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encoding/base64: add constant-time behavior, enabled by default #73909

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encoding/base64: add constant-time behavior, enabled by default #73909

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65 changes: 43 additions & 22 deletions src/encoding/base64/base64.go
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@ import (
* Encodings
*/

type MappingFunc func (in uint) byte
// 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
Expand All @@ -26,6 +27,8 @@ type Encoding struct {
decodeMap [256]uint8 // mapping of symbol byte value to symbol index
padChar rune
strict bool
encodeMapFunc MappingFunc // optional mapping function to replace table look-ups when encoding
decodeMapFunc MappingFunc // optional mapping function to replace table look-ups when decoding
}

const (
Expand Down Expand Up @@ -83,6 +86,10 @@ func NewEncoding(encoder string) *Encoding {
}
e.decodeMap[encoder[i]] = uint8(i)
}
var encodeFunc = e.encodeMapDefault
e.encodeMapFunc = encodeFunc
var decodeFunc = e.decodeMapDefault
e.decodeMapFunc = decodeFunc
return e
}

Expand All @@ -104,6 +111,17 @@ func (enc Encoding) WithPadding(padding rune) *Encoding {
return &enc
}

// WithDecodeMappingFunc sets the value fo encodeMapFunc
func (enc Encoding) WithDecodeMappingFunc(f MappingFunc) *Encoding {
enc.decodeMapFunc = f
return &enc
}
// WithEncodeMappingFunc sets the value fo encodeMapFunc
func (enc Encoding) WithEncodeMappingFunc(f MappingFunc) *Encoding {
enc.encodeMapFunc = f
return &enc
}

// Strict creates a new encoding identical to enc except with
// strict decoding enabled. In this mode, the decoder requires that
// trailing padding bits are zero, as described in RFC 4648 section 3.5.
Expand All @@ -116,11 +134,15 @@ func (enc Encoding) Strict() *Encoding {
}

// StdEncoding is the standard base64 encoding, as defined in RFC 4648.
var StdEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/")
var StdEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/").
WithEncodeMappingFunc(StandardBase64Encode).
WithDecodeMappingFunc(StandardBase64Decode)

// URLEncoding is the alternate base64 encoding defined in RFC 4648.
// It is typically used in URLs and file names.
var URLEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_")
var URLEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_").
WithEncodeMappingFunc(UrlSafeBase64Encode).
WithDecodeMappingFunc(UrlSafeBase64Decode)

// RawStdEncoding is the standard raw, unpadded base64 encoding,
// as defined in RFC 4648 section 3.2.
Expand Down Expand Up @@ -157,10 +179,10 @@ func (enc *Encoding) Encode(dst, src []byte) {
// 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]
dst[di+0] = enc.encodeMapFunc(val>>18&0x3F)
dst[di+1] = enc.encodeMapFunc(val>>12&0x3F)
dst[di+2] = enc.encodeMapFunc(val>>6&0x3F)
dst[di+3] = enc.encodeMapFunc(val&0x3F)

si += 3
di += 4
Expand All @@ -176,8 +198,8 @@ func (enc *Encoding) Encode(dst, src []byte) {
val |= uint(src[si+1]) << 8
}

dst[di+0] = enc.encode[val>>18&0x3F]
dst[di+1] = enc.encode[val>>12&0x3F]
dst[di+0] = enc.encodeMapFunc(val>>18&0x3F)
dst[di+1] = enc.encodeMapFunc(val>>12&0x3F)

switch remain {
case 2:
Expand Down Expand Up @@ -330,8 +352,7 @@ func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err err
}
in := src[si]
si++

out := enc.decodeMap[in]
out := enc.decodeMapFunc(uint(in))
if out != 0xff {
dbuf[j] = out
continue
Expand Down Expand Up @@ -529,14 +550,14 @@ func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
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]],
enc.decodeMapFunc(uint(src2[0])),
enc.decodeMapFunc(uint(src2[1])),
enc.decodeMapFunc(uint(src2[2])),
enc.decodeMapFunc(uint(src2[3])),
enc.decodeMapFunc(uint(src2[4])),
enc.decodeMapFunc(uint(src2[5])),
enc.decodeMapFunc(uint(src2[6])),
enc.decodeMapFunc(uint(src2[7])),
); ok {
byteorder.BEPutUint64(dst[n:], dn)
n += 6
Expand All @@ -554,10 +575,10 @@ func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
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]],
enc.decodeMapFunc(uint(src2[0])),
enc.decodeMapFunc(uint(src2[1])),
enc.decodeMapFunc(uint(src2[2])),
enc.decodeMapFunc(uint(src2[3])),
); ok {
byteorder.BEPutUint32(dst[n:], dn)
n += 3
Expand Down
94 changes: 94 additions & 0 deletions src/encoding/base64/mapping.go
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Original file line number Diff line number Diff line change
@@ -0,0 +1,94 @@
// Copyright 2025 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package base64 implements base64 encoding as specified by RFC 4648.
package base64

func (enc Encoding) encodeMapDefault(in uint) byte {
return enc.encode[in]
}

func (enc Encoding) decodeMapDefault(in uint) byte {
return enc.decodeMap[in]
}

func StandardBase64Decode(in uint) byte {
ch := int(in)
ret := -1

// if (ch > 0x40 && ch < 0x5b) ret += ch - 0x41 + 1; // -64
ret += (((0x40 - ch) & (ch - 0x5b)) >> 8) & (ch - 64)

// if (ch > 0x60 && ch < 0x7b) ret += ch - 0x61 + 26 + 1; // -70
ret += (((0x60 - ch) & (ch - 0x7b)) >> 8) & (ch - 70)

// if (ch > 0x2f && ch < 0x3a) ret += ch - 0x30 + 52 + 1; // 5
ret += (((0x2f - ch) & (ch - 0x3a)) >> 8) & (ch + 5)

// if (ch == 0x2b) ret += 62 + 1
ret += (((0x2a - ch) & (ch - 0x2c)) >> 8) & 63

// if (ch == 0x2f) ret += 63 + 1;
ret += (((0x2e - ch) & (ch - 0x30)) >> 8) & 64

return byte(ret)
}

func StandardBase64Encode(in uint) byte {
src := int(in)
diff := int(0x41)

// if (in > 25) diff += 0x61 - 0x41 - 26; // 6
diff += ((25 - src) >> 8) & 6;

// if (in > 51) diff += 0x30 - 0x61 - 26; // -75
diff -= ((51 - src) >> 8) & 75;

// if (in > 61) diff += 0x2b - 0x30 - 10; // -15
diff -= ((61 - src) >> 8) & 15;

// if (in > 62) diff += 0x2f - 0x2b - 1; // 3
diff += ((62 - src) >> 8) & 3
return byte(src + diff)
}

func UrlSafeBase64Decode(in uint) byte {
ch := int(in)
ret := -1

// if (ch > 0x40 && ch < 0x5b) ret += ch - 0x41 + 1; // -64
ret += (((0x40 - ch) & (ch - 0x5b)) >> 8) & (ch - 64)

// if (ch > 0x60 && ch < 0x7b) ret += ch - 0x61 + 26 + 1; // -70
ret += (((0x60 - ch) & (ch - 0x7b)) >> 8) & (ch - 70)

// if (ch > 0x2f && ch < 0x3a) ret += ch - 0x30 + 52 + 1; // 5
ret += (((0x2f - ch) & (ch - 0x3a)) >> 8) & (ch + 5)

// if (ch == 0x2c) ret += 62 + 1;
ret += (((0x2c - ch) & (ch - 0x2e)) >> 8) & 63

// if (ch == 0x5f) ret += 63 + 1;
ret += (((0x5e - ch) & (ch - 0x60)) >> 8) & 64

return byte(ret)
}


func UrlSafeBase64Encode(in uint) byte {
src := int(in)
diff := int(0x41)
// if (src > 25) diff += 0x61 - 0x41 - 26; // 6
diff += ((25 - src) >> 8) & 6

// if (src > 51) diff += 0x30 - 0x61 - 26; // -75
diff -= ((51 - src) >> 8) & 75

// if (src > 61) diff += 0x2d - 0x30 - 10; // -13
diff -= ((61 - src) >> 8) & 13

// if (src > 62) diff += 0x5f - 0x2b - 1; // 3
diff += ((62 - src) >> 8) & 49
return byte(src + diff)
}