Android Security Mechanisms

: name=”com . e x a m p l e . app . p e r m i s s i o n . PERMISSION1” : l a b e l =” @ s t r i n g / p e r m i s s i o n 1 l a b e l ” : d e s c r i p t i o n =” @ s t r i n g / p e r m i s s i o n 1 d e s c ” : p e r m i s s i o n G r o u p =”com . e x a m p l e . app . p e r m i s s i o n −g r o u p . TEST GROUP” : p r o t e c t i o n L e v e l =” s i g n a t u r e ” />

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Outline

Android Permissions Cryptographic Providers Network Security Bibliography Keywords

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JCA Provider Architecture

I

Java Cryptography Architecture (JCA) I I I

I

Cryptographic Service Provider (CSP) I I I I

I

Extensible cryptographic provider framework Set of APIs - major cryptographic primitives Applications specify an algorithm, do not depend on particular provider implementation Package with implementation of cryptographic services Advertises the implemented services and algorithms JCA maintains a registry of providers and their algorithms Providers in a order of preference

Service Provider Interface (SPI) I I

Common interface for implementations of a specific algorithm Abstract class implemented by provider

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JCA Engine Classes I

JCA engines provide: I I

I

Cryptographic operations (encrypt/decrypt, sign/verify, hash) Generation or conversion of cryptographic material (keys, parameters) Management and storage of cryptographic objects (keys, certificates)

I

Decouple client code from algorithm implementation

I

Static factory method getInstance()

I

Request implementation indirectly s t a t i c EngineClassName g e t I n s t a n c e ( S t r i n g a l g o r i t h m ) throws NoSuchAlgorithmException s t a t i c EngineClassName g e t I n s t a n c e ( S t r i n g algorithm , S t r i n g p r o v i d e r ) throws NoSuchAlgorithmException , NoSuchProviderException s t a t i c EngineClassName g e t I n s t a n c e ( S t r i n g algorithm , P r o v i d e r p r o v i d e r ) throws NoSuchAlgorithmException

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Message Digest

I

Hash function M e s s a g e D i g e s t md = M e s s a g e D i g e s t . g e t I n s t a n c e ( ”SHA− 2 5 6 ” ) ; byte [ ] data = getMessage ( ) ; b y t e [ ] h a s h = md . d i g e s t ( d a t a ) ;

I

Data provided in chuncks using update() then call digest()

I

If data is short and fixed - hashed in one step using digest()

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Signature I

Digital signature algorithms based on asymmetric encryption

I

Algorithm name: with

I

Sign: b y t e [ ] d a t a = ” m e s s a g e t o be s i g n e d ” . g e t B y t e s ( ” A S C I I ” ) ; S i g n a t u r e s = S i g n a t u r e . g e t I n s t a n c e ( ” SHA256withRSA ” ) ; s . i n i t S i g n ( privKey ) ; s i g . update ( data ) ; byte [ ] s i g n a t u r e = s i g . sign ( ) ;

I

Verify: S i g n a t u r e s = S i g n a t u r e . g e t I n s t a n c e ( ” SHA256withRSA ” ) ; s . i n i t V e r i f y ( pubKey ) ; s . update ( data ) ; boolean v a l i d = s . v e r i f y ( signature ) ;

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Cipher

I

Encryption and decryption operations

I

Encryption:

S e c r e t key = getSecretKey ( ) ; C i p h e r c = C i p h e r . g e t I n s t a n c e ( ” AES/CBC/ PKCS5Padding ” ) ; b y t e [ ] i v = new b y t e [ c . g e t B l o c k S i z e ( ) ] ; SecureRandom s r = new SecureRandom ( ) ; sr . nextBytes ( iv ) ; I v P a r a m e t e r S p e c i v p = new I v P a r a m e t e r S p e c ( i v ) ; c . i n i t ( C i p h e r . ENCRYPT MODE, key , i v p ) ; b y t e [ ] d a t a = ” Message t o e n c r y p t ” . g e t B y t e s ( ”UTF−8”); byte [ ] c i p h e r t e x t = c . doFinal ( data ) ;

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Cipher

I

Decryption:

C i p h e r c = C i p h e r . g e t I n s t a n c e ( ” AES/CBC/ PKCS5Padding ” ) ; c . i n i t ( C i p h e r . DECRYPT MODE, key , i v p ) ; byte [ ] data = c . doFinal ( c i p h e r t e x t ) ;

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MAC

I

Message Authentication Code algorithms

SecretKey key = getSecretKey ( ) ; Mac m = Mac . g e t I n s t a n c e ( ” HmacSha256 ” ) ; m. i n i t ( k e y ) ; b y t e [ ] d a t a = ” Message ” . g e t B y t e s ( ”UTF−8”); b y t e [ ] hmac = m. d o F i n a l ( d a t a ) ;

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KeyGenerator

I

Generates symmetric keys

I

Additional checks for weak keys

I

Set key parity when necessary

I

Takes advantage of the cryptographic hardware

K e y G e n e r a t o r kg = K e y G e n e r a t o r . g e t I n s t a n c e ( ” HmacSha256 ” ) ; S e c r e t K e y k e y = kg . g e n e r a t e K e y ( ) ;

K e y G e n e r a t o r kg = K e y G e n e r a t o r . g e t I n s t a n c e ( ” AES ” ) ; kg . i n i t ( 2 5 6 ) ; S e c r e t K e y k e y = kg . g e n e r a t e K e y ( ) ;

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KeyPairGenerator

I

Generates public and private keys

K e y P a i r G e n e r a t o r kpg = K e y P a i r G e n e r a t o r . g e t I n s t a n c e ( ”RSA ” ) ; kpg . i n i t i a l i z e ( 1 0 2 4 ) ; K e y P a i r p a i r = kpg . g e n e r a t e K e y P a i r ( ) ; PrivateKey p riv = pair . getPrivate ( ) ; P u b l i c K e y pub = p a i r . g e t P u b l i c ( ) ;

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Android JCA Providers I

Harmony’s Crypto Provider I I I

I

Android’s Bouncy Castle Provider I I I

I

I

Limited JCA provider part of the Java runtime library SecureRandom (SHA1PRNG), KeyFactory (DSA) MessageDigest (SHA-1), Signature (SHA1withDSA) Full-featured JCA provider Part of the Bouncy Castle Crypto API Cipher, KeyGenerator, Mac, MessageDigest, SecretKeyFactory, Signature, CertificateFactory Large number of algorithms

AndroidOpenSSL Provider I I I I

Native code, performance reasons Covers most functionality of Bouncy Castle Preferred provider Implementation uses JNI to access OpenSSL’s native code

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Outline

Android Permissions Cryptographic Providers Network Security Bibliography Keywords

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SSL/TLS

I

Secure Sockets Layer (SSL) and Transport Layer Security (TLS)

I

SSL is the predecesor of TLS

I

Secure point-to-point communication protocols

I

Authentication, Message confidentiality and integrity for communication over TCP/IP

I

Combination of symmetric and asymmetric encryption for confidentiality and integrity

I

Public key certificates for authentication

I

Java Secure Socket Extension (JSSE)

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Authentication

I

Based on public key cryptography and certificates

I

Both ends presents its certificate

I

If trusted, they negotiate a shared key for securing the communication using pairs of public/private keys

I

JSSE delegates trust decisions to TrustManager and authentication key selection to KeyManager

I

Each SSLSocket has access to them through SSLContext

I

TrustManager has a set of trusted CA certificates (trust anchors)

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Obtain Trust Anchors

I

Default JSSE TrustManager initialized using the system trust store I

/system/etc/security/cacerts.bks

T r u s t M a n a g e r F a c t o r y tmf = T r u s t M a n a g e r F a c t o r y . g e t I n s t a n c e ( TrustManagerFactory . getDefaultAlgorithm ( ) ) ; tmf . i n i t ( ( K e y S t o r e ) n u l l ) ; X509TrustManager xtm = ( X509TrustManager ) tmf . getTrustManagers ( ) [ 0 ] ; f o r ( X 5 0 9 C e r t i f i c a t e c e r t : xtm . g e t A c c e p t e d I s s u e r s ( ) ) { S t r i n g c e r t S t r = ”S : ” + c e r t . g e t S u b j e c t D N ( ) . getName ( ) + ”\ n I : ” + c e r t . g e t I s s u e r D N ( ) . getName ( ) ; Log . d (TAG, c e r t S t r ) ; }

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Use your own Trust Store I

Generate your trust store using Bouncy Castle and openSSL in comand line

I

Preferred HTTPS API

K e y S t o r e l o c a l T r u s t S t o r e = K e y S t o r e . g e t I n s t a n c e ( ”BKS ” ) ; I n p u t S t r e a m i n = g e t R e s o u r c e s ( ) . openRawResource ( R . raw . m y t r u s t s t o r e ) ; l o c a l T r u s t S t o r e . l o a d ( i n , TRUSTSTORE PASSWORD . t o C h a r A r r a y ( ) ) ; T r u s t M a n a g e r F a c t o r y tmf = T r u s t M a n a g e r F a c t o r y . g e t I n s t a n c e ( TrustManagerFactory . getDefaultAlgorithm ( ) ) ; tmf . i n i t ( t r u s t S t o r e ) ; S S L C o n t e x t s s l C t x = S S L C o n t e x t . g e t I n s t a n c e ( ” TLS ” ) ; s s l C t x . i n i t ( n u l l , tmf . g e t T r u s t M a n a g e r s ( ) , n u l l ) ; URL u r l = new URL( ” h t t p s : / / m y s e r v e r . com ” ) ; HttpsURLConnection u r l C o n n e c t i o n = ( HttpsURLConnection ) u r l urlConnection . setSSLSocketFactory ( sslCtx . getSocketFactory ( ) ) ;

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Outline

Android Permissions Cryptographic Providers Network Security Bibliography Keywords

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Bibliography

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Android Security Internals, Nikolay Elenkov

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http://nelenkov.blogspot.ro/2011/12/ using-custom-certificate-trust-store-on.html

I

https://github.com/nelenkov/custom-cert-https

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Outline

Android Permissions Cryptographic Providers Network Security Bibliography Keywords

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Keywords

I

Permissions

I

Java Cryptography Architecture

I

Protection levels

I

Cryptographic Service Provider

I

Static enforcement

I

Engine classes

I

Dynamic enforcement

I

Java Secure Socket Extension

I

Custom permissions

I

Trust Store

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