Category Archives: protocol

Protocol Cryptanalysis

SSLv3 considered to be insucure – How the POODLE attack works in detail

POODLE is a recent attack on SSLv3. This article will explain the attack in detail:

The POODLE attack on SSL Version 3, that sometimes allows an attacker to decrypt a single byte of an SSLv3 protected conversation. Repeating the attack might allow an attacker to decrypt multiple bytes of a secret (for example Session-Cookie, Password…) that is repeatedly send.

 When can the POODLE attack be applied?

The POODLE attack can be applied if:

  • A network connection is secured using SSLv3
  • A block cipher is used for the connection
  • The attacker is an active adversary in the path between the client and the server and he is able to intercept and modify network traffic

To improve the success probability, the attacker should be able to:

  • Force the client to repeatedly send a secret over that network connection
  • Force the client to vary the position of that secret in the transmission

What can an attacker gain using the POODLE attack?

An attacker will be able to decrypt a single byte of the encrypted content with a probability of 1/256. By repeating the attack (the same secret is send again), he will be able to decrypt that single byte with probability 1. By varying the position of a secret content, he will be able to fully recover that secret.

What cannot be done using POODLE?

Using POODLE, it is not possible to:

  • Impersonate an SSL/TLS Server
  • Recover the secret key of an SSL/TLS Server
  • Modify any data that is transmitted

What is the weak point of SSLv3 that is exploited?

When using a block cipher for SSv3, the data to be transmitted is protected against unauthorized modification by adding a MAC to that data first. The result will most likely not have a length that is an integral multiple of the block length of the block cipher. To expand the data to be encrypted to a proper length, a padding is added. The last byte of the padding contains the length of the padding, so that the padding can be properly removed. The data is then encrypted with the block cipher in CBC mode.

Assuming that the last block of the ciphertext (C_i) is entirely filled with padding. The last byte of plaintext in this block will be the length of the padding l. An attacker might now replace that block C_i with a previous block in that session C_j. The server will decrypt that block by computing P_i = D(C_j) \oplus C_{i-1}. If the last byte of P_i is now l, the server will correctly decode the length of the padding and will decode the entire ciphertext correctly. If the last byte differs, the server will very likely fail to decode that ciphertext correctly and close the connection.

This can be used to decode the last bytes of arbitrary ciphertext blocks in a conversation by repeating the process. The attacker might also shift the secret in a conversation to fully decode it.

What countermeasures are possible?

As long as SSLv3 is not used (also, SSLv2 is very weak and should not be used anymore), the attack is not possible. I consider disabling SSLv3 on the server or the client side (best is both sides) to be the best solution. If for any reasons, SSLv3 still needs to be used, you might disable all block cipher suites. However, the only stream cipher available in SSLv3 is RC4, which is also broken. I would currently assume that it takes more repeated conversations in SSLv3 using RC4 to recover a secret than it takes to decrypt a secret using POODLE. However, attacks against SSLv3 using RC4 can be done passively.

Will I notice the attack?

As long as logging is used on the client or the server, the attack will very likely produce a lot of error messages related to MAC verification failures or incorrectly terminated connections.

Which methods are available to shift a secret in a conversation?

That depends on the protocol. If the secret is for example a session cookie, the request URL in an HTTP request might be altered to have a varying length. As a result, this will also shift the position of a session cookie in the transmission.

Which methods are available to make a client to repeat a conversation?

That also depends on the protocol. For POP3/IMAP, this might happen automatically. For HTTP, the attacker might run javascript code or similar things in the victims browser to make it reload a certain URL. You don’t need an exploit for the browser to do this.

SSL/TLS broken again – A weakness in the RC4 stream cipher

A few days ago, a new attack against SSL/TLS has been published by Nadhem AlFardan, Dan Bernstein, Kenny Paterson, Bertram Poettering and Jacob Schuldt. Many attacks on SSL/TLS in the past relied on the protocol design itself, broken implementations leaking side channels, or the X.509 certificate system and improper issuing of certificates. In contrast to these, the new attack is focused on the RC4 stream cipher, that can be used against SSL/TLS.

RC4 usage in SSL/TLS

SSL/TLS doesn’t rely on a single cryptographic primitive. Instead when a new SSL/TLS session is established, both parties negotiate a ciphersuite. A ciphersuite consists of a key exchange algorithm, an encryption method and an integrity protection method. To agree on a common ciphersuite, the SSL/TLS client sends a list of all ciphersuites it supports, and the server choses one of the methods from the list, usually the first one which is also supported by the server. The RC4 stream cipher is one of the possible choices for the encryption method, and also wildly supported and used. Compared to many other encryption methods, RC4 is very fast in software, very easy to implement, and also very efficient because it doesn’t require any padding as many encryption methods based on block ciphers do.

What makes RC4 a bad stream cipher

So what would be expect from a (secure) stream cipher? A stream cipher should take a key, and transform this key into a (pseudo-) random sequence of bytes, chosen from a uniform distribution. However, RC4 has also serious weaknesses. For example, RC4 is also used in the famous WEP protocol, to protect WiFi networks. Here, many similar RC4 keys are used that differ only in the first 3 bytes. These 3 bytes of the key are used as an initialization vector for the cipher, and are transmitted in clear with the encrypted packet. The remaining bytes of the key are shared among all packets, but should only be known to the operator of the network. What has been shown for WEP is, that an attacker, who knows the first 3 bytes of the key, and also has access to some bytes of the key stream can predict the next bytes of the key. The probability for predicting a single byte correct is only slightly above a random guess, but repeating the procedure for many packets (like 10.000) will reveal the secret key that protects the network with almost 100% probability. In a nutshell, if you know a part of an RC4 key, and some bytes of the keystream, then you can predict parts of the remaining key better than just guessing.

How can this be applied to SSL/TLS

But for SSL/TLS, the situation is entirely different. Here, a new key is chosen almost randomly for every new connection and no parts of the key are shown to an attacker. As a result, the methods used to attack RC4 in WEP cannot be applied for SSL/TLS connections. But there are more problems in RC4:  Even if a key for RC4 is randomly chosen, the keystream bytes of RC4 have some biases. For example the second by of output will be 0 with a probability of more than 1/256. What recently has been discovered is, that much more of such biases exist. If a plaintext is transmitted over SSL/TLS, one gets a small hint about the plaintext. If the same plaintext is transmitted over and over again using TLS with RC4 encryption, one can recover the first bytes of the plaintext using these biases in the keystream generated by RC4. No assumptions about other plaintext or keystream bytes must be made, and no knowledge of parts of the key is required. Also this works independant of the implementation used, because it doesn’t require any timings or similar side channels from the implementation.

Consequences of the attack

The number of sessions required depends on how much is known about the plaintext, and what should be recovered. A full plaintext can be recovered using 2^{30} sessions. If only a single byte needs to be recovered, about 2^{24} sessions might be sufficient, depending on the position of the byte in the plaintext. If one only wants to distinguish two plaintexts only, then less than 2^{24} sessions might be enough.


One may ask now how to counter the attack. One of the best solutions would be to just simply disable RC4 support on a client or on a server. As long as a client and a server still share another common ciphersuite, they will still operate properly. Alternatively, both the server and the client can be patched to send empty application layer records, until the first 256 or 512 bytes of output of RC4 have been used. Currently, the attack works best with the first bytes of output of RC4, but less well with the following bytes.

Final remarks

I have also previously been active in research on RC4 and WEP attacks.

Secure Function Evaluation – There is an issue with OTR and plausible denability

OTR is a crypto overlay protocol for instant messaging. Instead of encrypting the connection to an instant messaging service like Gtalk, MSN, Skype or ICQ, OTR encrypts messages send over an arbitrary instant messaging service end-to-end. The message leaves your messaging client encrypted, and is later decrypted by the receivers client. Only the communicating clients are in possession of the keys necessary to decrypt the message, and the instant messaging service cannot read the message in clear.

Plausible deniability

Between other very nice properties, the OTR protocol also offers Plausible Deniability as well as Authenticity. This means, that when Alice and Bob are chatting, Alice and Bob can be sure that the messages they receive have really been send by their chat partners, and have not been altered. On the other hand, both Alice and Bob cannot prove to a third party, that any of their chat partner send a message with a specific content.

There is a trivial attack on these kinds of protocols. Assume that Alice chats with Bob, and Bob always asks Alice, to help him rob a bank. Now Alice would like to prove to a judge, that Bob really asks her to rob a Bank. A trivial way of doing this is handing over all keys of Alice to the Judge, so that the Judge can impersonate Alice and say Hi to Bob. Because Bob thinks, he is talking to Alice, he asks her again to rob a bank.

However, Alice might not be willing to hand over her keys to a Judge. Recently, greg found out, that there is a way how Alice can prove to a Judge, that Bob told her to rob a bank, without handing over her private keys. His approach uses Secure Function Evaluation: The concept of secure function evaluation is known for some time now: Assume that you have a function or an arbitrary computer program, that processes two inputs a and b, and generates an output c. Then two parties can jointly compute that function, each providing one of the inputs. The other party doesn’t learn anything about the inputs, and both parties get the output c.

Breaking plausible deniability

Effectively, Alice and the Judge can now jointly compute Alice sides of the protocol. Alice provides as input her private key, and the Judge provides all other inputs, including the messages send by Bob. The output of the jointly computed function can be either the short term communication keys, which Alice and Bob are using in the conversation, or the decrypted protocol messages send by Bob. In fact, this very generic approach can still be optimized exploiting some properties that are specific to the OTR protocol and the DH key exchange used in OTR.


I assume that it will be very hard to counter this type of attack, because secure function evaluation is a very generic method, that is not bound to any specific properties of OTR.

However, please keep in mind, that this attack is only possible while Bob is still chatting with Alice. As soon as the communication is over, Alice cannot decide to go evil afterwards. Also, while Alice is able to prove the authenticity of the messages send by Bob to the Judge, the Judge cannot prove the authenticity of these messages to another party like a jury.

Sovereign Keys – A proposal for fixing attacks on CAs and DNSSEC

The EFF presented their proposal how to improve the security of SSL/TLS and the internet PKI infrastructure. To understand their proposal, one needs to understand how PKI in the internet works today:

Public Key Cryptography

In cryptography, you can usually encrypt and decrypt  data. In the past, encryption and decryption used the same key. Starting from the 70s, a new class of encryption/decryption algorithms was invented, the public key encryption algorithm. Instead of using the same key for en- and decryption, these algorithms use different keys for en- and decryption. During key generation, two keys are generated: A public key, that is used to encrypt data, and can be given out to everybody in the word, and a corresponding secret key, that must be kept hidden by the owner. Everybody who has access to the public key can encrypt data, but only the owner of the secret key is able to decrypt it.

There are many algorithms, for example RSA and ElGamal are the most famous public key encryption algorithms, while other algorithms like McEliece and Rabin are less well known.

Besides encryption, there are also digital signature algorithms. Again, a public and a private key is generated. The private key can be used to generate a digital signature on a document. The public key can then be used to verify the signature on the document. A signature on a document should guarantee that the document was really signed by the holder of the private key, and was not altered afterwards.


These ideas sound simple at the first look, but in practice, getting a public key of a person or company is not that easy. Just publishing your public key in some kind of web forum or on your facebook page is not enough. Everybody would be able to create a facebook page for another person, and then posting a fake public key on that page, or under that persons name on a web forum. So we need a way to establish a binding between a public key and a person or identity (a company name, a domain name or an email address). One solution would be to meet everybody in person who you want to communicate with, but it doesn’t scale well, and not everybody wants to fly to San Jose, California, just to get the public key for

For these job, Public Key Infrastructue (PKI) and X.509 Certificates have been invented. A Certification Authority (CA) is an organization, that verifies the identity of a person, and that this person is in possession of a private key. After this has been confirmed, the CA issues a X.509 certificate. That certificate contains the corresponding public key of that person, and it’s identity, and this information is signed using the CAs private key. Everybody who thinks that this CA does a good job in verifying the identity of persons, and is in possession of that CAs public key can verify that signature. As from now on, one only needs to trust a CA. One can simply give away the certificate issued by a CA, and everybody can get the public key from the certificate, and verify that it really belongs to that person, by verifying the signature of the CA. Today, there are hundreds of CAs active on the internet, and every web browser comes with a pre-installed list of trustworthy CAs and their public keys.


To encrypt HTTP traffic and to prove the autenticity of a website, the SSL/TLS protocol was created. When a session to a web server is established, the web server usually provides a digital certificate containing the public key of that web server. The web browser verifies the signature on that certificate, and that the identity in that certificate matches with the servers name it want’s to connect to. If everything is fine, the public key in that certificate is used to establish a secure session with that web server using some kind of key derivation scheme. (I won’t go into detail here)

The current state of PKI in the internet

At the first look, this sounds like a perfect solution. Whenever I want to talk privately with paypal, I just point my web browser to, it automatically connects to the server, gets a certificate, verifies that is has been correctly signed by a trustworthy CA, and the identidy in the certificate matches the expected servers hostname.

Attack vectors

However, there are multiple problems with that system. Just to mention one example: There are hundred of CAs active in the internet, and your web browser trusts every single one of them. Every CA is allowed to issue a certificate for every domain name in the internet. For example the national Chinese stat CA is allowed to issue a certificate for, which is the website of the ministry of defense of the united states of america. Also, the verification done by most CAs is minimal. For many CAs, it is sufficient if you can receive a mail for hostmaster@domain.tld, to get a certificate for domain.tld. There are multiple ways how you can attack this:


First of all, you may find a bug in the CAs website or email server, that allows you to get access to the certificate issuing software, bypassing these checks.


Also, you might be able to attack a DNS server serving the zone-file for domain.tld, that allows you to reroute mail for hostmaster@domain.tld on the DNS level. This allows you to get a certificate for domain.tld too.


Routers, especially those using BGB or a similar protocol might be tricked into rerouting the traffic for the mail server of domain.tld to your network. This way, you can intercept the mail and get your certificate too.


Besides that, weak cryptography  algorithms like MD5 have been used by some CAs, and this has been used to generate a rouge certificate too.

The EFF solution

To improve the security of PKI, the EFF has presented a proposal: Sovereign Keys

Sovereign Keys should make it harder for an attacker to generate a new certificate for an HTTPS website, without the cooperation of the legitimate site operator. The main building block of Sovereign Keys are so called timeline servers. These timeline servers are append-only databases, meaning that one can only add entries to the database, but never modify or delete them. These timeline servers could be operated by different entities like the EFF itself, or Mozilla, Google or Microsoft.

To use Sovereign Keys, the side administrator obtains an X.509 certificate as usual. Then he generates a new key, the so called sovereign key. He uploads the key with the certificate to a timeline server. The server operator checks, if that certificate is really issued by a valid CA and no other sovereign key has been added previously, and adds the sovereign key with the hostname of the certificate to the database.

When a client connects to the website, he also requests all database entries belonging to that hostname from a timeline server. In parallel to that, a SSL/TLS connection is established. The Server delivers the server certificate to the client, with an additional signature created with the sovereign key. The client can then check, if this signature can be verified with the sovereign key retrieved from the timeline server.

More details

The full protocol is a little bit more complex, because it needs to deal with revocation, privacy, mirroring and load balancing the timeline servers and many more things. It has not yet been finalized, but a draft of the protocol can be downloaded from:;a=blob_plain;f=sovereign-key-design.txt;hb=master


For me, this looks like one of two solutions you need to improve the general security of SSL/TLS. Sovereign keys is a great solution for website operators that care about the security of their users. It will not help a user, if the website he connects to does not use it. For these cases, a different solution should be used, like checking if multiple computers in the internet get the same certificate from the server.