CMSC 414
Computer and Network Security
Lecture 11
Jonathan Katz
“Insecurity of 802.11”
 WEP encryption:
IV, RC4(IV | k)  (M, c(M))
 Is this secure against chosen-plaintext attacks?
– It is randomized…
 40-bit key (in some implementations)!
– Claims that, with IV, this gives a 64-bit effective key(!)
 And how is the IV chosen?
– Only 24 bits long -- IV repetitions are a problem!
– Reset to 0 upon re-initialization
– Some implementations increment the IV as a counter
“Insecurity of 802.11”
 A repeating IV allows the attacker to compute the
XOR of two plaintexts
– We have discussed already how this can be damaging
 Small IV space means the attacker can build a
dictionary of (IV, RC4(IV | k)) pairs
– If portions of some plaintexts known, this enables
determination of other plaintexts
“Insecurity of 802.11”
 Known-plaintext attacks discovered on this usage
of RC4
– Possible because the first byte of plaintext is a fixed,
known header!
 Chosen-plaintext attacks
– Send IP traffic/e-mail to the mobile host and watch it
get forwarded
– Transmit broadcast messages to access point
– Authentication spoofing
“Insecurity of 802.11”
 No cryptographic integrity protection
– The checksum is linear (i.e., c(xy) = c(x)c(y)) and
unkeyed, and therefore easy to attack
– Allows IP redirection attack
– Allows TCP “reaction” attacks
• Look at whether TCP checksum is valid
• Form of chosen-ciphertext attack
 Encryption used to provide authentication
of mobile station (access point sends nonce;
station returns an encryption of the nonce)
– Allows easy spoofing after eavesdropping
“Analysis of E-Voting System”
 This paper should scare you…
– Magnitude of possible attacks by voters
– Not just the security flaws, but also the reaction of
Diebold and govt. officials…
 Vulnerable to attacks by voters, as well as attacks
by insiders
 Security through obscurity did not help
– In this case, code was leaked
Desiderata?
 Security against voters
– No double voting
– No voting outside place of residence
– Unable to disrupt the election, or tamper with results
– Privacy of others’ votes
 Security against insiders (election officials, district
heads, programmers, tech staff, …)
– Privacy of votes, except end-of-day total
– Unable to disrupt the election, or tamper with results
 Public verifiability of the entire process
Overview of Diebold system
 Voting terminals initialized; ballots installed
 On voting day, voters given voting card
– Voter inserts card, gets ballot, makes choices
– After confirmation, voting card is “cancelled”
 Election is closed by inserting an admin card
– Results can be uploaded for tabulation
Poor cryptography
 Smartcards have no cryptographic functionality
– Possible to create home-made voting cards!
– Cast multiple votes by disabling “cancellation”, or
overwriting card
– Change party affiliation
 No cryptographic protection for admin cards
– Only a weak PIN…if any
– Possible to shut down the election!
 Bad audit mechanism for detecting over-voting
– Detected over-vote would nullify the election
“Analysis of E-Voting System”
 Most data stored without any integrity
• Possible to modify ballots, vote total, or even the software
 No authentication of data sent to back-end server
 Hard-coded, non-random DES key!
 CBC mode with IV = 0!
– Deterministic encryption…
– Linking voters to votes (encrypted votes stored
sequentially)
 CRC used instead of a secure MAC
 Poor random number generation
“…Attacks on SSH”
 Previous examples illustrated bad cryptography
 Here we will see an example of good
cryptography being ‘circumvented’
“…Attacks on SSH”
 Focus only on the symmetric-key encryption and
integrity protection mechanism for SSH packets
 Recall CBC mode: ci = Fk(pi  ci-1)
 Chosen-ciphertext attack on CBC mode…
SSH
 SSH uses a variant of CBC mode
 This variant can be proven secure against chosen-
ciphertext attacks
Proof model?
decryption of
ciphertext, or error
c’1, …, c’n
…
c1, …, cn
c’1, …, c’n
Even after this interaction,
adversary learns no information about original plaintext
Real world?
 Different error messages returned depending on
the error condition
– If packet_length not in correct range, terminate the
session and send SSH2_MSG_DISCONNECT
– If packet_length not a multiple of the block length,
terminate session with no error message
– Else accept packet_length bytes until MAC can be
checked; different error message sent in this case
 This does not match the formal model!
Real world?
 SSH sends/receives communication block-by-
block
…
c1, …, cn
c’1
c’2
c’3
c’n
decryption of
ciphertext, or error
This enables attacks!
 Focus on packet_length ciphertext block
– If SSH2_MSG_DISCONNECT message sent right
away, then the attacker learns that the most significant
14 bits of decrypted length field are not all 0
• Leaks that they are all 0 with probability 2-14
– If above check passes and length check does not fail,
then 4 least significant bits of decrypted length field
equal 12
• Happens with probability 2-4
– If above checks pass, then attacker injects blocks until
the MAC check fails
• Reveals exact value of decrypted length field
Side channel attacks
Side channel attacks
 We have seen already one example of how reality
can differ from the (standard) formal models used
in cryptography
 More generally, cryptographic analysis treats
primitives/protocols as black boxes
 In reality, primitives and protocols implemented in
the real world by hardware/software
– This may lead to (other) attacks ‘outside the model’
Side channel attacks
 CPU retains state in the form of caches, branch
prediction buffer, stack data, memory, disk…
 Interaction with users influences resource usage,
page protections, scheduling
 Timing attacks, power analysis, EM radiation,
heat/sound/disk access patterns
– Especially in embedded systems
Side channel attacks
 Side channel attacks may be used to break the
crypto
– E.g., timing attacks, power analysis
 Side channel attacks may also be used to
circumvent crypto entirely
– E.g., EM radiation from monitors/keyboards, extracting
keys from memory or data from disk
– (Really more of a systems issue than a crypto issue)
“Cold boot attacks”
 Attacks on disk encryption products, exploiting
poor key management along with the fact that
memory contents can be recovered
Basic setup
Encpw(k)
pw
data
k
RAM
Enck(data)
Encrypted
hard drive
What happens when computer is shut off (or put in standby)?
Setup 2
k
pw
ok
TPM
k
RAM
Before correct password entered, k is loaded into memory
Key observations
 If memory can be probed, possible to recover k
(and then read all data on hard drive) without the
correct password
– First case: k not scrubbed after power down
• Memory decays over time
• But not too quickly, and this process can be slowed by cooling
– Second case: k should be loaded only after successful
password is entered
 Video at http://citp.princeton.edu/memory/media/