The example below uses ASCII text for simplicity, but this is not a typical use case, as it can already be safely transferred across all systems that can handle Base64. This is the Base64 alphabet defined in RFC 4648 ยง4. For instance, uuencode uses uppercase letters, digits, and many punctuation characters, but no lowercase. The earliest instances of this type of encoding were created for dial-up communication between systems running the same OS, for example, uuencode for UNIX and BinHex for the TRS-80 (later adapted for the Macintosh), and could therefore make more assumptions about what characters were safe to use. Other variations share this property but differ in the symbols chosen for the last two values an example is UTF-7. For example, MIME's Base64 implementation uses A- Z, a- z, and 0- 9 for the first 62 values. This combination leaves the data unlikely to be modified in transit through information systems, such as email, that were traditionally not 8-bit clean. The general strategy is to choose 64 characters that are common to most encodings and that are also printable. The particular set of 64 characters chosen to represent the 64 digit values for the base varies between implementations. 4.11 Applications not compatible with RFC 4648 Base64.
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