# phpman > man > Crypt::Mode::CTR(3pm)

## NAME
    [Crypt::Mode::CTR](https://www.chedong.com/phpMan.php/perldoc/Crypt%3A%3AMode%3A%3ACTR/markdown) - Block cipher mode CTR [Counter mode]

## SYNOPSIS
       use [Crypt::Mode::CTR](https://www.chedong.com/phpMan.php/perldoc/Crypt%3A%3AMode%3A%3ACTR/markdown);
       my $m = [Crypt::Mode::CTR](https://www.chedong.com/phpMan.php/perldoc/Crypt%3A%3AMode%3A%3ACTR/markdown)->new('AES');

       #(en|de)crypt at once
       my $ciphertext = $m->encrypt($plaintext, $key, $iv);
       my $plaintext = $m->decrypt($ciphertext, $key, $iv);

       #encrypt more chunks
       $m->start_encrypt($key, $iv);
       my $ciphertext = $m->add('some data');
       $ciphertext .= $m->add('more data');

       #decrypt more chunks
       $m->start_decrypt($key, $iv);
       my $plaintext = $m->add($some_ciphertext);
       $plaintext .= $m->add($more_ciphertext);

## DESCRIPTION
    This module implements CTR cipher mode. NOTE: it works only with ciphers from CryptX
    ([Crypt::Cipher::NNNN](https://www.chedong.com/phpMan.php/perldoc/Crypt%3A%3ACipher%3A%3ANNNN/markdown)).

## METHODS
  new
     my $m = [Crypt::Mode::CTR](https://www.chedong.com/phpMan.php/perldoc/Crypt%3A%3AMode%3A%3ACTR/markdown)->new($cipher_name);
     #or
     my $m = [Crypt::Mode::CTR](https://www.chedong.com/phpMan.php/perldoc/Crypt%3A%3AMode%3A%3ACTR/markdown)->new($cipher_name, $ctr_mode, $ctr_width);
     #or
     my $m = [Crypt::Mode::CTR](https://www.chedong.com/phpMan.php/perldoc/Crypt%3A%3AMode%3A%3ACTR/markdown)->new($cipher_name, $ctr_mode, $ctr_width, $cipher_rounds);

     # $cipher_name .. one of 'AES', 'Anubis', 'Blowfish', 'CAST5', 'Camellia', 'DES', 'DES_EDE',
     #                 'KASUMI', 'Khazad', 'MULTI2', 'Noekeon', 'RC2', 'RC5', 'RC6',
     #                 'SAFERP', 'SAFER_K128', 'SAFER_K64', 'SAFER_SK128', 'SAFER_SK64',
     #                 'SEED', 'Skipjack', 'Twofish', 'XTEA', 'IDEA', 'Serpent'
     #                 simply any <NAME> for which there exists [Crypt::Cipher](https://www.chedong.com/phpMan.php/perldoc/Crypt%3A%3ACipher/markdown)::<NAME>
     # $ctr_mode ..... 0 little-endian counter (DEFAULT)
     #                 1 big-endian counter
     #                 2 little-endian + RFC3686 incrementing
     #                 3 big-endian + RFC3686 incrementing
     # $ctr_width .... counter width in bytes (DEFAULT = full block width)
     # $cipher_rounds ... optional num of rounds for given cipher

  encrypt
       my $ciphertext = $m->encrypt($plaintext, $key, $iv);

  decrypt
       my $plaintext = $m->decrypt($ciphertext, $key, $iv);

  start_encrypt
       $m->start_encrypt($key, $iv);

  start_decrypt
       $m->start_decrypt($key, $iv);

  add
       # in encrypt mode
       my $plaintext = $m->add($ciphertext);

       # in decrypt mode
       my $ciphertext = $m->add($plaintext);

## SEE ALSO
    *   CryptX, [Crypt::Cipher](https://www.chedong.com/phpMan.php/perldoc/Crypt%3A%3ACipher/markdown)

    *   [Crypt::Cipher::AES](https://www.chedong.com/phpMan.php/perldoc/Crypt%3A%3ACipher%3A%3AAES/markdown), [Crypt::Cipher::Blowfish](https://www.chedong.com/phpMan.php/perldoc/Crypt%3A%3ACipher%3A%3ABlowfish/markdown), ...

    *   <<https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Counter_.28CTR.29>>