pem.go

     1  // Copyright 2009 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 pem implements the PEM data encoding, which originated in Privacy
     6  // Enhanced Mail. The most common use of PEM encoding today is in TLS keys and
     7  // certificates. See RFC 1421.
     8  package pem
     9  
    10  import (
    11  	"bytes"
    12  	"encoding/base64"
    13  	"errors"
    14  	"io"
    15  	"slices"
    16  	"strings"
    17  )
    18  
    19  // A Block represents a PEM encoded structure.
    20  //
    21  // The encoded form is:
    22  //
    23  //	-----BEGIN Type-----
    24  //	Headers
    25  //	base64-encoded Bytes
    26  //	-----END Type-----
    27  //
    28  // where [Block.Headers] is a possibly empty sequence of Key: Value lines.
    29  type Block struct {
    30  	Type    string            // The type, taken from the preamble (i.e. "RSA PRIVATE KEY").
    31  	Headers map[string]string // Optional headers.
    32  	Bytes   []byte            // The decoded bytes of the contents. Typically a DER encoded ASN.1 structure.
    33  }
    34  
    35  // getLine results the first \r\n or \n delineated line from the given byte
    36  // array. The line does not include trailing whitespace or the trailing new
    37  // line bytes. The remainder of the byte array (also not including the new line
    38  // bytes) is also returned and this will always be smaller than the original
    39  // argument.
    40  func getLine(data []byte) (line, rest []byte) {
    41  	i := bytes.IndexByte(data, '\n')
    42  	var j int
    43  	if i < 0 {
    44  		i = len(data)
    45  		j = i
    46  	} else {
    47  		j = i + 1
    48  		if i > 0 && data[i-1] == '\r' {
    49  			i--
    50  		}
    51  	}
    52  	return bytes.TrimRight(data[0:i], " \t"), data[j:]
    53  }
    54  
    55  // removeSpacesAndTabs returns a copy of its input with all spaces and tabs
    56  // removed, if there were any. Otherwise, the input is returned unchanged.
    57  //
    58  // The base64 decoder already skips newline characters, so we don't need to
    59  // filter them out here.
    60  func removeSpacesAndTabs(data []byte) []byte {
    61  	if !bytes.ContainsAny(data, " \t") {
    62  		// Fast path; most base64 data within PEM contains newlines, but
    63  		// no spaces nor tabs. Skip the extra alloc and work.
    64  		return data
    65  	}
    66  	result := make([]byte, len(data))
    67  	n := 0
    68  
    69  	for _, b := range data {
    70  		if b == ' ' || b == '\t' {
    71  			continue
    72  		}
    73  		result[n] = b
    74  		n++
    75  	}
    76  
    77  	return result[0:n]
    78  }
    79  
    80  var pemStart = []byte("\n-----BEGIN ")
    81  var pemEnd = []byte("\n-----END ")
    82  var pemEndOfLine = []byte("-----")
    83  var colon = []byte(":")
    84  
    85  // Decode will find the next PEM formatted block (certificate, private key
    86  // etc) in the input. It returns that block and the remainder of the input. If
    87  // no PEM data is found, p is nil and the whole of the input is returned in
    88  // rest.
    89  func Decode(data []byte) (p *Block, rest []byte) {
    90  	// pemStart begins with a newline. However, at the very beginning of
    91  	// the byte array, we'll accept the start string without it.
    92  	rest = data
    93  	for {
    94  		if bytes.HasPrefix(rest, pemStart[1:]) {
    95  			rest = rest[len(pemStart)-1:]
    96  		} else if _, after, ok := bytes.Cut(rest, pemStart); ok {
    97  			rest = after
    98  		} else {
    99  			return nil, data
   100  		}
   101  
   102  		var typeLine []byte
   103  		typeLine, rest = getLine(rest)
   104  		if !bytes.HasSuffix(typeLine, pemEndOfLine) {
   105  			continue
   106  		}
   107  		typeLine = typeLine[0 : len(typeLine)-len(pemEndOfLine)]
   108  
   109  		p = &Block{
   110  			Headers: make(map[string]string),
   111  			Type:    string(typeLine),
   112  		}
   113  
   114  		for {
   115  			// This loop terminates because getLine's second result is
   116  			// always smaller than its argument.
   117  			if len(rest) == 0 {
   118  				return nil, data
   119  			}
   120  			line, next := getLine(rest)
   121  
   122  			key, val, ok := bytes.Cut(line, colon)
   123  			if !ok {
   124  				break
   125  			}
   126  
   127  			// TODO(agl): need to cope with values that spread across lines.
   128  			key = bytes.TrimSpace(key)
   129  			val = bytes.TrimSpace(val)
   130  			p.Headers[string(key)] = string(val)
   131  			rest = next
   132  		}
   133  
   134  		var endIndex, endTrailerIndex int
   135  
   136  		// If there were no headers, the END line might occur
   137  		// immediately, without a leading newline.
   138  		if len(p.Headers) == 0 && bytes.HasPrefix(rest, pemEnd[1:]) {
   139  			endIndex = 0
   140  			endTrailerIndex = len(pemEnd) - 1
   141  		} else {
   142  			endIndex = bytes.Index(rest, pemEnd)
   143  			endTrailerIndex = endIndex + len(pemEnd)
   144  		}
   145  
   146  		if endIndex < 0 {
   147  			continue
   148  		}
   149  
   150  		// After the "-----" of the ending line, there should be the same type
   151  		// and then a final five dashes.
   152  		endTrailer := rest[endTrailerIndex:]
   153  		endTrailerLen := len(typeLine) + len(pemEndOfLine)
   154  		if len(endTrailer) < endTrailerLen {
   155  			continue
   156  		}
   157  
   158  		restOfEndLine := endTrailer[endTrailerLen:]
   159  		endTrailer = endTrailer[:endTrailerLen]
   160  		if !bytes.HasPrefix(endTrailer, typeLine) ||
   161  			!bytes.HasSuffix(endTrailer, pemEndOfLine) {
   162  			continue
   163  		}
   164  
   165  		// The line must end with only whitespace.
   166  		if s, _ := getLine(restOfEndLine); len(s) != 0 {
   167  			continue
   168  		}
   169  
   170  		base64Data := removeSpacesAndTabs(rest[:endIndex])
   171  		p.Bytes = make([]byte, base64.StdEncoding.DecodedLen(len(base64Data)))
   172  		n, err := base64.StdEncoding.Decode(p.Bytes, base64Data)
   173  		if err != nil {
   174  			continue
   175  		}
   176  		p.Bytes = p.Bytes[:n]
   177  
   178  		// the -1 is because we might have only matched pemEnd without the
   179  		// leading newline if the PEM block was empty.
   180  		_, rest = getLine(rest[endIndex+len(pemEnd)-1:])
   181  		return p, rest
   182  	}
   183  }
   184  
   185  const pemLineLength = 64
   186  
   187  type lineBreaker struct {
   188  	line [pemLineLength]byte
   189  	used int
   190  	out  io.Writer
   191  }
   192  
   193  var nl = []byte{'\n'}
   194  
   195  func (l *lineBreaker) Write(b []byte) (n int, err error) {
   196  	if l.used+len(b) < pemLineLength {
   197  		copy(l.line[l.used:], b)
   198  		l.used += len(b)
   199  		return len(b), nil
   200  	}
   201  
   202  	n, err = l.out.Write(l.line[0:l.used])
   203  	if err != nil {
   204  		return
   205  	}
   206  	excess := pemLineLength - l.used
   207  	l.used = 0
   208  
   209  	n, err = l.out.Write(b[0:excess])
   210  	if err != nil {
   211  		return
   212  	}
   213  
   214  	n, err = l.out.Write(nl)
   215  	if err != nil {
   216  		return
   217  	}
   218  
   219  	return l.Write(b[excess:])
   220  }
   221  
   222  func (l *lineBreaker) Close() (err error) {
   223  	if l.used > 0 {
   224  		_, err = l.out.Write(l.line[0:l.used])
   225  		if err != nil {
   226  			return
   227  		}
   228  		_, err = l.out.Write(nl)
   229  	}
   230  
   231  	return
   232  }
   233  
   234  func writeHeader(out io.Writer, k, v string) error {
   235  	_, err := out.Write([]byte(k + ": " + v + "\n"))
   236  	return err
   237  }
   238  
   239  // Encode writes the PEM encoding of b to out.
   240  func Encode(out io.Writer, b *Block) error {
   241  	// Check for invalid block before writing any output.
   242  	for k := range b.Headers {
   243  		if strings.Contains(k, ":") {
   244  			return errors.New("pem: cannot encode a header key that contains a colon")
   245  		}
   246  	}
   247  
   248  	// All errors below are relayed from underlying io.Writer,
   249  	// so it is now safe to write data.
   250  
   251  	if _, err := out.Write(pemStart[1:]); err != nil {
   252  		return err
   253  	}
   254  	if _, err := out.Write([]byte(b.Type + "-----\n")); err != nil {
   255  		return err
   256  	}
   257  
   258  	if len(b.Headers) > 0 {
   259  		const procType = "Proc-Type"
   260  		h := make([]string, 0, len(b.Headers))
   261  		hasProcType := false
   262  		for k := range b.Headers {
   263  			if k == procType {
   264  				hasProcType = true
   265  				continue
   266  			}
   267  			h = append(h, k)
   268  		}
   269  		// The Proc-Type header must be written first.
   270  		// See RFC 1421, section 4.6.1.1
   271  		if hasProcType {
   272  			if err := writeHeader(out, procType, b.Headers[procType]); err != nil {
   273  				return err
   274  			}
   275  		}
   276  		// For consistency of output, write other headers sorted by key.
   277  		slices.Sort(h)
   278  		for _, k := range h {
   279  			if err := writeHeader(out, k, b.Headers[k]); err != nil {
   280  				return err
   281  			}
   282  		}
   283  		if _, err := out.Write(nl); err != nil {
   284  			return err
   285  		}
   286  	}
   287  
   288  	var breaker lineBreaker
   289  	breaker.out = out
   290  
   291  	b64 := base64.NewEncoder(base64.StdEncoding, &breaker)
   292  	if _, err := b64.Write(b.Bytes); err != nil {
   293  		return err
   294  	}
   295  	b64.Close()
   296  	breaker.Close()
   297  
   298  	if _, err := out.Write(pemEnd[1:]); err != nil {
   299  		return err
   300  	}
   301  	_, err := out.Write([]byte(b.Type + "-----\n"))
   302  	return err
   303  }
   304  
   305  // EncodeToMemory returns the PEM encoding of b.
   306  //
   307  // If b has invalid headers and cannot be encoded,
   308  // EncodeToMemory returns nil. If it is important to
   309  // report details about this error case, use [Encode] instead.
   310  func EncodeToMemory(b *Block) []byte {
   311  	var buf bytes.Buffer
   312  	if err := Encode(&buf, b); err != nil {
   313  		return nil
   314  	}
   315  	return buf.Bytes()
   316  }
   317
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