javascript format preserving

fpe.ts

import fe1 from 'node-fe1-fpe'

export interface IFPESetting {
  min: number
  max: number
  privateKey: string
  publicKey: string
}

export const encrypt = ({
  index,
  min,
  max,
  privateKey,
  publicKey,
}: IFPESetting & {
  index: number
}): number => {
  // Algorithm can be applied only when the difference
  // between the min and max values is at least 4.
  if (max - min < 3) return index

  // Algorithms can only be applied when the
  // range difference between min and max is prime number.
  // Therefore, when the range difference is not prime number,
  // the algorithm is applied while leaving the last index intact.
  // if ((max - min) % 2 === 0) {
  //   if (index === max) return index
  //   --max
  // }
  if ((max - min) % 2 === 0) {
    // console.log('enc yes')
    const middle = Math.ceil(min + (max - min) / 2)
    // console.log('encrypted middle', middle)
    if (index === middle) return max
    if (index === max) index = middle
    --max
  }

  // Algorithm does not apply to index
  // values that are not in the range.
  if (index < min || index > max) return index

  return fe1.encrypt(max - min + 1, index - min, privateKey, publicKey) + min
}

export const decrypt = ({
  index,
  min,
  max,
  privateKey,
  publicKey,
}: IFPESetting & {
  index: number
}): number => {
  // Algorithm can be applied only when the difference
  // between the min and max values is at least 4.
  if (max - min < 3) return index

  // Algorithms can only be applied when the
  // range difference between min and max is prime number.
  // Therefore, when the range difference is not prime number,
  // the algorithm is applied while leaving the last index intact.

  const isNonPrime = (max - min) % 2 === 0
  if (isNonPrime) {
    if (index > max - 1) {
      return Math.ceil(min + (max - min) / 2)
    }
    --max
  }

  // Algorithm does not apply to index
  // values that are not in the range.
  if (index < min || index > max) return index

  if (isNonPrime) {
    if (
      index ===
      encrypt({
        index: Math.ceil(min + (max - min) / 2),
        max,
        min,
        privateKey,
        publicKey,
      })
    )
      return max + 1
  }
  return fe1.decrypt(max - min + 1, index - min, privateKey, publicKey) + min
}

const settings: IFPESetting = {
  min: -5,
  max: 100,
  privateKey: 'some private string',
  publicKey: 'some public string',
}

for (let i = -8; i <= 102; i++) {
  const encrypted = encrypt({
    index: i,
    ...settings,
  })
  const decrypted = decrypt({
    index: encrypted,
    ...settings,
  })
  console.log(i, encrypted, decrypted)
}