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The bit patterns produced by the encoding rules described above are unlikely, in general, to correspond to printable characters. But sometimes it's convenient to represent the encoded data in printable form. This can be done by converting it to base64 encoding.
Base64 is a scheme for converting binary data to printable ASCII characters, namely the upper- and lower-case Roman alphabet characters (A–Z, a–z), the numerals (0–9), and the "+" and "/" symbols, with the "=" symbol as a special suffix code.
The data's original length must be a multiple of eight bits. To convert data to base64, the first byte is placed in the most significant eight bits of a 24-bit buffer, the next in the middle eight, and the third in the least significant eight bits. If there are fewer than three bytes to encode, the corresponding buffer bits will be zero. The buffer is then used, six bits at a time, most significant first, as indices into the string "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/" and each character thus selected is output.
This is repeated until the input data is exhausted. If at the end only one or two input bytes remain, then only the first two or three characters of the output are used, and the output is then padded with two or one "=" characters respectively. This prevents anyone adding extra bits to the reconstructed data.
The resulting base64-encoded data is approximately 33% longer than the original data, and typically appears as seemingly random characters.
Base64 encoding is specified in full in RFC 1421 and RFC 2045.
Many of the commands provided by the openssl.exe utility can convert between normal binary encoding and base64 encoding.
