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https://github.com/rfjakob/gocryptfs/issues/299 :
In GCM mode the auth tags are at the end of each block, but in
SIV mode the auth tags follow immediately after the nonce. As a
result, in AES-SIV mode the output of gocryptfs-xray is misleading
and does not actually print the auth tag, but just the last
16-byte of the ciphertext.
diff --git a/gocryptfs-xray/xray_main.go b/gocryptfs-xray/xray_main.go index 74c9fb3..5a81caf 100644 ---
a/gocryptfs-xray/xray_main.go +++ b/gocryptfs-xray/xray_main.go @@ -16,9 +16,10 @@ import (
)
const ( - ivLen = contentenc.DefaultIVBits / 8 - blockSize = contentenc.DefaultBS + ivLen + cryptocore.AuthTagLen - myName =
"gocryptfs-xray" + ivLen = contentenc.DefaultIVBits / 8 + authTagLen = cryptocore.AuthTagLen + blockSize = contentenc.DefaultBS +
ivLen + cryptocore.AuthTagLen + myName = "gocryptfs-xray"
)
func errExit(err error) { @@ -26,13 +27,18 @@ func errExit(err error) { os.Exit(1)
}
-func prettyPrintHeader(h *contentenc.FileHeader) { +func prettyPrintHeader(h *contentenc.FileHeader, aessiv bool) { id :=
hex.EncodeToString(h.ID)
- fmt.Printf("Header: Version: %d, Id: %s\n", h.Version, id) + msg := "Header: Version: %d, Id: %s" + if aessiv { + msg += ",
assuming AES-SIV mode" + } + fmt.Printf(msg+"\n", h.Version, id)
}
func main() { dumpmasterkey := flag.Bool("dumpmasterkey", false, "Decrypt and dump the master key") + aessiv :=
flag.Bool("aessiv", false, "Assume AES-SIV mode instead of AES-GCM")
flag.Parse() if flag.NArg() != 1 { fmt.Fprintf(os.Stderr, "Usage: %s [OPTIONS] FILE\n"+ @@ -54,7 +60,7 @@ func main() { if
*dumpmasterkey {
dumpMasterKey(fn)
} else {
- inspectCiphertext(fd) + inspectCiphertext(fd, *aessiv)
}
}
@@ -72,7 +78,7 @@ func dumpMasterKey(fn string) {
}
}
-func inspectCiphertext(fd *os.File) { +func inspectCiphertext(fd *os.File, aessiv bool) { headerBytes := make([]byte,
contentenc.HeaderLen) n, err := fd.ReadAt(headerBytes, 0) if err == io.EOF && n == 0 {
@@ -88,34 +94,30 @@ func inspectCiphertext(fd *os.File) { if err != nil { errExit(err)
}
- prettyPrintHeader(header) + prettyPrintHeader(header, aessiv) var i int64 + buf := make([]byte, blockSize) for i = 0; ;
i++ {
- blockLen := int64(blockSize) off := contentenc.HeaderLen + i*blockSize - iv := make([]byte, ivLen) - _, err := fd.ReadAt(iv, off)
- if err == io.EOF { - break - } else if err != nil { + n, err := fd.ReadAt(buf, off) + if err != nil && err != io.EOF {
errExit(err)
}
- tag := make([]byte, cryptocore.AuthTagLen) - _, err = fd.ReadAt(tag, off+blockSize-cryptocore.AuthTagLen) - if err == io.EOF { -
fi, err2 := fd.Stat() - if err2 != nil { - errExit(err2) - } - _, err2 = fd.ReadAt(tag, fi.Size()-cryptocore.AuthTagLen) - if err2
!= nil { - errExit(err2) - } - blockLen = (fi.Size() - contentenc.HeaderLen) % blockSize - } else if err != nil { - errExit(err) +
if n == 0 && err == io.EOF { + break + } + // A block contains at least the IV, the Auth Tag and 1 data byte + if n <
ivLen+authTagLen+1 { + errExit(fmt.Errorf("corrupt block: truncated data, len=%d", n)) + } + data := buf[:n] + // Parse block data +
iv := data[:ivLen] + tag := data[len(data)-authTagLen:] + if aessiv { + tag = data[ivLen : ivLen+authTagLen]
}
fmt.Printf("Block %2d: IV: %s, Tag: %s, Offset: %5d Len: %d\n", - i, hex.EncodeToString(iv),
hex.EncodeToString(tag), off, blockLen) + i, hex.EncodeToString(iv), hex.EncodeToString(tag), off, len(data))
}
}
diff --git a/gocryptfs-xray/xray_tests/aessiv_fs.masterkey.txt b/gocryptfs-xray/xray_tests/aessiv_fs.masterkey.txt new file mode
100644 index 0000000..70835ac --- /dev/null +++ b/gocryptfs-xray/xray_tests/aessiv_fs.masterkey.txt @@ -0,0 +1,5 @@ +Your master key
is: + + 29dd219d-e227ff20-8474469d-9fc9fdc6- + b434ab35-404e808c-489d441e-2c1003f2 + diff --git
a/gocryptfs-xray/xray_tests/aessiv_fs.xray.txt b/gocryptfs-xray/xray_tests/aessiv_fs.xray.txt new file mode 100644 index
0000000..6a48079 --- /dev/null +++ b/gocryptfs-xray/xray_tests/aessiv_fs.xray.txt @@ -0,0 +1,3 @@ +Header: Version: 2, Id:
c2f21142e108952a47edfe16053d2bb9, assuming AES-SIV mode +Block 0: IV: 7621fdc35be7671ac6f369214436e8ff, Tag:
e8108c158b22cad6bb3296645357eb75, Offset: 18 Len: 4128 +Block 1: IV: f096d86a4dc3461ef17655cfcf865b13, Tag:
925f23d647e4ab7add2c8d36362cc5a9, Offset: 4146 Len: 936 diff --git a/gocryptfs-xray/xray_tests/aessiv_fs/Ldq-c4ADpM5iGSSrPjUAqQ
b/gocryptfs-xray/xray_tests/aessiv_fs/Ldq-c4ADpM5iGSSrPjUAqQ new file mode 100644 index 0000000..bfd4dfe Binary files /dev/null and
b/gocryptfs-xray/xray_tests/aessiv_fs/Ldq-c4ADpM5iGSSrPjUAqQ differ diff --git a/gocryptfs-xray/xray_tests/aessiv_fs/gocryptfs.conf
b/gocryptfs-xray/xray_tests/aessiv_fs/gocryptfs.conf new file mode 100644 index 0000000..9b8b95f --- /dev/null +++
b/gocryptfs-xray/xray_tests/aessiv_fs/gocryptfs.conf @@ -0,0 +1,21 @@ +{ + "Creator": "gocryptfs v1.7-beta1-7-g6b94f5e", +
"EncryptedKey": "D0kHfg/pryMO9Ydo15EwpYjNHf3iWKq2GJyNocbjwJt9blEeMoLD5DnoARuDzQs54hblw+9MHwFjCSHYmJrFbA==", + "ScryptObject": { +
"Salt": "ehn0LM/Hy/4QkXAMCZq3c3p0O9G7gu5e3OQSR8MiJ6c=", + "N": 65536, + "R": 8, + "P": 1, + "KeyLen": 32 + }, + "Version": 2, +
"FeatureFlags": [ + "GCMIV128", + "HKDF", + "DirIV", + "EMENames", + "LongNames", + "Raw64", + "AESSIV" + ] +} diff --git
a/gocryptfs-xray/xray_tests/aessiv_fs/gocryptfs.diriv b/gocryptfs-xray/xray_tests/aessiv_fs/gocryptfs.diriv new file mode 100644
index 0000000..dd57ce1 --- /dev/null +++ b/gocryptfs-xray/xray_tests/aessiv_fs/gocryptfs.diriv @@ -0,0 +1 @@ +.¨Í1Aiõ&Á4öÉ \
No newline at end of file diff --git a/gocryptfs-xray/xray_tests/xray_test.go b/gocryptfs-xray/xray_tests/xray_test.go index
a3374b0..8e5fc0c 100644 --- a/gocryptfs-xray/xray_tests/xray_test.go +++ b/gocryptfs-xray/xray_tests/xray_test.go @@ -24,3 +24,20 @@
func TestAesgcmXray(t *testing.T) {
fmt.Printf("have:\n%s", string(out))
}
}
+ +func TestAessivXray(t *testing.T) { + expected, err := ioutil.ReadFile("aessiv_fs.xray.txt") + if err != nil { + t.Fatal(err) + }
+ cmd := exec.Command("../gocryptfs-xray", "-aessiv", "aessiv_fs/Ldq-c4ADpM5iGSSrPjUAqQ") + out, err := cmd.CombinedOutput() + if
err != nil { + t.Fatal(err) + } + if bytes.Compare(out, expected) != 0 { + t.Errorf("Unexpected output") +
fmt.Printf("expected:\n%s", string(expected)) + fmt.Printf("have:\n%s", string(out)) + } +}
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Example output for a file encrypted in reverse mode:
Header: Version: 2, Id: 0b7f5e2574e4afa859a9bb156a2e7772
Block 0: IV: 0b7f5e2574e4afa859a9bb156a2e7773, Tag: bf39279ac6b1ccd852567aaf26ee386b, Len: 4128
Block 1: IV: 0b7f5e2574e4afa859a9bb156a2e7774, Tag: a4f0f9cde7f70a752254aa8fe7718699, Len: 4128
Block 2: IV: 0b7f5e2574e4afa859a9bb156a2e7775, Tag: b467b153016fc1d531818b65ab9e24f6, Len: 4128
Block 3: IV: 0b7f5e2574e4afa859a9bb156a2e7776, Tag: 1fcb7ffd8f1816fbe807df8148718a5c, Len: 4128
Block 4: IV: 0b7f5e2574e4afa859a9bb156a2e7777, Tag: a217e7933ef434c9f03ad931bb5fde9b, Len: 4128
Block 5: IV: 0b7f5e2574e4afa859a9bb156a2e7778, Tag: f3e6240d75cd66371a0b301111d6f1fc, Len: 4128
Block 6: IV: 0b7f5e2574e4afa859a9bb156a2e7779, Tag: bc85d322ebc7761ae5ef114ea3903a56, Len: 4128
Block 7: IV: 0b7f5e2574e4afa859a9bb156a2e777a, Tag: efda01c6b794690f939a12d6d49ac3af, Len: 4128
Block 8: IV: 0b7f5e2574e4afa859a9bb156a2e777b, Tag: b198329d489d1392080f710206932ff0, Len: 2907
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