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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/1NWVvbe2ZB_kxAiBEAfwoRae8Nd_3ZQye/view?usp=sharing
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• Place/upvote questions in slido
while listening to lecture video
• We will together discuss these
during every week lecture Q&A
(every Monday, 17-18:00)
www.crcs.cz/rsa @CRoCS_MUNI
COURSE TRIVIA:
PV204_00_COURSEOVERVIEW_2021.PPT
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AUTHENTICATION & AUTHORIZATION
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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)
• …
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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: 9,490,577,236
Collection #1: 772,904,991 accounts!
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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 (inputs 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: 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
– Gawker/root.com leak: 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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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
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https://www.reddit.com/r/crypto/comments/3dz285/password_hashing_competition_phc_has_selected/
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Argon2
• Password hashing competition (PHC) winner, 2013
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https://www.reddit.com/r/crypto/comments/3dz285/password_hashing_competition_phc_has_selected/
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Problem solved?
24
PV204 Authentication and passwords
https://www.ietf.org/mail-archive/web/cfrg/current/msg08439.html
Problem: situation with PHC winner
still unclear in 2019 
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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!&
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Remote password managers
Google:
Sfdlk2c&432mo%
Skype:
*(&21mefd872!&
KeePass+Dropbox
LastPass
1Password
MozillaSync
Firefox Lockwise
…
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• Firefox Lockwise https://www.mozilla.org/en-US/firefox/lockwise/
– Part of the standard Firefox installation, sync between devices
– Automatically checks for password leakage (Firefox Monitor)
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But passwords are encrypted, right?
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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
• Treat storage service as untrusted and perform security sensitive
operations on client
• Make necessary trusted component as small as possible
• Prevent offline brute-force, but don’t expect strong password from
user
– add entropy from other source
• Make transmitted sensitive values short-lived
• 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/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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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’s service showcased in 2013
• Lack of details until iOS Security report 02/2014
– https://www.apple.com/business/docs/iOS_Security_Guide.pdf
• https://blog.cryptographyengineering.com/2016/08/13/is-apples-cloudkey-vault-crypto/
(M.Green)
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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
– But limited number of tries allowed
• Trusted component of iCloud realized via internal HSM
– 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
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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
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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
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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 project (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
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FIDO U2F devices
• Why have button? Is missing display problem?
• Recent problem: 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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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
• An API for accessing Public Key Credentials Level 1
• https://www.w3.org/TR/webauthn/
• Similar, but more complex standard than U2F => expect additional
problems (not yet scrutinized enough)
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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)
• Mandatory reading: UCAM-CL-817
– At least chapters: II. Benefits, V. Discussion
– Whole report is highly recommended
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• Place/upvote questions in slido
while listening to lecture video
• We will together discuss these
during every week lecture Q&A
(every Monday, 17-18:00)