“Defeating SSL” Impact of Hash

collisions on cyber security

By vaibhav

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Secure Sockets Layer

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Objective• 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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when in doubt , ask

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Cryptographic Hash • 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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Hash Functions• Compress an arbitrary finite length m-bit input

message into a fixed n-bit output value called hash.

hash

if h = H(m) then,• h is called the "hash" of m,• m is called a "preimage" of h

Data Message Digest

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Properties of (good)Hash Function

• (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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Lets talk about SSL first

• 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 requestoro verify domain ownership for SSL certso revoke bad certificates

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Site NameCN

Check Expiry

Check Signature

Signin CA in trust

store

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What if …Root CA

Intermediate I CA

Intermediate II CA

Hack.org

Hack.org is a valid certificate issued by intermediate II CA

What if hack.org issues a certificate for richest-bank.com ?

Richest-bank.com

Chain verification algorithm as described before would validate this certificate too.

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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 Richest-bank, 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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Obtaining certificates

Public Key

Domain name

User Identity

User

Public Private Key pair generation

CSR generated

CSR sent to CA

Validates user identity and

domain ownership

Create and signs certificate

User installs private key and certificate on a web server

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Certificate request format

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Certificate structure• the "to-be-signed" part, consisting of:

o serial numbero 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-be-signed“ part

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Secure websites and certificates

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Attack Scenario using Rogue CA certificate

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Revisiting MD5 and MD5 Collision

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Overview of MD5Hash function MD5 designed in 1991• Iterative design using compression function.

• Collision different messages , same hash

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MD5 Collisions in 2004

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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MD5 Collisions in 2007

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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Generating Colliding Certificates

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History of colliding certificates

Certificates with colliding to-be-signed parts• generate a pair of certificates• sign the legitimate certificate• copy the signature into the rogue certPrevious work• Different RSA public keys in 2005

o using 2004 collision attack• Different identities in 2006

o using chosen-prefix collisionso 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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Why RapidSSL?• 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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Collision generation and RSA keys

• Based on the 2007 chosen-prefix 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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Another Attack on SSL

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Background• 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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ASN1.0 and Certificates

• 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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Malformed Request• 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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Defeating SSL• 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

char *destination string match

w w w . b a n k . c o m /0 w w w

w w w . b a n k . c o m /0

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Question ?

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References• http://www.win.tue.nl/hashclash/rogue-ca/• http://

conf.isi.qut.edu.au/auscert/proceedings/2006/gauravaram06collision.pdf

• https://www.blackhat.com/presentations/bh-dc-09/Marlinspike/BlackHat-DC-09-Marlinspike-Defeating-SSL.pdf