A brief but deep into to Cryptography for developers.

1. TIMS Crypto Academy
Brief but deep intro to Cryptography for TIMS Developers
Paul Gillingwater
October 2015

2. Public Key (Asymmetric) encryption
• Relies on mathematical functions which are simple in one direction,
but extremely difficult to solve in reverse – e.g., multiplying together
two large prime numbers is simple and fast – but finding the two
prime factors of such a product is hard, and can take many decades
• Another popular source of such encryption is Elliptic Curve – finding
the discrete logarithm of a random elliptic curve with regard to one of
its base points (ECDLP). Larger curves make more difficult solutions.
• Third is Lattice based algorithms – using discrete points in a non-
Euclidian space. These *may* be better resistant to quantum
solutions, unlike prime factors and ECC

3. Public Keys & Private Keys
• RSA is first popular algorithm based on prime product factor problem
• Primes must be chosen with good randomness, and have similar
lengths
• Compute the product p*q – this gives the key length
• Derive two keys mathematically – one private, and one public
• The public key may be freely shared, while private is kept secret
• This method is known as “asymmetric encryption”
• It’s not perfect – bad keys result in poor protection. Attacks in future
based on quantum computing might be effective. Key length critical!

4. Using Public Keys
• Use PUB to encrypt a plain text – the recipient must use the matching
PRIV to decrypt the text
• Use PRIV to sign a text (which may or may not be encrypted) – the
recipient uses matching PUB to validate the signature (which is based
on an encrypted hash of the message)
• Usually used for short messages, or for protecting a secret key
• Examples include: Diffie-Hellman key exchange, DSA/DSS, RSA & ECC
• N.B. Key length is not comparable between Symmetric and
Asymmetric algorithms

5. Symmetric Encryption
• Uses a single shared secret key for both encrypting and decrypting
• Problem: how to transfer the secret key securely
• Popular algorithms: Twofish, AES (Rijndael), TDES, RC4, IDEA
• Two types: block and stream ciphers. Block works with fixed size
groups of bits, while stream does each bit sequentially
• This method is *much* faster than public key algorithms
• Typically, modern systems combine both methods, public and secret
key – start with asymmetric for key distribution, then switch to
symmetric for the session (e.g. SSL, PGP)

6. Hashes and Digests
• Algorithm used to computer a fixed-length unique code based on
arbitrary input length – from 1 byte to petabytes if desired
• Hash lengths typically 256 bits, 512 bits, etc.
• These are one-way functions – impossible to retrieve original text
from a given hash, and very unlikely in extreme to find collision
• Much stronger than checksums, but more complex to compute
• Algorithms include MD5, SHA1, SHA2 (latter preferred, typically with
256, 384 or 512 bits)
• Not the same as HMAC or TMAC (see later)

7. Using XOR and One Time Pads
• Simplest method of encryption, but can be strongest – but only if the
key chosen is perfectly secure and perfectly random
• Protects a stream of bits by flipping bit depending on key bit
• Key length must be identical to plain text length
• This is the basis of one-time pads
• Problem: distribution of and security of key material
• Ideally, keys should be generated by a truly random physical process,
and not generated by an algorithm

8. DES, 3DES and TDES
• DES was one of earliest symmetric key algorithms [Data Encryption
Standard]
• Original size of 56 bits was overtaken by technology – so Triple DES
was designed using the same encryption three times
• Heavily used in online payment industry (EMV)
• Most cryptographers prefer AES as replacement

9. Public Key Infrastructure
• Centralized database which stores Public Keys for users and devices
• Certificate Authority (CA) manages certificates which contain public
and private keys (often with HSM anchor)
• Registration Authority (RA) verifies identities of users/devices
• CA’s sign and publish the public keys of entities to whoever wants it
• CA is trusted third party – its private key is used to sign the
certificates, which means the trust can be verified
• OCSP is popular protocol to check revocation status
• CA’s handle whole certificate life cycle management

10. Managing Trust with PKI
• Importance of separation of duties (each person has half of key)
• Centralized trust makes for a single point of failure – very top-down
• Hardware Security Module used to protect private keys of CA
• Standards include PKCS#11 and PKCS#15 for certificate operations
• Certificates can be issued with different purposes – e.g., signing
email, encrypting data, etc.
• Certificates based on X.509 standard – these usually do NOT contain
the private keys, which are stored separately in HSM, or in extreme
cases never leave the smart card which generates them

11. Public Key Topics
• Certificate Pinning – a method of keeping a local copy of a certificate,
which can be relied upon, and which can detect unauthorized
changes
• Certificate Stapling is used with OCSP to improve performance of
revocation checking; signed cert attached in SSL startup sequence
• PKCS#11 is API for handling certificates, e.g. with CA’s, HSMs, smart
cards, etc. Designed for all crypto-related functions, including
generating, reading, enrolling, creating and deleting such objects.
• PKCS#8 is standard for storing private keys
• PKCS#15 is standard for smart card objects

12. Standards and Certification
• Evaluation Assurance Levels:
• 1.1 EAL1: Functionally Tested
• 1.2 EAL2: Structurally Tested
• 1.3 EAL3: Methodically Tested and Checked
• 1.4 EAL4: Methodically Designed, Tested and Reviewed
• 1.5 EAL5: Semiformally Designed and Tested
• 1.6 EAL6: Semiformally Verified Design and Tested
• 1.7 EAL7: Formally Verified Design and Tested
• Requires published Security Target for evaluation
• Based on Common Criteria ISO15408 international standard

13. Other Relevant Standards
• FIPS140-2 Specifications for Cryptographic Modules
• ISO17799, replaced by ISO27002 – Information Security Management
• PCI/DSS – Payment Card recommendations (EMV)
• NIST (whole range of various US government standards)
• ANSI X9 series (mostly encryption and key exchange)
• ITU SG17 – Identity Management standards
• ETSI – Identity and Access Management working group
• FIDO Alliance – token for authentication
• OIX – Open Identity Exchange framework (LoA)

14. PGP/GNU Privacy Guard
• Public key encryption and signing using chain of peer-to-peer trust
• Allows creation of keys, which can then be used for encryption and
message confirmation through digital signatures
• Uses standard implementation of popular algorithms (e.g. IDEA)

15. Smart Cards
• Tiny computer embedded in protected chip which runs simple
programs in response to requests (typically, PKCS#11 or PKCS#15)
• Designed to be tamper-resistant – destroys data if broken into
• Contains protected areas which cannot be read by external system
• May require use of PIN, BIO (MoC) or other protections before
releasing sensitive information
• Can perform operations such as signing or encryption/decryption,
based on requests sent to it by driver
• Also can generate its own keys; private key never leaves the chip

16. Smart Card Interfaces
• ISO7816 – uses contacts to interface with a card
• ISO14443 – uses NFC (based on RFID) to communicate with smart
card device
• In both cases, ADPU commands used to request services from cards
• EMV Cards have Card Verification Methods (e.g., local PIN check)
• Cards can detect tampering, e.g. velocity checking
• Some cards support key rotation (replacing or phasing out old keys if
they are compromised)
• Keys managed with ISO11568

17. ePP and ICAO9303
• Methods of mutual authentication of readers and cards, as well as
means to protect card or document reading
• BAC – Basic authentication, requires access to OCR of MRZ as key
• SAC – Supplementary Access Control; anti-eavesdropping used in
parallel with EAC (replaces BAC)
• EAC – Extended Access Control, requires cryptographic checking of
keys used by reader before access is permitted
• Two requirements for EAC: Chip and Terminal authentication (mutual)
• Based on PKI with CVC certificates (available as PKD for offline checks)

18. Network Security and Encryption
• SSL/TLS
• IPSec and VPN implementation
• SSH (tunneling too)
• FIDO tokens and U2F/UAF
• WEP; WPA2/PSK
• JWE/JWT

19. Any questions?