AES ECB iOS Encrypt

Although AES/ECB is not really recommended. This post explains why there's an alignment error and how to deal with it.

Alignment error almost means there is something wrong with the sizes.

Why is there an error?

A block cipher works on units of a fixed size (known as a block size), but messages come in a variety of lengths. So some modes (namely ECB and CBC) require that the final block be padded before encryption. (Source: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Electronic_Codebook_(ECB) )

Since AES ECB doesn't do the padding for you, you have to process the raw data before encrypting it.

Of course you can use kCCOptionPKCS7Padding when permitted, but since this thread is talking about ECB mode and alignment error, let's just focus on how to pad the data.

How many bytes should I pad?

The number of plain data bytes has to be an integral multiple of the current algorithm's block size.

That is, if your block size is kCCBlockSizeAES128 (16), you have to pad your data into "the nearest multiple of the block size" (16*n).

For example,

if your data is "abc" (3 bytes), then you have to pad your data into 16 bytes;

if your data is "1234567890123456" (16 bytes), then you have to pad it into 32 bytes.

0  ~ 15 bytes => pad to 16 bytes
16 ~ 31 bytes => pad to 32 bytes
32 ~ 47 bytes => pad to 48 bytes
... and so on

What is the value of the bytes to be padded?

If you have to pad 1 byte, then the 1 byte you pad would be '01';
If you have to pad 2 bytes, then the 2 bytes you pad would be '02';
...
If you have to pad 16 bytes, then the 16 bytes you pad would be '10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10'

Example of raw data and padded data before encryption

Example 1. (under block size 16)

Original data string: "abc" (3 bytes)
Data Bytes: "61 62 63"
Padded Data Bytes: "61 62 63 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d"

Explanation:
Data are 3 bytes, so the nearest multiple of the block size is 16 bytes.
So there are 13 bytes to be padded. 
And 13 in hex would be 0xd, so '0d' is padded for 13 times.

Let's have another example of data that is exactly a multiple of block size.

Example 2. (under block size 16)

Original data string: "1234567890123456" (16 bytes)
Data Bytes: "31 32 33 34 35 36 37 38 39 30 31 32 33 34 35 36"
Padded Data Bytes: "31 32 33 34 35 36 37 38 39 30 31 32 33 34 35 36 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10"

Explanation:
Data are 16 bytes, so the nearest multiple of the block size is 32 bytes.
So there are 16 bytes to be padded. 
And 16 in hex would be 0x10, so '10' is padded for 16 times.

To pad the data in Objective-C

Here is an example of how to pad your data when using AES/ECB encryption mode:

+ (NSData *)addPaddingBeforeEncryptWithAESECB:(NSData *) data{
    //Length has to be the nearest multiple of the block size
    int cipherLen = (int)(data.length/kAlgorithmBlockSize + 1)*kAlgorithmBlockSize;
    NSMutableData *newData = [NSMutableData dataWithLength:cipherLen];
    newData = [data mutableCopy];

    //How many bytes to be padded
    int bytesToAddOn = kAlgorithmBlockSize - data.length%kAlgorithmBlockSize;

    //Each byte in hex
    char byteToAdd = bytesToAddOn & 0xff;

    char *buffer = malloc(bytesToAddOn * sizeof byteToAdd);
    memset (buffer, byteToAdd, sizeof (char) * bytesToAddOn);

    [newData appendBytes:buffer length:bytesToAddOn];
    return newData;
}

Full example of AES ECB encryption:

+ (NSData *)encryptDataWithAESECB:(NSData *)data
                              key:(NSData *) key
                            error:(NSError **)error {

    size_t outLength;

    int cipherLen = (int)(data.length/kAlgorithmBlockSize + 1)*kAlgorithmBlockSize;
    NSMutableData *cipherData = [NSMutableData dataWithLength:cipherLen];
    NSData *newData = [self addPaddingBeforeEncryptWithAESECB:data];

    CCCryptorStatus result = CCCrypt(kCCEncrypt, // operation
                                     kAlgorithm, // Algorithm
                                     kCCOptionECBMode, // Mode
                                     key.bytes, // key
                                     key.length, // keylength
                                     0,// iv 
                                     newData.bytes, // dataIn
                                     newData.length, // dataInLength,
                                     cipherData.mutableBytes, // dataOut
                                     cipherData.length, // dataOutAvailable
                                     &outLength); // dataOutMoved


    if (result == kCCSuccess) {
        cipherData.length = outLength;
    }else {
        if (error) {
            *error = [NSError errorWithDomain:kRNCryptManagerErrorDomain code:result userInfo:nil];
        }
        return nil;
    }
    return cipherData;
}

What's next?

You have to strip off the extra bytes before AES ECB decryption.