1. An apparatus comprising: a key input circuit configured to receive a first cryptographic key; and a reverse key expansion circuit configured to generate a second cryptographic key based on the first cryptographic key, wherein each of the first cryptographic key and the second cryptographic key comprises a plurality of words, and wherein the reverse key expansion circuit comprises a first word circuit configured to generate the first word of the second cryptographic key based on i) the first word of the first cryptographic key and ii) the last two words of the first cryptographic key, and a remaining word circuit configured to generate the remaining words of the second cryptographic key, the remaining word circuit comprising at least one word circuit configured to generate a word of the second cryptographic key based on the corresponding word, and the immediately previous word, of the first cryptographic key, wherein the at least one word circuit comprises a first exclusive-OR circuit configured to generate an exclusive-OR of the corresponding word, and the immediately previous word, of the first cryptographic key.