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Modular exponentiation is a cornerstone operation to several public-key cryptosystems. It is performed using successive modular multiplications. This operation is time consuming for large operands, which is always the case in cryptography. For software or hardware fast cryptosystems, one needs thus to reduce the total number of modular multiplication required. Existing methods attempt to reduce this number by partitioning the exponent in constant or variable size windows. In this paper, we propose a fast and compact reconfigurable hardware for computing modular exponentiation using the m-ary method. The cryptographic hardware is low-cost and concise and therefore can be embedded in almost all electronic devices that use encrypted data.