Encrypt/decrypt JS Affine cipher - can you tell what such code does: // below?

JavaScript
1

encrypt/decrypt JS Affine cipher - can you tell what such code does: // below???
addkey = addkey + (f.add.options[i].text)(f.add.options[i].selected)
multkey = multkey + (f.mult.options[i].text)(f.mult.options[i].selected)

// code works … this is Not my code but from:
http://practicalcryptography.com/ciphers/affine-cipher/

===================================

<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Untitled Document</title>
   
<script type="text/javascript"> 
function Encrypt(f) {
	var word, newword, code, newcode, newletter
	var addkey, multkey
	word = f.p.value
	word = word.toLowerCase()
	word = word.replace(/\W/g, "")
	addkey = 0
	
	for (i=0; i < f.add.options.length; i++) {
		addkey = addkey + (f.add.options[i].text)*(f.add.options[i].selected)
	}
	
	multkey = 0
	
	for (i=0; i < f.mult.options.length; i++) {
		multkey = multkey + (f.mult.options[i].text)*(f.mult.options[i].selected)
	}
	
	newword = ""
	
	for (i = 0; i < word.length; i++) {
		code = word.charCodeAt(i) - 97
		newcode = ( (multkey*code + addkey) % 26 ) + 97
		newletter = String.fromCharCode(newcode)
		newword = newword + newletter
	}
	
	f.c.value = newword + " "
}

function Decrypt(f) {
	var word, newword, code, newcode, newletter
	var addkey, multkey, multinverse
	
	word = f.c.value
	word = word.toLowerCase()
	word = word.replace(/\W/g, "")
	addkey = 0
	
	for (i=0; i < f.add.options.length; i++) {
		addkey = addkey + (f.add.options[i].text)*(f.add.options[i].selected)
	}
	
	multkey = 0
	
	for (i=0; i < f.mult.options.length; i++) {
		multkey = multkey + (f.mult.options[i].text)*(f.mult.options[i].selected)
		//if (i==3) alert(multkey +" + "+f.mult.options[i].text + " * " + f.mult.options[i].selected+" = "+(f.mult.options[i].text) * ( f.mult.options[i].selected));
	}
	
	multinverse = 1
	
	for (i=1; i <= 25; i = i + 2) {
		if ( (multkey*i) % 26 == 1 ) { multinverse = i }
	}
	
	newword = ""
	
	for (i = 0; i < word.length; i++) {
		code = word.charCodeAt(i) - 97
		newcode = ( (multinverse*(code + 26 - addkey)) % 26 ) + 97
		newletter = String.fromCharCode(newcode)
		newword = newword + newletter
	}
	
	f.p.value = newword.toLowerCase()
}

</script>
</head>

<body><form>Plaintext<br> 
  <textarea name="p" rows="4" cols="50" wrap="soft">Defend the east wall of the castle</textarea> 
  <p>a =
  <select name="mult" size="1"> 
  <option>1</option> 
  <option>3</option> 
  <option>5</option> 
  <option>7</option> 
  <option>9</option> 
  <option>11</option> 
  <option>15</option> 
  <option>17</option> 
  <option>19</option> 
  <option>21</option> 
  <option>23</option> 
  <option>25</option> 
  </select> 
b =
  <select name="add" size="1"> 
  <option>0</option> 
  <option>1</option> 
  <option>2</option> 
  <option>3</option> 
  <option>4</option> 
  <option>5</option> 
  <option>6</option> 
  <option>7</option> 
  <option>8</option> 
  <option>9</option> 
  <option>10</option> 
  <option>11</option> 
  <option>12</option> 
  <option>13</option> 
  <option>14</option> 
  <option>15</option> 
  <option>16</option> 
  <option>17</option> 
  <option>18</option> 
  <option>19</option> 
  <option>20</option> 
  <option>21</option> 
  <option>22</option> 
  <option>23</option> 
  <option>24</option> 
  <option>25</option> 
  </select> 
  </p> 
  <p><input name="btnEn" value="v Encrypt v" onclick="Encrypt(this.form)" type="button"> 
  <input name="btnDe" value="^ Decrypt ^" onclick="Decrypt(this.form)" type="button"></p> 
  <p>Ciphertext<br> 
  <textarea name="c" rows="4" cols="50" wrap="soft">wbgbuwyqbbhtynhkkzgyqbrhtykb</textarea> </p> 
</form>
</body>
</html>
2

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