openssl_public_decrypt

(PHP 4 >= 4.0.6, PHP 5, PHP 7, PHP 8)

openssl_public_decrypt — Decrypts data with public key

Description

openssl_public_decrypt(
    string $data,
    #[\SensitiveParameter] string &$decrypted_data,
    OpenSSLAsymmetricKey|OpenSSLCertificate|array|string $public_key,
    int $padding = OPENSSL_PKCS1_PADDING
): bool

openssl_public_decrypt() decrypts data that was previous encrypted via openssl_private_encrypt() and stores the result into decrypted_data.

You can use this function e.g. to check if the message was written by the owner of the private key.

Parameters

data
decrypted_data
public_key: public_key must be the public key corresponding that was used to encrypt the data.
padding: padding can be one of OPENSSL_PKCS1_PADDING, OPENSSL_NO_PADDING.

Return Values

Returns true on success or false on failure.

Changelog

Version	Description
8.0.0	`public_key` accepts an OpenSSLAsymmetricKey or OpenSSLCertificate instance now; previously, a resource of type `OpenSSL key` or `OpenSSL X.509` was accepted.

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User Contributed Notes 1 note

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Hernanibus ¶

7 years ago

Just a little note on  [P.Peyremorte]'s note in manual's openssl_private_encrypt.

"- openssl_private_encrypt can encrypt a maximum of 117 chars at one time."

This depends on the length of $key:

- For a 1024 bit key length => max number of chars (bytes) to encrypt = 1024/8 - 11(when padding used) = 117 chars (bytes).
- For a 2048 bit key length => max number of chars (bytes) to encrypt = 2048/8 - 11(when padding used) = 245 chars (bytes).
... and so on

By the way, if openssl_private_encrypt fails because of data size you won't get anything but just false as returned value, the same for openssl_public_decrypt() on decryption.

"- the encrypted output string is always 129 char length. If you use base64_encode on the encrypted output, it will give always 172 chars, with the last always "=" (filler)"

This again depends on the length of $key:

- For a 1024 bit key length => encrypted number of raw bytes is always a block of 128 bytes (1024 bits) by RSA design.
- For a 2048 bit key length => encrypted number of raw bytes is always a block of 256 bytes (2048 bits) by RSA design.
... and so on

About base64_encode output length, it depends on what you encode (meaning it depends on the bytes resulting after encryption), but in general the resulting encoded string will be about a 33% bigger (for 128 bytes bout 170 bytes and for 256 bytes about 340 bytes).

I would then generalize a little [P.Peyremorte]'s note by:
<?php
// given the variables as constants:

  //Block size for encryption block cipher
private $ENCRYPT_BLOCK_SIZE = 200;// this for 2048 bit key for example, leaving some room

  //Block size for decryption block cipher
private $DECRYPT_BLOCK_SIZE = 256;// this again for 2048 bit key

         //For encryption we would use:
function encrypt_RSA($plainData, $privatePEMKey)
  {
$encrypted = '';
$plainData = str_split($plainData, $this->ENCRYPT_BLOCK_SIZE);
    foreach($plainData as $chunk)
    {
$partialEncrypted = '';

//using for example OPENSSL_PKCS1_PADDING as padding
$encryptionOk = openssl_private_encrypt($chunk, $partialEncrypted, $privatePEMKey, OPENSSL_PKCS1_PADDING);

      if($encryptionOk === false){return false;}//also you can return and error. If too big this will be false
$encrypted .= $partialEncrypted;
    }
    return base64_encode($encrypted);//encoding the whole binary String as MIME base 64
}

//For decryption we would use:
protected function decrypt_RSA($publicPEMKey, $data)
  {
$decrypted = '';

//decode must be done before spliting for getting the binary String
$data = str_split(base64_decode($data), $this->DECRYPT_BLOCK_SIZE);

    foreach($data as $chunk)
    {
$partial = '';

//be sure to match padding
$decryptionOK = openssl_public_decrypt($chunk, $partial, $publicPEMKey, OPENSSL_PKCS1_PADDING);

      if($decryptionOK === false){return false;}//here also processed errors in decryption. If too big this will be false
$decrypted .= $partial;
    }
    return $decrypted;
  }
?>