A computer system , comprising: at least one processor; and a memory storing computer-executable instructions that, when executed by the at least one processor , perform a method , comprising: receiving a key for encrypting an additional authentication challenge; receiving (202; 306) one or more credentials to authenticate an application programming interface request received from a second computer system based on a first level of authentication; comparing identifying information known about the second computer system to information associated with the application programming interface request; in response to detecting one or more changes associated with the identity or behavior of the second computer system based on the comparison, generating the additional authentication challenge; encrypting the additional authentication challenge using the key; issuing the encrypted additional authentication challenge to the second computer system to further authenticate the application programming interface request; and processing the application programming interface request based on a result of the encrypted additional authentication challenge. at least one processor; and a memory storing computer-executable instructions that, when executed by the at least one processor , perform a method , comprising: receiving a key for encrypting an additional authentication challenge; receiving (202; 306) one or more credentials to authenticate an application programming interface request received from a second computer system based on a first level of authentication; comparing identifying information known about the second computer system to information associated with the application programming interface request; in response to detecting one or more changes associated with the identity or behavior of the second computer system based on the comparison, generating the additional authentication challenge; encrypting the additional authentication challenge using the key; issuing the encrypted additional authentication challenge to the second computer system to further authenticate the application programming interface request; and processing the application programming interface request based on a result of the encrypted additional authentication challenge. receiving a key for encrypting an additional authentication challenge; receiving (202; 306) one or more credentials to authenticate an application programming interface request received from a second computer system based on a first level of authentication; comparing identifying information known about the second computer system to information associated with the application programming interface request; in response to detecting one or more changes associated with the identity or behavior of the second computer system based on the comparison, generating the additional authentication challenge; encrypting the additional authentication challenge using the key; issuing the encrypted additional authentication challenge to the second computer system to further authenticate the application programming interface request; and processing the application programming interface request based on a result of the encrypted additional authentication challenge. generating the additional authentication challenge; encrypting the additional authentication challenge using the key; issuing the encrypted additional authentication challenge to the second computer system to further authenticate the application programming interface request; and processing the application programming interface request based on a result of the encrypted additional authentication challenge. <EOS>