1. CHAPTER 14
Key Management &
Exchange
Slides adapted from "Foundations of Security: What Every Programmer
Needs To Know" by Neil Daswani, Christoph Kern, and Anita Kesavan
(ISBN 1590597842; http://www.foundationsofsecurity.com). Except as
otherwise noted, the content of this presentation is licensed under the
Creative Commons 3.0 License.
2. Agenda
Key Management: process of generating,
storing, agreeing upon and revoking keys
Generation: How should new keys be created?
Storage: How to securely store keys so they
can’t be stolen?
Agreement: How do 2 parties agree
on a key to protect secrets?
3. 14.1. Types of Keys
Encryption keys can be used to accomplish
different security goals
Identity Keys
Conversation or Session Keys
Integrity Keys
One Key, One Purpose: Don’t reuse keys!
4. 14.1.1. Identity Keys
Used to help carry out authentication
Authentication once per connection between two
parties
Generated by principal, long-lifetime (more bits)
Bound to identity with certificate (e.g. public keys
in asymmetric system)
5. 14.1.2. Conversation or Session
Keys
Helps achieve confidentiality
Used after 2 parties have authenticated
themselves to each other
Generated by key exchange protocol (e.g. Diffie-
Hellman algorithm)
Short-lifetime (fewer bits)
6. 14.1.3. Integrity Keys
Key used to compute Message Authentication
Codes (MACs)
Alice and Bob share integrity key
Can use to compute MACs on message
Detect if Eve tampered with message
Integrity keys used in digital signatures
7. 14.2. Key Generation
Key generated through algorithms (e.g. RSA)
Usually involves random # generation as a step
But for IBE, also need PKG, master key
Avoid weak keys (e.g. in DES keys of all 1s or
0s, encrypting twice decrypts)
Don’t want keys stolen: After generation
Don’t store on disk connected to network
Also eliminate from memory (avoid core dump attack)
Generating keys from passwords: Use
password-based encryption systems (e.g. PKCS
#5) to guard against dictionary attacks
8. 14.2.1. Random Number
Generation
Ex: Alice & Bob use RSA to exchange a secret
key for symmetric crypto (faster)
Alicegenerates random # k
Sends to Bob, encrypted with his public key
Then use k as key for symmetric cipher
But if attacker can guess k, no secrecy
Active eavesdropper can even modify/inject data
into their conversation
Problem: Generating hard to guess random #s
9. 14.2.2. The rand() function
How about using rand() function in C?
Uses linear congruential generator
After some time, output repeats predictably
Can infer seed based on few outputs of rand()
Allows attacker to figure out all past & future output
values
No longer unpredictable
Don’t use for security applications
10. 14.2.3. Random Device Files
Virtual devices that look like files: (e.g. on Linux)
Reading from file provides unpredictable random bits
generated based on events from booting
/dev/random – blocks until random bits available
/dev/urandom – doesn’t block, returns what’s there
$ head -c 20 /dev/random > /tmp/bits # read 20 chars
$ uuencode --base64 /tmp/bits printbits # encode,
print
begin-base64 644 printbits
bj4Ig9V6AAaqH7jzvt9T60aogEo===== # random output
11. 14.2.4. Random APIs
Windows OS: CryptGenKey() – to securely
generate keys
Java: SecureRandom class in java.security
package (c.f. AESEncrypter example, Ch. 12)
Underlying calls to OS (e.g. CryptGenKey() for
Windows or reads from /dev/random for Linux)
No guarantees b/c cross-platform
But better than java.util.Random
12. 14.3. Key (Secret) Storage
Secret to store for later use
Cryptographickey (private)
Password or any info system’s security depends on
Recall Kerchoff’s principle: security should
depend not on secrecy of algorithm, but on
secrecy of cryptographic keys
Options for storing secrets?
13. 14.3.1. Keys in Source Code
Ex: Program storing a file on disk such that no
other program can touch itMight use key to
encrypt file: Where to store it?
Maybe just embed in source code? Easy since
you can use at runtime to decrypt.
Can reverse-engineer binary to obtain the key
(even if obfuscated) e.g. strings utility outputs
sequence of printable chars in object code
15. 14.3.2. Storing the Key in a File
on Disk
Alternative to storing in source code, could store
in file on disk
Attacker with read access could
Findfiles with high entropy (randomness)
These would be candidate files to contain keys
C.f. “Playing Hide and Seek with Stored Keys”
(Shamir and van Someren)
16. 14.3.3. “Hard to Reach” Places
Store in Windows Registry instead of file?
Part of OS that maintains config info
Not as easy for average user to open
But regedit can allow attacker (or slightly
above-average user) to read the registry
Also registry entries stored on disk
Attacker with full read access can read them
Registry not the best place to store secrets
17. 14.3.4. Storing Secrets in
External Devices (1)
Store secrets in device external to computer!
Keywon’t be compromised even if computer is
Few options: smart card, HSMs, PDAs, key disks
Smart Card (contains tamper-resistant chip)
Limited CPU power, vulnerable to power attacks
Must rely on using untrusted PIN readers
Attacker observes power of circuits,
computation times to extract bits of
the key
18. 14.3.4. Storing Secrets in
External Devices (2)
Hardware Security Module (HSM)
Device dedicated to storing crypto secrets
External device, add-on card, or separate machine
Higher CPU power, key never leaves HSM
(generated and used there)
PDA or Cell phone
No intermediate devices like PIN readers
More memory, faster computations
Can have security bugs of their own
19. 14.3.4. Storing Secrets in
External Devices (3)
Key Disk
USB, non-volatile memory, 2nd factor
No CPU, not tamper-resistant
No support for authentication
Ex: IronKey, secure encrypted flash drive
External Devices & Keys
Allows key to be removed from host system
Problem: connected to compromised host
Advantage: if crypto operation done on device & key
never leaves it, damage limited
Can attack only while connected, can’t steal key
20. 14.4. Key Agreement and
Exchange
Keys have been generated and safely stored,
now what?
If
Alice & Bob both have it, can do symmetric crypto
Otherwise, have to agree on key
How to create secure communication channel
for exchange?
Few Options
Use Asymmetric Keys
Diffie-Hellman (DH) Key Exchange
21. 14.4.1. Using Asymmetric Keys
Public-key crypto much more computationally
expensive than symmetric key crypto
Use RSA to send cryptographically random
conversation key k
Use k as key for faster symmetric ciphers (e.g.
AES) for rest of conversation
22. A Word of Caution
In the following example, as well as some other
examples presented in later chapters, the simple
toy protocols that we discuss are for instructive
and illustration purposes only. They are
designed to make concepts easy to understand,
and are vulnerable to various types of attacks
that we do not necessarily describe. Do not
implement these protocols as is in software.
23. 14.4.1. Key Exchange Example
I am Bob. My public key is XYZ.
Alice Bob
{CK=8a6cd93b2b4f8803}RSA(XYZ)
{Hello Alice}AES(8a6cd93b2b4f8803)
Asymmetric (e.g. RSA)
{k} PK(B)
Alice {data} k Bob
{data} k
Symmetric (e.g. AES)
24. 14.4.2. Diffie-Hellman (DH) (1)
Key exchange (over insecure channel) without
public-key certificates?
DH: use public parameters g, p
Large prime number p
Generator g (of Zp = {1, …, p-1}), i.e. powers g, g2, …,
gp-1 produce all these elements
Alice & Bob generate rand #s a, b respectively
Using g, p, a, b, they can create a secret known
only to them (relies on hardness of solving the
discrete log problem)
25. 14.4.2. Diffie-Hellman (DH) (2)
Alice Bob
Choose a ga mod p Choose b
gb mod p
Compute (g b ) a Compute
mod p (g a ) b mod p
Secret Key = g ab mod p
Eve can compute (ga)(gb)= ga+b mod p but that’s not the secret key!
26. 14.4.2. Man-in-the-Middle
Attack against DH
Alice Mallory Bob
Choose a ga Choose m Choose b
gm
gb
Compute (gb)m Compute (gm)b
gm Compute (ga)m
Compute (gm)a
Secret Key = g am Secret Key = g bm
Mallory can see all communication between Alice & Bob!
27. 14.4.2. Detecting Man-in-the-
Middle Attack
Both Alice and Bob can compute hash of the
shared secret h(gab) and display it
Mallory’s secret key w/ Bob different from key
with Alice (gam ≠ gbm), hashes also different
If hashes don’t correspond, then there must
have been a man in the middle!
Proposed by P. Zimmermann in VoIP protocol
ZRTP
28. Summary
Key Management consists of generation,
storage, and agreement/exchange
Key Types: Identity, Session, Integrity
Key Generation
Problem is random number generation
Use device files or SecureRandom Java API
Key Storage: use external devices
Key Exchange:
Public-keycrypto
DH – guard against man-in-the-middle attack
Editor's Notes
Welcome to SEC103 on Secure Programming Techniques. In this course, I assume that you have some background in computer security, but now you want to put that background to use. For example, in the Computer Security Principles and Introduction To Cryptography courses, we cover topics such concerning trust and encryption. In this course, we put these principles into to practice, and I’ll show you have to write secure code that builds security into your applications from the ground up.
Because encryption can be used for purposes other than confidentiality, security systems use encryption in many places. We have seen encryption used to achieve confidentiality and authentication. However, encryption can also be used as a primitive to achieve other security goals as well. In some systems, when different algorithms are often used for different purposes, but all of these algorithms require us to use keys. As a result, it is often useful to make a distinction between different types of keys that are used for different purposes. Since asymmetric encryption is very costly/expensive (RSA encryption takes 1000 times as long as DES encryption), it is typically only used for authentication. Symmetric encryption, due to its relative efficiency, can be used for authentication as well as confidentiality purposes. Authentication typically happens once per connection set up between two parties, and some keys might be used in order to accomplish the authentication. Keys that are used to accomplish authentication are typically referred to as “identity” keys. Identity keys (such as a user’s private key) are generated by the principal whom that key authenticates. Since they are used to authenticate a principal, and the lifetime of a principal might be long (i.e., up to 100 years), we use key lengths that would generally not be break-able within that timeframe. On the other hand, we might use a key to encrypt the contents of a conversation with another party, and this type of key is often referred to as a “conversation key” or a “session key.” Since we may only care about protecting the contents of a particular conversation for say 1 year or 5 years, we can use a key length for a conversation key that is shorter than a identity key. In addition, either party may generate the conversation key (assuming that both parties are, to an extent, “trusted”), or the conversation key may partly be generated by each party.
We might be able to prevent a man-in-the-middle attack by using public-key crypto to do the key exchange. In the protocol above, the first two messages are still susceptible to spoofing.
In this slide, we will talk about a protocol called Diffie-Hellman (DH) Key Exchange which allows two parties to agree upon a key without ever meeting, and without relying on an alternate secure channel. In DH, both Alice and Bob execute the protocol using some public parameters g and p that everyone knows. P is a (large) prime number, and g is a “generator.” This means that as we raise g to successive powers g^1, g^2, we get back all numbers 0 to p-1. After these two public parameters have been chosen, any two parties that know the public parameters can participate in a key exchange. In DH, Alice generates a random number a, and Bob generates a random number b. Parameters a and b are used to create a key that will be known to only Alice and Bob even if a passive eavesdropper can view the contents of their conversation. This happens as follows: after Alice and Bob choose a and b, respectively, they do not transmit a or b to each other. Instead, Alice transmits g^a mod p to Bob and Bob transmits g^b mod p to Alice. Alice takes the g^b mod p that she received, and raises it to the power a thereby computing g^b^a mod p. Bob takes the g^a mod p that he received, and raises it to the power b thereby computing g^a^b mod p. Now, Alice and Bob both know g^a^b (since g^b^a = g^a^b), and that is the key that they now share. Can Eve possibly have computed g^a^b by eavesdropping? They answer is no. Eve saw g^a and g^b go across the wire, and Eve could multiply these two numbers together, but that will give Eve g^(a+b); not g^a^b! As a result, Alice and Bob are able to agree upon a key even if the channel is susceptible to eavesdropping! However, while DH is not susceptible to passive eavesdropping in with the attacker can only listen to the information going by on the wire, DH is susceptible to active eavesdropping in which the attacker can not only listen to the information going by on the wire, but is also capable of modifying the messages.
Let’s look at how an attacker can construct a “man-in-the-middle” attack against DH. Let’s say that Mallory is able to listen to the communications being exchanged between Alice and Bob and is able to modify the contents. We will see that Mallory will be able to fool Alice in thinking that she has successfully participated in a key exchange with Bob, and that Bob can be fooled into thinking that he has successfully participated in a key exchange with Alice. The protocol starts off as before, with Alice choosing a secret random number, a. Alice sends g^a to Bob, but Mallory intercepts it. Mallory chooses a secret random number, m, and sends g^m to Bob instead of g^a. As a result, Bob receives g^m. Bob also attempts to engage in the protocol as expected: he generates g^b and attempts to send it to Alice. Unfortunately, Mallory intercepts g^b as well, and replaces it with g^m, sending g^m to Alice. Alice ends up computing g^m^a, and Bob ends up computing g^m^b. Mallory can compute both g^m^a and g^m^b. As a result, Mallory will be able to see any message that Alice attempts to send to Bob using the secret key g^m^a, and will be able to see any message that Bob attempts to send to Alice using the secret key g^m^b. Mallory is the “man-in-the-middle” and can impersonate either Alice or Bob. While we have shown that we can do a key exchange using DH, this method is unfortunately susceptible to a man-in-the-middle attack. How can we do a key exchange that is resilient to this attack?
