Ask for a reversible encryption algorithm that requires a bit of high

Requirements:
1. The length of the encryption is as long as the length before encryption
2. Only letters from A to Z and numbers 0 to 9 are allowed in the encrypted string
3. According to the secret key can be counted back
The string before encryption is also a combination of letters and numbers
such as the string before encryption: Us3kqvu

Reply to discussion (solution)

You can try Caesar encryption and fence encryption, or use both of these methods.

Caesar Password

$key = ' 3ae '; $s = caesar_encode (' Us3kqvu ', $key); echo "$s \ n"; Echo Caesar_decode ($s, $key); function Caesar_encode ($s, $k)  {$k = "$k"; for ($i =0; $i
 
  Ciphertext: Ljubhml
  
Original: Us3kqvu
  
 
  
 
  
Caesar Password

  
 $key = ' 3ae '; $s = caesar_encode (' Us3kqvu ', $key); echo "$s \ n"; Echo Caesar_decode ($s, $  Key), function Caesar_encode ($s, $k) {$k = "$k"; for ($i =0; $i 
    
      ciphertext: Ljubhml 
     
 Original: Us3kqvu 
     
 is there a large probability of the repetition of this encryption and decryption method 
     
 
 
     
 Caesar password is implemented by moving character Map mappings, there is no possibility of duplication in theory 
 
 so-called mobile Character Map mapping is like 
 original ABCD 
 Move distance 4 
 Ciphertext efgh 
 
 
     
 One byte has 8 bits, 6 bits has 2^6=64 Possible, the letter + number Total 62 
 So there is a mapping table can be compressed into 6 bits 
 
 6 bit to 6 bits of the bit operation is more, any choice, and then reverse mapping to alphanumeric 
 
 The process above is reversible 
 
     
 Caesar encryption and fence encryption 
 
     
 