Previous first-order Differential Power Analysis (DPA) attacks have depended on knowledge of the target algorithm's input or output. This paper describes a first-order DPA attack against the Advanced Encryption Standard (AES) in counter mode, in which the initial counter, input values, and output values are all unknown. The attack proceeds as follows. Suppose the input data to an algorithm is unknown, but can be expressed as single secret constant summed with known, variable data. The known, variable part of the data is used to mount a DPA attack, and the secret constant is treated as part of the key to be recovered.