What does “ECC Prime and Binary Field keys up to 640 bits” mean?

The elliptic-curve operations defined in normal mathematics use real numbers. Operations over real numbers are slow and inaccurate due to rounding errors, whereas cryptographic operations must be fast and accurate. To make operations on elliptic-curve accurate and more efficient, the elliptic-curve cryptography is defined over finite fields, also called Galois fields in honor of the founder of finite field theory, Évariste Galois.

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