John Mwaniki / 23 Jun 2023

How to Encrypt and Decrypt text using RSA in PHP

RSA is an asymmetric encryption algorithm that is widely used in various applications, including secure email communication, secure web browsing (HTTPS), and secure file transfer.

Earlier on in another article, I explained more about the RSA algorithm and how to generate RSA private and public keys on Windows and Linux.

In this article, we will explore how to encrypt and decrypt text using the RSA encryption algorithm in PHP.

In case you do not already have the encryption key(s) with you or are not aware of how to obtain them, please visit that article for more information.

How to generate RSA private and public keys in your PC

I will use the keys below that I have already generated for encryption and decryption demonstration.

Public Key (publickey.pem)

Private Key (privatekey.pem)

Either of the keys can be used to encrypt the data, while the other key is used to decrypt it. For instance, if the public key is used for encryption, the corresponding private key must be used to decrypt the data and vice versa.

Encrypting data with the Public key

Encryption with the public key is the most common method and the applicable one when sending sensitive/confidential information across a network such as the Internet. This is because using this method, only the recipient (who possesses the private key) can decrypt and read the data.

First, we assign the public key to a variable by copying the file's content and pasting it directly to the variable value as a string, or by using the openssl_pkey_get_public() or get_file_contents() functions to get the public key from the publickey.pem file.

$publickey = openssl_pkey_get_public("file://path/to/publickey.pem");
// Or
$publickey = get_file_contents("path/to/publickey.pem");
// Or
$publickey = "Paste the file content here";

We then use the openssl_public_encrypt() function to encrypt the data with the public key.

Syntax

openssl_public_encrypt($data, $encrypted_data, $publickey, $padding)

This function returns true on success or false on failure.

Parameters Description

Parameter	Requirement	Description
data	Required	Specifies the plaintext data to be encrypted.
encrypted_data	Required	Specifies a variable that will hold the result of the encryption (ie the encrypted data).
publickey	Required	Specifies the public key to be used in the encryption.
padding	Optional	Specifies a padding scheme to be used together with RSA encryption. Its value can be either OPENSSL_PKCS1_PADDING, OPENSSL_SSLV23_PADDING, OPENSSL_PKCS1_OAEP_PADDING, or OPENSSL_NO_PADDING.

Example

<?php
$publickey = openssl_pkey_get_public("file://publickey.pem");
$mytext = "The quick brown fox jumps over the lazy dog";
openssl_public_encrypt($mytext, $encrypted, $publickey);
echo $encrypted;
?>

Output:
�Nm�s�,We�W��x8\�vu��(��La��ڧ��?��p��t�n��L��C3i�[&��+t�Db�i5n?ml�dL}�`ԻWya��ީ�P+��ʛ��Q�I��q=��6��y�ۓ��vJ�cQj�N?�L��Q�l�q5*��.b�T��L��j��I��a��|]�fSv��.��ѥ��]��$��пi�S�i�Ԥ��F_� e��:?�yf7�

The above example shows that the output (ciphertext) looks scrambled and incoherent. The PHP script and the public key file are in the same directory. The default padding scheme in RSA encryption is OPENSSL_PKCS1_PADDING. So from the fact that we omitted the padding parameter in the example, the default was used.

To use a different padding scheme (or none) in the encryption, make sure to specify its respective constant as the fourth parameter.

Example

<?php
$publickey = get_file_contents("publickey.pem");
$mytext = "The quick brown fox jumps over the lazy dog";
openssl_public_encrypt($mytext, $encrypted, $publickey, OPENSSL_PKCS1_OAEP_PADDING);
echo $encrypted;
?>

Output:
9�+F92d�bj�A0c�_��Hz�#��'�*麐C%�@e��6@?�=��u/��5 ��Q �k�t�� H�1Eչ��}�B�T�;��b�k.R��R`rqAc� 'mχO�>��pzu�"�0E?��m[%<�-G�/�,�b�=��i3�>�"��1qG=8�.oѯ{yēI�4�AXQazK�c�z�V4/��**�=��*�4>$RP��He�d�@��j��!��

It is important to note that RSA encryption padding is randomized. The same message encrypted multiple times will look different each time.

Decrypting data with Private Key

To decrypt the data encrypted with the public key, you need to use the openssl_private_decrypt() function with the corresponding private key.

Syntax

openssl_private_decrypt($encrypted_data, $decrypted_data, $privatekey, $padding)

This function returns true on success or false on failure.

Parameters Description

Parameter	Requirement	Description
encrypted_data	Required	Specifies the ciphertext to be decrypted.
decrypted_data	Required	Specifies a variable that will hold the result of the decryption process (ie the plaintext data).
privatekey	Required	Specifies the private key corresponding to the public key used in the encryption.
padding	Optional	Specifies a padding scheme to be used together with RSA decryption. It MUST be the same that was used during the encryption. Its value can be either OPENSSL_PKCS1_PADDING, OPENSSL_SSLV23_PADDING, OPENSSL_PKCS1_OAEP_PADDING, or OPENSSL_NO_PADDING.

Example

<?php
$publickey = openssl_pkey_get_public("file://publickey.pem");
$privatekey = openssl_pkey_get_private("file://privatekey.pem");
$mytext = "The quick brown fox jumps over the lazy dog";

echo "Before encryption: ".$mytext."<br><br>";

openssl_public_encrypt($mytext, $encrypted, $publickey);
echo "Encrypted data:<br>".$encrypted."<br><br>";

openssl_private_decrypt($encrypted, $decrypted, $privatekey);
echo $decrypted;
?>

Output:
Before encryption: The quick brown fox jumps over the lazy dog

Encrypted data:
iן�^�s PP_W�5ܬmp��A�o�K��4��ȫ��0w��1�T��}W��=R[S7a�Ilz�p��ـ��)Y�^?R��/hC��`��L�褧��E A�y�/5抩F�F�x�x��n��\�/iN��_�H�<�3�p|bS��㸑Q�Þ��^-0oݟ�߻�� ;�y��q��x�fӋ�}��v��_��z0�7��Gkx�@�A ��c5認4

The quick brown fox jumps over the lazy dog

Encrypting data with the Private key

Encryption with the private key is done only to verify the identity of the sender but does not protect the data from being read in transit. This is because the encrypted data (ciphertext) is usually sent together with the public key (for decrypting) to the recipient. Thus anyone who accesses the data can use the public key to decrypt and read it.

Encryption of data with a private key is done using the openssl_private_encrypt() function.

Syntax

openssl_private_encrypt($data, $encrypted_data, $privatekey, $padding)

Parameters Description

Parameter	Requirement	Description
data	Required	Specifies the plaintext data to be encrypted.
encrypted_data	Required	Specifies a variable that will hold the result of the encryption.
privatekey	Required	Specifies the private key to be used in encrypting the data.
padding	Optional	Specifies a padding scheme to be used together with RSA encryption. Its value can be either OPENSSL_PKCS1_PADDING or OPENSSL_NO_PADDING.

Example

<?php
$privatekey = openssl_pkey_get_private("file://privatekey.pem");
$mytext = "The quick brown fox jumps over the lazy dog";
openssl_private_encrypt($mytext, $encrypted, $privatekey);
echo $encrypted;
?>

Output:
>��dd6#�� bQ��@ @��>vI��m^��ᓺ��|�}�� hgM打��PG1R��>�,HQ!8�}0��fA�"��x2x��!֘s�>��ȁq݅��{s��)|�s��3r��j|��>��E+��Tq�8��!�آ��bH��~k�� w�;}W��N��S�OMD,��OX7��5�OCY�

In this example, the private key file and the PHP script are located in the same directory. Same as in the public key encryption, the default padding scheme is OPENSSL_PKCS1_PADDING, which is only the available one or no padding.

Decrypting data with Public Key

To decrypt the data encrypted with the private key, you will need to use the openssl_public_decrypt() function with the corresponding public key.

Syntax

openssl_public_decrypt($encrypted_data, $decrypted_data, $publickey, $padding)

This function returns true on success or false on failure.

Parameters Description

Parameter	Requirement	Description
encrypted_data	Required	Specifies the ciphertext to be decrypted.
decrypted_data	Required	Specifies a variable that will hold the result of the decryption process.
publickey	Required	Specifies the public key corresponding to the private key used in the encryption.
padding	Optional	Specifies a padding scheme to be used together with RSA decryption. It MUST be the same that was used during the encryption. Its value can be either OPENSSL_PKCS1_PADDING or OPENSSL_NO_PADDING.

Example

<?php
$publickey = openssl_pkey_get_public("file://publickey.pem");
$privatekey = openssl_pkey_get_private("file://privatekey.pem");
$mytext = "The quick brown fox jumps over the lazy dog";

echo "Before encryption: ".$mytext."<br><br>";

openssl_private_encrypt($mytext, $encrypted, $privatekey);
echo "Encrypted data:<br>".$encrypted."<br><br>";

openssl_public_decrypt($encrypted, $decrypted, $publickey);
echo $decrypted;
?>

Output:
Before encryption: The quick brown fox jumps over the lazy dog

Encrypted data:
>��dd6#�� bQ��@ @��>vI��m^��ᓺ��|�}�� hgM打��PG1R��>�,HQ!8�}0��fA�"��x2x��!֘s�>��ȁq݅��{s��)|�s��3r��j|��>��E+��Tq�8��!�آ��bH��~k�� w�;}W��N��S�OMD,��OX7��5�OCY�

The quick brown fox jumps over the lazy dog

That's it!

Now you know how to encrypt and decrypt data in PHP and safeguard it from unauthorized access (or verify the sender's identity) using the RSA algorithm.

How to Encrypt and Decrypt text using RSA in PHP

Encrypting data with the Public key

Decrypting data with Private Key

Encrypting data with the Private key

Decrypting data with Public Key

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How to Encrypt and Decrypt text using RSA in PHP

Encrypting data with the Public key

Decrypting data with Private Key

Encrypting data with the Private key

Decrypting data with Public Key

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