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Base64 With OpenSSL C API

OpenSSL has the ability to perform Base64 encodings and decodings. There seems to be many queries for working examples on how to use this functionality. Unfortunately, the example on the OpenSSL site is quite obtuse, and every other example I have come accross does not work. So here is some working code. Enjoy!

Get The Code

You can download this entire gist here. It consists of the following files:

  • Base64Decode.c - the decode function (takes Base64 encoded string as input).
  • Base64Encode.c - the encode function (takes a "normal" string as input).
  • Main.c - the main c file that demonstrates usage of the functionality in the two files above.
  • Makefile - the C makefile. Compilation has been tested on a linux ubuntu distribution, and links with lcrypto for opensll and lm for math.

Base64 Encoding

Note the following:

  • Given a string of length n, the resulting Base64 string is length $4 *\lceil \frac{n}{3} \rceil $. This is performed on line 12.
  • On line 13, *buffer is malloc'd to encodedSize+1. The +1 is because an extra character is needed for the NULL character ('\0') at the end of the string.

Base64 Decoding

Note the following:

  • It is important to set the flag BIO_FLAGS_BASE64_NO_NL. If this is not done, the read operation will block until a newline character (\n) is encountered.
  • The function calcDecodeLength will, given a Base64 encoded input string, calculate the length of the decoded string. Base64 encodes a "normal" 8 bit character string by using only 6 bits (hence only $2^6=64$ characters are needed). Therefore every 4 characters of Base64 decodes to three decoded characters, and multiplying the length of the Base64 string by $\frac{3}{4}$ will typically suffice. There are however two exceptions due to padding denoted by the = character. For more information, read decoding base64 with padding.


The above functionality is used like so:

Compile it with this MakeFile:

Memory Stuff

The memory for buffer in both functions is created on the heap using malloc. Therefore, it must be managed. This is a tiny example, and the program ends before any memory leaks become a problem, but in production code, remember to free the heap memory occupied by buffer after it has been used. This is done with the free command.


The above functions should perform better error checking if used in production. It also only works for encoding and decoding of a strings (although it is not too difficult to get it to work for files as well). This should give the inquisitive (and frustrated) programmer a base from which to work from.

Published In Blog
Tags: developmentOpenSSL

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