COMP043 Cryptography Cryptographic Attacks
Ciphertext Attacks Types of attacks on a cipher, mostly designed to discover the key: ciphertext only: the cryptanalyst has only the cipher text known-plaintext: the cryptanalyst knows both the ciphertext and its plaintext chosen-plaintext: the cryptanalyst selects the plaintext used to create the ciphertext
Ciphertext Attacks (cont.) Adaptive-chosen-plaintext: the plaintext is selected based on previous plaintext-ciphertext pairs ( as opposed to batch CPA) Chosen-ciphertext: the cryptanalyst selects the ciphertext to decode to known plaintext Adaptive chosen-ciphertext: like ACP
Infrastructure and other attacks Brick attacks Physical coercion to obtain key Infrastructure attacks Attack the PKI, steal private keys Software bugs and poor design Exploit implementation errors Side channel attacks MITM attacks
Ciphertext Only More common in classical cryptography Manual ciphers were subject to frequency analysis Modern ciphers mostly immune to Known Ciphertext
Ciphertext Only Considered successful if any info about plaintext determined E.g. detecting null messages inserted in a stream to defeat traffic flow analysis Microsoft PPTP VPN was vulnerable due to reuse of RC4 key Actually a related key attack
Related-key Attack Several different keys whose values are initially unknown Some mathematical relationship WEP encryption uses RC4 stream cypher RC4 key for each packet is an IV concatenated with WEP key IV's repeat (24 bits) Hence keys repeat
WEP Attack 24 bit IV allows around 16 million values Due to birthday paradox it is likely there is a duplicate key in any set of 4000 packets This makes attacks possible Some weak RC4 keys allowed for recovery of WEP key
Birthday Paradox Two people have a 1/365 chance of sharing a birthday With three people ~ 3/365 With four, ~ 6/365 In a group of 23 people the chance is > 50% With 57 > 99%
Birthday Paradox Not really a paradox, just surprising Lots of crypto attacks based on Birthday Paradox Often called birthday attacks
DNS Birthday Cache Poisoning Attack Sending spoofed replies to a DNS request that was sent to an authoritative server in response to a query you sent to a caching server Reply must have same port # and transaction ID as request ID only 16 bits Send lots of queries to cache and lots of replies Good chance one of the replies will match a request generated by one of the queries
Known Plaintext Attack Common in classical cryptography German weather reports in second world war Can be used to crack encrypted zip archives if you have plaintext of one file Note CBC mode will defeat this attack
CBC Mode
Known Plaintext Attack on GSM GSM phones encrypt and sign using a symmetric key (shared secret) stored on SIM card with copy retained by the network Some still use 56 bit DES keys Malformed Class 2 SMS message (sent to SIM) will be rejected by SIM card with predictable signed message Attacker pre-calculates encrypted message hash for all possible keys (rainbow table) Looks up message signature to reveal key
Chosen Plaintext Attack Attacker can choose arbitrary plaintext and get ciphertext Goal is to determine the key Public key systems have to resist this form of attack
Chosen Plaintext Attack Version of dictionary attack can be used by preencrypting likely messages, then using ciphertext as lookup key Need some randomiser in key to resist Salt Note that our public key applications seldom encrypt real data
Adaptive Chosen Plaintext Attack As opposed to batch Attacker chooses plaintext based on progress of attack Differential Cryptanalysis is a form of ACPA
Differential Cryptanalysis Discovered by Adi Shamir (the S in RSA) in late '80s Each known plaintext has carefully crafted difference from other plaintext Designed to reveal statistical failings in algorithm DES found to be remarkably and specifically resistant Later revealed that IBM (and NSA) knew about the attack since 1974
Chosen Ciphertext Attack El Gamal can be trivially defeated by CCA RSA is vulnerable E(P1) x E(P2) = E(P1 x P2) Using some complex math, attacker can recover plaintext from plaintext of a carefully constructed ciphertext Usually some complex padding is inserted to make this fail Early SSL systems did this wrong PKCS#1 has a padding scheme that avoided this problem
Adaptive Chosen Ciphertext Attack PKCS#1 proved susceptible to an adaptive chosen ciphertext attack in 1998, which resulted in the exposure of session keys A new scheme, Optimal Asymmetric Encryption Padding was developed to thwart this attack As were new versions of PKCS#1 from RSA labs
Lunchtime Attacks Attack types are interesting as theoretical measures of a cryptosystem's strength All systems must resist cyphertext only Should resist known plaintext Resistance to chosen plaintext implies resistance to known plaintext Chosen ciphertext is pretty strong condition
Lunchtime Attacks Lunchtime attacks suppose a limited number of chosen ciphertexts As if attacker had access to cryptosystem while user was at lunch
Infrastructure Attacks NSA is believed to have stolen private keys from CA's and other large X509 users to compromise SSL/TLS and ipsec Luxury car key codes compromised by reverse engineering Windows software used to program dongles that use secret crypto algorithm
Heartbleed Software Bugs Stupid idea in the first place Why have a heatbeat Stupid design Why allow variable length messages? Flawed implementation No bounds checking Poor oversight
Poor Design Android apps Signature and software stored in Java Beans file which is a form of zip archive Zip archives can be updated with new version of files contained in them Unzip extracts latest version of files But signature checker compares signature to oldest version Malicious replacement version appears to have correct signature
Poor Design SSH bruteforce ssh command allows config file options on the command line, for flexibility (rigidity is more secure) KbdInteractiveDevices option can specify a list of authentication methods, but you can specify the same option many times (bounds checking!) Specifying pam many times defeats limits on failed login, allowing password brute force attempts ssh okbdinteractivedevices=`perl e 'print "pam," x 10000'`
Side Channel Attacks Insecure channel Plain text The internet Secure Channel Ciphertext VPN SSL Side Channel Any other source of information
Types of Side Channel Attacks Timing attack Power monitoring attack TEMPEST Actually a defence Differential fault analysis Message Size Attacks
Timing Attack Some key values and plain text values will take longer to compute With lots of chosen plaintext, attackers can use timing attacks to determine key values one bit at a time Diffie-Hellman and RSA are vulnerable due to the complex math involved
Power Monitoring Attack Like timing attack, but this time watch power consumption Smart cards are vulnerable because power can be carefully monitored Sometimes DES internals are vulnerable Differential PMA collects many power consumption records and compares them statistically DPMA harder to defend against
Laptop Power Monitoring Attack Researchers monitoring fluctuations in the ground potential of laptops determined what instructions were executed Wire to chassis Monitor ground cable in USB, CAT5 cable Touch the chassis Can determine 4096 bit RSA keys while encrypting or signing Have achieved same result listening to generated noise
TPM attacks Power monitoring attacks have been used to retrieve keys from TPM modules Rumors are that the CIA has developed an attack of this sort that defeats bitlocker
TEMPEST TEMPEST is a military standard for shielding against compromising emanations (CE) Variations include Electrical Mechanical Acoustical
CE Electrical Printers, Monitors, Keyboards In 1985 CE from monitors were intercepted from 100s of meters away using TV and $15 of equipment LEDs on routers Mechanical Lasers on window panes Acoustical In cold war, soviets would bug IBM Selectric typewriters
Message Size Reveals Information in TLS encrypted web services Web 2.0 basically uses AJAX code rather than static HTML page Many small encrypted interactions with server But address of page is known, and with message size information attacker can determine what menu and other choices were made
Message Size Example Google search query completion With first letter typed server returns completion list Size of list determines letter typed Subsequent letters evoke new completion lists
Fault Analysis Analyse behaviour of cryptosystem under duress Power glitches, high temperature etc. Certain areas of the computation more likely to fail For DES, around 200 single flipped bits will reveal the secret key
MITM Attacker (Eve) intercepts communications between two endpoints (Bob and Alice) to monitor, or modify communications Effective against Diffie-Hellman if endpoints aren't authenticated An interesting MITM attack has been devised to defeat PIN-and-chip credit cards
Why Does Eve Want to Intercept Bob and Alice's Messages?
MITM vs PIN and Chip Smartcard communicates with Terminal Attack sits between card and terminal Terminal communicates with bank Three steps Card Authentication Card holder Authentication Transaction Authentication
Chip and PIN MTM To Bank Terminal MITM Card
Card Holder Authentication Card asks terminal if it supports PIN Some don't MITM modifies terminal's reply to tell card that terminal doesn't want PIN But tells terminal to get PIN Then drops it Card now happy that signature will be captured But terminal thinks cardholder authenticated by PIN
Transaction Authentication Now terminal thinks authenticated PIN transaction is taking place It gets card to supply cryptographic authentication for transaction using keys on card Bank thinks PIN authenticated transaction took place But cardholder not authenticated at all
FDE Attacks The Elephant in the room
FDE Attacks, Cold Boot Cold Boot Attack Computer is powered on with decryption keys in memory, but locked Reboot quickly to OS on portable device Decryption keys are still readable in memory Or use USB device that can read memory with malicious drivers
FDE Attacks, Evil Maid Maid can compromise computer, then return to collect data Installs malicious bootloader that will collect and store password at boot time Maid returns next day to collect password TPM can try to defend but...
FDE Attacks, Stoned Boot Malware is incorporated into the boot software (MBR) Gets loaded before anti-malware Gets loaded before software signature checking TPM can protect against this, but...
TPM protection PCT Extend TPM has several Platform Configuration Registers (PCR) Set to zero on power-up Extend function is only way to modify Function passes value to TPM and ID's a PCR TPM hashes value with current PCR value to create new PCR value Only way to replicate is to execute same sequence of extend functions
TPM protection Seal Function Seal function of TPM can can encrypt a value, such as a decryption key, in relation to some PCR values Cypher text can be decrypted by the TPM But only if the PCR's have the same value as when the key was sealed
TPM Protection Boot Attacks TPM protection against boot attacks uses the PCR's and seal function Boot code, and maybe unencrypted boot partition, is used during boot to extend PCR's Seal function encrypts decryption key with reference to these PCR's Any corruption in boot sequence will prevent decryption
TPM protection Evil Maid Attack Even with all this, evil maid can install corrupted bootloader that: Captures and stores credentials Restores original uncompromized boot path Reboots
FDE Cipher Text Attacks FDE does not employ MACS to authenticate the cipher text No place to store them or IV's So encrypted sectors can be modified without detection Effect of a bit flip on decrypted plaintext is to randomise one block but flip just one bit in another Attacker knows lots of plaintext and can maybe corrupt with appropriate cypher text changes
CBC Mode
Decrypting CBC mode with corrupted ciphertext
Another thing: CBC= ECB? Because of lack of MAC and no stored IV, CBC mode for FDE is like a symmetric cipher in ECB mode with block size = sector size So we could evaluate the crypto-system in that light Considerations like diffusion could be applied to the system as a whole
h Diffusion Good ciphers have good diffusion, meaning that a one bit change in plaintext changes every bit in cipher text with 50% probability Same applies to decryption This diffusion applies only to the small blocks encrypted by the basic AES or other symmetric key cipher But in FDE we might like to see diffusion apply to an entire sector, not just a block Harder then to make small changes to plaintext through small changes to ciphertext
Elephant Diffuser Bitlocker developers wanted diffusion to apply to entire sectors Looked at some encryption algorithms that did this using block size = sector size Bear and Lion Beast Too slow Too slow and poor diffusion So they added a diffuser called the elephant diffuser
End of the Elephant Despite lots of high praise, the elephant was dropped with W8 Replaced with AES in XTS mode Full sector diffusion really not needed Just need to avoid the 1 bit flip Diffuser was overkill Did it weaken the system? Developers said it couldn't be weaker than AES + CBC on its own but... Not standards compliant (FIPS)
XTS Decryption with Corrupted Cyphertext
Today's Lab Encrypting a container with bitlocker Next week is lab test Following week is written test