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(PHP ActiveX) Diffie-Hellman Key Exchange (DH)Diffie-Hellman key exchange (DH) is a cryptographic protocol that allows two parties that have no prior knowledge of each other to jointly establish a shared secret key. This example demonstrates how two parties (Alice and Bob) can compute an N-bit shared secret key without the key ever being transmitted.
<?php // This example requires the Chilkat API to have been previously unlocked. // See Global Unlock Sample for sample code. // Create two separate instances of the DH object. // For versions of Chilkat < 10.0.0, use new COM('Chilkat_9_5_0.Chilkat.Dh') $dhBob = new COM("Chilkat.Dh"); // For versions of Chilkat < 10.0.0, use new COM('Chilkat_9_5_0.Chilkat.Dh') $dhAlice = new COM("Chilkat.Dh"); // The DH algorithm begins with a large prime, P, and a generator, G. // These don't have to be secret, and they may be transmitted over an insecure channel. // The generator is a small integer and typically has the value 2 or 5. // The Chilkat DH component provides the ability to use known // "safe" primes, as well as a method to generate new safe primes. // This example will use a known safe prime. Generating // new safe primes is a time-consuming CPU intensive task // and is normally done offline. // Bob will choose to use the 2nd of our 8 pre-chosen safe primes. // It is the Prime for the 2nd Oakley Group (RFC 2409) -- // 1024-bit MODP Group. Generator is 2. // The prime is: 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 } $dhBob->UseKnownPrime(2); // The computed shared secret will be equal to the size of the prime (in bits). // In this case the prime is 1024 bits, so the shared secret will be 128 bytes (128 * 8 = 1024). // However, the result is returned as an SSH1-encoded bignum in hex string format. // The SSH1-encoding prepends a 2-byte count, so the result is going to be 2 bytes // longer: 130 bytes. This results in a hex string that is 260 characters long (two chars // per byte for the hex encoding). // Bob will now send P and G to Alice. $p = $dhBob->P; $g = $dhBob->G; // Alice calls SetPG to set P and G. SetPG checks // the values to make sure it's a safe prime and will // return 0 if not. $success = $dhAlice->SetPG($p,$g); if ($success != 1) { print 'P is not a safe prime' . "\n"; exit; } // Each side begins by generating an "E" // value. The CreateE method has one argument: numBits. // It should be set to twice the size of the number of bits // in the session key. // Let's say we want to generate a 128-bit session key // for AES encryption. The shared secret generated by the Diffie-Hellman // algorithm will be longer, so we'll hash the result to arrive at the // desired session key length. However, the length of the session // key we'll utlimately produce determines the value that should be // passed to the CreateE method. // In this case, we'll be creating a 128-bit session key, so pass 256 to CreateE. // This setting is for security purposes only -- the value // passed to CreateE does not change the length of the shared secret // that is produced by Diffie-Hellman. // Also, there is no need to pass in a value larger // than 2 times the expected session key length. It suffices to // pass exactly 2 times the session key length. // Bob generates a random E (which has the mathematical // properties required for DH). $eBob = $dhBob->createE(256); // Alice does the same: $eAlice = $dhAlice->createE(256); // The "E" values are sent over the insecure channel. // Bob sends his "E" to Alice, and Alice sends her "E" to Bob. // Each side computes the shared secret by calling FindK. // "K" is the shared-secret. // Bob computes the shared secret from Alice's "E": $kBob = $dhBob->findK($eAlice); // Alice computes the shared secret from Bob's "E": $kAlice = $dhAlice->findK($eBob); // Amazingly, kBob and kAlice are identical and the expected // length (260 characters). The strings contain the hex encoded bytes of // our shared secret: print 'Bob's shared secret:' . "\n"; print $kBob . "\n"; print 'Alice's shared secret (should be equal to Bob's)' . "\n"; print $kAlice . "\n"; // To arrive at a 128-bit session key for AES encryption, Bob and Alice should // both transform the raw shared secret using a hash algorithm that produces // the size of session key desired. MD5 produces a 16-byte (128-bit) result, so // this is a good choice for 128-bit AES. // To produce the session key: // For versions of Chilkat < 10.0.0, use new COM('Chilkat_9_5_0.Chilkat.Crypt2') $crypt = new COM("Chilkat.Crypt2"); $crypt->EncodingMode = 'hex'; $crypt->HashAlgorithm = 'md5'; $sessionKey = $crypt->hashStringENC($kBob); print '128-bit Session Key:' . "\n"; print $sessionKey . "\n"; // Encrypt something... $crypt->CryptAlgorithm = 'aes'; $crypt->KeyLength = 128; $crypt->CipherMode = 'cbc'; // Use an IV that is the MD5 hash of the session key... $iv = $crypt->hashStringENC($sessionKey); // AES uses a 16-byte IV: print 'Initialization Vector:' . "\n"; print $iv . "\n"; $crypt->SetEncodedKey($sessionKey,'hex'); $crypt->SetEncodedIV($iv,'hex'); // Encrypt some text: $crypt->EncodingMode = 'base64'; $cipherText64 = $crypt->encryptStringENC('The quick brown fox jumps over the lazy dog'); print $cipherText64 . "\n"; $plainText = $crypt->decryptStringENC($cipherText64); print $plainText . "\n"; ?> |
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