Crypt::AuthEnc::OCB(3)

NAME
Crypt::AuthEnc::OCB - Authenticated encryption in OCBv3 mode

SYNOPSIS
 ### OO interface
 use Crypt::AuthEnc::OCB;

 my $key = '...';
 my $nonce = '...';
 my $tag_len = 16;
 my $expected_tag = '...';

 # encrypt and authenticate
 my $ae_enc = Crypt::AuthEnc::OCB->new("AES", $key, $nonce, $tag_len);
 $ae_enc->adata_add('additional_authenticated_data1');
 $ae_enc->adata_add('additional_authenticated_data2');
 my $ct = $ae_enc->encrypt_add('data1');
 $ct .= $ae_enc->encrypt_add('data2');
 $ct .= $ae_enc->encrypt_add('data3');
 $ct .= $ae_enc->encrypt_last('rest of data');
 my $tag = $ae_enc->encrypt_done();

 # decrypt and verify
 my $ae_dec = Crypt::AuthEnc::OCB->new("AES", $key, $nonce, $tag_len);
 $ae_dec->adata_add('additional_authenticated_data1');
 $ae_dec->adata_add('additional_authenticated_data2');
 my $pt = $ae_dec->decrypt_add('ciphertext1');
 $pt .= $ae_dec->decrypt_add('ciphertext2');
 $pt .= $ae_dec->decrypt_add('ciphertext3');
 $pt .= $ae_dec->decrypt_last('rest of data');
 $ae_dec->decrypt_done($expected_tag) or die "decrypt failed"; # constant-time tag check

 #or, if you need the computed tag, compare it with slow_eq from Crypt::Misc
 #(Perl's 'eq' is not constant-time and leaks timing information)
 my $computed_tag = $ae_dec->decrypt_done();
 die "decrypt failed" unless Crypt::Misc::slow_eq($computed_tag, $expected_tag);

 ### functional interface
 use Crypt::AuthEnc::OCB qw(ocb_encrypt_authenticate ocb_decrypt_verify);

 my $key = '...';
 my $nonce = '...';
 my $adata = '...';
 my $plaintext = '...';
 my $tag_len = 16;

 my ($ciphertext, $tag) = ocb_encrypt_authenticate('AES', $key, $nonce, $adata, $tag_len, $plaintext);
 my $decrypted = ocb_decrypt_verify('AES', $key, $nonce, $adata, $ciphertext, $tag);

DESCRIPTION
This module implements OCB v3 according to <https://www.rfc-editor.org/rfc/rfc7253>.

This is a stateful API. Build one message by calling, in order: "new", optional "adata_add", zero or more "encrypt_add" or "decrypt_add" calls for full blocks, one optional "encrypt_last" or "decrypt_last" for the final partial block, then "encrypt_done" or "decrypt_done".

Use a fresh object per message. When verifying, decrypt_done($expected_tag) is the safer one-step form; decrypt_done() without arguments only returns the calculated tag. The first "encrypt_done" / "decrypt_done" call finalizes the object. After that, further "adata_add", "encrypt_add", "encrypt_last", "decrypt_add", "decrypt_last", "encrypt_done", and "decrypt_done" calls croak.

EXPORT
Nothing is exported by default.

You can export selected functions:

  use Crypt::AuthEnc::OCB qw(ocb_encrypt_authenticate ocb_decrypt_verify);

FUNCTIONS

  ocb_encrypt_authenticate
 my ($ciphertext, $tag) = ocb_encrypt_authenticate($cipher, $key, $nonce, $adata, $tag_len, $plaintext);

 # $cipher .. [string] 'AES' or name of any other cipher with 16-byte block len
 # $key ..... [binary string] AES key of proper length (128/192/256 bits)
 # $nonce ... [binary string] unique nonce/salt (no need to keep it secret)
 # $adata ... [binary string] additional authenticated data
 # $tag_len . [integer] required length of output tag

Use tag lengths from 4 to 16 bytes. Out-of-range values passed to this functional helper are normalized to 16.

  ocb_decrypt_verify
  my $plaintext = ocb_decrypt_verify($cipher, $key, $nonce, $adata, $ciphertext, $tag);
  # on error returns undef

METHODS
Unless noted otherwise, assume $ae is an existing AEAD object created via "new", for example:

 my $ae = Crypt::AuthEnc::OCB->new($cipher, $key, $nonce);

  new
 my $ae = Crypt::AuthEnc::OCB->new($cipher, $key, $nonce, $tag_len);

 # $cipher .. [string] 'AES' or name of any other cipher with 16-byte block len
 # $key ..... [binary string] AES key of proper length (128/192/256 bits)
 # $nonce ... [binary string] unique nonce/salt (no need to keep it secret)
 # $tag_len . [integer] required length of output tag

IMPORTANT: The length of the tag is a security parameter and a zero-length tag is legal (it simply provides no authenticity), which is the caller's choice.

  adata_add
Can be called only before the first "encrypt_add", "encrypt_last", "decrypt_add", or "decrypt_last". Returns the object itself (for chaining).

 $ae->adata_add($adata);                        #can be called multiple times

  encrypt_add
Returns a binary string of ciphertext (raw bytes).

 my $ciphertext = $ae->encrypt_add($data);      # can be called multiple times

 # note: \ must be a multiple of the block length (16 for AES)

  encrypt_last
 my $ciphertext = $ae->encrypt_last($data);

  encrypt_done
Returns the authentication tag as a binary string (raw bytes). This call finalizes the current message.

 my $tag = $ae->encrypt_done();                 # returns $tag value

  decrypt_add
Returns a binary string of plaintext (raw bytes).

 my $plaintext = $ae->decrypt_add($ciphertext); # can be called multiple times

 # note: \ must be a multiple of the block length (16 for AES)

  decrypt_last
 my $plaintext = $ae->decrypt_last($data);

  decrypt_done
Without argument returns the computed tag as a binary string. With $tag argument returns 1 (success) or 0 (failure). This call finalizes the current message.

 my $tag = $ae->decrypt_done;           # returns $tag value
 #or
 my $result = $ae->decrypt_done($tag);  # returns 1 (success) or 0 (failure)

  blocksize
 my $bs = $ae->blocksize;   # always returns 16

Returns the block size of the underlying cipher (always 16, since OCB requires a 128-bit block cipher such as AES).

  clone
Returns a copy of the AEAD object in its current state.

 my $ae_new = $ae->clone;

SEE ALSO

•   CryptX, Crypt::AuthEnc::CCM, Crypt::AuthEnc::GCM, Crypt::AuthEnc::EAX

•   <https://en.wikipedia.org/wiki/OCB_mode>

•   <https://www.rfc-editor.org/rfc/rfc7253>