Transcript Computer Science Information Security Management -- Cryptography Fall, 2005 CPSC499 Information Security Management Summary • • • • Symmetric Encryption Public Encryption Digital Signature Key Distribution Fall, 2005 CPSC499 Information Security Management Computer Science.

Computer Science
Information Security Management
-- Cryptography
Fall, 2005
CPSC499 Information Security Management
Summary
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Symmetric Encryption
Public Encryption
Digital Signature
Key Distribution
Fall, 2005
CPSC499 Information Security Management
Computer Science
Basic Terminology
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Computer Science
plaintext - the original message
ciphertext - the coded message
cipher - algorithm for transforming plaintext to ciphertext
key - info used in cipher known only to sender/receiver
encipher (encrypt) - converting plaintext to ciphertext
decipher (decrypt) - recovering ciphertext from plaintext
cryptography - study of encryption principles/methods
cryptanalysis (codebreaking) - the study of principles/
methods of deciphering ciphertext without knowing key
• cryptology - the field of both cryptography and
cryptanalysis
Fall, 2005
CPSC499 Information Security Management
The language of cryptography
Alice’s
K encryption
A
key
plaintext
encryption
algorithm
ciphertext
Computer Science
Bob’s
K decryption
B key
decryption plaintext
algorithm
symmetric key crypto: sender, receiver keys identical
public-key crypto: encryption key public, decryption key
secret (private)
Fall, 2005
CPSC499 Information Security Management
Symmetric Encryption
Computer Science
• or conventional / secret-key / single-key
• sender and recipient share a common key
• all classical encryption algorithms are
private-key
• was only type prior to invention of publickey in 1970’s
Fall, 2005
CPSC499 Information Security Management
Symmetric Cipher Model
Fall, 2005
CPSC499 Information Security Management
Computer Science
Symmetric Key Cryptography
KA-B
KA-B
plaintext
message, m
Computer Science
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: K A-B
• e.g., key is knowing substitution pattern in mono
alphabetic substitution cipher
Fall, 2005
CPSC499 Information Security Management
Requirements
Computer Science
• two requirements for secure use of
symmetric encryption:
– a strong encryption algorithm
– a secret key known only to sender / receiver
Y = EK(X)
X = DK(Y)
• assume encryption algorithm is known
• implies a secure channel to distribute key
Fall, 2005
CPSC499 Information Security Management
Cryptography
• can characterize by:
– type of encryption operations used
• substitution / transposition / product
– number of keys used
• single-key or private / two-key or public
– way in which plaintext is processed
• block / stream
Fall, 2005
CPSC499 Information Security Management
Computer Science
More Definitions
Computer Science
• unconditional security
– no matter how much computer power is
available, the cipher cannot be broken since
the ciphertext provides insufficient information
to uniquely determine the corresponding
plaintext
• computational security
– given limited computing resources (eg time
needed for calculations is greater than age of
universe), the cipher cannot be broken
Fall, 2005
CPSC499 Information Security Management
Classical Substitution Ciphers
Computer Science
• where letters of plaintext are replaced by
other letters or by numbers or symbols
• or if plaintext is viewed as a sequence of
bits, then substitution involves replacing
plaintext bit patterns with ciphertext bit
patterns
Fall, 2005
CPSC499 Information Security Management
Caesar Cipher
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Computer Science
earliest known substitution cipher
by Julius Caesar
first attested use in military affairs
replaces each letter by 3rd letter later
example:
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
Fall, 2005
CPSC499 Information Security Management
Caesar Cipher
Computer Science
• can define transformation as:
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
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
• mathematically give each letter a number
a b c
0 1 2
n o
13 14
d e f
3 4 5
p q
15 16
g h i
6 7 8
r s
17 18
j k l m
9 10 11 12
t u v w x y Z
19 20 21 22 23 24 25
• then have Caesar cipher as:
C = E(p) = (p + k) mod (26)
p = D(C) = (C – k) mod (26)
Fall, 2005
CPSC499 Information Security Management
Cryptanalysis of Caesar Cipher
Computer Science
• only have 26 possible ciphers
– A maps to A,B,..Z
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could simply try each in turn
a brute force search
given ciphertext, just try all shifts of letters
do need to recognize when have plaintext
eg. break ciphertext “ERE L ORYH BRX
DOLFH"
Fall, 2005
CPSC499 Information Security Management
Symmetric Encryption Example
-- Substitution
KA-B
KA-B
plaintext
message, m
encryption ciphertext
algorithm
K (m)
decryption plaintext
algorithm
A-B
• Plaintext
• KA-B
• Ciphertext
• Encryption algorithm
• Decryption algorithm
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Computer Science
CPSC499 Information Security Management
Monoalphabetic Ciphers
Computer Science
• “Cipher” line can be any permutation of the
26 alphabetic char
• Statistical analysis
– Letters “e” and “t” are the most frequent
occurring letters
– Two and three letter occurrences of letters
appear quite often together, like “the”, “in”
– Guess the appearance of the words
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CPSC499 Information Security Management
Types of Cryptanalytic Attacks
Computer Science
• ciphertext only
– only know algorithm / ciphertext, statistical, can identify plaintext
• known plaintext
– know/suspect plaintext & ciphertext to attack cipher
• chosen plaintext
– select plaintext and obtain ciphertext to attack cipher
Fall, 2005
CPSC499 Information Security Management
Brute Force Search
Computer Science
• always possible to simply try every key
• most basic attack, proportional to key size
• assume either know / recognise plaintext
Fall, 2005
CPSC499 Information Security Management
Chosen-Plaintext Attack
Computer Science
PIN is encrypted and
transmitted to bank
cipher(key,PIN)
Crook #1 changes
his PIN to a number
of his choice
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Crook #2 eavesdrops
on the wire and learns
ciphertext corresponding
to chosen plaintext PIN
… repeat for any PIN value
CPSC499 Information Security Management
Polyalphabetic encryption
Computer Science
• monoalphabetic ciphers + Caesar cipher.
• Two Caesar ciphers (k=5, k=19)
• Repeating pattern c1, c2, c2, c1, c2
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CPSC499 Information Security Management
Transposition Ciphers
Computer Science
• now consider classical transposition or
permutation ciphers
• these hide the message by rearranging
the letter order
• without altering the actual letters used
• can recognise these since have the same
frequency distribution as the original text
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CPSC499 Information Security Management
Rail Fence cipher
Computer Science
• write message letters out diagonally over a
number of rows
• then read off cipher row by row
• eg. write message out as:
m e m a t r h t g p r y
e t e f e t e o a a t
• giving ciphertext
MEMATRHTGPRYETEFETEOAAT
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CPSC499 Information Security Management
Row Transposition Ciphers
Computer Science
• a more complex scheme
• write letters of message out in rows over a
specified number of columns
• then reorder the columns according to
some key before reading off the rows
Key:
4 3 1 2 5 6 7
Plaintext: a t t a c k p
o s t p o n e
d u n t i l t
w o a m x y z
Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ
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CPSC499 Information Security Management
Product Ciphers
Computer Science
• ciphers using substitutions or transpositions are
not secure because of language characteristics
• hence consider using several ciphers in
succession to make harder, but:
– two substitutions make a more complex substitution
– two transpositions make more complex transposition
– but a substitution followed by a transposition makes a
new much harder cipher
• this is bridge from classical to modern ciphers
Fall, 2005
CPSC499 Information Security Management
Simple Idea: One-Time Pad
Computer Science
------------- = 10111101…

10001111…
= 00110010…
Key is a never-repeating bit
sequence as long as plaintext
Encrypt by bitwise XOR of
plaintext and key:
ciphertext = plaintext  key

10111101…
00110010…
Decrypt by bitwise XOR of
ciphertext and key:
ciphertext  key =
(plaintext  key)  key =
plaintext  (key  key) =
plaintext
Cipher achieves perfect secrecy if and only if
there are as many possible keys as possible plaintexts, and
every key is equally likely (Claude Shannon’s result)
Fall, 2005
CPSC499 Information Security Management
Advantages of One-Time Pad
Computer Science
• Easy to compute
– Encryption and decryption are the same operation
– Bitwise XOR is very cheap to compute
• As secure as possible
– Given a ciphertext, all plaintexts are equally likely,
regardless of attacker’s computational resources
– …as long as the key sequence is truly random
• True randomness is expensive to obtain in large
quantities
– …as long as each key is same length as plaintext
• But how does the sender communicate the key to
receiver?
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CPSC499 Information Security Management
Problems with One-Time Pad
Computer Science
• Key must be as long as plaintext
– Impractical in most realistic scenarios
– Still used for diplomatic and intelligence traffic
• Does not guarantee integrity
– One-time pad only guarantees confidentiality
– Attacker cannot recover plaintext, but can
easily change it to something else
• Insecure if keys are reused
– Attacker can obtain XOR of plaintexts
Fall, 2005
CPSC499 Information Security Management
Modern Block Ciphers
Computer Science
• will now look at modern block ciphers
• one of the most widely used types of
cryptographic algorithms
• provide secrecy and/or authentication
services
• in particular will introduce DES (Data
Encryption Standard)
Fall, 2005
CPSC499 Information Security Management
Block vs Stream Ciphers
Computer Science
• block ciphers process messages into
blocks, each of which is then en/decrypted
• like a substitution on very big characters
– 64-bits or more
• stream ciphers process messages a bit or
byte at a time when en/decrypting
• many current ciphers are block ciphers
• hence are focus of course
Fall, 2005
CPSC499 Information Security Management
Block Ciphers
Computer Science
• Operates on a single chunk (“block”) of plaintext
– For example, 64 bits for DES
– Same key is reused for each block (can use short keys)
• Result should look like a random permutation
– As if plaintext bits were randomly shuffled
• Only computational guarantee of secrecy
– Not impossible to break, just very expensive
• If there is no efficient algorithm (unproven
assumption!), then can only break by brute-force,
try-every-possible-key search
– Time/cost of breaking the cipher exceeds the value
and/or useful lifetime of protected information
Fall, 2005
CPSC499 Information Security Management
Permutation
Computer Science
1
2
3
4
1
2
3
4
CODE becomes DCEO
• For N-bit input, N! possible permutations
• Idea: split plaintext into blocks, for each block use secret
key to pick a permutation, rinse and repeat
– Without the key, permutation should “look random”
Fall, 2005
CPSC499 Information Security Management
Block Cipher Operation (Simplified)
Block of plaintext
S
S
S
S
S
S
S
S
repeat for several rounds
S
S
S
S
Computer Science
Key
Add some secret key bits
to provide confusion
Each S-box permutes
its input bits in a
“random-looking” way
to provide diffusion
(spread plaintext bits
throughout ciphertext)
Procedure must be reversible
Block of ciphertext
(for decryption)
Fall, 2005
CPSC499 Information Security Management
Block Cipher Principles
Computer Science
• needed since must be able to decrypt ciphertext
to recover messages efficiently
• block ciphers look like an extremely large
substitution
• instead create from smaller building blocks
• using idea of a product cipher (substitution
followed by transposition)
Fall, 2005
CPSC499 Information Security Management
Claude Shannon and SubstitutionPermutation Ciphers
Computer Science
• in 1949 Claude Shannon introduced idea of
substitution-permutation (S-P) networks
– modern substitution-transposition product cipher
• these form the basis of modern block ciphers
• S-P networks are based on the two primitive
cryptographic operations we have seen before:
– substitution (S-box)
– permutation (P-box)
• provide confusion and diffusion of message
Fall, 2005
CPSC499 Information Security Management
Confusion and Diffusion
Computer Science
• cipher needs to completely obscure
statistical properties of original message
• a one-time pad does this
• more practically Shannon suggested
combining elements to obtain:
• diffusion – dissipates statistical structure
of plaintext over bulk of ciphertext
• confusion – makes relationship between
ciphertext and key as complex as possible
Fall, 2005
CPSC499 Information Security Management
Data Encryption Standard (DES)
Computer Science
• most widely used block cipher in world
• adopted in 1977 by NBS (now NIST
http://www.itl.nist.gov/fipspubs/fip462.htm )
• encrypts 64-bit data using 56-bit key
• has widespread use
• has been considerable controversy over
its security
Fall, 2005
CPSC499 Information Security Management
DES History
Computer Science
• IBM developed Lucifer cipher
– by team led by Feistel
– used 64-bit data blocks with 128-bit key
• then redeveloped as a commercial cipher
with input from NSA and others
• in 1973 NBS issued request for proposals
for a national cipher standard
• IBM submitted their revised Lucifer which
was eventually accepted as the DES
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CPSC499 Information Security Management
DES Encryption
Computer Science
Fall, 2005
CPSC499 Information Security Management
Strength of DES – Key Size
Computer Science
• 56-bit keys have 256 = 7.2 x 1016 values
• brute force search looks hard
• recent advances have shown is possible
– in 1997 on Internet in a few months
– in 1998 on dedicated h/w (EFF) in a few days
– in 1999 above combined in 22hrs!
• still must be able to recognize plaintext
Fall, 2005
CPSC499 Information Security Management
Design Principles
Computer Science
• block size
– increasing size improves security, but slows cipher
• key size
– increasing size improves security, makes exhaustive key
searching harder, but may slow cipher
• number of rounds
– increasing number improves security, but slows cipher
• subkey generation
– greater complexity can make analysis harder, but slows
cipher
• round function
– greater complexity can make analysis harder, but slows
cipher
• fast software en/decryption & ease of analysis
– are more recent concerns for practical use and testing
Fall, 2005
CPSC499 Information Security Management
Confidentiality using Symmetric
Encryption
Computer Science
• have two major placement alternatives
• link encryption
– encryption occurs independently on every link
– implies must decrypt traffic between links
• end-to-end encryption
– encryption occurs between original source
and final destination
– need devices at each end with shared keys
Fall, 2005
CPSC499 Information Security Management
Placement of Encryption
Computer Science
• can place encryption function at various
layers in OSI Reference Model
– link encryption occurs at layers 1 or 2
– end-to-end can occur at layers 3, 4, 6, 7
– as move higher less information is encrypted
but it is more secure though more complex
with more entities and keys
Fall, 2005
CPSC499 Information Security Management
Summary
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Symmetric encryption
Public encryption
Digital Signature
Key distribution
Fall, 2005
CPSC499 Information Security Management
Computer Science
Private-Key Cryptography
Computer Science
• traditional private/secret/single key
cryptography uses one key
• shared by both sender and receiver
• if this key is disclosed communications are
compromised
• also is symmetric, parties are equal
• hence does not protect sender from
receiver forging a message & claiming is
sent by sender
Fall, 2005
CPSC499 Information Security Management
Public-Key Cryptography
Computer Science
• probably most significant advance in the
3000 year history of cryptography
• uses two keys – a public & a private key
• asymmetric since parties are not equal
• uses clever application of number
theoretic concepts to function
• complements rather than replaces private
key crypto
Fall, 2005
CPSC499 Information Security Management
Public-Key Cryptography
Computer Science
• public-key/two-key/asymmetric cryptography
involves the use of two keys:
– a public-key, which may be known by anybody, and
can be used to encrypt messages, and verify
signatures
– a private-key, known only to the recipient, used to
decrypt messages, and sign (create) signatures
• is asymmetric because
– those who encrypt messages or verify signatures
cannot decrypt messages or create signatures
Fall, 2005
CPSC499 Information Security Management
Public-Key Cryptography
Fall, 2005
CPSC499 Information Security Management
Computer Science
Public-Key Characteristics
Computer Science
• Public-Key algorithms rely on two keys
with the characteristics that it is:
– computationally infeasible to find decryption
key knowing only algorithm & encryption key
– computationally easy to en/decrypt messages
when the relevant (en/decrypt) key is known
– either of the two related keys can be used for
encryption, with the other used for decryption
(in some schemes)
Fall, 2005
CPSC499 Information Security Management
Public-Key Cryptosystems
Fall, 2005
CPSC499 Information Security Management
Computer Science
Public-Key Applications
Computer Science
• can classify uses into 3 categories:
– encryption/decryption (provide secrecy)
– digital signatures (provide authentication)
– key exchange (of session keys)
• some algorithms are suitable for all uses,
others are specific to one
Fall, 2005
CPSC499 Information Security Management
Security of Public Key Schemes
Computer Science
• like private key schemes brute force exhaustive
search attack is always theoretically possible
• but keys used are too large (>512bits)
• security relies on a large enough difference in
difficulty between easy (en/decrypt) and hard
(cryptanalysis) problems
• more generally the hard problem is known, its
just made too hard to do in practise
• requires the use of very large numbers
• hence is slow compared to secret key schemes
Fall, 2005
CPSC499 Information Security Management
Public key encryption algorithms
Computer Science
Requirements:
1
need
+
KB ( ) and KB ( )
- +
K (K (m)) = m
B B
.
.
such that
2 given public key KB+ , it should
be impossible to compute
private key KB
RSA: Rivest, Shamir, Adelson algorithm
Fall, 2005
CPSC499 Information Security Management
RSA: Choosing keys
Computer Science
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).
Fall, 2005
+
KB
-
KB
CPSC499 Information Security Management
RSA: Encryption, decryption
Computer Science
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
Fall, 2005
CPSC499 Information Security Management
RSA example:
Computer Science
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:
Fall, 2005
letter
m
me
l
12
1524832
c
17
d
c
481968572106750915091411825223071697
c = me mod n
17
m = cd mod n letter
12
l
CPSC499 Information Security Management
RSA: Why is that
d
m = (m e mod n) mod n
Computer Science
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
Fall, 2005
CPSC499 Information Security Management
RSA: another important property
Computer Science
The following property will be very useful later:
-
+
B
B
K (K (m))
+ = m = K (K (m))
B B
use public key
first, followed
by private key
use private key
first, followed
by public key
Result is the same!
Fall, 2005
CPSC499 Information Security Management
Summary
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Symmetric encryption
Public encryption
Digital Signature
Key distribution
Fall, 2005
CPSC499 Information Security Management
Computer Science
Digital Signatures
Computer Science
Cryptographic technique analogous to
hand-written signatures.
• 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
Fall, 2005
CPSC499 Information Security Management
Digital Signatures
Computer Science
Simple digital signature for message m:
• Bob
signs m by encrypting with
- his private
key 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
Fall, 2005
K B Bob’s private
key
Public key
encryption
algorithm
CPSC499 Information Security Management
-
K B(m)
Bob’s message,
m, signed
(encrypted) with
his private key
Digital Signatures (more)
Computer Science
-
• 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.
Fall, 2005
CPSC499 Information Security Management
Internet checksum: poor crypto hash function
Computer Science
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
Fall, 2005
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!
CPSC499 Information Security Management
Message Digests
Computer Science
Computationally expensive
to public-key-encrypt long
messages
Goal: fixed-length, easy- tocompute digital
“fingerprint”
• apply hash function H to
m, get fixed size message
digest, H(m).
Fall, 2005
large
message
m
H: Hash
Function
H(m)
Hash function
properties:
• many-to-1
• produces fixed-size msg digest
(fingerprint)
• given message digest x,
computationally infeasible to
find m such that x = H(m)
CPSC499 Information Security Management
Digital signature = signed message digest
Computer Science
Bob sends digitally signed
message:
large
message
m
H: Hash
function
Bob’s
private
key
+
Fall, 2005
-
KB
Alice verifies signature and
integrity of digitally
signed message:
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
H(m)
H(m)
CPSC499 Information Security Management
+
KB
digital
signature
(decrypt)
equal
?
Digital Envelopes
-- Symmetric + Asymmetric
Computer Science
• Generate a secret key (session key) at random.
• Encrypt the message using the session key and
symmetric algorithm.
• Encrypt the session key with the recipient’s
public key. This becomes the “digital envelope”.
• Send the encrypted message and the digital
envelope to the recipient.
• Figure …
Fall, 2005
CPSC499 Information Security Management
Summary
•
•
•
•
Symmetric encryption
Public encryption
Digital Signature
Key distribution
Fall, 2005
CPSC499 Information Security Management
Computer Science
Key Distribution
Computer Science
• symmetric schemes require both parties to
share a common secret key
• issue is how to securely distribute this key
• often secure system failure due to a break
in the key distribution scheme
Fall, 2005
CPSC499 Information Security Management
Key Distribution
•
Computer Science
given parties A and B have various key
distribution alternatives:
1. A can select key and physically deliver to B
2. third party can select & deliver key to A & B
3. if A & B have communicated previously can
use previous key to encrypt a new key
4. if A & B have secure communications with a
third party C, C can relay key between A & B
Fall, 2005
CPSC499 Information Security Management
Trusted Intermediaries
Symmetric key problem:
• How do two entities
establish shared secret key
over network?
Solution:
• trusted key distribution
center (KDC) acting as
intermediary between
entities
Fall, 2005
Computer Science
Public key problem:
• When Alice obtains
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)
CPSC499 Information Security Management
Key Distribution Center (KDC)
Computer Science
• 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
Fall, 2005
CPSC499 Information Security Management
KX-KDC
KY-KDC
KB-KDC
KZ-KDC
Key Distribution Center (KDC)
Computer Science
Q: How does KDC allow Bob, Alice to determine shared
symmetric secret key to communicate with each other?
KDC
generates
R1
KA-KDC(A,B)
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
Fall, 2005
CPSC499 Information Security Management
Key Management (public)
Computer Science
• public-key encryption helps address key
distribution problems
• have two aspects of this:
– distribution of public keys
– use of public-key encryption to distribute
secret keys
Fall, 2005
CPSC499 Information Security Management
Distribution of Public Keys
• can be considered as using one of:
– Public announcement
– Publicly available directory
– Public-key authority
– Public-key certificates
Fall, 2005
CPSC499 Information Security Management
Computer Science
Public Announcement
Computer Science
• users distribute public keys to recipients or
broadcast to community at large
– eg. append PGP keys to email messages or
post to news groups or email list
• major weakness is forgery
– anyone can create a key claiming to be
someone else and broadcast it
– until forgery is discovered can masquerade as
claimed user
Fall, 2005
CPSC499 Information Security Management
Certification Authorities
Computer Science
• Certification authority (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
Fall, 2005
+
KB
digital
signature
(encrypt)
CA
private
key
K-
CA
CPSC499 Information Security Management
+
KB
certificate for
Bob’s public key,
signed by CA
Certification Authorities
Computer Science
• When Alice wants Bob’s public key:
– gets Bob’s certificate (Bob or elsewhere).
– apply CA’s public key to Bob’s certificate,
get Bob’s public key
+
KB
digital
signature
(decrypt)
CA
public
key
Fall, 2005
+
K CA
CPSC499 Information Security Management
Bob’s
public
+
key
KB
A certificate contains:
Computer Science
• 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
Fall, 2005
CPSC499 Information Security Management
Summary
•
•
•
•
Symmetric encryption
Public encryption
Digital Signature
Key distribution
Fall, 2005
CPSC499 Information Security Management
Computer Science