Patent ID: 7359508

Claim:
A method of securely implementing a public-key cryptography algorithm in a microprocessor-based system, the public key being composed of an integer n that is a product of two large prime numbers p and q, and of a public exponent e, said algorithm also including a private key, said method determining a set E comprising a predetermined number of prime numbers e i that can correspond to the value of the public exponent e, and comprising the following steps: a) computing a value Φ = ∏ ei ∈ E ⁢ ei such that Φ/e i is less than Φ(n) for any e i belonging to E, where Φ is the Euler totient function; b) applying the value Φ to a predetermined computation involving, as a modular product, only the modular product of Φ multiplied by said private key of the algorithm; c) for each e i , testing whether the result of said predetermined computation is equal to a value Φ/ i : if so, then attributing the value e i to e, and storing e; otherwise, indicating that the computations of the cryptography algorithm using the value e cannot be performed; and d) performing a cryptographic operation on data using the stored value for e.