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// Copyright 2017 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 cryptobyte contains types that help with parsing and constructing
// length-prefixed, binary messages, including ASN.1 DER. (The asn1 subpackage
// contains useful ASN.1 constants.)
//
// The String type is for parsing. It wraps a []byte slice and provides helper
// functions for consuming structures, value by value.
//
// The Builder type is for constructing messages. It providers helper functions
// for appending values and also for appending length-prefixed submessages –
// without having to worry about calculating the length prefix ahead of time.
//
// See the documentation and examples for the Builder and String types to get
// started.
package cryptobyte // import "golang.org/x/crypto/cryptobyte"
// String represents a string of bytes. It provides methods for parsing
// fixed-length and length-prefixed values from it.
type String []byte
// read advances a String by n bytes and returns them. If less than n bytes
// remain, it returns nil.
func (s *String) read(n int) []byte {
if len(*s) < n {
return nil
}
v := (*s)[:n]
*s = (*s)[n:]
return v
}
// Skip advances the String by n byte and reports whether it was successful.
func (s *String) Skip(n int) bool {
return s.read(n) != nil
}
// ReadUint8 decodes an 8-bit value into out and advances over it.
// It reports whether the read was successful.
func (s *String) ReadUint8(out *uint8) bool {
v := s.read(1)
if v == nil {
return false
}
*out = uint8(v[0])
return true
}
// ReadUint16 decodes a big-endian, 16-bit value into out and advances over it.
// It reports whether the read was successful.
func (s *String) ReadUint16(out *uint16) bool {
v := s.read(2)
if v == nil {
return false
}
*out = uint16(v[0])<<8 | uint16(v[1])
return true
}
// ReadUint24 decodes a big-endian, 24-bit value into out and advances over it.
// It reports whether the read was successful.
func (s *String) ReadUint24(out *uint32) bool {
v := s.read(3)
if v == nil {
return false
}
*out = uint32(v[0])<<16 | uint32(v[1])<<8 | uint32(v[2])
return true
}
// ReadUint32 decodes a big-endian, 32-bit value into out and advances over it.
// It reports whether the read was successful.
func (s *String) ReadUint32(out *uint32) bool {
v := s.read(4)
if v == nil {
return false
}
*out = uint32(v[0])<<24 | uint32(v[1])<<16 | uint32(v[2])<<8 | uint32(v[3])
return true
}
func (s *String) readUnsigned(out *uint32, length int) bool {
v := s.read(length)
if v == nil {
return false
}
var result uint32
for i := 0; i < length; i++ {
result <<= 8
result |= uint32(v[i])
}
*out = result
return true
}
func (s *String) readLengthPrefixed(lenLen int, outChild *String) bool {
lenBytes := s.read(lenLen)
if lenBytes == nil {
return false
}
var length uint32
for _, b := range lenBytes {
length = length << 8
length = length | uint32(b)
}
if int(length) < 0 {
// This currently cannot overflow because we read uint24 at most, but check
// anyway in case that changes in the future.
return false
}
v := s.read(int(length))
if v == nil {
return false
}
*outChild = v
return true
}
// ReadUint8LengthPrefixed reads the content of an 8-bit length-prefixed value
// into out and advances over it. It reports whether the read was successful.
func (s *String) ReadUint8LengthPrefixed(out *String) bool {
return s.readLengthPrefixed(1, out)
}
// ReadUint16LengthPrefixed reads the content of a big-endian, 16-bit
// length-prefixed value into out and advances over it. It reports whether the
// read was successful.
func (s *String) ReadUint16LengthPrefixed(out *String) bool {
return s.readLengthPrefixed(2, out)
}
// ReadUint24LengthPrefixed reads the content of a big-endian, 24-bit
// length-prefixed value into out and advances over it. It reports whether
// the read was successful.
func (s *String) ReadUint24LengthPrefixed(out *String) bool {
return s.readLengthPrefixed(3, out)
}
// ReadBytes reads n bytes into out and advances over them. It reports
// whether the read was successful.
func (s *String) ReadBytes(out *[]byte, n int) bool {
v := s.read(n)
if v == nil {
return false
}
*out = v
return true
}
// CopyBytes copies len(out) bytes into out and advances over them. It reports
// whether the copy operation was successful
func (s *String) CopyBytes(out []byte) bool {
n := len(out)
v := s.read(n)
if v == nil {
return false
}
return copy(out, v) == n
}
// Empty reports whether the string does not contain any bytes.
func (s String) Empty() bool {
return len(s) == 0
}
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