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articles:base64 [2015-01-31 21:01 SLT] sei info on padding |
articles:base64 [2015-07-10 04:15 SLT] (current) sei Add nav |
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| =====Base64===== | =====Base64===== | ||
| - | Base64 is one way of storing sequences of characters in strings, that represent a sequence of bytes. | + | Base64 is one way of storing sequences of characters in strings, that represent a sequence of bytes. It's similar to hexadecimal, which provides 4 bits per character, and thus it needs 2 characters per byte. Base64 is more compact: with base64 only 4 characters are needed for every 3 bytes, or ~1.333 characters per byte on average. |
| The representation uses the characters A to Z, a to z, 0 to 9, the plus sign (+) and the slash (/), in that order. Each character represents a 6-bit binary number from ''000000'' to ''111111'', which is part of a byte. So A represents ''000000'', B ''000001'', ..., Z ''011001'', a ''011010'', ..., z ''110011'', 0 ''110100'', ..., 9 ''111101'', + represents ''111110'', and finally / represents ''111111''. | The representation uses the characters A to Z, a to z, 0 to 9, the plus sign (+) and the slash (/), in that order. Each character represents a 6-bit binary number from ''000000'' to ''111111'', which is part of a byte. So A represents ''000000'', B ''000001'', ..., Z ''011001'', a ''011010'', ..., z ''110011'', 0 ''110100'', ..., 9 ''111101'', + represents ''111110'', and finally / represents ''111111''. | ||
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| These sequences of six bits are parts of bytes. The first character represents the leftmost 6 bits of the first byte; the second character represents the rightmost two bits of the first byte, plus the leftmost 4 bits of the second byte; the third character represents the rightmost 4 bits of the second byte, plus the leftmost two bits of the third byte; the fourth character represents the rightmost 6 bits of the third byte, and then the cycle repeats. | These sequences of six bits are parts of bytes. The first character represents the leftmost 6 bits of the first byte; the second character represents the rightmost two bits of the first byte, plus the leftmost 4 bits of the second byte; the third character represents the rightmost 4 bits of the second byte, plus the leftmost two bits of the third byte; the fourth character represents the rightmost 6 bits of the third byte, and then the cycle repeats. | ||
| - | So, every four Base64 characters represent three bytes. Let's see for example what sequence of bytes does the string "z9+A" represent. z is ''110011'', 9 is ''111101'', + is ''111110'' and A is '000000'' so the bit string is: | + | So, every four Base64 characters represent three bytes. Let's see for example what sequence of bytes does the string "z9+A" represent. z is ''110011'', 9 is ''111101'', + is ''111110'' and A is ''000000'' so the bit string is: |
| 110011 111101 111110 000000 | 110011 111101 111110 000000 | ||
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| which is the byte sequence 0xCF, 0xDF, 0x80. Therefore we can say that the Base64 string "z9+A" represents the byte sequence 0xCF 0xDF 0x80. | which is the byte sequence 0xCF, 0xDF, 0x80. Therefore we can say that the Base64 string "z9+A" represents the byte sequence 0xCF 0xDF 0x80. | ||
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| - | It's more compact than hexadecimal, which provides 4 bits per character, and thus it needs 2 characters per byte. With base64 only 4 characters are needed for every 3 bytes, or ~1.333 characters per byte on average. | ||
| The standard that specifies how Base64 is encoded requires that Base64 strings have a number of characters that is a multiple of four. When it is not, equal signs (=) are appended until it is. LSL is permissive with respect to padding, and accepts unpadded strings. When it generates Base64 strings, it generates them with padding. | The standard that specifies how Base64 is encoded requires that Base64 strings have a number of characters that is a multiple of four. When it is not, equal signs (=) are appended until it is. LSL is permissive with respect to padding, and accepts unpadded strings. When it generates Base64 strings, it generates them with padding. | ||