（RHCA）Red Hat Enterprise Linux RHS333: Red Hat Enterprise Security: Network Services

Unit 3

Cryptography

Objectives

Upon completion of this unit, you should be able to: Network communication security Introduction into cryptography OpenSSL

Network Security

Communication channels are often vulnerable to a “man in the middle” attack Eavesdropping Data injection Session hijacking Authentication and encryption of messages can protect data integrity and confidentiality

Switches and Security

A network switch does not provide security Provides improved bandwidth management Attacks on network switches include MAC Flooding MAC Duplication ARP Redirection Port security, static ARP entries, and arptables may provide some defense

Cryptography

Key building block of secure protocols Authentication and confidentiality of data Encryption does not automatically imply authentication, and vice versa Generally does not hide the fact that communication is taking place, or with whom Cryptography does not solve every problem May be easier to break host security than break an encrypted communication

Cryptography

Kerckhoffs' Principle Security must depend on secrecy of the key, not on secrecy of the encryption method Harder to replace algorithms than secret keys Secure authentication and integrity can be more important than secure encryption Cryptography can help with both Authentication may not automatically handle blocked or repeated messages

OpenSSL

libcrypto: Generic encryption support Provides cryptographic functions libssl: Support for TLS/SSL protocols Connection-based security for authentication, confidential data transfer, session integrity openssl: Multi-purpose encryption utility Generate encryption keys, digital certificates Encrypt/decrypt data manually

Symmetric Encryption

Secret key used to both encrypt and decrypt Block ciphers encrypt fixed-size blocks of plaintext as fixed-size blocks of ciphertext Block cipher modes are used to encrypt data larger than a single block (ECB, CBC, etc.) Algorithms: DES, 3DES, Blowfish, AES, etc. Utilities: gpg, openssl enc

Cryptographic Hashes

Converts an input string of any length to an output string of fixed length One-way: not feasible to get plaintext from hash Collision-free: not feasible to find two strings that hash to the same output Algorithms: CRC-32, MD5, SHA-1, etc. CRC-32 is not cryptographically secure! Utilities: sha1sum, md5sum, cksum, openssl dgst

Message Authentication Codes

Secret key maps plaintext to random output Used for preventing message tampering Attacker needs secret key to forge MAC value CBC-MAC: use block cipher to construct Encrypt the message in CBC mode and throw away all but the last block of output HMAC: use keyed cryptographic hash

User Authentication

Cryptographic hash of account password is stored in on-line database Made more difficult to break by adding random “salt” to password MD5-based hash by default, old modified DES version also available System hashes password given to login If passwords match, user is authenticated Utilities: passwd, openssl passwd

Asymmetric Encryption

Public key to encrypt, private key to decrypt Based on trapdoor one-way functions Partial solution to key distribution problem Can give the public key to everybody Algorithms: RSA, ElGamal Utilities: gpg, openssl rsautl

Digital Signatures

Private key to encrypt, public key to decrypt Any holder of the public key can decrypt the message, but only a holder of the private key could have encrypted it For speed and improved security, hash the plaintext and sign that, then encrypt Algorithms: RSA, ElGamal, DSA DSA is intended for digital signatures only

Key Distribution

Diffie-Hellman key exchange Client and server agree on shared secret without transmitting it over the network Use digital signatures to authenticate messages Public key encryption key exchange Client generates symmetric session keys, sends to server encrypted with server's public key Still need to verify that the public key belongs to the other host

Digital Certificates

Trusted third party digitally signs public key Certificate Authority (CA) has a public key that is known by everyone involved The resulting digital certificate contains Server's public key and expiration date Information about the owner of the key Information about the CA and the CA's signature Information on how the certificate may be used

TLS/SSL Handshake

Generating a RSA Private Key

Generates private key file, includes information needed to compute public key make *.key in /etc/pki/tls/certs openssl genrsa command Can encrypt the key for theft protection Service will prompt for password when started Attacker may be able to recover the password from main memory in any case

Generating a Certificate Signing request (CSR)

CA creates the final certificate from the Certificate Signing Request Does not contain the private key! CA must verify the identity of the CSR owner before signing Need a key pair for the CSR make *.csr creates key if necessary openssl req command

Creating a CA-signed Certificate

$ openssl ca -in my.csr -out my.crt Verify information, then approve Every certificate issued should have a unique serial number (stored in index.txt) Keep a copy of the final CRT file so the certificate can be revoked if necessary

Certificate Expiration

Expiration is a safety measure to help reduce the risk of a private key compromise Short expiration periods are inconvenient, but speed recovery if the key is compromised CA certificates typically have a long lifetime Individual certificates have shorter lifetime Be sure to deploy new certificates well before the old ones expire!

Certificate Revocation

Revocation of a compromised key is hard One solution is a certificate revocation list (CRL) Requires periodic downloads of the latest CRL Many users or clients do not bother! Some key distribution systems use very short expiration times and simply allow certificates to expire naturally

Managing a CRL

To revoke a certificate: $ openssl ca -revoke 01.crt To build an up to date CRL from index.txt: $ openssl ca -gencrl -out my.crl Some programs can automatically download and import the latest CRL from a web URL Mozilla, expects the CRL in DER format

Operational Issues

Programs and users fail to check identities Programs and users fail to use CRLs A third-party CA can issue bogus certificates If standard CA is not to be trusted, may need to be disabled manually A private CA must make sure all clients have its CA certificate installed

GnuPG

GnuPG is a powerful encryption tool To generate a key: gpg --gen-key Exchange public keys with others to encrypt email and verify signatures

End of Unit 3

Questions and Answers Summary Network communication vulnerabilities Introduction to cryptography Working with digital certificates for TLS/SSL Operational issues with TLS