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I have two BASE64 encoded strings and I would like to get the BASE64 encoding of the binary concatenation of the two string using just the command line.

Example:

> $ echo -n "\x01\x02" |base64                                                          
AQI=

> $ echo -n "\x03\x04" |base64                                                              
AwQ=

> $ echo -n "\x01\x02\x03\x04" |base64
AQIDBA==

So the input values to my problem would be AQI= and AwQ=, the desired output is AQIDBA==

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2 Answers 2

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2

With bash:

str1=$(echo -ne "\x01\x02" | base64)
str2=$(echo -ne "\x03\x04" | base64)
if [[ $str1 =~ =$ ]; then
    concat=$( { base64 -d <<<"$str1"; base64 -d <<<"$str2"; } | base64 )
else
    concat="${str1}${str2}"
fi
printf '%s\n' "$concat"

The point being that if str1 doesn't end in in = then the Base64 form has no padding, so it can be concatenated just as it is. Otherwise the string needs to be re-encoded.

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  • Surely there's a way to combine padded base64 strings without decoding the whole thing. Base64 encodes chunks of data (I think 4 bytes?), and later chunks have no effect on earlier chunks. In the case that the first string contains padding, you can just decode from the second-last chunk boundary of the first string and append the decoded second string before re-encoding. If you're dealing with several MB or GB of strings this makes a big difference (in terms of processing time and buffer size). For more than 2 strings this can be done recursively. Commented Feb 17, 2020 at 17:23
  • @ATLief Yes, you only need to decode the last 4 characters in the first string, and you can also avoid decoding the second string with the use of a suitably large translation table for 1- and 2-bits shifts. Would that be an optimization? Absolutely. Would that matter in the context of the actual question above, of strings of length 2 (or at least "short enough")? Err... ROFL. See also: straw man argument. Commented Feb 18, 2020 at 18:50
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It's probably easiest to decode the inputs and encode again.

Since this was tagged , you likely have the GNU coreutils version of base64, which can decode multiple back-to-back inputs, so you can just concatenate the encoded strings first:

$ echo "AQI=AwQ=" | base64 -d | base64
AQIDBA==

Other decoders (e.g. the one on macOS, or Perl's MIME::Base64) don't do that, but instead may error out, or just silently drop the latter part.

So make sure to verify that the above works correctly before using it. If doesn't work on your system (or you don't like concatenating the encoded strings for some other reason), then run the decoder separately for each string:

$ (echo "AQI=" |base64 -d ; echo "AwQ=" |base64 -d) | base64
AQIDBA==

I'm assuming here that the two encoded strings are known separately, without being concatenated beforehand. This is the case in the question as posted anyway.

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  • 2
    This takes advantage of a glitch in base64. If won't work with other Base64 decoders / encoders (f.i. with mimencode -u / mimencode). That's because = is only valid at the end of the string, as padding. Commented Jun 8, 2017 at 12:36
  • glitch or a feature? In this case it happens to do exactly what is wanted. Commented Jun 8, 2017 at 18:22
  • As I said: AQI=AwQ= is invalid Base64 data, I can't really see how that could be a feature. I can however easily imagine it being a parser glitch. :) In fact I'm not convinced off hand it's going to work in all cases. You have only tested it with a single pair of 2-byte strings, where all bytes have a zero high nibble. At the very least you need to test more corner cases. shrug Commented Jun 8, 2017 at 20:22
  • @SatoKatsura, is it forbidden for a decoder to decode multiple back-to-back base64 strings, then? Calling it a glitch and assuming it's broken is quite a leap, but you're of course free to show an input it breaks with. Commented Jun 8, 2017 at 21:18
  • As an example of what @SatōKatsura says, this doesn't work on the base64 binary that ships with macOS: you'll receive base64: stdin: (null): error decoding base64 input stream Commented Aug 16, 2024 at 3:24

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