Adversaries may reduce the level of effort required to decrypt data transmitted over the network by reducing the cipher strength of encrypted communications.(Citation: Cisco Synful Knock Evolution) Adversaries can weaken the encryption software on a compromised network device by reducing the key size used by the software to convert plaintext to ciphertext (e.g., from hundreds or thousands of bytes to just a couple of bytes). As a result, adversaries dramatically reduce the amount of effort needed to decrypt the protected information without the key. Adversaries may modify the key size used and other encryption parameters using specialized commands in a Network Device CLI introduced to the system through Modify System Image to change the configuration of the device. (Citation: Cisco Blog Legacy Device Attacks)
Defenders may observe attempts to alter cryptographic settings on network devices that reduce key strength or allowable cipher suites. Suspicious indicators include configuration changes that downgrade encryption algorithms, key length parameters, or the disabling of strong encryption in favor of legacy ciphers. These activities often appear as CLI commands modifying crypto policies, firmware changes affecting crypto libraries, or unexpected updates to key management files. Correlation across device config logs and traffic analysis showing weaker ciphers provides higher confidence of malicious key space reduction.