October 7, 2003Serguei A. Mokhov, [email protected] 1 Cryptographic Protocols and Possible...

October 7, 2003 Serguei A. Mokhov, [email protected]

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Cryptographic Protocols andPossible Attacks

SOEN321- Information-Systems Security

Revision: 1.1

Date: November 25, 2004


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Contents

• Security Flaws in Cryptographic Protocols– Freshness Flaws– Oracle Flaws– Type Flaws– Implementation-Dependent Flaws– Elementary Flaws– Others


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Security Flaws

• A flaw is a protocol property that contradicts the security requirements.

• A security flaw is a part of a program that can cause the system to violate its security requirements.

• Finding security flaws, then, demands some knowledge of the system security requirements. These requirements vary according to the system and the application [Landweher, Bull, McDermott and Choi].

• The proof of a flaw is commonly known as an “attack” and it is generally presented as actions performed on the protocol.


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Flow Types

• Freshness

• Oracle

• Type

• Implementation-Dependent

• Others…


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Freshness Flaws

• Freshness flaws appear when critical messages are used in the protocol without including freshness information such as nonces and/or timestamps.

• This lack can be exploited by an intruder to do a masquerade by replaying messages belonging to previous runs.


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Freshness Flaws

• A classical example of a freshness flaw occurs in the symmetric-key protocol proposed by Needham-Shroeder:– Message 1 A -> S : A,B,Na

– Message 2 S -> A : {Na,B, kab, {kab,A}kbs }kas

– Message 3 A -> B : {kab,A}kbs

– Message 4 B -> A : {Nb}kab

– Message 5 A -> B : {Nb + 1}kab


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Freshness Flaws (2)

• This protocol aims to provide a mutual authentication between two principals A and B.

• Each principal shares a secret key with a trusted server S.

• This protocol was thought to be correct until 1981 when the basic weakness was pointed out by Denning and Sacco.

• The main problem of this protocol is that the principal playing the role B cannot detect whether the message {kab,A} sent by the principal playing the role A at step 3 has been recently created or not since it does not contain any freshness information.


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Freshness Flaws (3)

• Suppose, for example, that an intruder can compromise one previously distributed key k’ab (by using cryptanalysis for example) and it replays the appropriate message to the principal playing the role B in step 3.

• In this case, the principal playing the role B will accept this key as a new one and it replays by the message {Nb}k’ab

• Hence, the intruder can intercept this message and impersonate A’s reply by sending the message {Nb + 1}k’ab


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Freshness Flaws (4)

• To fix this weakness, Denning and Sacco have proposed to add a timestamp to the messages used at step 2 and step 3:– Message 1 A -> S : A,B,Na

– Message 2 S -> A : {T,Na,B, kab, {kab,A, T}kbs}kas

– Message 3 A -> B : {kab,A, T}kbs

– Message 4 B -> A : {Nb}kab


• Needham and Shroeder have proposed a solution based on the use of nonces. The two proposed solutions seem to resolve the problem, however there is no correction proof for any one of those new versions.


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Oracle Flaws

• Oracle flaws occur when the cryptographic protocol dialog allows an adversary to know some secret information or to foretell the content of some encrypted messages.

• Two subclasses of oracle flaws are distinguished:– Single oracle flaws and,– Multi-role oracle flaws.


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Single Oracle Flaws

• It consists of oracle flaws that occur when the protocol does not allow principals to change their roles from one protocol run to another.

• The most famous example of a single role oracle flaw was given by Rivest, Shamir, and Adelman. It consists of the following three-steps protocol:– Message 1 A -> B : {M}ka

– Message 2 B -> A : {{M}ka}kb

– Message 3 A -> B : {M}kb

• We assume that the encrypting function is commutative i.e. {{M}ka}kb ={{M}kb}ka


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Single Oracle Flaws (2)

• The goal of this protocol is to transfer secret messages from one principal to another without the help of a trusted server.

• In step one, the principal playing the role A encrypts the messages M under its secret key ka (can be randomly generated) then sends the result to the principal playing the role B.

• In the second step, the principal playing the role B encrypts the received message with its secret key kb and sends the result to the principal playing the role A.

• Finally, the principal playing the role A decrypts the message {{M}ka}kb to obtain the message {M}kb (this can be achieved under the commutative assumption) which is sent to the principal playing the role B.


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Single Oracle Flaws (3)

• This protocol can be attacked as follows:– Message 1 A -> I(B) : {M}ka

– Message 2 I(B) -> A : {M}ka

– Message 3 A -> I(B) : M

• At step one, the intruder intercepts the message {M}ka which is supposed to be sent to the principal playing the role B.

• At step two, the intruder sends the intercepted message to the principal playing the role A as a B’s response.

• Finally, the principal playing the role A decrypts the received message and sends the result (M) to the principal playing the role B.

• However, the intruder intercepts this message; hence, it learns the information that was supposed to be secret.


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Multi-Role Oracle Flaws

• Multi-role oracle flaws occur when the protocol assumptions allow principals to change their role from one run to another.

• In this case, an intruder has more chance to attack the protocol.

• In fact, the intruder can participate in many runs executed concurrently; hence, messages of one run can be used to form messages that will be used in another run.


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Multi-Role Oracle Flaws (2)

• A good example of multi-roles oracle flaws is:– Message 1 A -> B : {Na}kab

– Message 2 B -> A : {Na + 1}kab

• The objective of this protocol is to convince the principal playing role A that the principal playing role B is operational.


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Multi-Role Oracle Flaws (3)• At step one, the principal playing role A sends a challenge, the

nonce Na encrypted using the key kab.• The principal playing role B can easily give a response ({Na

+1}kb) to this challenge at step two since it knows the key kab.

• This protocol can be attacked as follows:– Message 1.1 A -> I(B) : {Na}kab

– Message 2.1 I(B) -> A : {Na}kab

– Message 2.2 A -> I(B) : {Na + 1}kab

– Message 1.2 I(B) -> A : {Na + 1}kab

• At step one of the first protocol run, the intruder intercepts the message {Na}kab and uses it as its own challenge in the first step of the second protocol run.


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Multi-Role Oracle Flaws (4)

• Therefore, it is not surprising that the principal playing the role A will answer by sending the message {Na + 1}kab in step two of the second protocol run.

• Furthermore, this message is also the necessary one to finish the first run.

• Finally, the principal playing the role A is convinced that the principal playing the role B is operational, however this principal may not exist any longer in the system.


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Type Flaws

• The extraction of message components requires a full knowledge about their types.

• In fact, a message is implemented in a concrete level as a sequence of bits, then to extract the value of the first component, for example, we need its type (length).

• Such information can be implicit if the receiver has a previous knowledge about the message’s components, their types and their positions.

• Another solution is to represent types explicitly in the transmitted data structure.

• In this case, the receiver does not need to know previously the types since it will find them embedded within the received message.


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Type Flaws (2)

• Type flaws occur when an adversary can induce the receiver to infer message component types which are different from their real one.

• The Andrew Secure RPC (From Andrew File System) Protocol, presented below, provides a good example for this class of flaws.– Message 1 A -> B : A, {Na}kab

– Message 2 B -> A : {Na + 1,Nb}kab


– Message 4 B -> A : {k’ab,N’b}kab


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Type Flaws (3)

• In step one, the principal playing the role A sends its identity and a challenge {Na}kab to indicate to the principal playing the role B that it wishes to communicate with it.

• At the second step, the principal playing the role B sends the message {Na + 1,Nb}kab which is a challenge to the principal playing the role A.

• At step three, the principal playing the role A replies to the challenge of the principal playing the role B by sending the message {Nb + 1}kab.

• At the last step, the principal playing the role B creates a session key k’ab, concatenates it with N’b, an identifier for a future communication, encrypts the result with the key kab and sends it to principal playing the role A.


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Type Flaws (4)

• Suppose that nonces and keys have the same length (x bits).

• This protocol can be attacked as follows:– an intruder I can intercept the message

{Na+1,Nb}kab sent at the second step and send it in step four as B’s reply.

– In this case, the principal playing the role A will consider the value of Na + 1 as the value of the key kab.


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Type Flaws (5)

• The complete attack is:– Message 1 A -> B : A, {Na}kab

– Message 2 B -> A : {Na + 1,Nb}kab


– Message 4 I(B) -> A : {Na + 1,Nb}kab


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Binding Flaws

• In public key cryptography, it would be catastrophic if a principal misjudges the key of another.

• In fact, a public key is used to send secret information, since only the principal having the appropriate private key can decrypt the encrypted message.

• However, if, for example, an intruder I having a public key ki can convince a principal A that B’s public key is ki, then the intruder can read all secret messages (encrypted by ki) coming from A and going to B.

• To avoid such a flaw, a veritable binding between agents and public keys must be established.


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Binding Flaws (2)

• In general, with a distributed systems, a trusted server takes in charge the key distribution task.

• Each principal uses an authentication protocol to get public keys of other principals from the server.

• However, if the authentication protocol is not carefully designed, binding flaws can take place.


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Binding Flaws (3)

• A good illustrative example of this class of flaws is given hereafter:– Message 1 A -> S : A,B,Na

– Message 2 S -> A : S, {S,A,Na, kb}ks-1

• Here, the principal playing the role A wishes to know the public key of the principal playing the role B with the help of the trusted server S.

• At step one, the principal playing the role A sends its identity, the identity of the principal playing the role B and a nonce Na to the server S.

• In step two, the server replies by a message containing its identity, A’s identity, the nonce Na (to ensure the freshness of the message) and the public key of the principal playing the role B.

• All these components are concatenated and encrypted under S’s private key (signature) allowing the principal playing the role A to be sure about the origin of the message.


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Binding Flaws (4)

• As shown by Hwang and Chen, this protocol can be attacked as follows:– Message 1.1 A -> I(S) : A,B,Na

– Message 2.1 I(A) -> S : A, I,Na

– Message 2.2 S -> I(A) : S, {S,A,Na, ki}ks-1

– Message 1.2 I(S) -> A : S, {S,A,Na, ki}ks-1

• At step one of the first protocol run, the intruder I intercepts the message “A,B,Na”, substitutes the identity of B by its identity and sends the result as the first message of a new run of the protocol (Message 2.1).

• At step 2.2, the server replies by a message containing I’s public key, since it thinks that the principal playing the role A is asking for this public key.

• Finally, the intruder replays S’s message to the principal playing the role A. Thus, a binding flaw occurs, since the principal playing the role A thinks that the public key of the principal playing the role B is ki.


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Binding Flaws (5)

• To avoid this flaw, Hwang and Chen proposed the following modification:– Message 1 A -> S : A,B,Na

– Message 2 S -> A : S, {S,A,Na,B, kb}ks-

1


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Repudiation Flaws• We say that a cryptographic protocol contains a

repudiation flaw if at least one principal is able to deny its participation in any run of this protocol.

• A popular example of this category of flaws was given by the coin-flip protocol proposed by Toussaint.

• This protocol can be used by two principals to toss a coin over a “phone” as follows:– B sends his choice of Heads or Tails to A.– A

• chooses a key ka.• sends the message {ka, Heads}ka , {ka, Tails}ka to B.

– B chooses arbitrary one of {ka, Heads}ka and {ka, Tails}ka and sends his choice, say X, to A.


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Repudiation Flaws (2)– A decrypts X, compares the result with B’s initial choice and sends

the key ka to B.– B decrypts X and compares the result with his initial choice.

• The probability that the principal A wins is equal to B’s one (1/2) as is shown by Toussaint.

• However, in this protocol, the result of the game is known by A before B.

• Then, if the principal A discovers that he has lost, he can abort the protocol at step four and never reveal the key ka to B at the last step.

• In other terms, the principal A can deny his participation in this protocol run and a repudiation flaw occurs.


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Implementation-Dependent Flaws

• Cryptosystems used within cryptographic protocols are supposed to be perfect, modulo a set of properties containing at least integrity and confidentiality.

• However, some examples show that these conditions are not sufficient for some protocols, because their security can be severely affected by the implementation approach adopted for cryptographic functions.

• The interaction between cryptosystems and cryptographic protocols did not have the chance to be deeply studied and it is still an open area of research.

• However, it is clear that speaking about the security of a protocol combination with respect to a specific cryptosystem is better then speaking about the security of a protocol in absolute.


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Implementation-Dependent Flaws (2)

• To be convinced by the severity of this problem let us see the example proposed by Massey as shown below:– Message 1 A -> B : {M}ka

– Message 2 B -> A : {{M}ka}kb

– Message 3 A -> B : {M}kb

• Suppose that we use the XOR function to cipher messages.• Hence, if k is a key and M is a message, encrypting M under

k turns to do the simple following operation: {M}k = M k.

• Since k k = 0 (0 0 = 0 and 1 1 = 0 ), the deciphering transformation is performed by using the same operation: {{M}k}k = M k k = M.


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Implementation-Dependent Flaws (3)

• The intent of this protocol is to transmit a secret message M from a principal playing the role A to a principal playing the role B.

• However, if we compute the XOR of the three messages used in this protocol:– ({M}ka {{M}ka}kb {M}kb),

– then the result is M (the message which is supposed to be secret).


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Other Flaws

• Elementary Flaws:– Some cryptographic protocols provide only a marginal

protection against an adversary. In general, this category of protocols is breakable with a little effort.

– A little protection or a non-protection of a protocol leads in almost all the cases to so-called elementary flaws.

– A simple example of these flaws can be given by the following protocol:

• Message 1 A -> B : {Na, kab}ka-1

• Message 2 B -> A : {Na}kab


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Other Flaws (2)

• Password Guessing Flaws:– Password guessing flaws occur if it is easy to

an adversary to guess some secret key.– An intruder can do an exhaustive search in a

word space smaller than the whole key space to look for keys that are not randomly selected.

– This category of flaws is independent from the protocol design but it is related to cryptographic techniques used to generate keys.


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Other Flaws (3)

• Calculi Flaws:– Normally, after receiving a message, the

receiver does some verification in order to know if this received message is the good expected one or not.

– However, if these computations are not completed or they are not correctly done, then a calculi flaw could arise.


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References

• Dr. Mourad Debbabi• http://www.ciise.concordia.ca/~debbabi/inse7100.html

October 7, 2003Serguei A. Mokhov, [email protected] 1 Cryptographic Protocols and Possible...

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