AES Encryption for an NSString?

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.

Welcome to SO; congrats for your first question, very clearly and well written. — Cyrille, Oct 05 '12 at 21:05

score 0 · Answer 1 · edited May 23 '17 at 11:45

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

AES Encryption for an NSString?

1 Answers1