---
name: "Crypt::Mode::CTR"
section: "3pm"
platform: "linux"
package: "perl-cryptx"
version: "0.091-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/Crypt::Mode::CTR"
---

# Crypt::Mode::CTR(3pm)

Block cipher mode CTR \[Counter mode\]

## NAME

Crypt::Mode::CTR - Block cipher mode CTR \[Counter mode\]

## SYNOPSIS

```
   use Crypt::Mode::CTR;
   my $m = Crypt::Mode::CTR->new('AES');
   my $key = '1234567890123456';
   my $iv = '1234567890123456';
   my $plaintext = 'example plaintext';
   my $chunk1 = 'example ';
   my $chunk2 = 'plaintext';
   # encrypt or decrypt in one call
   my $single_ciphertext = $m->encrypt($plaintext, $key, $iv);
   my $single_plaintext = $m->decrypt($single_ciphertext, $key, $iv);
   # encrypt more chunks
   $m->start_encrypt($key, $iv);
   my $chunked_ciphertext = '';
   $chunked_ciphertext .= $m->add($chunk1);
   $chunked_ciphertext .= $m->add($chunk2);
   # decrypt more chunks
   $m->start_decrypt($key, $iv);
   my $chunked_plaintext = '';
   $chunked_plaintext .= $m->add($chunked_ciphertext);
```

## DESCRIPTION

This module implements CTR cipher mode. **Note:** It works only with ciphers from CryptX (Crypt::Cipher::NNNN).

## METHODS

Unless noted otherwise, assume `$m` is an existing mode object created via `"new"`, for example:

```
 my $m = Crypt::Mode::CTR->new('AES');
```

### new

```
 my $m = Crypt::Mode::CTR->new($cipher_name);
 #or
 my $m = Crypt::Mode::CTR->new($cipher_name, $ctr_mode, $ctr_width);
 #or
 my $m = Crypt::Mode::CTR->new($cipher_name, $ctr_mode, $ctr_width, $cipher_rounds);
 # $cipher_name .. [string] 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'
 #                 or any <NAME> for which there is a Crypt::Cipher::<NAME> module
 # $ctr_mode ..... [integer] CTR_COUNTER_LITTLE_ENDIAN    (0) - little-endian counter (DEFAULT)
 #                 CTR_COUNTER_BIG_ENDIAN       (1) - big-endian counter
 #                 CTR_COUNTER_LITTLE_ENDIAN | 2 (2) - little-endian + RFC 3686 initial-counter-incrementing
 #                 CTR_COUNTER_BIG_ENDIAN | 2    (3) - big-endian + RFC 3686 initial-counter-incrementing
 # $ctr_width .... [integer] counter width in bytes (DEFAULT = full block width, e.g. 16 for AES)
 # $cipher_rounds ... [integer] optional, number of rounds for given cipher
```

### encrypt

Encrypts the plaintext in a single call. Returns the ciphertext as a binary string. The plaintext scalar is converted to bytes using Perl's usual scalar stringification. Defined scalars, including numbers and string-overloaded objects, are accepted. `"undef"` is treated as an empty string and may emit Perl's usual "uninitialized value" warning.

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

### decrypt

Decrypts the ciphertext in a single call. Returns the plaintext as a binary string. The ciphertext scalar is converted to bytes using Perl's usual scalar stringification. Defined scalars, including numbers and string-overloaded objects, are accepted. `"undef"` is treated as an empty string and may emit Perl's usual "uninitialized value" warning.

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

### start\_encrypt

Initializes encryption mode. Returns the object itself.

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

### start\_decrypt

Initializes decryption mode. Returns the object itself.

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

### add

Feeds data to the encryption or decryption stream. Returns a binary string.

Each argument is converted to bytes using Perl's usual scalar stringification. Defined scalars, including numbers and string-overloaded objects, are accepted. `"undef"` is treated as an empty string and may emit Perl's usual "uninitialized value" warning.

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

### finish

CTR is a streaming mode and does not use padding, so `"finish"` returns an empty string. It exists for API consistency with Crypt::Mode::CBC and Crypt::Mode::ECB and may be safely called or omitted.

```
   $m->start_encrypt($key, $iv);
   my $ciphertext = '';
   $ciphertext .= $m->add($chunk1);
   $ciphertext .= $m->add($chunk2);
   $ciphertext .= $m->finish;   # returns ''
```

## SEE ALSO

- •

  CryptX, Crypt::Cipher

- •

  Crypt::Cipher::AES, Crypt::Cipher::Blowfish, ...

- •

  \<https://en.wikipedia.org/wiki/Block\_cipher\_mode\_of\_operation#Counter\_.28CTR.29\>