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  // ReadUint48 decodes a big-endian, 48-bit value into out and advances over it.
    85  // It reports whether the read was successful.
    86  func (s *String) ReadUint48(out *uint64) bool {
    87  	v := s.read(6)
    88  	if v == nil {
    89  		return false
    90  	}
    91  	*out = uint64(v[0])<<40 | uint64(v[1])<<32 | uint64(v[2])<<24 | uint64(v[3])<<16 | uint64(v[4])<<8 | uint64(v[5])
    92  	return true
    93  }
    94  
    95  // ReadUint64 decodes a big-endian, 64-bit value into out and advances over it.
    96  // It reports whether the read was successful.
    97  func (s *String) ReadUint64(out *uint64) bool {
    98  	v := s.read(8)
    99  	if v == nil {
   100  		return false
   101  	}
   102  	*out = uint64(v[0])<<56 | uint64(v[1])<<48 | uint64(v[2])<<40 | uint64(v[3])<<32 | uint64(v[4])<<24 | uint64(v[5])<<16 | uint64(v[6])<<8 | uint64(v[7])
   103  	return true
   104  }
   105  
   106  func (s *String) readUnsigned(out *uint32, length int) bool {
   107  	v := s.read(length)
   108  	if v == nil {
   109  		return false
   110  	}
   111  	var result uint32
   112  	for i := 0; i < length; i++ {
   113  		result <<= 8
   114  		result |= uint32(v[i])
   115  	}
   116  	*out = result
   117  	return true
   118  }
   119  
   120  func (s *String) readLengthPrefixed(lenLen int, outChild *String) bool {
   121  	lenBytes := s.read(lenLen)
   122  	if lenBytes == nil {
   123  		return false
   124  	}
   125  	var length uint32
   126  	for _, b := range lenBytes {
   127  		length = length << 8
   128  		length = length | uint32(b)
   129  	}
   130  	v := s.read(int(length))
   131  	if v == nil {
   132  		return false
   133  	}
   134  	*outChild = v
   135  	return true
   136  }
   137  
   138  // ReadUint8LengthPrefixed reads the content of an 8-bit length-prefixed value
   139  // into out and advances over it. It reports whether the read was successful.
   140  func (s *String) ReadUint8LengthPrefixed(out *String) bool {
   141  	return s.readLengthPrefixed(1, out)
   142  }
   143  
   144  // ReadUint16LengthPrefixed reads the content of a big-endian, 16-bit
   145  // length-prefixed value into out and advances over it. It reports whether the
   146  // read was successful.
   147  func (s *String) ReadUint16LengthPrefixed(out *String) bool {
   148  	return s.readLengthPrefixed(2, out)
   149  }
   150  
   151  // ReadUint24LengthPrefixed reads the content of a big-endian, 24-bit
   152  // length-prefixed value into out and advances over it. It reports whether
   153  // the read was successful.
   154  func (s *String) ReadUint24LengthPrefixed(out *String) bool {
   155  	return s.readLengthPrefixed(3, out)
   156  }
   157  
   158  // ReadBytes reads n bytes into out and advances over them. It reports
   159  // whether the read was successful.
   160  func (s *String) ReadBytes(out *[]byte, n int) bool {
   161  	v := s.read(n)
   162  	if v == nil {
   163  		return false
   164  	}
   165  	*out = v
   166  	return true
   167  }
   168  
   169  // CopyBytes copies len(out) bytes into out and advances over them. It reports
   170  // whether the copy operation was successful
   171  func (s *String) CopyBytes(out []byte) bool {
   172  	n := len(out)
   173  	v := s.read(n)
   174  	if v == nil {
   175  		return false
   176  	}
   177  	return copy(out, v) == n
   178  }
   179  
   180  // Empty reports whether the string does not contain any bytes.
   181  func (s String) Empty() bool {
   182  	return len(s) == 0
   183  }
   184
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