which of the following?

In SPA attacks, an attacker directly observes a system’s power consumption. The amount of power consumed varies depending on the microprocessor instruction performed. Large features such as DES rounds, RSA operations, etc. may be identified, since the operations
performed by the microprocessor vary significantly during different parts of these operations. At higher magnification, individual instructions can be differentiated. SPA analysis can, for example, be used to break RSA implementations by revealing differences between
multiplication and squaring operations. Similarly, many DES implementations have visible differences within permutations and shifts (e.g., the PC1 permutation or rotates of the C and D registers), and can thus be broken using SPA. While Cryptography Research found many smartcards to be vulnerable to SPA analysis, it is not particularly difficult to build SPA-resistant devices.

Source: https://cryptome.org/jya/dpa.htm