Encryption Recap: A Refresher on Key Concepts DO and DON'T for developers A while ago, I had chance to collect information and share this PDF summarizing common encryption terminology within my teams. This covers algorithms, and best practices that many developers may find helpful as a refresher or intro. Let me know if any part needs further explanation.

1. Data Encryption
Recap
Thomas Kim

2. Security &
Encryption
• Priority of security ‘was’ low than features
• There is NO 100% secure
• Security is not just cost, it is everything and
everywhere
• Encryption is the minimum defence, when
other security fails
• Encryption is minimum requirement for
any services or apps

3. Common
Terminology
• Number of Keys : Symmetric vs. Asymmetric
• Data Processing Unit : Stream vs. Block
• Data Recovery Capability : One Way vs. Both
Way

4. Hash
Functions
• Turn arbitrary size of input to fixed size of
output
• Guaranteed same output for same input
• It is fast, used for fast search as hash table
• Digest : output of hashing

5. Hash
Collision
• h(M) = H
• h() : hash function
• M : input
• H : hash (digest)
• Collision: different input,
same hash (MD5, SHA1)

6. Requirement of
Encrypting
Hash Function
• Pre-image Resistance
• 2nd Pre-image Resistance
• Collision Resistance

7. Pre-image
Resistance
With given hash H, difficult to find out original
input
h(M) = H
H = ‘aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d’
M = ‘hello’

8. 2nd Pre-image
Resistance
With given (M), ensure there is no other
input (M`) to have the same h
h(M) = H
H = ‘aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d’
M = ‘hello’, M`=?

9. Collision
Resistance
• Ensure mathematically ‘nearly’ impossible
to have two M and M` that has same h
• Finding arbitrary M1, M2 that produce the
same h
• Stability of hash measured to the half of
the bit size of the algorithm (SHA1 =
80bit, SHA256 = 128bit)

10. Rainbow
Attack
• A type of Brute-force attack
• Using pre-calculated rainbow table to
match result H
• If H is the same, then M is out of the
rainbow table
• Prevent the attack by adding salt

11. Hashing
Algorithms
• MD5, SHA1, SHA2 (SHA256, SHA384, SHA512)
• MD5 (128bit) : not secure
• SHA1(160bit) : not recommended as long term
key (i.e. digital signature, used as one and only
algorithm for password encryption), still OK for
transient keys (session, git commit hash)

12. Symmetric-key
Algorithm
• One secret key for encryption and
decryption
• DES, 3DES, AES, IDEA, RC4, RC5
• Speedy and Easy to implement
• Hard to transfer secret keys each
other
• Key management is even harder
n(n-1)/2

13. Stream
Cipher
•Make a symmetric key
•Bit-wise XOR
•RC4, AS/2
•Speedy, no longer used

14. Block
Cipher
• encrypt/decrypt by data block
• symmetric algorithm
• DES : not recommended
• AES : adopted by NIST, 128/192/256
• Camellia : used for TLS session
• implementations by size of block
and key length
https://www.youtube.com/watch?v=gP4PqVGudtg

15. Padding
• Input data is NOT always the
multiples of block size
(i.e.) PKCS7/PKCS5 Padding : if lack
of 3 bytes, put 03 03 03

16. Mode of
Operations
• Define the rules between each blocks
• ECB, CBC, CFR, etc.

17. Mode of Operation
ECB
https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation
• Electronic Code Book
• No mode of operations
• Each block can be
encrypted/decrypted
• Critical issues (guess input)
• Shouldn’t be used with
symmetric key encryption
(i.e. AES/ECB NOT secure)

18. Mode of Operation
CBC
• Cipher Block Chaining
• Enhanced security
• Uses previous block as input to
produce the next block
• For 1st block, use IV
(Initialization Vector), hard to
guess
• Recommended for symmetric
key encryption (AES/CBC)
https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation

19. CBC
private static final String key = "aesEncryptionKey"; // 16 bytes
private static final String IV = "encryptionIntVec"; // 16 bytes
private static final String UTF8 = "UTF-8";
public static String encrypt(String value) {
try {
IvParameterSpec iv = new IvParameterSpec(IV.getBytes(UTF8));
SecretKeySpec spec = new SecretKeySpec(key.getBytes(UTF8), "AES");
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING");
cipher.init(Cipher.ENCRYPT_MODE, spec, iv);
byte[] encrypted = cipher.doFinal(value.getBytes());
return Base64.encodeBase64String(encrypted);
}
catch (Exception ex) {
ex.printStackTrace();
}
return null;
}
• Key : should be loaded
from secure storage
• IV : secure random value
• Key, IV are required for
encryption/decryption

20. Comparison
: Mode of Operations
•ECB
•CBC
ECB CBC
Original

21. PBKDF2
• Password Based Key Derivation Function 2
• Widely used for user password encryption
• Prevent brute-force attack by iteration (key
stretching)
1. Generate random key based on password
2. Adding salt
3. Iterate enough times to produce hash

22. Public Key
Encryption
• Asymmetric Key Algorithm
• Encrypt with Public Key
• Decrypt with Private Key
• Resolve the Difficulty of Key Sharing
• Used for
- Authentication
- Non-Repudiation
- Digital Signature

23. Public Key
Cryptography
RSA
• Rivest, Shamir, Adleman
• uses HUGE prime numbers as keys
• Much calculation, slow
ECDSA
• Elliptic Curve Digital Signature Algorithm
• Bitcoin
DSA
• Digital Signature Algorithm
13 = A * B
A=? and B=?
472,882,027 = A * B
A=? and B=?

24. Key
Exchange
• Key agreement
- Procedure to get agreement on key exchange
- Diffie-Hellman Algorithm (SSH, SSL)
(https://www.youtube.com/watch?v=wLFztjQDdzI)))
• Key Encipherment
- RSA Algorithm
1. Receiver generate symmetric key
2. Encrypt the symmetric key with sender’s public key
3. Transfer to the sender

25. SSL
TLS
• Session Key: symmetric key for a session
• SSL Hands-shake: key exchange procedure for SSL session
(Diffie-Hellman)
• SSL uses symmetric key (session key) throughout the session
• Session key cache for speed up
• TLS 1.2/1.3
• Excessive session timeout NOT recommended

26. PGP
• Pretty Good Privacy
• 1991 by Phil Zimmermann
• Used for Email Encryption
• Public Key Repository (http://pgp.mit.edu)
• GPG Tools
• Lack of Certified Authority

27. CA
Certificate Authority
SSL
Certificate
SSL certificate issued by CA
• Public Key Certificate
• CA certify ownership of Public Key
• CA sign Public Key by its own Private Key
• validate SSL certificate by CA public key on establishing SSL session
• start to trust owner of SSL certificate certified by a CA
• Verification Domain Ownership by certificate chain
Self-Signed SSL Certificate
• certified by its own CA
• NO trust from browsers

28. Authentication
vs.
Authorization
Authentication
• validate a user (or entity) is right one
• By password, biometry (fingerprint, face/palm/
iris scan, voice signature), smart card, OTP, etc.
Authorization
• Decide whether allow or not (permission)
• Authentication followed by Authorization

29. HSM
Hardware Security Module
• Security Compliance
• Embedded circuit (or software) to perform
cryptographic calculation
• Key management
• No access of key from outside
• Self destroy keys on unauthorized
disassemble attempts*
• Keep information safe
• Cloud-based HSM available (AWS, Azure,
etc.)

30. Rules of
Thumb
PLEASE DON’T DO
• DO NOT try to invent new encryption algorithm by yourself
• DO NOT use AES/ECB, instead AES/CBC
• DO NOT save AES Keys and IVs as file
• DO NOT use Self-signed certificate (if possible)
PLEASE DO
• PBKDF2 for user password
• Use salt on one way hashing to avoid rainbow attack
• Use key stretching (hash iteration) to avoid brute-force attack
• Consider key strength and hash iteration based on life of data and importance
• Use HSM for Super sensitive data

31. Thanks
Q/A