Calculate RSA d parameter

CalcRSAd.fs

let rec eGCD a b =
    if a = 0 then b, 0, 1
    else
        let gcd, x, y = eGCD (b % a) a
        gcd, y - (b / a) * x, x
        
let calcRSAd e n =
    let gcd, x, _ = eGCD e n
    if gcd = 1 then x + n
    else failwith "e and n must be coprime."
    
// Example
// 7 * d mod 288 = 1
// let d = calcRSAd 7 288 --> d = 247