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// Copyright 2011 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 packet
import (
"crypto/cipher"
"crypto/openpgp/error"
"crypto/openpgp/s2k"
"io"
"os"
"strconv"
)
// This is the largest session key that we'll support. Since no 512-bit cipher
// has even been seriously used, this is comfortably large.
const maxSessionKeySizeInBytes = 64
// SymmetricKeyEncrypted represents a passphrase protected session key. See RFC
// 4880, section 5.3.
type SymmetricKeyEncrypted struct {
CipherFunc CipherFunction
Encrypted bool
Key []byte // Empty unless Encrypted is false.
s2k func(out, in []byte)
encryptedKey []byte
}
func (ske *SymmetricKeyEncrypted) parse(r io.Reader) (err os.Error) {
// RFC 4880, section 5.3.
var buf [2]byte
_, err = readFull(r, buf[:])
if err != nil {
return
}
if buf[0] != 4 {
return error.UnsupportedError("SymmetricKeyEncrypted version")
}
ske.CipherFunc = CipherFunction(buf[1])
if ske.CipherFunc.keySize() == 0 {
return error.UnsupportedError("unknown cipher: " + strconv.Itoa(int(buf[1])))
}
ske.s2k, err = s2k.Parse(r)
if err != nil {
return
}
encryptedKey := make([]byte, maxSessionKeySizeInBytes)
// The session key may follow. We just have to try and read to find
// out. If it exists then we limit it to maxSessionKeySizeInBytes.
n, err := readFull(r, encryptedKey)
if err != nil && err != io.ErrUnexpectedEOF {
return
}
err = nil
if n != 0 {
if n == maxSessionKeySizeInBytes {
return error.UnsupportedError("oversized encrypted session key")
}
ske.encryptedKey = encryptedKey[:n]
}
ske.Encrypted = true
return
}
// Decrypt attempts to decrypt an encrypted session key. If it returns nil,
// ske.Key will contain the session key.
func (ske *SymmetricKeyEncrypted) Decrypt(passphrase []byte) os.Error {
if !ske.Encrypted {
return nil
}
key := make([]byte, ske.CipherFunc.keySize())
ske.s2k(key, passphrase)
if len(ske.encryptedKey) == 0 {
ske.Key = key
} else {
// the IV is all zeros
iv := make([]byte, ske.CipherFunc.blockSize())
c := cipher.NewCFBDecrypter(ske.CipherFunc.new(key), iv)
c.XORKeyStream(ske.encryptedKey, ske.encryptedKey)
ske.CipherFunc = CipherFunction(ske.encryptedKey[0])
if ske.CipherFunc.blockSize() == 0 {
return error.UnsupportedError("unknown cipher: " + strconv.Itoa(int(ske.CipherFunc)))
}
ske.CipherFunc = CipherFunction(ske.encryptedKey[0])
ske.Key = ske.encryptedKey[1:]
if len(ske.Key)%ske.CipherFunc.blockSize() != 0 {
ske.Key = nil
return error.StructuralError("length of decrypted key not a multiple of block size")
}
}
ske.Encrypted = false
return nil
}
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