# Cryptography and Key Management Basics

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1 Cryptography and Key Management Basics Erik Zenner Technical University Denmark (DTU) Institute for Mathematics DTU, Oct. 23, 2007 Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

2 Plan for Today 1 Talk 1: Cryptography and Key Management Basics (Erik Zenner) 2 Talk 2: Public Key Infrastructure (Christian D. Jensen) 3 Discussion Identify open questions If you have questions, don t hesitate to ask (anytime). Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

3 1 Cryptographic Basics Goals, Algorithms, and Keys Symmetric vs. Asymmetric Cryptography Important Examples 2 Key Management Key Setup Key Life-Cycle 3 Final Remarks Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

4 Outline Cryptographic Basics 1 Cryptographic Basics Goals, Algorithms, and Keys Symmetric vs. Asymmetric Cryptography Important Examples 2 Key Management Key Setup Key Life-Cycle 3 Final Remarks Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

5 Protection Goals Cryptographic Basics Goals, Algorithms, and Keys Cryptography is not only about encryption. There exist many potential protection goals: Confidentiality Data Authentication Integrity Authenticity Non-Repudiation Entity Authentication Key Establishment Anonymity... Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

6 Cryptographic Basics From Algorithm to Solution Goals, Algorithms, and Keys Cryptography is only about the lowest layers when building a security solution. Higher layers are typically handled by Security Engineers. Layer Algorithm / Primitive Scheme Protocol (math) Protocol (tech) Implementation Deployment Example AES, RSA AES-128-CTR, OAEP Diffie-Hellman, Kerberos SSL/TLS, IPSec OpenSSL (C/C++) Portalen Single Sign-on Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

7 Cryptographic Keys Cryptographic Basics Goals, Algorithms, and Keys Standard Assumption: The attacker knows everything about the security solution with the exception of the key. (Kerckhoffs Principle) Why? Protecting keys is easier than protecting whole implementations. Managing keys (generating, exchanging, storing, changing...) is easier than managing whole implementations. If only the key is secret, all other aspects of the security solution can be publicly scrutinised. Consequence: Protect the key by all means! Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

8 Cryptographic Basics Purpose of Cryptographic Keys Goals, Algorithms, and Keys The following is a categorisation of cryptographic keys according to what they are used for: Data key: Directly used for the cryptographical purpose, e.g. encryption or authentication. Key-encryption key: Used to encrypt other keys, e.g. in key exchange or key storage. Master key: Used to generate other keys, using a key derivation function (KDF). E.g.: Session Key := KDF(Master Key, Session Number). Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

9 Symmetric Keys Cryptographic Basics Symmetric vs. Asymmetric Cryptography Cryptographic operations typically involve a sender and a receiver (can be the same person). Symmetric Keys: Sender and receiver use the same key (traditional case). Properties: Short keys ( bit) Fast algorithms Special case: Passwords. Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

10 Asymmetric Keys Cryptographic Basics Symmetric vs. Asymmetric Cryptography Asymmetric Keys: Sender and receiver use different keys: Public key: publicly available (e.g. for encryption) Private key: personal secret (e.g. for decryption) Properties: Long keys (e.g. RSA: bit) Slow algorithms Advantage: Makes key transport easy if implemented properly. Remark: Public keys are known to the attacker, i.e. no real keys. Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

11 Cryptographic Basics Example 1: Hybrid Encryption Symmetric vs. Asymmetric Cryptography Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

12 Cryptographic Basics Example 2: Digital Signature Symmetric vs. Asymmetric Cryptography Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

13 Cryptographic Basics Algorithm Classification Important Examples If we organise cryptographic algorithms and protocols by protection goals and symmetric vs. asymmetric keys, we obtain the following table: Symmetric Asymmetric Confidentiality Sym. Encryption Asym. Encryption Data Authentication MAC Digital Signatures Entity Authentication Challenge/Response, Challenge/Response, Passwords Zero Knowledge Key Establishment var. var. Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

14 Important Examples Cryptographic Basics Important Examples The following are examples for such algorithms and protocols: Symmetric Encryption: AEA (AES), DEA (DES), RC4 Asymmetric Encryption: RSA, ElGamal MAC: HMAC, CBC-MAC Digital Signatures: RSA, DSA (DSS), ECDSA Entity Authentication: Password, PIN, OTP, Biometrics, Kerberos, Needham-Schroeder Key Establishment: Diffie-Hellman, IKE, Kerberos, Needham-Schroeder, TTP, Public-Key Infrastructure (PKI) Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

15 Outline Key Management 1 Cryptographic Basics Goals, Algorithms, and Keys Symmetric vs. Asymmetric Cryptography Important Examples 2 Key Management Key Setup Key Life-Cycle 3 Final Remarks Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

16 Key Generation Key Management Key Setup Any secret key material has to be generated. Main options: Generated by one party, then sent to the other (key transport). Generated by all parties working together (key agreement). Generated by a trusted third party and sent to all parties. The form of the key material depends on its use (e.g., RSA keys are very different from AES-128 keys). See the relevant standard for details of format and generation. With the exception of passwords, key generation typically requires some kind of random input. Random number generation Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

17 Key Management Random Number Generation Key Setup Three types of random number generators (often confused): Statistical random number generator: Deterministic algorithm, not cryptographically secure (e.g., rand() from stdlib.h in C/C++). Never use this for cryptographic purposes! Cryptographic random number generator: Deterministic algorithm, cryptographically secure. Be very careful to seed correctly! Be careful to protect the inner state against attacker! Real random number generator: Uses measurements of physical processes to generate real randomness. Too expensive for most applications. Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

18 Key Exchange Key Management Key Setup In addition to being generated, the key also needs to be distributed to all legitimate parties. How to prevent others from seeing the key? How to authenticate the legitimate parties (sender and receiver)? How to distribute the key to the legitimate parties? How to verify that the legitimate parties received the key? If done remotely: Use cryptography (many different solutions). Sometimes easier: Personal key exchange. Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

19 Key Storage Key Management Key Life-Cycle Keys have to be stored somehow. Problems include: How to store keys such that only legitimate parties have access? Use more keys? Special case: Passwords (not stored in hardware) How to make backups such that lost keys can be recovered? Prioritise: Availability or security? Backups have to be secured, too! Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

20 Key Expiration Key Management Key Life-Cycle Keys can (in fact: should) expire sometime. Problems include: How to keep track of key expiration? Inform all users. Set up new key. What happens after expiration? Archive old key material? How? Delete old key material? How? Remember all copies! Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

21 Key Compromise Key Management Key Life-Cycle Worst case: Key has been compromised because 1 an attacker has potentially had access to the key, or 2 the corresponding cryptographic algorithm was broken. What do we have to do? Key must no longer be used in the future. Key Expiration (see above) All concerned parties have to be informed. Key Revocation (see talk 2) Old documents have to be protected. Re-Encryption? Re-Signing? Destruction of old documents? Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

22 Outline Final Remarks 1 Cryptographic Basics Goals, Algorithms, and Keys Symmetric vs. Asymmetric Cryptography Important Examples 2 Key Management Key Setup Key Life-Cycle 3 Final Remarks Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

23 How to Proceed Final Remarks No international standards on key management. Probably to come in the next years No one size fits all solutions. You have to know the usage scenario. Never build your own cryptographic solutions! Use off-the-shelf (or off-the-standard) products. If in doubt, ask cryptographers or IT security engineers. Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

24 Final Remarks References / Further Reading The following books and references could be useful: N. Ferguson, B. Schneier: Practical Cryptography. Wiley, A. Menezes, P.C. van Oorschot, S.A. Vanstone: Handbook of Applied Cryptography. (parts of chapters 10,12,13; available online) NIST SP : Recommendation for Key Management. (3 parts; available online) Erik Zenner (DTU-MAT) Cryptography and Key Management Basics DTU, Oct. 23, / 24

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