The Amazon S3 proposal for uploading via POST describes how to assemble a policy document that can be used to create a time-sensitive signature. The obvious advantage to this method is that you don’t have to worry about someone stealing your secret AWS key or uploading random files without your permission.

Here is the example policy document from the proposal:

{ "expiration": "2007-12-01T12:00:00.000Z",
  "conditions": [
    {"acl": "public-read" },
    {"bucket": "johnsmith" },
    ["starts-with", "$key", "user/eric/"],
    ["content-length-range", 2048, 20971520]
  ]
}

This Policy document is Base64 encoded and the Signature is the HMAC of the Base64 encoding.

The application I am developing at work requires this signed policy method of uploading files to S3, however I needed to do it with PHP4 and preferably without any extra PEAR packages. This posed somewhat of a challenge, as all the tutorials I found on the web explained how to sign the policy using the PEAR Crypt_HMAC package or some feature of PHP5.

I eventually figured it out, and I’m here to show you how. The two functions used were found on the web (I don’t remember exactly where) and worked perfectly for my situation.

(Note: I had a lot of trouble saving the contents of the following code in WordPress due to some Apache mod_security settings configured on my server.)

/*
 * Calculate HMAC-SHA1 according to RFC2104
 * See http://www.faqs.org/rfcs/rfc2104.html
 */
function hmacsha1($key,$data) {
    $blocksize=64;
    $hashfunc='sha1';
    if (strlen($key)>$blocksize)
        $key=pack('H*', $hashfunc($key));
    $key=str_pad($key,$blocksize,chr(0x00));
    $ipad=str_repeat(chr(0x36),$blocksize);
    $opad=str_repeat(chr(0x5c),$blocksize);
    $hmac = pack(
                'H*',$hashfunc(
                    ($key^$opad).pack(
                        'H*',$hashfunc(
                            ($key^$ipad).$data
                        )
                    )
                )
            );
    return bin2hex($hmac);
}

/*
 * Used to encode a field for Amazon Auth
 * (taken from the Amazon S3 PHP example library)
 */
function hex2b64($str)
{
    $raw = '';
    for ($i=0; $i < strlen($str); $i+=2)
    {
        $raw .= chr(hexdec(substr($str, $i, 2)));
    }
    return base64_encode($raw);
}

/* Create the Amazon S3 Policy that needs to be signed */
$policy = '{ "expiration": "2007-12-01T12:00:00.000Z",
  "conditions": [
    {"acl": "public-read" },
    {"bucket": "johnsmith" },
    ["starts-with", "$key", "user/eric/"],
    ["content-length-range", 2048, 20971520]
  ]';

/*
 * Base64 encode the Policy Document and then
 * create HMAC SHA-1 signature of the base64 encoded policy
 * using the secret key. Finally, encode it for Amazon Authentication.
 */
$base64_policy = base64_encode($policy);
$signature = hex2b64(hmacsha1($secretkey, $base64_policy));

That’s it! This method doesn’t require PHP5 and doesn’t require any additional PEAR packages.