This paper considers the use of electromagnetic pulses (EMP) to inject transient faults into the calculations of a hardware and a software AES. A pulse generator and a 500μm- diameter magnetic coil were used to inject the localized EMP disturbances without any physical contact with the target. EMP injections were performed against a software AES running on a CPU , and a hardware AES (with and without countermeasure) embedded in a FPGA. The purpose of this work was twofold: (a) reporting actual faults injection induced by EMPs in our targets and describing their main properties; (b) explaining the coupling mechanism between the antenna used to produce the EMP and the targeted circuit, which causes the faults. The obtained results revealed a localized effect of the EMP since the injected faults were found dependent on the spatial position of the antenna on top of the circuit's surface. The assumption that EMP faults are related to the violation of the target's timing constraints was also studied and ascertained thanks to the use of a countermeasure based on monitoring such timing violations.