Modular multiplication is the most dominant arithmetic operation in elliptic curve cryptography (ECC), that is a type of public-key cryptography. Montgomery multiplier is commonly used to compute the modular multiplications and requires scalability because the bit length of operands varies depending on its security level. In addition, ECC is performed in
GF(
P) or
GF(2
n), and unified architecture for multipliers in
GF(
P) and
GF(2
n) is required. However, in previous works, changing frequency is necessary to deal with delay-time difference between
GF(
P) and
GF(2
n) multipliers because the critical path of the
GF(
P) multiplier is longer. This paper proposes unified dual-radix architecture for scalable Montgomery multiplications in
GF(
P) and
GF(2
n). This proposed architecture unifies four parallel radix-2
16 multipliers in
GF(
P) and a radix-2
64 multiplier in
GF(2
n) into a single unit. Applying lower radix to
GF(
P) multiplier shortens its critical path and makes it possible to compute the operands in the two fields using the same multiplier at the same frequency so that clock dividers to deal with the delay-time difference are not required. Moreover, parallel architecture in
GF(
P) reduces the clock cycles increased by dual-radix approach. Consequently, the proposed architecture achieves to compute a
GF(
P) 256-bit Montgomery multiplication in 0.28μs. The implementation result shows that the area of the proposal is almost the same as that of previous works: 39kgates.
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