Base64
什么是 Base64
Base64 是一种基于 64 个可打印字符来表示二进制数据的表示方法。
HTTP 的全称是 HyperText Transfer Protocol（超文本传输协议），最初设计的目的确实是：
- 传输 超文本（HTML）
但实际上作为应用层协议，HTTP 可以传输任意类型的数据。
- HTTP Header 不能直接放图片字节进去，只能用 ASCII 字符（Base64 就是 ASCII-safe）
- Email 内容（MIME 协议）SMTP 协议历史原因只支持 7-bit ASCII，不能发图片，怎么办？Base64！
- JSON / XML 这些格式都是文本格式，不能放二进制文件进去，必须转 Base64 编码后作为字符串存放
Base64要求把每三个8Bit的字节转换为四个6Bit的字节(3*8 = 4*6 = 24)，然后把6Bit再添两位高位0，组成四个8Bit的字节，也就是说，转换后的字符串理论上将要比原来的长1/3。
例如：
11111111,11111111,11111111 -> 00111111,00111111,00111111,00111111
知道了 byte 的表示，但是还需要一张表，作为 base64 的编码表。
base64 中的 64 表示 64 个字符：
// StdEncoding is the standard base64 encoding, as defined in RFC 4648.
var StdEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/")
// URLEncoding is the alternate base64 encoding defined in RFC 4648.
// It is typically used in URLs and file names.
var URLEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_")
Golang 中的 Base64 编码
在 Go 语言中，可以使用 encoding/base64 包来实现 Base64 编码和解码。
// An Encoding is a radix 64 encoding/decoding scheme, defined by a
// 64-character alphabet. The most common encoding is the "base64"
// encoding defined in RFC 4648 and used in MIME (RFC 2045) and PEM
// (RFC 1421). RFC 4648 also defines an alternate encoding, which is
// the standard encoding with - and _ substituted for + and /.
type Encoding struct {
encode [64]byte // mapping of symbol index to symbol byte value
decodeMap [256]uint8 // mapping of symbol byte value to symbol index
padChar rune
strict bool
}
/*
* Encoder
*/
// Encode encodes src using the encoding enc,
// writing [Encoding.EncodedLen](len(src)) bytes to dst.
//
// The encoding pads the output to a multiple of 4 bytes,
// so Encode is not appropriate for use on individual blocks
// of a large data stream. Use [NewEncoder] instead.
func (enc *Encoding) Encode(dst, src []byte) {
if len(src) == 0 {
return
}
// enc is a pointer receiver, so the use of enc.encode within the hot
// loop below means a nil check at every operation. Lift that nil check
// outside of the loop to speed up the encoder.
_ = enc.encode
di, si := 0, 0
n := (len(src) / 3) * 3
for si < n {
// Convert 3x 8bit source bytes into 4 bytes
val := uint(src[si+0])<<16 | uint(src[si+1])<<8 | uint(src[si+2])
dst[di+0] = enc.encode[val>>18&0x3F]
dst[di+1] = enc.encode[val>>12&0x3F]
dst[di+2] = enc.encode[val>>6&0x3F]
dst[di+3] = enc.encode[val&0x3F]
si += 3
di += 4
}
remain := len(src) - si
if remain == 0 {
return
}
// Add the remaining small block
val := uint(src[si+0]) << 16
if remain == 2 {
val |= uint(src[si+1]) << 8
}
dst[di+0] = enc.encode[val>>18&0x3F]
dst[di+1] = enc.encode[val>>12&0x3F]
switch remain {
case 2:
dst[di+2] = enc.encode[val>>6&0x3F]
if enc.padChar != NoPadding {
dst[di+3] = byte(enc.padChar)
}
case 1:
if enc.padChar != NoPadding {
dst[di+2] = byte(enc.padChar)
dst[di+3] = byte(enc.padChar)
}
}
}
encode 代码实现比较简单，但有几个注意点:
_ = enc.encode在循环外，避免每次循环都进行 nil 检查（hot loop）val := uint(src[si+0])<<16 | uint(src[si+1])<<8 | uint(src[si+2])将 3 个字节转换为 24 位dst[di+0] = enc.encode[val>>18&0x3F]将 24 位 0-5 位转换为 base64 编码dst[di+1] = enc.encode[val>>12&0x3F]将 24 位 6-11 位转换为 base64 编码dst[di+2] = enc.encode[val>>6&0x3F]将 24 位 12-17 位转换为 base64 编码dst[di+3] = enc.encode[val&0x3F]将 24 位 18-23 位转换为 base64 编码
Golang 中的 Base64 解码
解码函数是 Decode 函数，它将 Base64 编码的字符串解码为原始数据。
// An Encoding is a radix 64 encoding/decoding scheme, defined by a
// 64-character alphabet. The most common encoding is the "base64"
// encoding defined in RFC 4648 and used in MIME (RFC 2045) and PEM
// (RFC 1421). RFC 4648 also defines an alternate encoding, which is
// the standard encoding with - and _ substituted for + and /.
type Encoding struct {
encode [64]byte // mapping of symbol index to symbol byte value
decodeMap [256]uint8 // mapping of symbol byte value to symbol index
padChar rune
strict bool
}
// Decode decodes src using the encoding enc. It writes at most
// [Encoding.DecodedLen](len(src)) bytes to dst and returns the number of bytes
// written. The caller must ensure that dst is large enough to hold all
// the decoded data. If src contains invalid base64 data, it will return the
// number of bytes successfully written and [CorruptInputError].
// New line characters (\r and \n) are ignored.
func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
if len(src) == 0 {
return 0, nil
}
// Lift the nil check outside of the loop. enc.decodeMap is directly
// used later in this function, to let the compiler know that the
// receiver can't be nil.
_ = enc.decodeMap
si := 0
for strconv.IntSize >= 64 && len(src)-si >= 8 && len(dst)-n >= 8 {
src2 := src[si : si+8]
if dn, ok := assemble64(
enc.decodeMap[src2[0]],
enc.decodeMap[src2[1]],
enc.decodeMap[src2[2]],
enc.decodeMap[src2[3]],
enc.decodeMap[src2[4]],
enc.decodeMap[src2[5]],
enc.decodeMap[src2[6]],
enc.decodeMap[src2[7]],
); ok {
binary.BigEndian.PutUint64(dst[n:], dn)
n += 6
si += 8
} else {
var ninc int
si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
n += ninc
if err != nil {
return n, err
}
}
}
for len(src)-si >= 4 && len(dst)-n >= 4 {
src2 := src[si : si+4]
if dn, ok := assemble32(
enc.decodeMap[src2[0]],
enc.decodeMap[src2[1]],
enc.decodeMap[src2[2]],
enc.decodeMap[src2[3]],
); ok {
binary.BigEndian.PutUint32(dst[n:], dn)
n += 3
si += 4
} else {
var ninc int
si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
n += ninc
if err != nil {
return n, err
}
}
}
for si < len(src) {
var ninc int
si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
n += ninc
if err != nil {
return n, err
}
}
return n, err
}
// assemble32 assembles 4 base64 digits into 3 bytes.
// Each digit comes from the decode map, and will be 0xff
// if it came from an invalid character.
func assemble32(n1, n2, n3, n4 byte) (dn uint32, ok bool) {
// Check that all the digits are valid. If any of them was 0xff, their
// bitwise OR will be 0xff.
if n1|n2|n3|n4 == 0xff {
return 0, false
}
return uint32(n1)<<26 |
uint32(n2)<<20 |
uint32(n3)<<14 |
uint32(n4)<<8,
true
}
// assemble64 assembles 8 base64 digits into 6 bytes.
// Each digit comes from the decode map, and will be 0xff
// if it came from an invalid character.
func assemble64(n1, n2, n3, n4, n5, n6, n7, n8 byte) (dn uint64, ok bool) {
// Check that all the digits are valid. If any of them was 0xff, their
// bitwise OR will be 0xff.
if n1|n2|n3|n4|n5|n6|n7|n8 == 0xff {
return 0, false
}
return uint64(n1)<<58 |
uint64(n2)<<52 |
uint64(n3)<<46 |
uint64(n4)<<40 |
uint64(n5)<<34 |
uint64(n6)<<28 |
uint64(n7)<<22 |
uint64(n8)<<16,
true
}
Decode 函数实现比较简单:
- 读取 8 个字节，通过 assemble64 函数将 8 个字节转换为 6 个字节，然后写入到 dst 中。
- 如果读取的字节数不足 8 个字节，则通过 assemble32 函数将 4 个字节转换为 3 个字节，然后写入到 dst 中。
- 如果读取的字节数不足 4 个字节，则通过 assemble32 函数将 4 个字节转换为 3 个字节，然后写入到 dst 中。