Convert RSA public key, from XML to PEM (PHP) - php

Question

How convert RSA public key, from XML to PEM (PHP)?

Answer 1

I'm assuming that by XML format, you mean XML DSig RSAKeyValue, and that by PEM format you mean what OpenSSL exports in between -----BEGIN PUBLIC KEY----- and -----END PUBLIC KEY-----.
You need first to extract the modulus and public exponent from the XML.
<RSAKeyValue>
<Modulus>xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W
jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRgBUwUlV
5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U=
</Modulus>
<Exponent>AQAB</Exponent>
</RSAKeyValue>
You can easily convert these into a bit string using base64_decode.
Once this is done, you need to build the ASN.1 public key structure somehow.
What OpenSSL exports between BEGIN/END PUBLIC KEY is an X.509 SubjectPublicKeyInfo structure.
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING }
The subjectPublicKey is made of a sequnce is described in the PKCS#1 spec:
RSAPublicKey ::= SEQUENCE {
modulus INTEGER,
publicExponent INTEGER
}
The algorithm (an AlgorithmIdentifier) is also described in the PKCS#1 spec (see section A.1):
rsaEncryption
OBJECT IDENTIFIER ::= { pkcs-1 1 }
This structure needs to be serialized in DER form, then base64-encoded and then placed between the BEGIN/END delimiters.
I don't know of any PHP library to do the ASN.1/DER encoding unfortunately (the rest is relatively easy, but dealing with ASN.1 tends to be tedious).
The PHP/PEAR Crypt_RSA module can construct RSA public keys from modulus and exponent, but its toString() method uses a custom format (just the base64-encoding of the result of PHP serialize on the array structure, which has nothing to do with the ASN.1/DER encoding).

Answer 2

we know
.pem - (Privacy Enhanced Mail) Base64
encoded DER certificate, enclosed
between "-----BEGIN CERTIFICATE-----"
and "-----END CERTIFICATE-----"
X.509
The SignatureValue element contains
the Base64 encoded signature result -
the signature generated with the
parameters specified in the
SignatureMethod element - of the
SignedInfo element after applying the
algorithm specified by the
CanonicalizationMethod.
XML_Signature
so we end up with
$xml = simplexml_load_file($xmlFile); // or simplexml_load_string
$pem = "-----BEGIN CERTIFICATE-----\n";
$pem .= $xml->SignatureValue;
$pem .= "\n-----END CERTIFICATE-----";
// save to file
if your xml-file isn't a XML_Signature
$xml = simplexml_load_file($xmlFile); // or simplexml_load_string
$pem = "-----BEGIN CERTIFICATE-----\n";
$pem .= $xml->nodeWithWantedValue; // use base64_encode if needed
$pem .= "\n-----END CERTIFICATE-----";

Answer 3

Here's an example of how to read XML RSA keys in PHP:

Answer 4

There is no standard for storing RSA public keys in XML. So the manner of conversion will depend on the XML you have.

Answer 5

Just for completeness, here is a working example of creating the PEM from modulus in python. You could call it in a subprocess from PHP if necessary.
The meat of the solution is:
def big_endian(n):
s = '%x' % n
if len(s) & 1:
s = '0' + s
return s.decode('hex')
from M2Crypto import RSA
e = E_PREFIX + big_endian(public_exponent)
n = N_PREFIX + big_endian(modulus)
new = RSA.new_pub_key((e,n))
new.save_key('foo.pub')
Where E_PREFIX and N_PREFIX are constants that (as far as I can tell) depend on the exponent and key length. Here is a quick table I have constructed:
E_PREFIX = '\x00\x00\x00\x01' # 0x3 (3)
E_PREFIX = '\x00\x00\x00\x03' # 0x10001 (65537)
N_PREFIX = '\x00\x00\x00!\x00' # 256-bit
N_PREFIX = '\x00\x00\x00A\x00' # 512-bit (default)
N_PREFIX = '\x00\x00\x00\x81\x00' # 1024-bit
N_PREFIX = '\x00\x00\x01\x01\x00' # 2048-bit
N_PREFIX = '\x00\x00\x02\x01\x00' # 4096-bit
If someone knows a more general way to compute the prefixes, do tell.

Answer 6

Maybe you should have a look here
Extract the two base64-encoded strings, convert and pass to PEAR::Crypt_RSA, then export as text file, then openssl convert?
Check this too