gocryptfs-xray tool does not correctly print auth tags of encrypted files in AES-SIV mode #299
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rfjakob
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Jan 4, 2019
#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 @@ +�.¨Í1A�iõ&Á�4�öÉ \ 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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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.
The differences also are not properly documented yet:
https://github.com/rfjakob/gocryptfs/blob/master/Documentation/file-format.md
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