Concatenate two Base64 encoded strings

Question

I want to decode two Base64 encoded strings and combine them to make one 128 bit string. I am able to decode the Base64 encoded strings. Can some one guide me on how to combine these two decoded strings?

This is the code I used for decoding the two encoded strings.

NSData *decodedData_contentKey = [[NSData alloc] initWithBase64EncodedString:str_content options:0];
NSString *decodedString_contentKey = [[NSString alloc] initWithData:decodedData_contentKey encoding:NSUTF8StringEncoding];
NSLog(@"%@", decodedString_contentKey);

Thanks.

"128 bit string" makes no sense. What is the problem? Where is the test data and incorrect results? Where is the code to combine the strings? If you just want to concatenate two strings use the `NSString` method: `stringByAppendingString:`. — zaph, Sep 21 '14 at 07:22
Note that encoding != encryption, changed tags to reflect this. — Maarten Bodewes, Sep 21 '14 at 12:48

score 1 · Answer 1 · edited May 23 '17 at 10:33

Base 64 is a statically sized encoding of octets/bytes into characters/text: 6 bits of a byte are represented as a printable ASCII character. Hence the name: 2^6 = 64, it uses a alphabet of 64 characters to encode the binary data (+ plus a delimiter character: '=' that does not contain encoded bits).

UTF-8 - used in your sample code - on the other hand is a character-encoding. It is used to encode characters in octets. So character encoding works the other way around. What you are actually doing is to decode the characters back from the bytes. UTF-8 does not use 128 bit values, nor does is it statically sized; multiple bytes may be used to represent one character. It will likely fail when it comes across an octet or octets that do not combine into a valid character encoding.

There is no such thing as base 128 encoding. Please think of what you are trying to accomplish and ask a new question that we can decode, if you get stuck.

GUESSED ANSWER:

Base 64 encoding will output 64 bits (8 bytes) of ASCII text for each 6 bytes. Therefore, if you want 128 bit (16 bytes) of encoding output, you simply have to input 12 bytes. As the base 64 encoding restarts at each 4 character boundary however (4 * 8 = 32 bits of encoding, each 8 bit character represents 6 bits, 4 * 6 = 24 bits of data, 24 bits is 3 bytes -> each 4 character string holds precisely 3 bytes of input), you can simply concatenate the two base 64 strings without decoding.

Concatenate two Base64 encoded strings

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