By vaibhav

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• Background Information of SSL , MD5 &
“Certificate”\”Public Key” Infrastructure
• Attack scenario on core assumption of SSL i.e. collision resistance of hash function
• Attack scenarios on the specification\ implementation of SSL
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• Serves an essential role within a wide range of security applications.
• Like
(a) digital signature generation and verification
(b) session key establishment
(c) management of password schemes
(d) commitment schemes in cryptographic protocols
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• Compress an arbitrary finite length m-bit input message into a fixed n-bit output value called hash.
Data
Message
Digest if h = H(m) then,
• h is called the "hash" of m,
• m is called a "preimage" of h
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• (practicality) computation of hash can be done efficiently
• (preimage resistance) given h, it is hard to compute a preimage of h
• (second preimage resistance) given m, it is hard to compute a second preimage of m
• (collision resistance) it is hard to compute a collision for H
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• Ensures secrecy ,authenticity, and integrity.
• Safeguarding communication from both the passive and active adversaries.
• SSL rely heavily on the x509* certificate structure.
• For SSL protocols , it is the “common name” field in the subject of an x509 certificate that is used to identify entities presenting certificates.
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X509 ? Digital certificates ?
What ?
• ITU-T standard for the public key infrastructure.
• X.509 specifies standard formats for public key certificates
• Public key certificates are structured according to version3 of X.509 specification.
• A public key certificate uses a digital signature to bind a public key with an identity.
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Certification Authorities & Hierarchy
• Browsers ship with a list of trusted CA certificate.
o Firefox 3 includes 135 trusted CA certs.
• CAs’ responsibilities: o verify the identity of the requestor o verify domain ownership for SSL certs o revoke bad certificates
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Check
Signature
Signin CA in trust store
Check
Expiry
CN Site Name
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Root CA
Intermediate I
CA
Hack.org is a valid certificate issued by intermediate II CA
What if hack.org issues a certificate for richest-bank.com ?
Intermediate
II CA
Chain verification algorithm as described before would validate this certificate too.
Hack.org
Richest-bank.com
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Something must be wrong, but...
• All the signatures are valid.
• Nothing has expired.
• The chain is intact.
• The root CA is embedded in the browser and trusted.
But we just created a valid certificate for Richestbank, and we're not Richest-bank?
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X509v3 extensions provide a extension to tackle with this.
Basic Constraints: critical CA:FALSE
But …
• Most CAs didn't explicitly set basicConstraints: CA=False
• Whether the field was there or not, most SSL implementations didn't bother to check it.
Hacker moxie marlinspike a tool, sslsniff, to attack this vulnerability.
Eventually Microsoft released a patch to address this issue.
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User
User installs private key and certificate on a web server
Public Private
Key pair generation
Create and signs certificate
User Identity CSR generated
Validates user identity and domain ownership
Domain name
Public Key
CSR sent to CA
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• the "to-be-signed" part, consisting of: o serial number o validity period o issuer name o subject o subject public key o "basic constraints" field, containing
• a bit indicating whether this is a CA certificate or a user certificate
• a path length field
• the "signature" part, containing a digital signature, produced by CA`s private key, over the "to-besigned“ part
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Attack Scenario using Rogue
CA certificate
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Hash function MD5 designed in 1991
• Iterative design using compression function.
• Collision  different messages , same hash
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2004: First MD5 collision attack
• Only difference between messages in random looking 128 collision bytes
• Currently < 1 second on commodity PC
MD5( ) = MD5( )
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2007: Stronger collision attack
• Chosen-Prefix Collisions
• Messages can differ freely up to the random looking 716 collision bytes
• Currently approx. 1 day on PS3+PC
MD5( ) = MD5( )
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Certificates with colliding to-be-signed parts
• generate a pair of certificates
• sign the legitimate certificate
• copy the signature into the rogue cert
Previous work
• Different RSA public keys in 2005 o using 2004 collision attack
• Different identities in 2006 o using chosen-prefix collisions o the theory is well known since 2007
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Colliding certificates in 2006
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Rogue CA certificate
CA bit
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Action Items for generating hash colliding certificates
• Find CA which issues MD5 signed certificate
• Predict the Validity and Serial Number
• Construct structure and content rogue Certificate such that real certificate( constructed by CA) and rouge CA cert(Constructed by Hackers) are perfectly aligned.
• Compute the collision blocks
• Create RSA key pair such that it includes collision block in it.
• Construct CSR and send it to CA for signing
Detailed view
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• Out of 9000 MD5 certificates collected 97% of those were issued by RapidSSL.
• RapidSSL issues exactly 6 seconds after “accept” button is clicked and expires in one year.
• RapidSSL uses sequential serial numbers and on weekend approximately 1000 certificates are issued.
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Predicting the serial number
• Get the serial number S on Friday
• Predict the value for time T on Sunday to be S+1000
• Generate the collision bits
• Shortly before time T buy enough certs to increment the counter to S+999
• Send colliding request at time T and get serial number S+1000
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• Based on the 2007 chosenprefix collisions paper with new improvements
• 1-2 days on a cluster of 200
PlayStation 3’s
• Equivalent to 8000 desktop CPU cores or $20,000 on Amazon
EC2
• takes couple of minutes to calculate RSA key pair such as it contains collision blocks
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• As mentioned earlier SSL handshake uses “common name” of certificate and compares with site name.
• Before year 2000 actual people were involved while dealing with certificate request.
• Entities are validated based on proof of ownership of the domain listed in the “common name” field.
• Now a days a simple lookup in WHOIS database for the root domain listed and sending a confirmation mail would complete the verification part.
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• Certificates are formatted using ASN1.0 notation.
• Supports different type of strings, all represented as
PASCAL strings.
• Represented in memory by the length of the string followed by the string data.
• NULL character has no special meaning, like C strings
Example :
0x05
(length)
0x44 ( D ) 0x41 (A ) 0x00
(NULL)
0x54 ( T ) 0x41 ( A )
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• One can create a certificate request with common name as www.richest-bank.com\0www.hack.com
• CA for verification would do WHOIS
• issues the certificate with embedded NULL to the owner of hack.com.
• Spoof www.richest-bank.com
and use NULL embedded certificate
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• This is how comparison function would be implemented for CN verification.
char *destination = getDomainWeAreConnectingTo(); char *commonName = getCommonNameFromCertificate();
Bool everythingIsOk = (strcmp(destination, commonName) == 0); char *commonName w w w .
b a n k .
c o m /0 w w w char *destination w w w .
b a n k .
string match c o m /0
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• http://www.win.tue.nl/hashclash/rogue-ca/
• http://conf.isi.qut.edu.au/auscert/proceedings/200
6/gauravaram06collision.pdf
• https://www.blackhat.com/presentations/bh-dc-
09/Marlinspike/BlackHat-DC-09-Marlinspike-
Defeating-SSL.pdf
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