Covert timing channels convey information by modulating some aspect of system behavior over time, so that the program receiving the information can observe system behavior and infer protected information.
In some instances, knowing when data is transmitted between parties can provide a malicious user with privileged information. Also, externally monitoring the timing of operations can potentially reveal sensitive data. For example, a cryptographic operation can expose its internal state if the time it takes to perform the operation varies, based on the state.
Covert channels are frequently classified as either storage or timing channels. Some examples of covert timing channels are the system's paging rate, the time a certain transaction requires to execute, and the time it takes to gain access to a shared bus.
Example Language: Python
def validate_password(actual_pw, typed_pw):
if len(actual_pw) <> len(typed_pw):
for i in len(actual_pw):
if actual_pw[i] <> typed_pw[i]:
In this example, the attacker can observe how long an authentication takes when the user types in the correct password. When the attacker tries his own values, he can first try strings of various length. When he finds a string of the right length, the computation will take a bit longer because the for loop will run at least once. Additionally, with this code, the attacker can possibly learn one character of the password at a time, because when he guesses the first character right, the computation will take longer than when he guesses wrong. Such an attack can break even the most sophisticated password with a few hundred guesses. Note that, in this example, the actual password must be handled in constant time, as far as the attacker is concerned, even if the actual password is of an unusual length. This is one reason why it is good to use an algorithm that, among other things, stores a seeded cryptographic one-way hash of the password, then compare the hashes, which will always be of the same length.
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