Pure JS Implementation of RC4

RC4.js

class MyRC4 {

  constructor(key) {
    const keyBytes = Buffer.from(key)
    const keyLength = keyBytes.length
    this.m_state = Array(256)
    this.m_x = 0
    this.m_y = 0


    let counter

    for (let counter = 0; counter < 256; counter++) {
      this.m_state[counter] = counter;
    }

    // console.log(`Initial sBox after KSA: ${this.m_state}`)

    let index1 = 0
    let j = 0

    for (let counter = 0; counter < 256; counter++) {
      j = (j + this.m_state[counter] + keyBytes[counter % keyLength]) % 256
      // console.log(`counter: ${counter}, j: ${j}`)
      this.swapByte(counter, j)
      if (index1 >= keyLength) index1 = 0;
    }
    console.log(`Initial sBox: ${this.m_state}`)

  }

  swapByte(b1, b2) {
    let t1 = this.m_state[b1]
    this.m_state[b1] = this.m_state[b2]
    this.m_state[b2] = t1
  }

  processByte(inByte) {
    this.m_x++
    this.m_y += this.m_state[this.m_x]
    this.swapByte(this.m_x, this.m_y)
    return inByte ^ ((this.m_state[(this.m_state[this.m_x] + this.m_state[this.m_y]) & 256]) % 256)
  }

  processString(inString) {
    let inBytes = Buffer.from(inString)
    let idx1 = 0
    let idx2 = 0
    let strLength = inBytes.length
    let output = Buffer.alloc(strLength)
    let s = this.m_state
    let x = this.m_x
    let y = this.m_y

    while (strLength--) {
      x = (x + 1) & 0xff
      let a = s[x]
      y = (y + a) & 0xff
      let b = s[y]
      s[x] = b
      s[y] = a
      output[idx1++] = inBytes[idx2++] ^ s[(a+b) & 0xff]
    }

    this.m_x = x
    this.m_y = y
    return output
  }

}