AES Encryption for an NSString? - objective-c

Question

I am implementing for iOS some decryption code for a message originating on a server over which I have no control. So the decryption requirements are:
Cipher Method : AES256
Cipher Mode: ECB
Padding: PKCS5Padding
Since my initial trials failed to decrypt. So I played around with some test vectors to see the code that i use was right,
This is the code that encrypts the data:
NSString+AESCrypt.h
-------------------
#import <Foundation/Foundation.h>
#import "NSData+AESCrypt.h"
#interface NSString (AESCrypt)
- (NSString *)AES256EncryptWithKey:(NSString *)key;
- (NSString *)AES256DecryptWithKey:(NSString *)key;
#end
NSString+AESCrypt.m
-------------------
#import "NSString+AESCrypt.h"
#implementation NSString (AESCrypt)
- (NSString *)AES256EncryptWithKey:(NSString *)key
{
NSData *plainData = [self dataUsingEncoding:NSUTF8StringEncoding];
NSData *encryptedData = [plainData AES256EncryptWithKey:key];
NSString *encryptedString = [encryptedData base64Encoding];
return encryptedString;
}
- (NSString *)AES256DecryptWithKey:(NSString *)key
{
NSData *encryptedData = [NSData dataWithBase64EncodedString:self];
NSData *plainData = [encryptedData AES256DecryptWithKey:key];
NSString *plainString = [[NSString alloc] initWithData:plainData encoding:NSUTF8StringEncoding];
return [plainString autorelease];
}
#end
NSData+AESCrypt.h
-------------------
#import <Foundation/Foundation.h>
#interface NSData (AESCrypt)
- (NSData *)AES256EncryptWithKey:(NSString *)key;
- (NSData *)AES256DecryptWithKey:(NSString *)key;
+ (NSData *)dataWithBase64EncodedString:(NSString *)string;
- (id)initWithBase64EncodedString:(NSString *)string;
- (NSString *)base64Encoding;
- (NSString *)base64EncodingWithLineLength:(NSUInteger)lineLength;
- (BOOL)hasPrefixBytes:(const void *)prefix length:(NSUInteger)length;
- (BOOL)hasSuffixBytes:(const void *)suffix length:(NSUInteger)length;
#end
NSData+AESCrypt.m
-------------------
#import "NSData+AESCrypt.h"
#import <CommonCrypto/CommonCryptor.h>
static char encodingTable[64] =
{
'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P',
'Q','R','S','T','U','V','W','X','Y','Z','a','b','c','d','e','f',
'g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v',
'w','x','y','z','0','1','2','3','4','5','6','7','8','9','+','/'
};
#implementation NSData (AESCrypt)
- (NSData *)AES256EncryptWithKey:(NSString *)key
{
// 'key' should be 32 bytes for AES256, will be null-padded otherwise
char keyPtr[kCCKeySizeAES256 + 1]; // room for terminator (unused)
bzero( keyPtr, sizeof( keyPtr ) ); // fill with zeroes (for padding)
// fetch key data
[key getCString:keyPtr maxLength:sizeof( keyPtr ) encoding:NSUTF8StringEncoding];
NSUInteger dataLength = [self length];
//See the doc: For block ciphers, the output size will always be less than or
//equal to the input size plus the size of one block.
//That's why we need to add the size of one block here
size_t bufferSize = dataLength + kCCBlockSizeAES128;
void *buffer = malloc( bufferSize );
size_t numBytesEncrypted = 0;
CCCryptorStatus cryptStatus = CCCrypt( kCCEncrypt, kCCAlgorithmAES128, kCCOptionECBMode + kCCOptionPKCS7Padding,
keyPtr, kCCKeySizeAES256,
NULL /* initialization vector (optional) */,
[self bytes], dataLength, /* input */
buffer, bufferSize, /* output */
&numBytesEncrypted );
if( cryptStatus == kCCSuccess )
{
//the returned NSData takes ownership of the buffer and will free it on deallocation
return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
}
free( buffer ); //free the buffer
return nil;
}
- (NSData *)AES256DecryptWithKey:(NSString *)key
{
// 'key' should be 32 bytes for AES256, will be null-padded otherwise
char keyPtr[kCCKeySizeAES256+1]; // room for terminator (unused)
bzero( keyPtr, sizeof( keyPtr ) ); // fill with zeroes (for padding)
// fetch key data
[key getCString:keyPtr maxLength:sizeof( keyPtr ) encoding:NSUTF8StringEncoding];
NSUInteger dataLength = [self length];
//See the doc: For block ciphers, the output size will always be less than or
//equal to the input size plus the size of one block.
//That's why we need to add the size of one block here
size_t bufferSize = dataLength + kCCBlockSizeAES128;
void *buffer = malloc( bufferSize );
size_t numBytesDecrypted = 0;
CCCryptorStatus cryptStatus = CCCrypt( kCCDecrypt, kCCAlgorithmAES128, kCCOptionECBMode + kCCOptionPKCS7Padding,
keyPtr, kCCKeySizeAES256,
NULL /* initialization vector (optional) */,
[self bytes], dataLength, /* input */
buffer, bufferSize, /* output */
&numBytesDecrypted );
if( cryptStatus == kCCSuccess )
{
//the returned NSData takes ownership of the buffer and will free it on deallocation
return [NSData dataWithBytesNoCopy:buffer length:numBytesDecrypted];
}
free( buffer ); //free the buffer
return nil;
}
#pragma mark -
+ (NSData *)dataWithBase64EncodedString:(NSString *)string
{
return [[[NSData allocWithZone:nil] initWithBase64EncodedString:string] autorelease];
}
- (id)initWithBase64EncodedString:(NSString *)string
{
NSMutableData *mutableData = nil;
if( string )
{
unsigned long ixtext = 0;
unsigned long lentext = 0;
unsigned char ch = 0;
unsigned char inbuf[4], outbuf[3];
short i = 0, ixinbuf = 0;
BOOL flignore = NO;
BOOL flendtext = NO;
NSData *base64Data = nil;
const unsigned char *base64Bytes = nil;
// Convert the string to ASCII data.
base64Data = [string dataUsingEncoding:NSASCIIStringEncoding];
base64Bytes = [base64Data bytes];
mutableData = [NSMutableData dataWithCapacity:base64Data.length];
lentext = base64Data.length;
while( YES )
{
if( ixtext >= lentext ) break;
ch = base64Bytes[ixtext++];
flignore = NO;
if( ( ch >= 'A' ) && ( ch <= 'Z' ) ) ch = ch - 'A';
else if( ( ch >= 'a' ) && ( ch <= 'z' ) ) ch = ch - 'a' + 26;
else if( ( ch >= '0' ) && ( ch <= '9' ) ) ch = ch - '0' + 52;
else if( ch == '+' ) ch = 62;
else if( ch == '=' ) flendtext = YES;
else if( ch == '/' ) ch = 63;
else flignore = YES;
if( ! flignore )
{
short ctcharsinbuf = 3;
BOOL flbreak = NO;
if( flendtext )
{
if( ! ixinbuf ) break;
if( ( ixinbuf == 1 ) || ( ixinbuf == 2 ) ) ctcharsinbuf = 1;
else ctcharsinbuf = 2;
ixinbuf = 3;
flbreak = YES;
}
inbuf [ixinbuf++] = ch;
if( ixinbuf == 4 )
{
ixinbuf = 0;
outbuf [0] = ( inbuf[0] << 2 ) | ( ( inbuf[1] & 0x30) >> 4 );
outbuf [1] = ( ( inbuf[1] & 0x0F ) << 4 ) | ( ( inbuf[2] & 0x3C ) >> 2 );
outbuf [2] = ( ( inbuf[2] & 0x03 ) << 6 ) | ( inbuf[3] & 0x3F );
for( i = 0; i < ctcharsinbuf; i++ )
[mutableData appendBytes:&outbuf[i] length:1];
}
if( flbreak ) break;
}
}
}
self = [self initWithData:mutableData];
return self;
}
#pragma mark -
- (NSString *)base64Encoding
{
return [self base64EncodingWithLineLength:0];
}
- (NSString *)base64EncodingWithLineLength:(NSUInteger)lineLength
{
const unsigned char *bytes = [self bytes];
NSMutableString *result = [NSMutableString stringWithCapacity:self.length];
unsigned long ixtext = 0;
unsigned long lentext = self.length;
long ctremaining = 0;
unsigned char inbuf[3], outbuf[4];
unsigned short i = 0;
unsigned short charsonline = 0, ctcopy = 0;
unsigned long ix = 0;
while( YES )
{
ctremaining = lentext - ixtext;
if( ctremaining <= 0 ) break;
for( i = 0; i < 3; i++ )
{
ix = ixtext + i;
if( ix < lentext ) inbuf[i] = bytes[ix];
else inbuf [i] = 0;
}
outbuf [0] = (inbuf [0] & 0xFC) >> 2;
outbuf [1] = ((inbuf [0] & 0x03) << 4) | ((inbuf [1] & 0xF0) >> 4);
outbuf [2] = ((inbuf [1] & 0x0F) << 2) | ((inbuf [2] & 0xC0) >> 6);
outbuf [3] = inbuf [2] & 0x3F;
ctcopy = 4;
switch( ctremaining )
{
case 1:
ctcopy = 2;
break;
case 2:
ctcopy = 3;
break;
}
for( i = 0; i < ctcopy; i++ )
[result appendFormat:#"%c", encodingTable[outbuf[i]]];
for( i = ctcopy; i < 4; i++ )
[result appendString:#"="];
ixtext += 3;
charsonline += 4;
if( lineLength > 0 )
{
if( charsonline >= lineLength )
{
charsonline = 0;
[result appendString:#"\n"];
}
}
}
return [NSString stringWithString:result];
}
#pragma mark -
- (BOOL)hasPrefixBytes:(const void *)prefix length:(NSUInteger)length
{
if( ! prefix || ! length || self.length < length ) return NO;
return ( memcmp( [self bytes], prefix, length ) == 0 );
}
- (BOOL)hasSuffixBytes:(const void *)suffix length:(NSUInteger)length
{
if( ! suffix || ! length || self.length < length ) return NO;
return ( memcmp( ((const char *)[self bytes] + (self.length - length)), suffix, length ) == 0 );
}
#end
I execute above function and write the resulting data to the log with this code:
NSString * _secret = #"6bc1bee22e409f96e93d7e117393172a";
NSString * _key = #"603deb1015ca71be2b73aef0857d77811f352c073b6108d72d9810a30914dff4";
NSString *encryptedString = [_secret AES256EncryptWithKey:_key];
NSLog(#"Encrypted ID : %#", encryptedString);
NSString *decryptedString = [encryptedString AES256DecryptWithKey:_key];
NSLog(#"Decrypted ID : %#", decryptedString);
As from test vector, the encrypted cipher should be this:
f3eed1bdb5d2a03c064b5a7e3db181f8
Result logs:
2011-10-19 13:32:41.640 Ticket[2215:707] Encrypted ID : XWLsnTQvocXNkAqVisEgWTCPdYR6KPoIojezjN3fn/wuytQkpUZnNbzUoT4peeTK
2011-10-19 13:32:41.641 Ticket[2215:707] Decrypted ID : 6bc1bee22e409f96e93d7e117393172a
I know that this Encrypted ID is in Base64, but still even if i convert it to HEX, the actual output varies from the result.
What option am I forgetting? Is the encoding of the NSData returned something else…?
So if someone could direct me on the right path that would be great, Cheers.

Answer 1

I htink the problem is that the test-vector-page assumes that these printed hex-values are binaries and not text: The string "6b" does look utf-8 encoded like this: 0x3662.
for getting the correct test-string you have to encode it first. your string should start with this: #"kÁ¾â....". That String will result in the right hexa-representation if it will be ecnoded with utf-8.
You should either test your aes-encryption with NSData initialized with an hex-string or you have to decode utf-8 first to pack all that into a string. But take care: there are often symbols which cannot be represented/printed and even worse: if your tesdata or the cypher contains the Zero-Byte then this will cause some problems because it is often used as a termination-symbol in strings which normally contains only readable charachters. (dont know how this interacts with NSString)
or use the first variant to create an NSData out of a hex-string and then convert that data to an NSString