https://crocs.fi.muni.cz @CRoCS_MUNI
PV204 Security technologies
Authentication: passwords, OTP, FIDO U2F
Petr Švenda svenda@fi.muni.cz @rngsec
Centre for Research on Cryptography and Security, Masaryk University
Please report any inaccuracies or suggestions for improvements here:
https://drive.google.com/file/d/1WNMU4fmG7sMMcuHHA-jgXQgG2nWkUCoJ/view?usp=sharing
IS,1998 2021
https://crocs.fi.muni.cz @CRoCS_MUNI2 PV204 Authentication and passwords
• Place/upvote questions in slido
while listening to lecture video
• We will together discuss these
during every week lecture Q&A
#pv204_2022#pv204_2024
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COURSE TRIVIA:
PV204_00_COURSEOVERVIEW_2024.PDF
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Basic terms
• Identification
– Establish what the (previously unknown) entity is
• Authentication
– Verify if entity is really what it claims to be
• Authorization (access control)
– Define an access policy to use specified resource
– Check if entity is allowed (authorized) to use resource
• Authentication may be required before an entity allowed to use
resource to which is authorized
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Options for authentication
• Something you:
1. Know (password, key)
2. Have (token, smartcard)
3. Are (biometrics)
• Combination of multiple options – two-factor authentication (or more)
1. Registration phase (how is new user added)
2. Verification phase (how is user’s claimed identity verified)
3. Recovery phase (what if user forgot/lost authentication credentials)
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PASSWORDS
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Mode of usage for passwords
• Verify by direct match (provided_password == expected_password?)
– Example: HTTP basic access authentication
– Be aware of plaintext storage on server
– Be aware of potential side-channels (mismatch on Xth character)
• Verify by match of derived value (hash(password | salt))
– Be aware of rainbow tables and brute-force crackers
• Derive key: Password → cryptographic key
– Example: key = PBKDF2(password)
• Used to establish authenticated key
– Example: Password + Diffie-Hellman → authenticated key…
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Problems associated with passwords
• How to create strong password?
• How to use password securely?
• How to store password securely?
• Same value is used for the long time (exposure)
• Value of password is independent from the target operation (e.g.,
authorization of bank transfer request)
• User usually can’t memorize long-enough password
• …
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Where the passwords can be compromised?
1. Client side (malware on user computer)
2. Database storage
– Cleartext storage
– Backup data (“tapes”)
– Server compromise, misconfiguration
3. Host machine (memory, history, cache)
4. Network transmission (network sniffer, proxy logs)
5. Hardcoded secrets (inside app binary)
• Difficult to detect compromise and change after the exposure
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https://haveibeenpwned.com/ (Troy Hunt)
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Total pwned accounts: 12,463,965,246
Collection #1: 772,904,991 accounts!
https://crocs.fi.muni.cz @CRoCS_MUNI
https://haveibeenpwned.com/Passwords
• Check how many times was given password found in leaked datasets
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password
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Password “hardening” ideas
1. Hash password by one-way function (shall be hard to invert)
2. Slowdown cracking attempts (less potential passwords tried)
3. Enable users to have long, random and unique passwords
4. Have unique password for every authentication attempt
5. Replace/complement passwords with something else (e.g.,
smartcard)
6. Bind response to server domain name (to prevent phishing)
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In follow-up slides, we will
discuss these ideas one by one
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IDEA: HASH PASSWORDS
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Joe; insecure
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(Hashed-)Password cracking
• Scenario: dump of database with password hashes, find original password
• Password cracking attacks
– Brute-force attack (up to 8 characters)
– Dictionary attack (passwords with higher probability tried first)
– Patterns: Dictionary + brute-force (Password[0-9]*)
– Rainbow tables (time-memory trade-off)
– Parallelization (many parallel cores)
– GPU/FPGA/ASIC speedup of cracking
• Tools
– Generic: Hashcat, John the Ripper, Brutus, RainbowCrack…
– Targeted to application: TrueCrack, Aircrack-NG…
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Password reality (from many breaches + pwd cracking)
• User has usually weak password
– >60% were (dictionary) brute-forced
• Server/service is frequently compromised
– Server-side compromises are now very frequent
• Users do not use unique passwords between services
– Gawker and root.com leaks: 76% had the exact same password
• Different authentication channels may not be independent
– Web-browsing + SMS on smart phones?
• Account recovery is often easier to guess than original password
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Total pwned accounts: 12,463,965,246
Collection #1: 772,904,991 accounts!
https://crocs.fi.muni.cz @CRoCS_MUNI
Insecure password handling … what is the attack?
• Verify by direct match (provided_password == expected_password?)
– Attack: compromise plain passwords on server
• pwdTagi = SHA-2(“password”)
– Same passwords from multiple users => same resulting pwdTag
– Attack: Large pre-computed “rainbow” tables allow for very quick check common passwords
• pwdTagi = SHA-2(“password” | salt)
– Use of rainbow tables “prevented” by addition of random (and potentially public) salt
– Attack: dictionary-based brute-force still possible
• pwdTagi = AES(“password”, secret_key)
– Attack: If secret_key is leaked => direct decryption of all stored pwdTags => passwords
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Some issues addressed by PAKE (Password
Authenticated Key Exchange) protocols – future lecture
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IDEA: SLOWDOWN CRACKING ATTEMPTS
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Derivation of secrets from passwords
• PBKDF2 function, widely used
– Password is key for HMAC
– Salt added
– Many iterations to slow derivation
• Problem with custom-build hardware (GPU, ASIC)
– Repeated iterations not enough to prevent bruteforce
– (or would be too slow on standard CPU – user experience)
• Solution: function which requires large amount of memory
PV204 Authentication and passwords
Source: https://nakedsecurity.sophos.com
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scrypt – memory hard function
• Design as a protection against cracking hardware (usable against
PBKDF2)
– GPU, FPGA, ASICs…
– https://github.com/wg/scrypt/blob/master/src/main/java/com/lambdaworks/crypto
/SCrypt.java
• Memory-hard function
– Force computation to hold r (parameter) blocks in memory
– Uses PBKDF2 as outer interface
• Improved version: NeoScrypt (uses full Salsa20)
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Reuse of external PBKDF2 structure
PV204 Authentication and passwords
https://www.reddit.com/r/crypto/comments/3dz285/password_hashing_competition_phc_has_selected/
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Argon2 – memory hard function
• Password hashing competition (PHC) winner (2013/2020)
– Large (configurable) memory size is required
• Memory hard functions are (slowly) replacing PBKDF2
• Now available in OpenSSL 3.2
• Why it slows down GPU cracking?
– GeForce RTX 4080 X3 16GB (9 728 cores, 16GB)
– GPU has thousands cores => thousands PBKDF2 passwords tested in parallel
– If Argon2 is used with 1GB memory required => max 16 passwords in parallel
• Why not parametrize with 16GB?
– Legitimate user must also have available memory (mobile phone…)
PV204 Authentication and passwords
https://www.reddit.com/r/crypto/comments/3dz285/password_hashing_competition_phc_has_selected/
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IDEA: LONG, RANDOM AND UNIQUE
PASSWORDS
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PASSWORD MANAGERS
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Evolution of password (managers)
1. Human memory only
2. Write it down on paper
3. Write it into file
4. Use local password manager
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Pαs$w0rd
Pαs$w0rd01
Google:
Sfdlk2c&
Skype:
*(&21mefd
Google:
Sfdlk2c&432mo%
Skype:
*(&21mefd872!&
Google:
Sfdlk2c&432mo%
Skype:
*(&21mefd872!&
https://crocs.fi.muni.cz @CRoCS_MUNI
Remote password managers
Google:
Sfdlk2c&432mo%
Skype:
*(&21mefd872!&
KeePass+Dropbox
LastPass
1Password
MozillaSync
Firefox Lockwise
…
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But passwords are encrypted, right?
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Devil is in the details
• How are passwords encrypted? (PBKDF2 or Argon2? Parameters?)
• How are legacy users handled? (possible smaller parameters)
• Is everything encrypted? (URL, notes, IPs…)
• Is recovery possible? How?
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PASSWORD MANAGER FOR MULTIPLE
DEVICES
Case study
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Functional and security assumptions
• Functional
– User stores fixed secrets (passwords…)
– User has multiple connected devices
– Easy to use ☺
• Security
– Service can’t be trusted
– User chooses weak password
– Devices can be lost (and later revoked)
– User has independent channel (phone)
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Main security design principles
I. Treat storage service as untrusted and perform security
sensitive operations on client
II. Make necessary trusted component as small as possible
III. Prevent offline brute-force, but don’t expect strong
password from user
– add entropy from other source
IV. Make transmitted sensitive values short-lived
V. (Trusted hardware can provide additional support)
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Public-key cryptography indirection
Google:
Sfdlk2c&432mo%
Skype:
*(&21mefd872!&
K = H(‘Password’)
K
Google:
Sfdlk2c&432mo%
K
Password
Priv_U
KEK
K
Pub_U
Password
KEK = H(‘Password’)
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Public-key crypto indirection
Google:
Sfdlk2c&432mo%
K
Priv_U
KEK
K
Pub_U
Password
KEK = H(‘Password’)
Public-key crypto
indirection allows for
asynchronous change of K
Long private key can be
also stored on Service
K’,K’’,K’’’…
[K’]Pub_U
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Weak password?
Google:
Sfdlk2c&432mo%
K
Priv_U
KEK
K
Pub_U
Password
KEK = H(‘Password’)
Password
KEK = H(‘Password’)
KEK
Priv_UK
K
Google:
Sfdlk2c&432mo%
Attacker has motivation
for attacking the Service!
Users tend to have
weak passwords…
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Trusted server/secure element
Google:
Sfdlk2c&432mo%
K
Priv_U
KEK
K
Pub_U
Password
KEK = H(‘Password’
User1:SecretData
User2:SecretData’
…
Separate trusted entities
provide additional data
| SecretData)
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Larger attack surface on
Trusted server
(connection from world)
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Google:
Sfdlk2c&432mo%
K
Priv_U
KEK
K
Pub_U
Password
KEK = H(‘Password’ | SecretData)
User1:SecretData
User2:SecretData’
…
SMS:
SecretData
SecretData
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Multiple devices
Google:
Sfdlk2c&432mo%
K
Priv_U
KEK
K
Pub_U
KEK
Dev1
KEK
Dev2
KEK
Dev3
Dev1 Dev2
Dev3
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Dev1 = H(‘Password1’|SecretData1)
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• Device management (new, remove, revoke)
• Device authentication
• Group management (users, boards, secrets)
• Password change, private key change
• Access recovery
• …
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Devil is in the details…
Other operations
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Do we have some implementations?
• Apple iCloud Keychain - service showcased in 2013
• Lack of details until iOS Security report 02/2014
– https://web.archive.org/web/20150319073804/https://www.apple.com/business/docs/iOS_Security_Guide.pdf
– https://blog.cryptographyengineering.com/2016/08/13/is-apples-cloud-key-vault-crypto/ (M.Green)
• Current platform details (Advanced Data Protection for iCloud)
– https://help.apple.com/pdf/security/en_US/apple-platform-security-guide.pdf
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Apple’s iCloud Keychain
• Multiple similarities to the described example
– Layer of indirection via asymmetric cryptography
– Support for multiple devices
– Asynchronous operations via application tickets
– Authorization and signature of additional devices
– User phone registered and required
• Still reliance on user’s (potentially weak) password for recovery
– But only limited number of tries allowed
• Trusted component via internal HSM (Hardware Security Module)
– Recovery mode with 4-digit code (default, can be set longer)
– HSM will decrypt recovery key only after code validation
– Note: only 4 digits is not an issue here – HSM enforce limited # retries
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IDEA: HAVE UNIQUE PASSWORD FOR
EVERY AUTHENTICATION ATTEMPT
ONE-TIME PASSWORDS: HOTP & TOTP
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ONE-TIME PASSWORDS
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Recall: Problems associated with passwords
• How to create secure password?
• How to use password securely?
• How to store password securely?
• Same value is used for the long time (exposure)
• Value of password is independent from target operation (e.g.,
authorization of request)
• …
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One-time passwords tries
to address these issues
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HMAC-based One-time Password Algorithm (RFC 4226)
• HMAC-based One-time Password Algorithm (HOTP)
– Secret key K
– Counter (challenge) C
– HMAC(K,C) = SHA1(K ⊕ 0x5c5c… ∥ SHA1(K ⊕ 0x3636… ∥ C))
– HOTP(K,C) = Truncate(HMAC(K,C)) & 0x7FFFFFFF
– 0x7FFFFFFF mask to drop most significant bit (portability)
– HOTP-Value = HOTP(K,C) mod 10d (d … # of digits)
• Many practical implementations
– E.g., Google Authenticator
• https://en.wikipedia.org/wiki/HOTP
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HOTP – items, operations
• Logical operations
1. Generate initial state for new user and distribute key
2. Generate HOTP code and update state (user)
3. Verify HOTP code and update state (auth. server)
• Security considerations of HOTP
– Client compromise
– Server compromise
– Repeat of counter/challenge
– Counter mismatch tolerance window
– Phishing – user enters HOTP code at phishing website
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Sylvain Maret
Time-based One-time Password Algorithm
• Very similar to HOTP
– Time used instead of counter
• Requires synchronized clocks
– In practice realized as time window
• Tolerance to gradual desynchronization possible
– Server keeps device’s desynchronization offset
– Updates with every successful login
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OCRA: OATH Challenge-Response Algorithm
• Initiative for Open Authentication (OATH)
• OCRA is authentication algorithm based on HOTP
• OCRA code = CryptoFunction(K, DataInput)
– K: a shared secret key known to both parties
– DataInput: concatenation of the various input data values
• Counter, challenges, H(PIN/Passwd), session info, H(time)
– Default CryptoFunction is HOTP-SHA1-6
– https://tools.ietf.org/html/rfc6287
• Don’t confuse with Oauth (delegation of authentication)
– The OAuth 2.0 Authorization Framework (RFC6749)
– TLS-based security protocol for accessing HTTP service
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Increased risk at *OTP verification server
• More secure against client compromise
– Using OTP instead of passwords, KDF(time|key),
• But what if server is compromised?
– database hacks, temporal attacker presence
– E.g., Heartbleed – dump of OTP keys
• Possible solution
– Trusted hardware on the server
– OTP code verified inside trusted environment
– OTP key never leaves the hardware
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Problems:
1. Is OTP code fresh?
2. Is OTP generated for correct
domain (not phishing)?
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Possible password replacements
• Cambridge’s TR – wide range of possibilities listed
– The quest to replace passwords: a framework for comparative evaluation of
Web authentication schemes
– https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-817.pdf
• Many different possibilities, but passwords are cheap to start with, a
lot of legacy code exists and no mechanism offers all benefits
• Mandatory reading: UCAM-CL-817
– At least chapters: II. Benefits, V. Discussion
– Whole report is highly recommended
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IDEA: REPLACE PASSWORD BY
SMARTCARD WITH ASYMMETRIC KEYPAIR,
CHALLENGE-RESPONSE PROTOCOL AND
PREVENT PHISHING
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FIDO U2F PROTOCOL
(U2F → FIDO2 → WEBAUTHN)
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Revision 1: ECC-based challenge-response
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https://developers.yubico.com/U2F/Protocol_details/Overview.html
Problems: phishing, MiTM…
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Revision 2: URI + TLS channel id added
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https://developers.yubico.com/U2F/Protocol_details/Overview.html
Problem: using same device =>
detectable by services (same kpub)
https://accounts.google.com/ServiceLogin
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Revision 3: Application-specific key added
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https://developers.yubico.com/U2F/Protocol_details/Overview.html
Problem: Undetectable device cloning
new key pair and key
handle for each
registration
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Revision 4: Authentication counter added
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https://developers.yubico.com/U2F/Protocol_details/Overview.html
Option: What if server wants to verify
device properties before register?
Incremental counter
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Revision 5: Device attestation added
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https://developers.yubico.com/U2F/Protocol_details/Overview.html
Attestation certificate
signed with TTP
ECDSA NIST secp256r1 used
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FIDO U2F – current state
• FIDO alliance of major companies
• U2F → FIDO2 → WebAuthn (more than “just” U2F)
• Original U2F protocol extended and moved under W3 as WebAuthn
– https://www.w3.org/TR/webauthn/
• Large selection of tokens now available (including open-hardware)
• Android added systematic support for FIDO U2F (02/2019)
– Android phone acts as U2F token
– https://www.wired.com/story/android-passwordless-login-fido2
• Google Smart Lock app on iOS uses secure enclave and acts as FIDO token
• Since iOS 13.3. USB, NFC, and Lightning FIDO2-compliant security keys in
Safari browser (12/2019)
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CTAP/WebAuthn stack
• WebAuthn Protocol
– Asymmetric crypto-based challenge-response protocol
– Browser inserts actual URL (origin) as a part of the challenge
– Private key stored and used (token, phone…)
– An API for accessing Public Key Credentials Level 2 (level=version)
• Official documentation: https://www.w3.org/TR/webauthn/
• Client to Authenticator Protocol (CTAP)
– Protocol between browser and authenticator
– Authenticator = initially hardware token, but now range of devices (phones,
calculators…)
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https://developers.yubico.com/U2F/Protocol_details/Overview.html
WebAuthn
Authenticator CTAP
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True2F FIDO U2F token
• Yubikey 4 has single master key
– To efficiently derive keypairs for separate Relying parties (Google, GitHub…)
– Inserted during manufacturing phase (what if compromised?)
• Additional SMPC protocols (protection against backdoored token)
– Secure Multi-Party Computation (SMPC) will be covered later
– Verifiable insertion of browser randomness into final keypairs
– Prevention of private key leakage via ECDSA padding
• Backward-compatible (Relying party, HW)
• Efficient: 57ms vs. 23ms to authenticate
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https://arxiv.org/pdf/1810.04660.pdf
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WebAuthn - evolution of U2F protocol
• An API for accessing Public Key Credentials Level 2
– Official documentation: https://www.w3.org/TR/webauthn/
– (Level means version here ☺)
• Similar, but more complex standard than U2F
• Client to Authenticator Protocol (CTAP)
– protocol for communication between browser and token (authenticator)
– USB, NFC, Bluetooth
• CTAP 2.2 adds support for the hybrid transport (FIDO Cross-Device
Authentication flow, aka Passkeys)
• Explanation, demo page https://webauthn.guide/#about-webauthn
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Missing piece? “passkeys”
• Authentication on PC with FIDO2 token
• Authentication on mobile phone (with or without token)
• Authentication on PC without FIDO2 token?
• Idea of “passkeys” (multi-device FIDO credentials)
– WebAuthn (“U2F”) protocol used for base authentication (private keys needed)
– Replace hardware token with mobile phone
– Connect mobile phone with PC using Bluetooth LE (BLE) or websockets
• Now supported natively by Apple (Keychain), Google (Password
Manager) and Microsoft (Hello)
– https://media.fidoalliance.org/wp-content/uploads/2022/03/How-FIDO-Addresses-a-Full-Range-of-Use-Cases-March24.pdf
– https://passkeys.dev/docs/reference/specs/
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FIDO U2F devices
• Why have button? Is missing display problem?
• Existing past attack: direct WebUSB API in Chrome
– Malware bypass U2F API checking the URL
– Legitimate URL is send from malicious page
– https://www.wired.com/story/chrome-yubikey-phishing-webusb/
– APDU-level communication: https://npmccallum.gitlab.io/post/u2f-protocol-
overview/
• Well known is Yubikey, but open-source hardware and/or softwareonly
implementations also possible
– https://github.com/conorpp/u2f-zero
– https://github.com/solokeys/solo
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Always dig for implementation details
• How are ECC keys generated and stored?
• Yubikey saves storage memory by deriving ECC private keys from
master secret instead of randomly generating new one
– Possible as the ECC private key is random value
• Device secret generated during manufacturing
• What is the possible attack
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https://developers.yubico.com/U2F/Protocol_details/Key_generation.html
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Summary
• Passwords have multiple issues, but are hard to be replaced
• Major server-side breaches now very common
• Important to use passwords securely (guidelines)
• One-time passwords and tokens getting more used
• Password manager with synchronization over multiple devices is not
straightforward, but doable (e.g., Apple’s iCloud Keychain)
• FIDO2/Webauthn is major authentication improvement – use it!
• Mandatory reading: UCAM-CL-817
– At least chapters: II. Benefits, V. Discussion
– Whole report is highly recommended
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Hierarchy of authentication and
key establishment goals
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Common (mis-)Assumptions
1. User has strong password
2. Server/service is hard to compromise
3. User have unique passwords
4. Different authentication channels are independent
5. Recovery
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Password cracking defenses
• Don’t transmit or store in plaintext
• Process password on client, transmit only digest
• Don’t encrypt, hash instead
• Use salt to prevent rainbow tables attack
• Use memory-hard KDF algorithms
– To slow down custom build hardware
– Use strong KDF to derive keys (PBKDF2→Argon2)
• Use password-authenticated key exchange instead of password check
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Handling passwords in source code
• Limiting memory exposure
– Load only when needed
– Erase right after use
– Pass by reference / pointer to prevent copy in memory
– Derive session keys
• Don’t hardcode password into application binary
• Nice presentation (K. Kohli, examples how NOT to):
http://www.slideshare.net/amiable_indian/insecure-implementation-of-security-
best-practices-of-hashing-captchas-and-caching-presentation
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Hard-coded password might be visible both in
application binary and memory
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Alternative to hardcoded passwords/keys
• Don’t use passwords ☺
• Ask the user for a password
• Keep secrets in a separate file
• Encrypt stored secrets
• Store secrets in protected database
• Use already existing authentication credentials
• CERN guidelines
– https://security.web.cern.ch/security/recommendations/en/password_alternative
s.shtml
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Group activity
• Form group of 3-4 members (mix, not your neighbours)
– Introduce yourself with your name
• Discuss and write down on paper:
– What method(s) you use for authentication (password…)
– Is server using other authentication factor?
– How you store the authentication secret? (brain-only…)
• Time limit: 5 minutes
• Now return back to your original seat (if you wish ☺)
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Activity:
• Think about one or two surprising things from this lecture
• I want to hear at least 5 of these, tell me please ☺
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