅威
Packet sniffing: countermeasures
パケットスニフィング：対策
all hosts in organization run software that
checks periodically if host interface in
promiscuous mode.
 one host per segment of broadcast media
(switched Ethernet at hub)
C
A

src:B dest:A
payload
B
8: Network Security
8-63
Internet security threats
インタネットセキュリティの脅威
IP Spoofing: IPスプーフィング（IP偽装）
can generate “raw” IP packets directly from
application, putting any value into IP source
address field
 receiver can’t tell if source is spoofed
 e.g.: C pretends to be B

C
A
src:B dest:A
Countermeasures?
payload
B
8: Network Security
8-64
Internet security threats
インタネットセキュリティの脅威
IP Spoofing: ingress filtering
IPスプーフィング：進入フィルタリング
routers should not forward outgoing packets
with invalid source addresses (e.g., datagram
source address not in router’s network)
 great, but ingress filtering can not be mandated
for all networks
C
A

src:B dest:A
payload
B
8: Network Security
8-65
Internet security threats
インタネットセキュリティの脅威
Denial of service (DOS):
サービス不能攻撃（DOS）：
 flood of maliciously generated packets “swamp”
receiver
 Distributed DOS (DDOS): multiple coordinated
sources swamp receiver
 e.g., C and remote host SYN-attack A
C
A
SYN
SYN
SYN
SYN
SYN
B
Countermeasures?
SYN
SYN
8: Network Security
8-66
Internet security threats
インタネットセキュリティの脅威
Denial of service (DOS): countermeasures
サービス不能攻撃（DOS）：対策
filter out flooded packets (e.g., SYN) before
reaching host: throw out good with bad
 traceback to source of floods (most likely an
innocent, compromised machine)

C
A
SYN
SYN
SYN
SYN
SYN
B
SYN
SYN
8: Network Security
8-67
Chapter 8 roadmap
8.1 What is network security?
8.2 Principles of cryptography
8.3 Authentication
8.4 Integrity
8.5 Key Distribution and certification
8.6 Access control: firewalls
8.7 Attacks and counter measures
8.8 Security in many layers 各レイヤにおけるセキュリティ
8.8.1. Secure email セキュアEメール
8.8.2. Secure sockets セキュアソケット
8.8.3. IPsec ネットワーク層セキュリティ
8.8.4. Security in 802.11 無線LANのセキュリティ
8: Network Security
8-68
Secure e-mail セキュアEメール

Alice wants to send confidential e-mail, m, to Bob.
アリスはボブへ、機密の電子メール(m)を送りたい
KS
m
K (.)
S
+
KS
+
.
K B( )
+
KS(m )
KS(m )
+
KB(KS )
.
KS( )
-
Internet
+
KB(KS )
KB
m
KS
-
.
K B( )
-
KB
Alice:




generates random symmetric private key, KS.
encrypts message with KS (for efficiency)
also encrypts KS with Bob’s public key.
sends both KS(m) and KB(KS) to Bob.
8: Network Security
8-69
Secure e-mail セキュアEメール

Alice wants to send confidential e-mail, m, to Bob.
KS
m
K (.)
S
+
KS
+
.
K B( )
+
KS(m )
KS(m )
+
KB(KS )
.
KS( )
-
Internet
+
KB(KS )
KB
m
KS
-
.
K B( )
-
KB
Bob:
 uses his private key to decrypt and recover KS
 uses KS to decrypt KS(m) to recover m
8: Network Security
8-70
Secure e-mail (continued)
セキュアEメール（続き）
• Alice wants to provide sender authentication
message integrity.
+
-
KA
m
H(.)
-
.
KA( )
-
-
KA(H(m))
KA(H(m))
+
Internet
m
KA
+
.
KA( )
m
H(m )
compare
.
H( )
H(m )
• Alice digitally signs message.
• sends both message (in the clear) and digital signature.
8: Network Security
8-71
Secure e-mail (continued)
セキュアEメール（続き）
• Alice wants to provide secrecy, sender authentication,
message integrity.
-
KA
m
.
H( )
-
.
KA( )
-
KA(H(m))
+
KS
.
KS( )
+
m
KS
+
.
K B( )
+
Internet
+
KB(KS )
KB
Alice uses three keys: her private key, Bob’s public
key, newly created symmetric key
8: Network Security
8-72
Pretty good privacy (PGP)
 Internet e-mail encryption
scheme, de-facto standard.
 uses symmetric key
cryptography, public key
cryptography, hash
function, and digital
signature as described.
 provides secrecy, sender
authentication, integrity.
 inventor, Phil Zimmerman,
was target of 3-year
federal investigation.
A PGP signed message:
---BEGIN PGP SIGNED MESSAGE--Hash: SHA1
Bob:My husband is out of town
tonight.Passionately yours,
Alice
---BEGIN PGP SIGNATURE--Version: PGP 5.0
Charset: noconv
yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJ
hFEvZP9t6n7G6m5Gw2
---END PGP SIGNATURE---
8: Network Security
8-73
Secure sockets layer (SSL)
セキュアソケットレイヤ（SSL）
 transport layer
security to any TCPbased app using SSL
services.
SSLサービスを使うTCPベース
のアプリへのトランスポート層
のセキュリティ
 used between Web
browsers, servers for
e-commerce (shttp).
 security services:



server authentication
data encryption
client authentication
(optional)
 server authentication:
サーバ認証：
 SSL-enabled browser
includes public keys for
trusted CAs.
 Browser requests
server certificate,
issued by trusted CA.
 Browser uses CA’s
public key to extract
server’s public key from
certificate.
 check your browser’s
security menu to see
its trusted CAs.
8: Network Security
8-74
SSL (continued) SSL（続き）
Encrypted SSL session:
暗号化SSLセッション：
 Browser generates
symmetric session key,
encrypts it with server’s
public key, sends
encrypted key to server.
 Using private key, server
decrypts session key.
 Browser, server know
session key

All data sent into TCP
socket (by client or server)
encrypted with session key.
 SSL: basis of IETF
Transport Layer
Security (TLS).
 SSL can be used for
non-Web applications,
e.g., IMAP.
 Client authentication
can be done with client
certificates.
8: Network Security
8-75
IPsec: Network Layer Security
IPsec: ネットワーク層セキュリティ
 Network-layer secrecy:
ネットワーク層セキュリティ：
 sending host encrypts the
data in IP datagram
 TCP and UDP segments;
ICMP and SNMP
messages.
 Network-layer authentication
ネットワーク層認証
 destination host can
authenticate source IP
address
 Two principle protocols:
２つの基本プロトコル
 authentication header
(AH) protocol
 encapsulation security
payload (ESP) protocol
 For both AH and ESP, source,
destination handshake:
AHとESPはともに、始点・終点間
のハンドシェイクを行う
 create network-layer
logical channel called a
security association (SA)
 Each SA unidirectional.
それぞれSAは単方向
 Uniquely determined by:
以下によって一意的に識別され
る
 security protocol (AH or
ESP)
 source IP address
 32-bit connection ID
8: Network Security
8-76
Authentication Header (AH) Protocol
AH（認証ヘッダ）プロトコル
 provides source
authentication, data
integrity, no
confidentiality
始点認証およびデータ保全性(
機密性なし)を提供
 AH header inserted
between IP header,
data field.
 protocol field: 51
 intermediate routers
process datagrams as
usual
IP header
AH header
AH header includes:
AHヘッダは以下を含む：
 connection identifier
 authentication data:
source- signed message
digest calculated over
original IP datagram.
 next header field:
specifies type of data
(e.g., TCP, UDP, ICMP)
data (e.g., TCP, UDP segment)
8: Network Security
8-77
ESP Protocol
ESP（暗号ペイロード）プロトコル
 provides secrecy, host
authentication, data
integrity.
セキュリティ、ホスト認証、データ保
全性を提供
 ESP authentication
field is similar to AH
authentication field.
 Protocol = 50.
 data, ESP trailer
encrypted.
 next header field is in ESP
trailer.
authenticated
encrypted
IP header
ESP
ESP
ESP
TCP/UDP segment
header
trailer authent.
8: Network Security
8-78
IEEE 802.11 security
IEEE 802.11セキュリティ

War-driving: drive around Bay area, see what 802.11
networks available?
ウォー・ドライブ：ベイエリアの周りをドライブした際、どんな802.11ネット
ワークが利用できるか？
More than 9000 accessible from public roadways
 85% use no encryption/authentication
 packet-sniffing and various attacks easy!
 Securing 802.11
 encryption, authentication
 first attempt at 802.11 security: Wired Equivalent
Privacy (WEP): a failure
 current attempt: 802.11i

8: Network Security
8-79
Wired Equivalent Privacy (WEP):
ap4.0
プロトコルap4.0の中でのような認証
 authentication as in protocol
host requests authentication from access point
 access point sends 128 bit nonce
 host encrypts nonce using shared symmetric key
 access point decrypts nonce, authenticates host
 no key distribution mechanism
 authentication: knowing the shared key is enough

8: Network Security
8-80
WEP data encryption
WEPデータ暗号化
 Host/AP share 40 bit symmetric key (semi



permanent)
Host appends 24-bit initialization vector (IV) to
create 64-bit key
64 bit key used to generate stream of keys, kiIV
kiIV used to encrypt ith byte, di, in frame:
ci = di XOR kiIV
IV and encrypted bytes, ci sent in frame
8: Network Security
8-81
802.11 WEP encryption
802.11 WEP暗号化
IV
(per frame)
KS: 40-bit
secret
symmetric
key
plaintext
frame data
plus CRC
key sequence generator
( for given KS, IV)
k1IV k2IV k3IV … kNIV kN+1IV… kN+1IV
d1
d2
d3 … dN
CRC1 … CRC4
c1
c2
c3 … cN
cN+1 … cN+4
802.11
IV
header
WEP-encrypted data
plus CRC
Figure 7.8-new1:
802.11encryption
WEP protocol
Sender-side
WEP
8: Network Security
8-82
Breaking 802.11 WEP encryption
802.11 WEP暗号化を突破（破壊）
Security hole: セキュリティホール：
 24-bit IV, one IV per frame, -> IV’s eventually reused
 IV transmitted in plaintext -> IV reuse detected
 Attack: 攻撃：
 Trudy causes Alice to encrypt known plaintext d1 d2
d3 d4 …
IV
 Trudy sees: ci = di XOR ki
Trudy knows ci di, so can compute kiIV
IV
IV
IV
 Trudy knows encrypting key sequence k1 k2 k3 …
 Next time IV is used, Trudy can decrypt!

8: Network Security
8-83
802.11i: improved security
802.11i: 改善されたセキュリティ
 numerous (stronger) forms of encryption
possible
 provides key distribution
 uses authentication server separate from
access point
8: Network Security
8-84
802.11i: four phases of operation
802.11i: オペレーションの４つの段階
STA:
client station
AP: access point
AS:
Authentication
server
wired
network
1 Discovery of
security capabilities
2 STA and AS mutually authenticate, together
generate Master Key (MK). AP servers as “pass through”
3 STA derives
Pairwise Master
Key (PMK)
4 STA, AP use PMK to derive
Temporal Key (TK) used for message
encryption, integrity
3 AS derives
same PMK,
sends to AP
8: Network Security
8-85
EAP: extensible authentication protocol
 EAP: end-end client (mobile) to authentication
server protocol
 EAP sent over separate “links”
mobile-to-AP (EAP over LAN)
 AP to authentication server (RADIUS over UDP)

wired
network
EAP TLS
EAP
EAP over LAN (EAPoL)
IEEE 802.11
RADIUS
UDP/IP
8: Network Security
8-86
Network Security (summary)
ネットワークセキュリティ（まとめ）
Basic techniques…... 基本テクニック
cryptography (symmetric and public) 暗号化
 authentication 認証
 message integrity メッセージ保全
 key distribution 鍵の配送

…. used in many different security scenarios
さまざまなセキュリティシナリオで使用
secure email セキュアEメール
 secure transport (SSL) セキュア転送
 IP sec
 802.11

8: Network Security
8-87

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