Question

The situation I’m trying to solve: in my Cocoa app, I need to encrypt a string with a symmetric cipher, POST it to PHP, and have that script decode the data. The process needs to work in reverse for returning an answer (PHP encodes, Cocoa decodes).

I’m missing something because even though I can get both the key and initialization vector (iv) to be the same in both PHP and Cocoa, the decoding never works when one app sends its encoded data to the other. Both work just fine encoding/decoding their own data (verified to make sure there wasn’t some PEBKAC issue at hand). I have a suspicion that there’s a padding issue someplace, I just don’t see it.

My cocoa app encodes using SSCrypto (which is just a handy-dandy wrapper around OpenSSL functions). The cipher is Blowfish, mode is CBC. (forgive the memory leaks, code has been stripped to the bare essentials)

NSData *secretText = [@'secretTextToEncode' dataUsingEncoding:NSUTF8StringEncoding]; NSData *symmetricKey = [@'ThisIsMyKey' dataUsingEncoding:NSUTF8StringEncoding];  unsigned char *input = (unsigned char *)[secretText bytes]; unsigned char *outbuf; int outlen, templen, inlen; inlen = [secretText length];  unsigned char evp_key[EVP_MAX_KEY_LENGTH] = {'\0'}; int cipherMaxIVLength = EVP_MAX_IV_LENGTH; EVP_CIPHER_CTX cCtx; const EVP_CIPHER *cipher = EVP_bf_cbc();  cipherMaxIVLength = EVP_CIPHER_iv_length( cipher ); unsigned char iv[cipherMaxIVLength];  EVP_BytesToKey(cipher, EVP_md5(), NULL, [symmetricKey bytes], [symmetricKey length], 1, evp_key, iv);  NSData *initVector = [NSData dataWithBytes:iv length:cipherMaxIVLength];  EVP_CIPHER_CTX_init(&cCtx);  if (!EVP_EncryptInit_ex(&cCtx, cipher, NULL, evp_key, iv)) {     EVP_CIPHER_CTX_cleanup(&cCtx);     return nil; } int ctx_CipherKeyLength = EVP_CIPHER_CTX_key_length( &cCtx ); EVP_CIPHER_CTX_set_key_length(&cCtx, ctx_CipherKeyLength);  outbuf = (unsigned char *)calloc(inlen + EVP_CIPHER_CTX_block_size(&cCtx), sizeof(unsigned char));  if (!EVP_EncryptUpdate(&cCtx, outbuf, &outlen, input, inlen)){     EVP_CIPHER_CTX_cleanup(&cCtx);     return nil; } if (!EVP_EncryptFinal(&cCtx, outbuf + outlen, &templen)){     EVP_CIPHER_CTX_cleanup(&cCtx);     return nil; } outlen += templen; EVP_CIPHER_CTX_cleanup(&cCtx);  NSData *cipherText = [NSData dataWithBytes:outbuf length:outlen];  NSString *base64String = [cipherText encodeBase64WithNewlines:NO]; NSString *iv = [initVector encodeBase64WithNewlines:NO]; 

base64String and iv are then POSTed to PHP that attempts to decode it:

<?php  import_request_variables( 'p', 'p_' );  if( $p_data != '' && $p_iv != '' ) {     $encodedData = base64_decode( $p_data, true );     $iv = base64_decode( $p_iv, true );      $td = mcrypt_module_open( MCRYPT_BLOWFISH, '', MCRYPT_MODE_CBC, '' );     $keySize = mcrypt_enc_get_key_size( $td );     $key = substr( md5( 'ThisIsMyKey' ), 0, $keySize );      $decodedData = mcrypt_decrypt(MCRYPT_BLOWFISH, $key, $encodedData, MCRYPT_MODE_CBC, $iv );     mcrypt_module_close( $td );      echo 'decoded: ' . $decodedData; } ?> 

decodedData is always gibberish.

I’ve tried reversing the process, sending the encoded output from PHP to Cocoa but EVP_DecryptFinal() fails, which is what leads me to believe there’s a NULL padding issue somewhere. I’ve read and re-read the PHP and OpenSSL docs but it’s all blurring together now and I’m out of ideas to try.

Answer 1

I think your problem is that the method of deriving the raw encryption key from the key string is different on the two sides. The php md5() function returns a hexadecimal string, i.e ‘a476c3…’ which you are chopping down to the key size, while EVP_BytesToKey() is a fairly complicated hash routine that return a raw byte string. It might, with the parameters supplied simplify down to a raw MD5 hash, but I can’t really tell. Either way, it’s going to be different from the php hash.

If you change the php to md5( ‘ThisIsMyKey’, TRUE ), that will give you a raw md5 hash. On the cocoa side of things, SSCrypto’s +getMD5ForData: method should generate the same one for the same string (text encoding issues aside).

Edit 1: If the php string and Cocoa data print out identically, they’re still different at the byte level. The php string is hex-encoded (i.e consists only of characters 0-9 and a-f) while the cocoa data is the raw bytes (although NSData helpfully prints out a hex-encoded string of its contents when NSLogged). You still need to add the second TRUE parameter to php’s md5() function to get the raw byte string.

Edit 2: OpenSSL 1.1.0c changed the digest algorithm used in some internal components. Formerly, MD5 was used, and 1.1.0 switched to SHA256. Be careful the change is not affecting you in both EVP_BytesToKey and commands like openssl enc.