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Base64

# Algorithm

Base64 is using 4 chars to represent 3 chars in original message.  

echo " 255 ^ 3 " | bc
# 16581375
echo " 64 ^ 4 " | bc
# 16777216

  So it is possible to represent 3 bytes with 4 chars (64 bases).

Wikipedia. The base64 encoding generates text that only contains upper case and lower case letters, (A–Z, a–z), numerals (0–9), and the "+", "/", and "=" symbols.