Crypt::Salsa20 - Encrypt data with the Salsa20 cipher
This document describes version 0.02 of Crypt::Salsa20, released August 3, 2013.
use Crypt::Salsa20; my $salsa20 = Crypt::Salsa20->new(-key => $key, -iv => $nonce); my $cryptor = $salsa20->cryptor; my $ciphertext = $cryptor->($plaintext); # Or use Crypt::CBC-like API: my $ciphertext = $salsa20->encrypt('plaintext'); my $plaintext = $salsa20->decrypt($plaintext);
Crypt::Salsa20 implements D. J. Bernstein's Salsa20 stream cipher (a.k.a. Snuffle 2005) in Perl. For more information on Salsa20, see his page at http://cr.yp.to/snuffle.html.
Salsa20 takes a 256 bit (or 128 bit) key and a 64 bit nonce (a.k.a. an IV or message number) and uses them to generate a stream of up to 2**70 bytes of pseudo-random data. That stream is XORed with your message. Because of that, encryption and decryption are the same operation.
It is critical that you never use the same nonce and key to encrypt two different messages. Because the keystream is completely determined by the key and nonce, reusing them means that you can cancel out the keystream by XORing the two ciphertexts together:
ciphertext1 ^ ciphertext2 keystream ^ message1 ^ keystream ^ message2 message1 ^ message2 ^ keystream ^ keystream message1 ^ message2 ^ 0 message1 ^ message2
The API is similar to that of the Crypt::CBC module, but there are some differences:
-literal_keyoption. The key is always interpreted as raw bytes (and must be either 16 or 32 bytes long). If you want to use a pasword hashing function, you have to supply your own.
startmethod does not require a parameter, and it is not necessary to call it at all.
finishmethod is available, but unnecessary. In Crypt::Salsa20 it does nothing and always returns the empty string.
Each attribute has a method of the same name. Calling the method with no parameter returns the current value of the attribute. Calling it with a parameter sets the attribute to that value (and returns the new value).
The encryption key is a 16 or 32 byte string (128 or 256 bits), with 32 bytes being the recommended size. It's always interpreted as raw bytes; if you want to use a pasword hashing function, you have to supply your own. Setting the key does not change the IV or reset the block counter.
The nonce (IV) is an 8 byte string (64 bits). The nonce does not need to be kept secret, but you must never encrypt two different messages with the same key and nonce, or you have catastrophically weakened the security of the cipher. You must supply an IV before encrypting or decrypting, but you can omit it from the constructor and call the
iv method instead. Setting the IV does not change the key, but it does reset the block counter.
The number of cipher rounds to use. The default is 20, which is the standard Salsa20 cipher. The standard variants are 8 or 12 rounds (Salsa20/8 or Salsa20/12), but any even integer will work.
$salsa20 = Crypt::Salsa20->new(-key => $key, ...);
This constructs a new Crypt::Salsa20 object, with attributes supplied as
key => value pairs. For compatibility with Crypt::CBC, attribute names may have a leading hyphen (but unlike Crypt::CBC the hyphen is not required).
The only required attribute at construction time is the key (but you must supply an IV before encrypting or decrypting).
Resets the internal block counter, starting the keystream over at the beginning. You should also change the IV, because using the same key and IV is a security breach.
For compatibility with the Crypt::CBC method of the same name, you can pass a parameter (e.g.
'encrypting'), but it is ignored. With Salsa20, encryption and decryption are the same operation, so there's no need to indicate which one you want.
This method is primarily for Crypt::CBC compatibility. Since with Salsa20 you don't need to specify whether you're encrypting or decrypting, and the
iv method also does everything
start does, you don't really need to call this method.
$ciphertext = $salsa20->crypt($plaintext); $plaintext = $salsa20->crypt($ciphertext);
Encrypts or decrypts the provided string.
Because encryption & decryption are the same operation, it is not necessary to call
start before calling
crypt, but you do need to have set the IV, either by passing it to the constructor or calling the
$remaining_ciphertext = $salsa20->finish;
This method exists solely for Crypt::CBC compatibility. It always returns the empty string.
$ciphertext = $salsa20->encrypt($plaintext);
Equivalent to calling
start and then
$plaintext = $salsa20->decrypt($ciphertext);
Equivalent to calling
start and then
crypt (the same as
$cryptor = $salsa20->cryptor; $ciphertext = $cryptor->($plaintext); $plaintext = $cryptor->($ciphertext);
This method is the most efficient way to use Crypt::Salsa20 if you are encrypting multiple chunks. It returns a coderef that encrypts or decrypts the text you pass it.
$cryptor->($text) is equivalent to
$salsa20->crypt($text), just faster.
The cryptor remains tied to the original object. Changing the key or IV affects it. But it is not necessary to save a reference to the original object if you don't plan to call any other methods.
Crypt::Salsa20 requires no configuration files or environment variables.
No bugs have been reported.
Christopher J. Madsen
<perl AT cjmweb.net>
Please report any bugs or feature requests to
<bug-Crypt-Salsa20 AT rt.cpan.org> or through the web interface at http://rt.cpan.org/Public/Bug/Report.html?Queue=Crypt-Salsa20.
You can follow or contribute to Crypt-Salsa20's development at http://github.com/madsen/crypt-salsa20.
This software is copyright (c) 2013 by Christopher J. Madsen.
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.
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