Software can do a number of things to thwart power analysis. Fundamentally, power analysis looks at power consumption to extract information about operations carried out inside the device. To make sense of the data it needs to know the correlation between device power consumption and the algorithm steps carried out by the device.
- make time matching of operations harder
- randomise order of S-box application in each round
- shuffle in dummy computations on additional unused values
- reduce leaked information from each operation by masking values during computations
Here's a good intro paper to look at for algorithm modifications
Less well known are changes to the mode of operation of cryptographic components. Power analysis on a given implementation may require hundreds to thousands of power traces on unique inputs. An attacker feeds plaintexts or ciphertexts of their choice into a keyed cryptographic function. Limiting the number of times a key can be used, or the range of inputs it will accept can limit the attacker's ability to extract information about a key.
As an example, when performing a key derivation using an attacker controlled Nonce from a master key. Instead of doing k=AES_enc(Nonce,K_master) which allows the attacker 2^128 queries of the key, the nonce can be broken into n bit parts and each part mixed with the master key in a single AES operation. Similarly, high level constructions that rotate keys frequently cost little but avoid giving the attacker time to extract information from any one key.
There's an app note from Microchip about that somewhere.
Finally, consider rate limiting. Make acquiring each power trace take longer. As an example, if a bank card is being used to perform a transaction requiring PIN entry, a software timer to delay cryptographic operations until 5-10 seconds after power on of the card might be acceptable.
Another option for rate limiting for online protocols is to use asymmetric cryptography to do an initial challenge response protocol before the device allows a sensitive key to be used. As an example, the device could store the root of a Merkle tree of 2^N hashed answer values. It randomly generates a challenge 0 ≤ challenge < 2^N and expects to receive the corresponding answer value and intermediate leaf hashes that can be used to reach the stored root hash. That's a simple bit of public key cryptography that only requires symmetric primitives. An attacker who has captured k challenge response pairs has their attack slowed by 2^N/k unless they're performing an online attack.
