How can I transform between the two styles of public key format, one "BEGIN RSA PUBLIC KEY", the other is "BEGIN PUBLIC KEY"

How can I transform between the two styles of public key format, one format is:

-----BEGIN PUBLIC KEY-----
...
-----END PUBLIC KEY-----

the other format is:

-----BEGIN RSA PUBLIC KEY-----
...
-----END RSA PUBLIC KEY-----

for example I generated id_rsa/id_rsa.pub pair using ssh-keygen command, I calculated the public key from id_rsa using:

openssl rsa -in id_rsa -pubout -out pub2

then again I calculated the public key from id_rsa.pub using :

ssh-keygen -f id_rsa.pub -e -m pem > pub1

the content is pub1 is :

and the content of pub2 is :

According to my understanding, pub1 and pub2 contain the same public key information, but they are in different format, I wonder how can I transform between the two format? Can anyone show me some concise introduction on the two formats?

Solution

Using phpseclib, a pure PHP RSA implementation...

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

$rsa = new Crypt_RSA();
$rsa->loadKey('-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA61BjmfXGEvWmegnBGSuS
+rU9soUg2FnODva32D1AqhwdziwHINFaD1MVlcrYG6XRKfkcxnaXGfFDWHLEvNBS
EVCgJjtHAGZIm5GL/KA86KDp/CwDFMSwluowcXwDwoyinmeOY9eKyh6aY72xJh7n
oLBBq1N0bWi1e2i+83txOCg4yV2oVXhBo8pYEJ8LT3el6Smxol3C1oFMVdwPgc0v
Tl25XucMcG/ALE/KNY6pqC2AQ6R2ERlVgPiUWOPatVkt7+Bs3h5Ramxh7XjBOXeu
lmCpGSynXNcpZ/06+vofGi/2MlpQZNhHAo8eayMp6FcvNucIpUndo1X8dKMv3Y26
ZQIDAQAB
-----END PUBLIC KEY-----');
$rsa->setPublicKey();

echo $rsa->getPublicKey(CRYPT_RSA_PUBLIC_FORMAT_PKCS1_RAW);

The base64-encoded stuff appears to match even though the header says BEGIN PUBLIC KEY and not BEGIN RSA PUBLIC KEY. So maybe just use str_replace to fix that and you should be good to go!