icloud keychain and ios 7 data protection presentation

iCloud Keychain and

iOS 7 Data ProtectionAndrey Belenko

Sr. Security Engineer @ viaForensics !

Alexey Troshichev@hackappcom founder

What is iCloud?

Whats inside?

Documents

Photos

Backups (SMS, application data, etc)

Keychain

Hackers view

Bruteforce protection?

Find My iPhone

Brought to you by hackapp.com

!

github.com/hackappcom/ibrute

@hackappcom

iCloud KeychainImage: Apple Inc.

Motivation

http://support.apple.com/kb/HT4865

Intercepting SSL

SSL Proxy (Burp, Charles, )

Root CA cert Proxy settings

Authentication

GET /authenticateAppleID, Password

DsID, mmeAuthToken, fmipAuthToken

icloud.com

/getAccountSettings

Setup Options

The Big Picture

*.keyvalueservice.icloud.com

*.escrowproxy.icloud.com

Keychain items (encrypted) Keybag (encrypted)

Some Secret

Key-Value Store Not new

Used extensively by many apps e.g. to keep preferences in sync across devices

iCloud Keychain utilises two stores:

com.apple.security.cloudkeychainproxy3 Syncing between devices

com.apple.sbd3 (securebackupd3) Copy to restore if no other devices

Escrow Proxy New; Designed to store precious secrets

Need to know iCSC to recover escrowed data

Need to receive SMS challenge

Must successfully complete SRP auth

User-Agent: com.apple.lakitu (iOS/OS X)

Image: mariowiki.com

Key-Value Store com.apple.security.cloudkeychainproxy3

S(usrPwd, D2_pub)

S(D2_priv, (D1_pub, D2_pub))

S(D1_priv, D1_pub)S(userPwd, D1_pub)

S(D1_priv, (D1_pub, D2_pub))S(userPwd, (D1_pub, D2_pub))

Key-Value Store com.apple.sbd3

Key Description

com.apple.securebackup.enabled Is Keychain data saved in KVS?

com.apple.securebackup.record Keychain records, encrypted

SecureBackupMetadata iCSC complexity, timestamp, country

BackupKeybag Keybag protecting Keychain records

BackupUsesEscrow Is keybag password escrowed?

BackupVersion Version, currently @1

BackupUUID UUID of the backup

4-digit iCSC [Default]


Random PasswordBL7Z-EBTJ-UBKD-X7NM-4W6D-J2N4



Keychain PasswordsyMa9ohCJtzzcVhE7sDVoCnb

Backup KeybagKey 1Key 2Key 3

AES-GCM 256 bit





AES-GCM 256 bit

AES-Wrap Keys RFC 3394





AES-GCM 256 bit





iCloud Security Code1234 PBKDF2 SHA-256 x 10000



AES-GCM 256 bit






AES-CBC 256 bit




AES-GCM 256 bit



Secure Remote Password Zero-knowledge password proof scheme

Combats sniffing/MITM

One password guess per connection attempt

Password verifier is not sufficient for impersonation

Escrow Proxy uses SRP-6a

Key Negotiationa random, A g^a

b random, B kv + g^bu H(A, B) u H(A, B)

x H(SALT, Password)S (B - kg^x) ^ (a + ux)

K H(S)S (Av^u) ^ b

K H(S)

Key VerificationM H(H(N) H(g), H(ID), SALT, A, B, K)

(Aborts if M is invalid)

ID, ASALT, B

MH(A, M, K)

Password verifier:!SALT randomx H(SALT,Password)v g^x

Agreed-upon parameters:!H one-way hash functionN, g group parametersk H(N, g)

Key Negotiationa random, A g^a

b random, B kv + g^bu H(A, B) u H(A, B)

x H(SALT, Password)S (B - kg^x) ^ (a + ux)

K H(S)S (Av^u) ^ b

K H(S)

Key VerificationM H(H(N) H(g), H(ID), SALT, A, B, K)

(Aborts if M is invalid)

ID, A, SMS CODESALT, B

M, SMS CODEH(A, M, K)

Password verifier:!SALT randomx H(SALT,Password)v g^x

Agreed-upon parameters:!H SHA-256N, g RFC 5054 w. 2048-bit groupk H(N, g)

Escrowed Data Recovery

*Dis

play

pur

pose

s on

ly

Escrowed Data Recovery/get_records

List of escrowed records

*Dis

play

pur

pose

s on

ly



/get_sms_targetsList of phone numbers*

*Dis

play

pur

pose

s on

ly




/generate_sms_challengeOK

*Dis

play

pur

pose

s on

ly





/srp_init [DsID, A, SMS CODE][UUID, DsID, SALT, B]

*Dis

play

pur

pose

s on

ly





/srp_init [DsID, A, SMS CODE][UUID, DsID, SALT, B]

/recover [UUID, DsID, M, SMS CODE][IV, AES-CBC(KSRP, Escrowed Record)]

*Dis

play

pur

pose

s on

ly

Escrow Proxy EndpointsEndpoint Description

get_club_cert [?] Obtain certificateenroll Submit escrow record

get_records List escrowed recordsget_sms_targets List SMS numbers for escrowed records

generate_sms_challenge Generate and send challenge codesrp_init First step of SRP protocolrecover Second step of SRP protocol

alter_sms_target Change SMS number

Escrow Record



AES-CBC 256 bit





AES-GCM 256 bit


Escrow Record



AES-CBC 256 bit


Key PBKDF2-SHA256(iCSC, 10000)

EscrowRecord AES-CBC(Key, RandomPassword)

Escrow RecordKey PBKDF2-SHA256(iCSC, 10000)


This is stored by Apple




iCSC is 4 digits by default




iCSC is 4 digits by default



Can you spot the problem yet?


Offline iCSC guessing is possible

Almost instant recovery [for default settings]

iCSC decrypts keybag password

Keybag password unlocks keybag keys

Keybag keys decrypt Keychain items

Apple, or other adversary with access to stored data, can near-

instantly decrypt master password and read synced iCloud

Keychain records !

(for default settings)

Setup Options

Complex iCSC



iCloud Security Codecorrect horse battery staple PBKDF2 SHA-256 x 10000

AES-CBC 256 bit




AES-GCM 256 bit


Complex iCSC

Mechanics are the same as with simple iCSC

Offline password recovery attack is still possible, although pointless if password is complex enough

Setup Options





AES-GCM 256 bit


iCloud Security Codecorrect horse battery staple PBKDF2 SHA-256 x 10000

AES-CBC 256 bit


Random iCSC





AES-GCM 256 bit


Random iCSC

Random iCSC

Escrow Proxy is not used

Random iCSC (or derived key) stored on the device [havent verified]

Setup OptionsiCloud

Keychain

Keychain Sync

Keychain Backup

Master Password

EscrowNo iCloud Security Code

Random iCloud Security CodeComplex iCloud Security CodeSimple iCloud Security Code

Conclusions

Image: Apple Inc.

Conclusions

Trust your vendor but verify his claims

Never ever use simple iCloud Security Code

Do not think that SMS Apple sends you is a 2FA

Yet, iCK is reasonably well engineered although not without shortcomings

Thank You! Questions are welcome :-)

!!

@abelenko @hackappcom

icloud keychain and ios 7 data protection presentation

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