Verify signatures with public key obtained through DNS TXT records.
Alice wants to send a message to Bob and prevent Carol from altering the message.
Alice generates an RSA key:
$ openssl genrsa -out private.key 1024
Alice extracts the public key:
$ openssl rsa -in private.key -text -pubout
... and stores it in a DNS TXT record of her domain:
$ORIGIN example.net.
[...]
foo IN TXT "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCyvgpjWK2w6FmlFxdxcSBwP0P50jsIUB3ZV0xYXolLF0jipXdjN/TgZjBAlRYlF7u/iXW7WmrbU/mwpdlyiK0+dZvwZMGUzqBOC1ULz5i8xPVidutkBhMMWFovipibnvU6TePJjMdeBTlEDpi1p4/V66yOxJ8sU1nYCTYJIqaX1QIDAQAB"
Alice signs the message with the private key:
$ php <<'EOT'
<?php
include_once("DNSPubKeyVerifier.php");
$message = "I love you!";
$signer = new DNSPubKeySigner("file://private.key");
$signature = $signer->sign($message);
printf("* Signature = '%s'\n", $signature);
EOT
* Signature = 'gbpMyBy6TiWzHB3IGPk4eQQMCrKxSDbi2CdN9+anbyuS/nHzAO3s/TDkTZP1d68qVOSs1f/fBxJDr84tSlCeYDInrpMl00Y5PPXKMXT8ce3O7IzBb6cz8IZ6IzbB4G+vyLV7LlmJ7TONxb8dngl+ZwuVJE/CKT8i2pXqAtcc4h0='
Alice sends the message and the signature to Bob.
Bob verifies the message signature with the public key from Alice's TXT DNS record of "foo.example.net":
$ php <<'EOT'
<?php
include_once("DNSPubKeyVerifier.php");
$message = "I love you!";
$signature = "gbpMyBy6TiWzHB3IGPk4eQQMCrKxSDbi2CdN9+anbyuS/nHzAO3s/TDkTZP1d68qVOSs1f/fBxJDr84tSlCeYDInrpMl00Y5PPXKMXT8ce3O7IzBb6cz8IZ6IzbB4G+vyLV7LlmJ7TONxb8dngl+ZwuVJE/CKT8i2pXqAtcc4h0=";
$verifier = new DNSPubKeyVerifier("foo.example.net");
$res = $verifier->verify($message, $signature);
printf("* Message is %s.\n", $res ? 'authentic' : 'NOT authentic');
EOT
* Message is authentic.
If Carol tampered the message, then verification will fail:
$ php <<'EOT'
<?php
include_once("DNSPubKeyVerifier.php");
$message = "I hate you!";
$signature = "gbpMyBy6TiWzHB3IGPk4eQQMCrKxSDbi2CdN9+anbyuS/nHzAO3s/TDkTZP1d68qVOSs1f/fBxJDr84tSlCeYDInrpMl00Y5PPXKMXT8ce3O7IzBb6cz8IZ6IzbB4G+vyLV7LlmJ7TONxb8dngl+ZwuVJE/CKT8i2pXqAtcc4h0=";
$verifier = new DNSPubKeyVerifier("foo.example.net");
$res = $verifier->verify($message, $signature);
printf("* Message is %s.\n", $res ? 'authentic' : 'NOT authentic');
EOT
* Message is NOT authentic.
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