Example - 1

Suppose memory access patterns for an encryption routine are dependent on the secret key.

An attacker can recover the key by knowing if specific memory locations have been accessed or not. The value stored at those memory locations is irrelevant. The encryption routine's memory accesses will affect the state of the processor cache. If cache resources are shared across contexts, after the encryption routine completes, an attacker in different execution context can discover which memory locations the routine accessed by measuring the time it takes for their own memory accesses to complete.

Example - 2

The following code checks validity of the supplied username and password and notifies the user of a successful or failed login.

print "Login Successful";
print "Login Failed - incorrect password";if (IsValidPassword($username, $password) == 1){}else{}
print "Login Failed - unknown username";my $username=param('username');my $password=param('password');if (IsValidUsername($username) == 1){}else{}

In the above code, there are different messages for when an incorrect username is supplied, versus when the username is correct but the password is wrong. This difference enables a potential attacker to understand the state of the login function, and could allow an attacker to discover a valid username by trying different values until the incorrect password message is returned. In essence, this makes it easier for an attacker to obtain half of the necessary authentication credentials.

While this type of information may be helpful to a user, it is also useful to a potential attacker. In the above example, the message for both failed cases should be the same, such as:

"Login Failed - incorrect username or password"

Example - 3

Non-uniform processing time causes timing channel.

Suppose an algorithm for implementing an encryption routine works fine per se, but the time taken to output the result of the encryption routine depends on a relationship between the input plaintext and the key (e.g., suppose, if the plaintext is similar to the key, it would run very fast).

In the example above, an attacker may vary the inputs, then observe differences between processing times (since different plaintexts take different time). This could be used to infer information about the key.

Artificial delays may be added to ensured all calculations take equal time to execute.