Chapter 4. Miscellaneous Cryptography

Chapter 4. Miscellaneous Cryptography
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Table of Contents

4.1. Crypt_Hash

4.1.1. Supported Algorithms and Dependencies
4.1.2. Example

4.2. Crypt_RSA

4.2.1. Dependencies
4.2.2. Examples
4.2.3. createKey()
4.2.4. setPrivateKeyFormat(), setPublicKeyFormat(), loadKey() and setPassword()
4.2.5. setPublicKey() and getPublicKey()
4.2.6. encrypt(), decrypt() and setEncryptionMode()
4.2.7. sign(), verify(), and setSignatureMode()
4.2.8. setHash(), setMGFHash() and setSaltLength()

4.1. Crypt_Hash

4.1.1. Supported Algorithms and Dependencies

The following algorithms are supported:

md2, md5, md5-96, sha1, sha1-96, sha256, sha384, and sha512

Crypt_Hash requires, minimally, PHP 4.3.0 (due to its use of sha1()). If sha384 or sha512 are being used and you're not running PHP 5.1.2 or greater then Math/BigInteger.php is also required.

Crypt_Hash uses the hash extension if it's available (> 5.1.2), mhash if it's not, and it's own internal implementation if not even mhash is available.

4.1.2. Example

<?php
include('Crypt/Hash.php');

$hash = new Crypt_Hash('sha1');
//$hash->setKey('abcdefg');
echo bin2hex($hash->hash('abcdefg'));
?>

If $hash->setKey() had been called $hash->hash() would have returned an HMAC.

4.2. Crypt_RSA

4.2.1. Dependencies

If you're running PHP 5, Crypt_RSA requires Math/BigInteger.php and Crypt/Hash.php. If you're running PHP 4, Crypt_RSA also requires PHP/Compat/Function/array_fill.php, PHP/Compat/Function/bcpowmod.php, and PHP/Compat/Function/str_split.php

4.2.2. Examples

Here's an example of how to encrypt / decrypt with Crypt_RSA:

<?php
include('Crypt/RSA.php');

$rsa = new Crypt_RSA();
extract($rsa->createKey());

$plaintext = 'terrafrost';

$rsa->loadKey($privatekey);
$ciphertext = $rsa->encrypt($plaintext);

$rsa->loadKey($publickey);
echo $rsa->decrypt($ciphertext);
?>

Here's an example of how to create / verify a signature with Crypt_RSA:

<?php
include('Crypt/RSA.php');

$rsa = new Crypt_RSA();
extract($rsa->createKey());

$plaintext = 'terrafrost';

$rsa->loadKey($privatekey);
$signature = $rsa->sign($plaintext);

$rsa->loadKey($publickey);
echo $rsa->verify($plaintext, $signature) ? 'verified' : 'unverified';
>

4.2.3. createKey()

createKey() takes three parameters - $bits, $timeout, and $primes. $timeout is present since creating a key has the potential to be fairly time consuming and will guarantee that createKey() does not run for more than $timeout seconds. $primes lets provide pre-computed prime numbers to speed things up.

extract($rsa->createKey()) creates three variables - $publickey, $privatekey, and $partialkey. If createKey hit the timeout then it'll return all the primes that it had managed to compute so that you might pass them back to createKey() on a subsequent call.

The exponent can be set by defining CRYPT_RSA_EXPONENT and multi-prime RSA can be utilized by adjusting CRYPT_RSA_SMALLEST_PRIME. Note that these must be done before a Crypt_RSA() object is initialized.

Smaller values for CRYPT_RSA_SMALLEST_PRIME result in increased speed at the cost of security.

4.2.4. setPrivateKeyFormat(), setPublicKeyFormat(), loadKey() and setPassword()

Crypt_RSA supports the following formats:

CRYPT_RSA_PRIVATE_FORMAT_PKCS1:

CRYPT_RSA_PRIVATE_FORMAT_PKCS1 (with password):

CRYPT_RSA_PUBLIC_FORMAT_PKCS1:

CRYPT_RSA_PUBLIC_FORMAT_OPENSSH:

ssh-rsa AAAAB3NzaC1yc2EAAAABJQAAAIB8eVx2ty44roLTatsZHdKBX9knbCdOIUGC6HkUd26gHotx
nPj0sgL8pwiIf9NyQrwbFB2gnvAcI8Gk2K6auULpxqP8LyM78dndxvXZa6TiBS4mrWWDn4dO/gpEn2kV
4QVCpLpf4FPO1j4ShHG5zwyUtdLg9R6i7B30iizYrRjn+w== phpseclib-generated-key

Passwords can be set via setPassword() and are only supported on private keys. CRYPT_RSA_PUBLIC_FORMAT_OPENSSH generates keys that are intended to go in $HOME/.ssh/authorized_keys for use with OpenSSH. Another format - CRYPT_RSA_PUBLIC_FORMAT_RAW - is stored as an array with two indexes - one for the modulus and one for the exponent. Indexes accepted by loadkey() are as follows:

e, exponent, publicExponent, modulus, modulo, n

loadKey() has two parameters - $key and the optional $type. The default type, if $type is not explicitely set, is CRYPT_RSA_PRIVATE_FORMAT_PKCS1. It should, at this point, be noted that Crypt_RSA treats public and private keys largelly identically. A key can be formatted as a CRYPT_RSA_PUBLIC_FORMAT_PKCS1 and still conform to the CRYPT_RSA_PRIVATE_FORMAT_PKCS1 format and vice versa. The only real difference between private keys and public keys is that private keys *can* contain their public key counterparts whereas public keys cannot. That said, this distinction is, for the most part, irrelevant and academic. For a more thorough discussion of this see setPublicKey() and getPublicKey().

4.2.5. setPublicKey() and getPublicKey()

As noted in setPrivateKeyFormat(), setPublicKeyFormat(), loadKey() and setPassword(), Crypt_RSA treats public and private keys largely identically. The only real difference is that some private key formats contain the public key within them whereas no public key format does. The reason you'd want to do this is for indexing purposes. For example, in SSH-2, RSA authentication works by sending your public key along with a signature created by your private key. The SSH-2 server then looks the public key up in an index of public keys to see if it's an allowed key and then verifies the signature. To that end, setPublicKey() defines the public key if it hasn't already been defined and getPublicKey() returns it. getPublicKey() has an optional parameter - $type - that sets the format.

4.2.6. encrypt(), decrypt() and setEncryptionMode()

Crypt_RSA supports two encryption modes - CRYPT_RSA_ENCRYPTION_OAEP and CRYPT_RSA_ENCRYPTION_PKCS1. CRYPT_RSA_ENCRYPTION_OAEP uses Optimal Asymmetric Encryption Padding and provides more security than CRYPT_RSA_ENCRYPTION_PKCS1.

Both CRYPT_RSA_ENCRYPTION_OAEP and CRYPT_RSA_ENCRYPTION_PKCS1 impose limits on how large the plaintext can be. If the plaintext exceeds these limits the plaintext will be split up such that each block falls within those limits.

4.2.7. sign(), verify(), and setSignatureMode()

Crypt_RSA supports two signature modes - CRYPT_RSA_SIGNATURE_PSS and CRYPT_RSA_SIGNATURE_PKCS1. The former is assumed to provide more security than the latter. See Examples for examples.

4.2.8. setHash(), setMGFHash() and setSaltLength()

In all likelihood, calling these functions will be unnecessary as the default values should be sufficient. None-the-less a discussion of them follows.

setHash() is used with signature production / verification and (if the encryption mode is CRYPT_RSA_ENCRYPTION_OAEP) encryption and decryption. If the specified hash isn't supported sha1 will be used.

setMGFHash() determines which hashing function should be used for the mask generation function as utilized in CRYPT_RSA_ENCRYPTION_OAEP and CRYPT_RSA_SIGNATURE_PSS. PKCS#1 recommends but does not require that the MGFHash and the Hash be set to the same thing.

setSaltLength() is only utilized with CRYPT_RSA_SIGNATURE_PSS. PKCS#1 recommends this value either be 0 (which is what it is by default) or the length of the output of the hash function as set via setHash()

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