string.go

     1  // Copyright 2017 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package cryptobyte contains types that help with parsing and constructing
     6  // length-prefixed, binary messages, including ASN.1 DER. (The asn1 subpackage
     7  // contains useful ASN.1 constants.)
     8  //
     9  // The String type is for parsing. It wraps a []byte slice and provides helper
    10  // functions for consuming structures, value by value.
    11  //
    12  // The Builder type is for constructing messages. It providers helper functions
    13  // for appending values and also for appending length-prefixed submessages –
    14  // without having to worry about calculating the length prefix ahead of time.
    15  //
    16  // See the documentation and examples for the Builder and String types to get
    17  // started.
    18  package cryptobyte // import "golang.org/x/crypto/cryptobyte"
    19  
    20  // String represents a string of bytes. It provides methods for parsing
    21  // fixed-length and length-prefixed values from it.
    22  type String []byte
    23  
    24  // read advances a String by n bytes and returns them. If less than n bytes
    25  // remain, it returns nil.
    26  func (s *String) read(n int) []byte {
    27  	if len(*s) < n || n < 0 {
    28  		return nil
    29  	}
    30  	v := (*s)[:n]
    31  	*s = (*s)[n:]
    32  	return v
    33  }
    34  
    35  // Skip advances the String by n byte and reports whether it was successful.
    36  func (s *String) Skip(n int) bool {
    37  	return s.read(n) != nil
    38  }
    39  
    40  // ReadUint8 decodes an 8-bit value into out and advances over it.
    41  // It reports whether the read was successful.
    42  func (s *String) ReadUint8(out *uint8) bool {
    43  	v := s.read(1)
    44  	if v == nil {
    45  		return false
    46  	}
    47  	*out = uint8(v[0])
    48  	return true
    49  }
    50  
    51  // ReadUint16 decodes a big-endian, 16-bit value into out and advances over it.
    52  // It reports whether the read was successful.
    53  func (s *String) ReadUint16(out *uint16) bool {
    54  	v := s.read(2)
    55  	if v == nil {
    56  		return false
    57  	}
    58  	*out = uint16(v[0])<<8 | uint16(v[1])
    59  	return true
    60  }
    61  
    62  // ReadUint24 decodes a big-endian, 24-bit value into out and advances over it.
    63  // It reports whether the read was successful.
    64  func (s *String) ReadUint24(out *uint32) bool {
    65  	v := s.read(3)
    66  	if v == nil {
    67  		return false
    68  	}
    69  	*out = uint32(v[0])<<16 | uint32(v[1])<<8 | uint32(v[2])
    70  	return true
    71  }
    72  
    73  // ReadUint32 decodes a big-endian, 32-bit value into out and advances over it.
    74  // It reports whether the read was successful.
    75  func (s *String) ReadUint32(out *uint32) bool {
    76  	v := s.read(4)
    77  	if v == nil {
    78  		return false
    79  	}
    80  	*out = uint32(v[0])<<24 | uint32(v[1])<<16 | uint32(v[2])<<8 | uint32(v[3])
    81  	return true
    82  }
    83  
    84  func (s *String) readUnsigned(out *uint32, length int) bool {
    85  	v := s.read(length)
    86  	if v == nil {
    87  		return false
    88  	}
    89  	var result uint32
    90  	for i := 0; i < length; i++ {
    91  		result <<= 8
    92  		result |= uint32(v[i])
    93  	}
    94  	*out = result
    95  	return true
    96  }
    97  
    98  func (s *String) readLengthPrefixed(lenLen int, outChild *String) bool {
    99  	lenBytes := s.read(lenLen)
   100  	if lenBytes == nil {
   101  		return false
   102  	}
   103  	var length uint32
   104  	for _, b := range lenBytes {
   105  		length = length << 8
   106  		length = length | uint32(b)
   107  	}
   108  	v := s.read(int(length))
   109  	if v == nil {
   110  		return false
   111  	}
   112  	*outChild = v
   113  	return true
   114  }
   115  
   116  // ReadUint8LengthPrefixed reads the content of an 8-bit length-prefixed value
   117  // into out and advances over it. It reports whether the read was successful.
   118  func (s *String) ReadUint8LengthPrefixed(out *String) bool {
   119  	return s.readLengthPrefixed(1, out)
   120  }
   121  
   122  // ReadUint16LengthPrefixed reads the content of a big-endian, 16-bit
   123  // length-prefixed value into out and advances over it. It reports whether the
   124  // read was successful.
   125  func (s *String) ReadUint16LengthPrefixed(out *String) bool {
   126  	return s.readLengthPrefixed(2, out)
   127  }
   128  
   129  // ReadUint24LengthPrefixed reads the content of a big-endian, 24-bit
   130  // length-prefixed value into out and advances over it. It reports whether
   131  // the read was successful.
   132  func (s *String) ReadUint24LengthPrefixed(out *String) bool {
   133  	return s.readLengthPrefixed(3, out)
   134  }
   135  
   136  // ReadBytes reads n bytes into out and advances over them. It reports
   137  // whether the read was successful.
   138  func (s *String) ReadBytes(out *[]byte, n int) bool {
   139  	v := s.read(n)
   140  	if v == nil {
   141  		return false
   142  	}
   143  	*out = v
   144  	return true
   145  }
   146  
   147  // CopyBytes copies len(out) bytes into out and advances over them. It reports
   148  // whether the copy operation was successful
   149  func (s *String) CopyBytes(out []byte) bool {
   150  	n := len(out)
   151  	v := s.read(n)
   152  	if v == nil {
   153  		return false
   154  	}
   155  	return copy(out, v) == n
   156  }
   157  
   158  // Empty reports whether the string does not contain any bytes.
   159  func (s String) Empty() bool {
   160  	return len(s) == 0
   161  }
   162
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