openssl_public_decrypt

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;
 }
?>