Backdoors with the MS Office file encryption master key and a proposal for a reliable file format

transcript

Backdoors with the MS Office file encryption master key

and a proposal for a reliable file format

2015/10/28, 29 CODE BLUE

Mitsunari Shigeo（@herumi）

• Attacking Excel files without a password

• This bug is fixed at 2015/Oct/13 (MS15-110)

• Comparison of password encrypted file formats

• How I found this bug

• Proposal for a backdoor-resistant format

Abstract

• R&D for cloud security and infrastructure at Cybozu Labs, Inc.

• Author of “Applied Cryptography for the Cloud“ (in Japanese)

• about PFS, ECC, IBE, ABE, FE, HE, ZKP,...

• http://herumi.github.io/ango/

• Microsoft MVP Developer Security (2015)

• Author of the fastest implementation of pairing

• https://github.com/herumi/ate-pairing

• Software implementation of an Attribute-Based Encryption Scheme, IEEE trans on computers, 2014

Mitsunari Shigeo(@herumi)

• Technical Fellow at Recruit Marketing Partners

• Visiting associate professor at Kochi National College of Technology

• Review board for CODE BLUE

• OWASP Japan advisory board

• Chairperson of SECCON CTF

• Leader of Shibuya Perl Mongers

• Microsoft MVP Developer Security in 2008

• Author of "How to Execute Arbitrary Code on x86 JIT Compliers"

• Translator of "Reading ECMA-262 Edition 5.1" (2013)

• Best award of CSSx2.0 at "Computer Security Symposium 2013"

Takesako Yoshinori (@takesako)

• Demo

• Situations

• MS Office file format

Agenda

• Password encryption

• Introduction to attack tools

• Comparison of attack time

• Demo

• Situations

Agenda

• The very basic way

• input : 𝑝𝑎𝑠𝑠 : password, 𝑚 : message

1. 𝑆𝐾 = 𝐻𝑎𝑠ℎ 𝑝𝑎𝑠𝑠

2. 𝑐 = 𝐸𝑛𝑐 𝑆𝐾 , 𝑚

3. output : 𝑐

• Vulnerable

• The same password always generates the same 𝑆𝐾.

Encrypted file with password

𝑝𝑎𝑠𝑠

𝑆𝐾

𝐻𝑎𝑠ℎ

𝐸𝑛𝑐

𝑚 depends on only 𝑝𝑎𝑠𝑠

• Password attack tool

• http://hashcat.net/oclhashcat/

• GPGPU based very fast engine

hashcat

• Number of attempts per second

• SHA1 : 4.2 × 1010times/sec on 8x NVidia Titan X

• SHA512 : 5.2 × 109 times/sec

• Time to detect password from 𝐻𝑎𝑠ℎ value

• 𝑝𝑎𝑠𝑠 ∶ assume [a-zA-Z0-9]; 62 letters

• 628 4.2 × 1010 = 1h27m to try all patterns for SHA1

Performance of hashcat

• Add salt

1. generate 𝑠𝑎𝑙𝑡 randomly

2. 𝑆𝐾 = 𝐻𝑎𝑠ℎ 𝑠𝑎𝑙𝑡, 𝑝𝑎𝑠𝑠

3. 𝑖𝑣 : Initialization Vector

4. c = 𝐸𝑛𝑐(𝑖𝑣, 𝑆𝐾 , 𝑚)

• Even the same password generates different 𝑆𝐾

• 𝐻𝑎𝑠ℎ 𝑠𝑎𝑙𝑡1 + ′abc′ ≠ 𝐻𝑎𝑠ℎ(𝑠𝑎𝑙𝑡2 + ′abc′)

• Stronger against rainbow tables attacks

More secure file formats

𝑝𝑎𝑠𝑠

𝐻𝑎𝑠ℎ

𝑠𝑎𝑙𝑡

𝑆𝐾 𝑚

𝐸𝑛𝑐

𝑖𝑣

• Iterate the hash function many times

• 𝑑1 = 𝐻𝑀𝐴𝐶(𝑝𝑎𝑠𝑠, 𝑠𝑎𝑙𝑡)

• 𝑛 is iteration count

• for 𝑖 = 1 to 𝑛 − 1:

• 𝑑𝑖+1 = 𝐻𝑀𝐴𝐶(𝑑𝑖 , 𝑠𝑎𝑙𝑡)

• Decrease attack ability to 1/𝑛

• PKCS#5(RFC 2898)

• Password-Based Cryptography Specification

• PBKDF2(password based key derivation functions)

• used by ZIP format, etc.

Key stretching

𝑝𝑎𝑠𝑠

𝐻𝑀𝐴𝐶

𝑑𝑖

𝑆𝐾 = 𝑑1 ⊕𝑑2 ⊕⋯

𝑠𝑎𝑙𝑡

𝑝𝑎𝑠𝑠

𝐻𝑀𝐴𝐶

𝑑𝑖+1

• Another password recovery tool

• http://passcovery.com/

Passcovery

• Brute-force attack time against 8-byte password

• by Passcovery on GeForce GTX860M 1019MHz

• Office 2013 docx format is strong

• Recently, a memory-hard function is recommended

• Argon2 is the winner of Password Hashing Competition at 2015/Jul/20

Compare attack time

File format # of tries/sec hash stretching days

ZIP(96-bit) 230000000 none 10 days

Office2003 doc 11000000 ? 220 days

ZIP(256-bit AES) 370000 1000 x HMAC SHA1 18 years

Office2007 docx 16000 50000 x SHA1 430 years

• Demo

• Secret key generator of the MS Office file format

• Introduction of my tool

• Demo

• Situations

Agenda

• [MS-OFFCRYPTO]

• Office Document Cryptography Structure

• [MS-CFB]

• Compound File Binary - file format

MS Office documents

• Hashing algorithm 𝐻(𝑝𝑎𝑠𝑠, 𝑠𝑎𝑙𝑡)

• ℎ0 = 𝐻𝑎𝑠ℎ(𝑠𝑎𝑙𝑡, 𝑝𝑎𝑠𝑠)

• 𝑛 : iteration count

• for 𝑖 = 1 to 𝑛: ℎ𝑖 = 𝐻𝑎𝑠ℎ < 𝑖 > +ℎ𝑖−1

• < 𝑖 > : 4-byte little endian

Secret key generator of MS Office

𝑝𝑎𝑠𝑠

ℎ𝑜

𝐻𝑎𝑠ℎ

𝑠𝑎𝑙𝑡

ℎ𝑖−1 𝑖

𝐻𝑎𝑠ℎ

ℎ𝑖 𝑛

𝑆𝐾 ≔ ℎ𝑛

• Use two kinds of secret key

• 𝐻 : the above iterated hash function

1. generate 𝑠 from 𝑝𝑎𝑠𝑠 and 𝑠𝑎𝑙𝑡

2. generate 𝑆𝐾 and encrypt it by 𝑠

3. encrypt 𝑚 by 𝑆𝐾

• I will explain latter

• two keys for key escrow

• Administrator can decrypt if password is lost

• encrypt 𝑆𝐾 with public key of Administrator in advance

• desabled (default)

MS Office Agile format

𝑝𝑎𝑠𝑠

𝑆𝐾

Enc by 𝑆𝐾

𝑆𝐾

Enc by 𝑠

𝑠𝑎𝑙𝑡

• https://github.com/herumi/msoffice/

• My tool to encrypt/decrypt MS Office files

• Supports Windows/Linux

• Supports OpenXML of Office 2007~

• Support Agile format of Office 2010～

• LibreOffice does not support the format yet

• Configurable secret key for Agile format

• Configurable iteration count for stretching

msoffice-crypt

• encrypt with password "test"

• decrypt with password "test"

• attack without password

decrypt easy.xlsx with "test" and get 𝑆𝐾

attack complex.xlsx by 𝑆𝐾 without password

msoffice-crypt -d complex.xlsx -by easy.xlsx -p test

msoffice-crypt -e plain.xlsx enc.xlsx –p test

msoffice-crypt -d enc.xlsx dec.xlsx –p test

• Demo

• Situations

• What happened?

• Some scenarios

Agenda

• Excel 2010/2013 does not update the secret key in the file when password is changed (Bug).

What happened?

We can attack them with this one secret key.

master file with pass

with pass1

with pass2

with pass3

save as...

have same secret key

• At HR

• prepares a master Excel file

• write pay slip into the file, change password, send to staff

Scenario 1（pay slip delivery）

master file with pass HR

can attack other files

• Owner

• A master Excel file encrypted with an easy pass

• Modify it and make an important file with strong pass

• Attacker

• Brute-force attack against easy pass and attack important file

Scenario 2（lost PC）

Brute-force attack to easy pass

save it with strong pass

write secret info.

can attack this

• Demo

• Situations

• Detail of MS Office file encryption structure

• Relation between password and secret key

Agenda

• Old format and new format

• MS OLE2のヘッダは"D0 CF 11 E0"

MS Office file encryption details

Office file type Format

doc, ppt, xls (old Office files) MS OLE2

plain docx, pptx, xlsx ZIP file of Open XML files

encrypted docx, pptx, xlsx MS OLE2 including a header and an encrypted ZIP file

ZIP files (Open XML)

Enc(ZIP files)

encrypted with AES

header

not encrypted

encrypted

• There is a directory structure in one file

Layout of encrypted docx

EncryptionPackage

EncryptionInfo

DataSpaces/

Version

DataSpaceMap

Transformation/

StrongEncryption

Transform/ Primary

DataSpaceInfo/ StrongEncryption

DataSpace

not used

encrypted main ZIP file

encryption information

• Standard encryption (~Office 2007)

• binary format

• supports only SHA-1

• spinCount(=# of iteration) is fixed to 50000

• Agile encryption (Office 2010~)

• XmlEncryptionDescriptor

• supports SHA-1, SHA256, etc.

• variable spinCount

Version of EncryptionInfo

XmlEncryptionDescriptor <encryption>

<keyData saltSize="16" blockSize="16" keyBits="256" hashSize="64"

cipherAlgorithm="AES" cipherChaining="ChainingModeCBC"

hashAlgorithm="SHA512" saltValue="..."/>

<keyEncryptors><keyEncryptor

uri="http://schemas.microsoft.com/office/2006/keyEncryptor/password">

<p:encryptedKey spinCount="100000"

saltSize="16" blockSize="16" keyBits="256" hashSize="64" cipherAlgorithm="AES"

cipherChaining="ChainingModeCBC" hashAlgorithm="SHA512" saltValue="..."

encryptedVerifierHashInput="..."

encryptedVerifierHashValue="..."

encryptedKeyValue="..."/>

</keyEncryptor></keyEncryptors>

</encryption>

Dependency of variables in encryption

generate encryptedKey.saltValue

encryptedVerifierHashValue

encryptedVerifierHashInput

encryptedKeyValue

encryptedHmacKey

encryptedHmacValue

password

gen. secretKey

EncryptionPackage Enc(ZIP file)

ZIP file gen.

keyData.saltValue

gen. verifierHashInput

verifierHashValue

gen. HmacKey

hash 30/54

Check password integrity

encryptedKeyValue

encryptedHmacKey

encryptedHmacValue

password

gen. secretKey

ZIP file gen.

keyData.saltValue

verifierHashValue

gen. HmacKey

hash 31/54

Check data integrity

encryptedKeyValue

encryptedHmacKey

encryptedHmacValue

password

gen. secretKey

ZIP file gen.

keyData.saltValue

verifierHashValue

gen. HmacKey

hash 32/54

Dependency of encryption

encryptedKeyValue

encryptedHmacKey

encryptedHmacValue

password

gen. secretKey

ZIP file gen.

keyData.saltValue

verifierHashValue

gen. HmacKey

irrelevant to pass 𝑐 = 𝐸𝑛𝑐(𝑆𝐾 , 𝑠𝑎𝑙𝑡,𝑚)

• Demo

• Situations

• Motivation

• CSPRG

• Example of known backdoor

• Hooking into MS Office's secret key generator

Agenda

• Normal encryptor

• main part to encrypt 𝑚

• You can not decrypt (𝑠𝑎𝑙𝑡, 𝑐) without 𝑆𝐾

• We can make a backdoor if the generator of 𝑆𝐾 is controlled.

generate 𝑠𝑎𝑙𝑡 randomly generate 𝑆𝐾 randomly

𝑐 = 𝐸𝑛𝑐 𝑆𝐾 , 𝑠𝑎𝑙𝑡,𝑚

Motivation

𝑚 (𝑠𝑎𝑙𝑡, 𝑐)

• Malicious (having backdoor）encryptor

• Malicious Eve prepares a master secret key 𝑋

• Eve embeds 𝑋 into an encryptor

• Eve can get 𝑆𝐾 = 𝐻 𝑋, 𝑠𝑎𝑙𝑡 by 𝑋 then decrypt it

• You can not notice that (𝑠𝑎𝑙𝑡, 𝑐) has a backdoor

An example of controlled 𝑆𝐾

𝑚 (𝑠𝑎𝑙𝑡, 𝑐) generate 𝑠𝑎𝑙𝑡 randomly

𝑆𝐾 = 𝐻 𝑋, 𝑠𝑎𝑙𝑡 𝑐 = 𝐸𝑛𝑐 𝑚 by (𝑠𝑎𝑙𝑡, 𝑆𝐾)

Eve 𝑋

𝑆𝐾

• 5 p.m. on Friday,

Make a proof of concept for MS Office

Hi Mitsunari, could you hook into the random

generator of MS Office?

I'll try it. By when do you need it?

Coming Monday.

• PRG（Pseudo Random Generator）

• used for game and simulation

• MT(Mersenne Twister) is popular

• CSPRG(Cryptographically Secure PRG)

• Nobody should be able to predict next bit from previous bits

• used for secret key generator

• MT is not CSPRG

• inner status is determined by 624x4-byte output

0 1 0 0 1 1 1 0 1 ?

known previous bits

next bit

• I strongly discourage you from implementing your own CSPRG

• Use CSPRG provided by system vendor instead

• /dev/urandom on Linux

• non blocking device for CSPRG

• entropy from mouse, keyboard, disk I/O and interrupts

Example of CSPRG

• Intel hardware CSPRG instruction

• Meets the NIST SP 800-90A standard

• Uses an on-chip non-deterministic entropy source

• Easy to use

• Difficult to use it safely

• Retry limit should be employed to prevent a busy loop (extremely rare)

rdrand

// uint64_t getRand(); getRand: .lp: rdrand rax // store random number in rax jnc .lp // retry if failure (rare) ret

• Output of /dev/urandom is fixed by only modified rdrand (kernel 3.8.13 by Taylor Hornby）

• rdrand() { return [edx] ^ 0x41414141; }

• Combine other entropy sources to avoid having a single point of failure

If rdrand has a backdoor?

• CSPRG of Microsoft CryptoAPI

• Used to generate salt and secret key

• Hook this function

• I expected that MS Office calls this to make a secret key

• If my hooked CryptGenRandom always return fixed bytes, then a secret key may be fixed.

• First, I tried to my (very old) DLL injection library

• Did not work as expected due to ASLR

CryptGenRandom()

• A library to hook Win32 APIs by MS Research

• http://research.microsoft.com/en-us/projects/detours/

• 64-bit version costs $9,999.95 (32-bit version is free)

• Usage

• Source code of dll to hook

Detours library (1/2)

BOOL HookCryptGenRandom(HCRYPTPROV, DWORD len, BYTE *p) { memset(p, 'a', len); return TRUE; } BOOL DllMain(HINSTANCE, DWORD reason, LPVOID) { if (reason == DLL_PROCESS_ATTACH) { orgFunc = DetourFindFunction("adapi32.dll", "CryptGenRandom"); DetourAttach(&orgFunc, HookCryptGenRandom); ...

• test.exe

• Run test.exe with withdll.exe in Detours

• CryptGenRandom is now hooked!

• But, the function is not called by MS Office...

Detours library (2/2)

int main() { RandomGenerator rg; // wrapper of CryptGenRandom() for (int i = 0; i < 3; i++) printf("%08x¥n", rg.get32()); }

>test.exe 812e1af0 // random ad990e76 865cb964

>withdll.exe /d:hook.dll test.exe 61616161 // "aaaa" 61616161 61616161

• Run Excel on debugger, see loaded DLLs

• I found rsaenh.dll

Trial and error (1/3)

• Extract symbols of rsaenh.dll

• dumpbin /exports rsaenh.dll

• What is CPGenRandom?

ordinal hint RVA name 1 0 0000230C CPAcquireContext 2 1 00003A80 CPCreateHash 3 2 0001CC1C CPDecrypt 4 3 0001DBC8 CPDeriveKey ... 11 A 00009A80 CPGenKey 12 B 00001D3C CPGenRandom

• CPxxx functions are obsolete

• CPxxx is renamed to Cryptxxx

• CPxxx is called from Cryptxxx

• Cryptxxx is hooked automatically if CPxxx is hooked

• Excel calls CPGenRandom

• Excel seems to generate secret key!

• However,

>msoffice-crypt –psk easy.xlsx –p test ... secretKey = 8BBE31319EA4CAB9F...33013EB8853F8C6A7F5

>msoffice-crypt –psk complex.xlsx –p testtest ... secretKey = 8BBE31319EA4CAB9F...33013EB8853F8C6A7F5

• Excel generates same secret key without hook

• MS Office Word, PowerPoint generate different key

• Only Excel has a bug

• About hooking CPGenRandom

• This hook was not enough to take control over the generator

• Hooking timer functions were not enough yet

• Details unknown because of no investigation made

It is a bug of Excel

• Demo

• Situations

• Check your Excel files

• Improved format

Agenda

• This bug is fixed at 2015/Oct/13 (MS15-110)

• But, the generated files won't be fixed

• Verify secret keys in your files by msoffice-crypt

• Re-encrypt files if you find the same secret keys

Check your Excel files

• What is a reliable format?

• MS Office format is secure enough, but it is difficult to prove no backdoor exists in the file

• The problem generally exists for tools provided in binary

• (again) malicious encryptor

• We want a format that is provably secure against the backdoor

Future work

𝑚 𝑠𝑎𝑙𝑡, 𝑐 , 𝑆𝐾 blackbox encryptor

𝑠𝑎𝑙𝑡 : random number 𝑋 : master secret key in the encryptor 𝑆𝐾 : 𝐻(𝑠𝑎𝑙𝑡, 𝑋) Eve gets 𝑆𝐾 by (𝑠𝑎𝑙𝑡, 𝑋)

• Generate 𝑆𝐾 with KDF(Key Derivation Function)

1. generate 𝑟0, 𝑟1 randomly

2. 𝑠𝑎𝑙𝑡 = 𝐻 𝑝𝑎𝑠𝑠, 𝑟0 , 𝐻 : KDF

3. 𝑆𝐾 = 𝐻(𝑝𝑎𝑠𝑠, 𝑟1)

4. 𝑐 = 𝐸𝑛𝑐 𝑟0, 𝑟1, 𝑚 by 𝑠𝑎𝑙𝑡, 𝑆𝐾

5. output: 𝑐, 𝑠𝑎𝑙𝑡

• Format validation

• Verify the output follows the correct steps

1. 𝑟0, 𝑟1, 𝑚 ≔ 𝐷𝑒𝑐 𝑐

2. 𝑠𝑎𝑙𝑡 =?𝐻(𝑝𝑎𝑠𝑠, 𝑟0)

3. 𝑆𝐾 =?𝐻(𝑝𝑎𝑠𝑠, 𝑟1)

A backdoor-resistant format

𝑆𝐾 𝑚

𝑟0 𝑟1 𝑝𝑎𝑠𝑠

𝐻 𝐻

𝑠𝑎𝑙𝑡

𝐸𝑛𝑐

• For a person who knows only (𝑠𝑎𝑙𝑡, 𝑐)

• same difficulty as previous formats

• For Eve

• same difficulty of the attack against KDF to get 𝑝𝑎𝑠𝑠 from 𝑠𝑎𝑙𝑡 when 𝑟0 is known

• Detection of maliciousness is easy if 𝑟0 is fixed

• seems to be safe if all 𝑟0 are different from each other when 10000 times encrypted

• then, the attack is 10000 times harder for Eve

Difficutly of the proposed format

𝑆𝐾 𝑚

𝑟0 𝑟1 𝑝𝑎𝑠𝑠

𝐻 𝐻

𝑠𝑎𝑙𝑡

𝐸𝑛𝑐

• Excel files can be attacked through their secret key without knowing the pass

• It is a bug of Excel and is fixed

• It can be applied to general password based encrypted file formats

Conclusion

Backdoors with the MS Office file encryption master key and a proposal for a reliable file format

Engineering