security through obscurity

Security Through Obscurity

Clark Thomborson

Version of7 December 2011

for Mark Stamp’s CS266 at SJSU

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Questions to be (Partially) Answered

What is security? What is obscurity? Is obscurity necessary for security? How can we obscure a computation or a

communication?

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The first step in wisdom is to know the things themselves; this notion consists in having a true idea of the

objects; objects are distinguished and known by

classifying them methodically and giving them appropriate names.

Therefore, classification and name-giving will be the foundation of our science.

[Carolus Linnæus, Systema Naturæ, 1735]

What is Security?(A Taxonomic Approach)

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Standard Taxonomy of Security

1. Confidentiality: no one is allowed to read, unless they are authorised.

2. Integrity: no one is allowed to write, unless they are authorised.

3. Availability: all authorised reads and writes will be performed by the system.

Authorisation: giving someone the authority to do something.

Authentication: being assured of someone’s identity. Identification: knowing someone’s name or ID#. Auditing: maintaining (and reviewing) records of

security decisions.

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A Hierarchy of Security

Static security: the Confidentiality, Integrity, and Availability properties of a system.

Dynamic security: the technical processes which assure static security. The gold standard: Authentication, Authorisation, Audit. Defense in depth: Prevention, Detection, Response.

Security governance: the “people processes” which develop and maintain a secure system. Governors set budgets and delegate their responsibilities

for Specification, Implementation, and Assurance.

A Full Range of Static Security

Confidentiality, Integrity, and Availability are properties of data objects, allowing us to specify “information security”.

What about computer security? Data + executables. Unix directories have “rwx” permission bits. If all executions are authorised, then the system has “X-ity”. GuiJu FangYuan ZhiZhiYe a new English word “guijuity”

Let’s use a classifier, rather than listing some classes! Confidentiality, Integrity, and Guijuity are Prohibitions (P+). Availability is a general Permission (P−), with 3 subclasses.

Security

P− P+

C I G

Security

AC I G W XR

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Prohibitions and Permissions

Prohibition: disallow an action. Permission: allow an action. There are two types of P-secure systems:

In a prohibitive system, all actions are prohibited by default. Permissions are granted in special cases, e.g. to authorised individuals.

In a permissive system, all actions are permitted by default. Prohibitions are special cases, e.g. when an individual attempts to access a secure system.

Prohibitive systems have permissive subsystems. Permissive systems have prohibitive subsystems.

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Recursive Security

Prohibitions, i.e. “Thou shalt not kill.” General rule: An action (in some range P−) is

prohibited, with exceptions (permissions) E1, E2, E3, ...

Permissions, i.e. a “licence to kill” (James Bond). General rule: An action in P+ is permitted, with

exceptions (prohibitions) E1, E2, E3, ... Static security is a hierarchy of controls on actions:

P+: permitted

E3

E1: prohibitedE2E11

E12

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Is Our Taxonomy Complete?

Prohibitions and permissions are properties of hierarchical systems, such as a judicial system. Most legal controls (“laws”) are prohibitive: they prohibit

certain actions, with some exceptions (permissions). Contracts are non-hierarchical, agreed between

peers, consisting of Obligations: requirements to act, i.e. prohibitions on

future inaction. Exemptions: exceptions to an obligation, i.e.

permissions for future inaction Obligations and exemptions are not P-security

rules. Obligations arise occasionally in the law, e.g. a doctor’s

“duty of care” or a trustee’s fiduciary responsibility.

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Obligations are forbidden inactions; Prohibitions are forbidden actions. When we take out a loan, we are obligated to repay it. We are

forbidden from never repaying. Exemptions are allowed inactions; Permissions are

allowed actions. In the English legal tradition, a court can not compel a person to give

evidence which would incriminate their spouse (husband or wife). This is an exemption from a general obligation to give evidence.

We have added a new level to our hierarchy!

Forbiddances and Allowances

S

Forbid Allow

PerPro Obl Exe

S

ExePro Per Obl


Reviewing our Questions

1. What is security? Three layers: static, dynamic, governance. A taxonomic structure for static security:

(forbiddances, allowances) x (actions, inactions). Four types of static security rules: prohibitions

(on reading C, writing I, executing G); permissions (R, W, X); obligations (OR, OW, OX), and exemptions (ER, EW, EX).

Most existing systems are underspecified on permissions, obligations, and exemptions.

2. What is obscurity?3. Is obscurity necessary for security?


Obscurity, Opacity, Steganography, Cryptography

Obscure: difficult to see Opaque: impossible to see through

Not antonyms, but connotative... Steganography: surreptitious communication

Axiomatically “obscure”, may be trustworthy. Goal: adversary is unaware of comms (“stealthy”)

Cryptography: secret communication Axiomatically “opaque”, may be untrustworthy. Goal: adversary is unable to interpret comms.


Unifying the Model

Transmitter (Alice) Receiver (Bob) Secret Message (M)Encryption: Alice sends e(M, k) to Bob on channel C. Bob computes M ← d(e(M, k), k’) using

secret k’. Charles, the adversary, knows C, e( ), d( ).


Steganographic Comms

Alice uses an obscure channel C. Bob must know “where and when” to look for

a message from Alice. Alice uses an obscure coding e( ).

Bob must know “how to interpret” Alice’s message.

Alice & Bob must be stealthy: Additional traffic on C must not be obvious. Interpretation of e( ) must not be obvious. e(M) must seem “normal” for C.


An Example: Stegoblogging

Alice gains write access to a disused (or new) blog or wiki X.

Alice selects “covertext” from an existing blog or wiki on a similar subject

Alice writes her “stegomessage”, one bit at a time, by selecting homonyms or misspellings from a dictionary for words in the covertext that are selected at random with low probability from the covertext.

Bob must know (or guess) X; he can find the covertext by googling on the “stegotext”; then he can read the stegomessage.

Bob leverages his prior knowledge of X: the stegomessage should be longer than a URL!


The Importance of Secrets

Charles has a feasible attack, if he locates the stegotext or can guess a cryptokey. He needs a very long sequence of cryptotext, if the

cipher and key are both “strong”. It is generally difficult or expensive for Alice and

Bob to establish the secret(s) required to set up their channel. Exceptions: A memory stick can hold many gigabytes (but how

can Alice transmit it securely to Bob?) Alice and Bob can use the Diffie-Hellman algorithm,

even if Charles is eavesdropping (but how can Alice be sure she’s talking to Bob?)


Evaluating Cryptosecurity

Cryptography is assumed secure in practice, but we can’t measure this security. Cryptographic methods are not used, unless they are trusted. Axiom 1:

the “crack rate” 1/t is very small. Big targets! Only a few methods in widespread use. Axiom 2: if anyone cracks a widely used cipher, we’ll soon know (time

parameter t’). Design implication: we need a backup cipher, and an ability to shift to it

quickly (parameter t”) Axiom 3: trusted ciphers will be created at rate > 1/t. Axiom 4: key secrecy is maintained (we need obscurity). Design implication: any single-key breach and rekeying should have

negligible cost. Then: the cost of cryptosecurity is B/t, where B is the cost of a breach

that persists for t’+t”.


Evaluating Insecurity

Steganography is assumed insecure in practice. If Bob knows where and when to look, and how to

interpret, why doesn’t Charles also know this? Bob must be stealthy when listening and

interpreting: Charles may learn. Axiom 1: our stegosystems will be cracked at rate

1/t (Poisson process). Design implication: we must shift stegosystems at

rate > 1/t. The cost of stegosecurity is B/t, where B is the cost

of each breach.


Practicalities

Available stegosystems may have such large 1/t that they’re uneconomic, even for systems with small B.

It may be impossible to purchase insurance to cover B for a system which relies on a highly trusted (“small 1/t”) cipher to attain its moderate B/t.

Implication: don’t rely solely on cryptography (or steganography)!


Defense in Depth

Ideally, security is preventative. A single preventive layer may be insufficient.

“Defence in depth” through Additional preventive layer(s); or Layer(s) that “respond” to a detected breach.

Goals of detect & respond systems To detect breaches more rapidly (reducing t’) To respond more appropriately (reducing B)


Security Techniques1. Prevention:

a) Deter attacks on forbiddances using encryption, obfuscation, cryptographic hashes, watermarks, or trustworthy computing.

b) Deter attacks on allowances using replication (or other resilient algorithmic techniques), obfuscation.

2. Detection:a) Monitor subjects (user logs). Requires user ID: biometrics, ID

tokens, or passwords.b) Monitor actions (execution logs, intrusion detectors). Requires

code ID: cryptographic hashing, watermarking.c) Monitor objects (object logs). Requires object ID: hashing,

watermarking.3. Response:

a) Ask for help: Set off an alarm (which may be silent –steganographic), then wait for an enforcement agent.

b) Self-help: Self-destructive or self-repairing systems. If these responses are obscure, they’re more difficult to attack.


Too Much to Think About!

We can’t discuss all uses of obscurity in security during a single seminar.

Let’s focus on a subset of the forbiddances: the guijuities. Obscurity is also helpful in assuring

exceptions. (Bureaucracies rely heavily on this technique ;-)


Opacity vs Obscurity in CIG

Confidentiality (access control on reads) Encryption vs. stegocommunication

Integrity (access control on writes) Cryptographic signatures vs. fragile watermarks

Guijuity (access control on executions) Homomorphic encryption vs. obfuscation Opacity is only feasible for very simple

computations (mul-adds, FSAs). In practice, we use obscurity to assure our

guijuities.


What is Obfuscation? Obfuscation is a semantics-preserving

transformation of computer code that renders it difficult to analyse – thus impossible to modify safely. This enforces guijuity on the current platform.

To secure guijuity in cases where the code itself is the protected resource, we need a ‘tether’. Tethered code uses a platform ID in its guijuity

decisions (e.g. license-enforcement).


How to Obfuscate Software? Lexical layer: obscure the names of variables,

constants, opcodes, methods, classes, interfaces, etc. (Important for interpreted languages and named interfaces.)

Data obfuscations: obscure the values of variables (e.g. by encoding

several booleans in one int; encoding one int in several floats; encoding values in enumerable graphs)

obscure data structures (e.g. transforming 2-d arrays into vectors, and vice versa).

Control obfuscations (to be explained later)


Attacks on Data Obfuscation An attacker may be able to discover the decoding

function, by observing program behaviour immediately prior to output: print( decode( x ) ), where x is an obfuscated variable.

An attacker may be able to discover the encoding function, by observing program behaviour immediately after input.

A sufficiently clever human will eventually de-obfuscate any code. Our goal is to frustrate an attacker who wants to automate the de-obfuscation process.

More complex obfuscations are more difficult to de-obfuscate, but they tend to degrade program efficiency and may enable pattern-matching attacks.


Cryptographic Obfuscations? Cloakware have patented a “homomorphic

obfuscation” method: add, mul, sub, and divide by constant, using the Chinese Remainder Theorem. W Zhu, in my group, fixed a bug in their division algorithm.

An ideal data obfuscator would have a cryptographic key that selects one of 264 encoding functions.

Fundamental vulnerability: The encoding and decoding functions must be included in the obfuscated software. Otherwise the obfuscated variables cannot be read and written. “White-box cryptography” is an obfuscated code that resists

automated analysis, deterring adversaries who would extract a working implementation of the keyed functions or of the keys themselves.


Practical Data Obfuscation Barak et al. have proved that “perfect obfuscation” is

impossible, but “practical obfuscation” is still possible. We cannot build a “black box” (as required to

implement an encryption) without using obfuscation somewhere – either in our hardware, or in software, or in both.

In practical obfuscation, our goal is to find a cost-effective way of preventing our adversaries from learning our secret for some period of time. This places a constraint on system design – we must be able

to re-establish security after we lose control of our secret. “Technical security” is insufficient as a response mechanism. Practical systems rely on legal, moral, and financial controls

to mitigate damage and to restore security after a successful attack.


Control Obfuscations Inline procedures Outline procedures Obscure method inheritances (e.g.

refactor classes) Opaque predicates:

Dead code (which may trigger a tamper-response mechanism if it is executed!)

Variant (duplicate) code Obscure control flow (“flattened” or

irreducible)


History of Software Obfuscation “Hand-crafted” obfuscations: IOCCC (Int’l Obfuscated C Code

Contest, 1984 - ); a few earlier examples. InstallShield (1987 - present). Automated lexical obfuscations since 1996: Crema,

HoseMocha, … Automated control obfuscations since 1996: Monden, … Opaque predicates since 1997: Collberg et al., … Commercial vendors since 1997: Cloakware, Microsoft (in

their compiler). Commercial users since 1997: Adobe DocBox, Skype, … Obfuscation is still a small field, with just a handful of

companies selling obfuscation products and services. There are only a few non-trivial results in conference or journal articles, and a few dozen patents.


Summary / Review

A taxonomy of static security:(forbiddance, allowance) x (action, inaction) =(prohibition, permission, obligation, exemption).

Some uses of opacity and obscurity, in the design of secure systems.

An argument that obscurity is necessary, in practice, for secure systems.


The Future?

What if our primary design goal were … Transparency (and translucency)?

• Our systems would assure integrity.• We’d know what happened, and could respond appropriately.

Predictability (and guessability)?• Our systems would assure availability.• We could hold each other accountable for our actions – fewer

excuses (“the dog ate it”, “the system crashed”). Opacity and obscurity are preventative, fearful.

Would it be brave, or would it be foolish, to design forward-looking systems by relying on transparency or predictability, instead of opacity?

security through obscurity

Documents

dynamic security

security governance

information security

computer security

software protection

records of security

prohibitions p

general permission p