Have you looked at BearSSL? It has constant time AES[1]-GCM[2] optimized for ARM Cortex M0 (32bit microprocessors that go into USB cables: http://www.cypress.co

Have you looked at BearSSL? It has constant time AES[1]-GCM[2] optimized for ARM Cortex M0 (32bit microprocessors that go into USB cables: http://www.cypress.com/products/ez-pd-ccg2-type-c-port-controller http://www.cypress.com/products/ez-pd-ccg2-type-c-port-contr...) 1 - https://bearssl.org/speed.html#symmetric-encryption https://bearssl.org/speed.html#symmetric-encryption 2 - https://bearssl.org/speed.html#mac https://bearssl.org/speed.html#mac