Your code is leaking the first allocation of output.
Besides that it looks mostly ok.
This is a complete end-to-end implementation using a SHA256 hash of the user's passphrase (in this case 'Salamander') and base64'ing the output. There is a PHP test implementation in the source that reconstructs the key then trims the PKCS7 padding before giving final output. A decryptor in Ruby follows, the PKCS7 padding removal happens automatically by the OpenSSL::Cipher.
Here you go:
// Crypto categories for iOS
#import <CommonCrypto/CommonCryptor.h>
#import <CommonCrypto/CommonDigest.h>
@interface NSData( Crypto )
- (NSData *) aesEncryptedDataWithKey:(NSData *) key;
- (NSString *) base64Encoding;
@end
@interface NSString( Crypto )
- (NSData *) sha256;
@end
// --------
@implementation NSData( Crypto )
- (NSData *) aesEncryptedDataWithKey:(NSData *) key {
unsigned char *buffer = nil;
size_t bufferSize;
CCCryptorStatus err;
NSUInteger i, keyLength, plainTextLength;
// make sure there's data to encrypt
err = ( plainTextLength = [self length] ) == 0;
// pass the user's passphrase through SHA256 to obtain 32 bytes
// of key data. Use all 32 bytes for an AES256 key or just the
// first 16 for AES128.
if ( ! err ) {
switch ( ( keyLength = [key length] ) ) {
case kCCKeySizeAES128:
case kCCKeySizeAES256: break;
// invalid key size
default: err = 1; break;
}
}
// create an output buffer with room for pad bytes
if ( ! err ) {
bufferSize = kCCBlockSizeAES128 + plainTextLength + kCCBlockSizeAES128; // iv + cipher + padding
err = ! ( buffer = (unsigned char *) malloc( bufferSize ) );
}
// encrypt the data
if ( ! err ) {
srandomdev();
// generate a random iv and prepend it to the output buffer. the
// decryptor needs to be aware of this.
for ( i = 0; i < kCCBlockSizeAES128; ++i ) buffer[ i ] = random() & 0xff;
err = CCCrypt( kCCEncrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,
[key bytes], keyLength, buffer, [self bytes], plainTextLength,
buffer + kCCBlockSizeAES128, bufferSize - kCCBlockSizeAES128, &bufferSize );
}
if ( err ) {
if ( buffer ) free( buffer );
return nil;
}
// dataWithBytesNoCopy takes ownership of buffer and will free() it
// when the NSData object that owns it is released.
return [NSData dataWithBytesNoCopy: buffer length: bufferSize + kCCBlockSizeAES128];
}
- (NSString *) base64Encoding {
char *encoded, *r;
const char eTable[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
unsigned i, l, n, t;
UInt8 *p, pad = '=';
NSString *result;
p = (UInt8 *) [self bytes];
if ( ! p || ( l = [self length] ) == 0 ) return @"";
r = encoded = malloc( 4 * ( ( n = l / 3 ) + ( l % 3 ? 1 : 0 ) ) + 1 );
if ( ! encoded ) return nil;
for ( i = 0; i < n; ++i ) {
t = *p++ << 16;
t |= *p++ << 8;
t |= *p++;
*r++ = eTable[ t >> 18 ];
*r++ = eTable[ t >> 12 & 0x3f ];
*r++ = eTable[ t >> 6 & 0x3f ];
*r++ = eTable[ t & 0x3f ];
}
if ( ( i = n * 3 ) < l ) {
t = *p++ << 16;
*r++ = eTable[ t >> 18 ];
if ( ++i < l ) {
t |= *p++ << 8;
*r++ = eTable[ t >> 12 & 0x3f ];
*r++ = eTable[ t >> 6 & 0x3f ];
} else {
*r++ = eTable[ t >> 12 & 0x3f ];
*r++ = pad;
}
*r++ = pad;
}
*r = 0;
result = [NSString stringWithUTF8String: encoded];
free( encoded );
return result;
}
@end
@implementation NSString( Crypto )
- (NSData *) sha256 {
unsigned char *buffer;
if ( ! ( buffer = (unsigned char *) malloc( CC_SHA256_DIGEST_LENGTH ) ) ) return nil;
CC_SHA256( [self UTF8String], [self lengthOfBytesUsingEncoding: NSUTF8StringEncoding], buffer );
return [NSData dataWithBytesNoCopy: buffer length: CC_SHA256_DIGEST_LENGTH];
}
@end
// -----------------
@implementation AppDelegate
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
NSData *plain = [@"This is a test of the emergency broadcast system." dataUsingEncoding: NSUTF8StringEncoding];
NSData *key = [NSData dataWithBytes: [[@"Salamander" sha256] bytes] length: kCCKeySizeAES128];
NSData *cipher = [plain aesEncryptedDataWithKey: key];
NSString *base64 = [cipher base64Encoding];
NSLog( @"cipher: %@", base64 );
// stuff the base64'ed cipher into decrypt.php:
// http://localhost/~par/decrypt.php?cipher=<base64_output>
/*
<?php
header( "content-type: text/plain" );
if ( ! ( $cipher = $_GET[ 'cipher' ] ) ) {
echo "no cipher parameter found";
return;
}
echo "cipher: $cipher\n";
$cipher = base64_decode( $cipher );
$iv = substr( $cipher, 0, 16 );
$cipher = substr( $cipher, 16 );
// use the full key (all 32 bytes) for aes256
$key = substr( hash( "sha256", "Salamander", true ), 0, 16 );
$plainText = mcrypt_decrypt( MCRYPT_RIJNDAEL_128, $key, $cipher, MCRYPT_MODE_CBC, $iv );
$plainTextLength = strlen( $plainText );
// strip pkcs7 padding
$padding = ord( $plainText[ $plainTextLength - 1 ] );
$plainText = substr( $plainText, 0, -$padding );
printf( "plaintext: %s\n", $plainText );
?>
*/
return YES;
}
@end
Decryption of the output of the above in Ruby:
require 'base64'
require 'openssl'
def decrypt( cipherBase64 )
cipher = Base64.decode64( cipherBase64 )
aes = OpenSSL::Cipher::Cipher.new( "aes-128-cbc" ).decrypt
aes.iv = cipher.slice( 0, 16 )
# don't slice the SHA256 output for AES256
aes.key = ( Digest::SHA256.digest( 'Salamander' ) ).slice( 0, 16 )
cipher = cipher.slice( 16..-1 )
return aes.update( cipher ) + aes.final
end
text = '3o4ARWOxwmLEPgq3SJ3A2ws7sUSxMvWSKbbs+oABsOcywk+9qPBoDjhLAfAW/n28pbnsT2w5QMSye6pz3Lz8xmg5BYL8HdfKwbS9EpTbaUc='
print decrypt( text ) + "\n"
