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openssl_public_decrypt

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

openssl_public_decryptDecrypts data with public key

Description

openssl_public_decrypt(
    string $data,
    string &$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.

See Also

add a note

User Contributed Notes 3 notes

up
4
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;
}
?>
up
-8
wfredkNOSPAM at L5DevelopmentNOSPAM dot com
22 years ago
Encrypt using private key, decrypt using public key.

Use this for posting signed messages: Anyone with access to
your public key can read it, but they can't create one with
your signature.

<?php
echo "Source: $source";
$fp=fopen("/path/to/private.key","r");
$priv_key=fread($fp,8192);
fclose($fp);
// $passphrase is required if your key is encoded (suggested)
$res = openssl_get_privatekey($priv_key,$passphrase);
/*
* NOTE: Here you use the returned resource value
*/
openssl_private_encrypt($source,$crypttext,$res);
echo
"String crypted: $crypttext";

$fp=fopen ("/path/to/certificate.crt","r");
$pub_key=fread($fp,8192);
fclose($fp);
openssl_get_publickey($pub_key);
/*
* NOTE: Here you use the $pub_key value (converted, I guess)
*/
openssl_public_decrypt($crypttext,$newsource,$pub_key);
echo
"String decrypt : $newsource";
?>
up
-21
billnet at tiscalinet dot it
22 years ago
<?php
$fp
=fopen ("/path/to/certificato.pem");
$pub_key=fread ($fp,8192);
openssl_get_publickey ($pub_key);
openssl_public_decrypt ($source,$finaltext,$pub_key);
echo
"String decrypt : $finaltext";
?>
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