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Cryptography in PHP: use cases


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Security is a very important aspect of web applications. In order to protect sensitive data we should use cryptography. But cryptography means security? Absolutely not, especially if developers do …

Security is a very important aspect of web applications. In order to protect sensitive data we should use cryptography. But cryptography means security? Absolutely not, especially if developers do not,especially if developers do not use it properly. In this talk I would like to present some best practices in PHP to implement secure cryptography using the extensions mcrypt, Hash and OpenSSL.

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  • 1. October 2011Cryptography in PHP:use casesEnrico ZimuelZend Technologies
  • 2. About me October 2011 • Enrico Zimuel (ezimuel) • Software Engineer since 1996 – Assembly x86, C/C++, Java, Perl, PHP • Enjoying PHP since 1999 • Senior PHP Engineer at Zend Technologies since 2008 • Author of two italian books aboutEmail: applied cryptography • B.Sc. Computer Science and Economics from University of Pescara (Italy)
  • 3. Summary October 2011● Cryptography in PHP● Some use cases: ● Safe way to store passwords ● Generate pseudo-random numbers ● Encrypt/decrypt sensitive data● Demo: encrypt PHP session data
  • 4. Cryptography in PHP October 2011● crypt()● Mcrypt● Hash● OpenSSL
  • 5. crypt() October 2011● One-way string hashing● Support strong cryptography ● bcrypt, sha-256, sha-512● PHP 5.3.0 – bcrypt support● PHP 5.3.2 – sha-256/512● Note: dont use PHP 5.3.7 (bug #55439)
  • 6. Mcrypt October 2011● Mcrypt is an interface to the mcrypt library● Supports the following encryption algorithms: ● 3DES, ARCFOUR, BLOWFISH, CAST, DES, ENIGMA, GOST, IDEA (non-free), LOKI97, MARS, PANAMA, RIJNDAEL, RC2, RC4, RC6, SAFER, SERPENT, SKIPJACK, TEAN, TWOFISH, WAKE, XTEA
  • 7. Hash October 2011● Enabled by default from PHP 5.1.2● Hash or HMAC (Hash-based Message Authentication Code)● Supported hash algorithms: MD4, MD5, SHA1, SHA256, SHA384, SHA512, RIPEMD, RIPEMD, WHIRLPOOL, GOST, TIGER, HAVAL, etc
  • 8. OpenSSL October 2011● The OpenSSL extension uses the functions of the OpenSSL project for generation and verification of signatures and for sealing (encrypting) and opening (decrypting) data● Public key cryptography (RSA algorithm)
  • 9. Which algorithm? October 2011● Some suggestions: ● Symmetric encryption: – Blowfish / Twofish – Rijndael (AES, FIST 197 standard since 2001) ● Hash: SHA-256, 384, 512 ● Public key: RSA
  • 10. Cryptography vs. Security October 2011● Cryptography doesnt mean security● Encryption is not enough● Bruce Schneier quotes: ● “Security is only as strong as the weakest link” ● “Security is a process, not a product”
  • 11. Cryptography vs. Security October 2011
  • 12. October 2011Use cases
  • 13. Use case 1: store a password October 2011● Scenario: ● Web applications with a protect area ● Username and password to login● Problem: how to safely store a password?
  • 14. Hash a password October 2011● Basic ideas, use of hash algorithms: ● md5($password) – not secure – Dictionary attack (pre-built) ● md5($salt . $password) – better but still insecure – Dictionary attacks: ● 700000000 passwords a second using CUDA (budget of 2000 $, a week) ● Cloud computing, 500000000 passwords a second (about $300/hour)
  • 15. bcrypt October 2011● Better idea, use of bcrypt algorithm: ● bcrypt prevent the dictionary attacks because is slow as hell ● Based on a variant of Blowfish ● Introduce a work factor, which allows you to determine how expensive the hash function will be
  • 16. bcrypt in PHP October 2011 ● Hash the password using bcrypt (PHP 5.3+)$salt = substr(str_replace(+, .,$salt = substr(str_replace(+, ., base64_encode($salt)), 0, 22); base64_encode($salt)), 0, 22);$hash = crypt($password,$2a$.$workload.$.$salt);$hash = crypt($password,$2a$.$workload.$.$salt);● $salt is a random string (it is not a secret!)● $workload is the bcrypts workload (from 10 to 31)
  • 17. bcrypt workload benchmark $workload time in sec October 2011 10 0.1 11 0.2 12 0.4 13 0.7 14 1.5Suggestion:Spend ≈ 1 sec (or more) 15 3 16 6 17 12 18 24.3 19 48.7 20 97.3 21 194.3 OS: Linux kernel 2.6.38CPU: Intel Core2, 2.1Ghz 22 388.2RAM: 2 GB - PHP: 5.3.6 … …
  • 18. bcrypt output October 2011 ● Example of bcrypts output:$2a$14$c2Rmc2Fka2hmamhzYWRmauBpwLLDFKNPTfmCeuMHVnMVaLatNlFZO ● c2Rmc2Fka2hmamhzYWRmau is the salt ● Workload: 14 ● Length of 60 btyes
  • 19. bcrypt authentication October 2011● How to check if a $userpassword is valid for a $hash value?if ($hash==crypt($userpassword,$hash)) { if ($hash==crypt($userpassword,$hash)) { echo The password is correct; echo The password is correct;} else { } else { echo The password is not correct!; echo The password is not correct!;}}
  • 20. Use case 2: generate random data in PHP October 2011● Scenario: ● Generate random passwords for – Login systems – API systems ● Problem: how to generate random data in PHP?
  • 21. Random number generators October 2011
  • 22. PHP vs. randomness October 2011● How generate a pseudo-random value in PHP?● Not good for cryptography purpose: ● rand() ● mt_rand()● Good for cryptography (PHP 5.3+): ● openssl_random_pseudo_bytes()
  • 23. rand() is real random? October 2011Pseudo-random bits rand() in PHP on Windows From website
  • 24. Use case 3: encrypt data October 2011● Scenario: ● We want to store some sensitive data (e.g. credit card numbers)● Problem: ● How to encrypt this data in PHP?
  • 25. Symmetric encryption October 2011● Using Mcrypt extension: ● mcrypt_encrypt(string $cipher,string $key, string $data,string $mode[,string $iv]) ● mcrypt_decrypt(string $cipher,string $key, string $data,string $mode[,string $iv])● What are these $mode and $iv parameters?
  • 26. Encryption mode October 2011● Symmetric encryption mode: ● ECB, CBC, CFB, OFB, NOFB or STREAM● We are going to use the CBC that is the most used and secure● Cipher-Block Chaining (CBC) mode of operation was invented in 1976 by IBM
  • 27. CBC October 2011 The Plaintext (input) is divided into blocks Block 1 Block 2 Block 3 ... Block 1 Block 2 Block 3The Ciphertext (output) is the concatenation of the cipher-blocks
  • 28. IV October 2011● Initialization Vector (IV) is a fixed-size input that is typically required to be random or pseudo● The IV is not a secret, you can send it in plaintext● Usually IV is stored before the encrypted message● Must be unique for each encrypted message
  • 29. Encryption is not enough October 2011● We cannot use only encryption to store sensitive data, we need also authentication!● Encryption doesnt prevent alteration of data ● Padding Oracle Attack (Vaudenay, EuroCrypt 2002)● We need to authenticate: ● MAC (Message Authentication Code) ● HMAC (Hash-based Message Authentication Code)
  • 30. HMAC October 2011● In PHP we can generate an HMAC using the hash_hmac() function: hash_hmac ($algo, $msg, $key) $algo is the hash algorithm to use (e.g. sha256) $msg is the message $key is the key for the HMAC
  • 31. Encryption + authentication October 2011● Three possible ways: ● Encrypt-then-authenticate ● Authenticate-then-encrypt ● Encrypt-and-authenticate● We will use encrypt-then-authenticate, as suggested by Schneier in [1]
  • 32. Demo: encrypt session data October 2011● Specific PHP session handler to encrypt session data using files● Use of AES (Rijndael 128) + HMAC (SHA-256)● Pseudo-random session key● The encryption and authentication keys are stored in a cookie variable● Source code:
  • 33. Conclusion (1) October 2011● Use standard algorithms for cryptography: ● AES (Rijndael 128), SHA-* hash family, RSA● Generate random data using the function: ● openssl_random_pseudo_bytes()● Store passwords using bcrypt: ● crypt($password, $2a$.$workload.$.$salt)
  • 34. Conclusion (2) October 2011● For symmetric encryption: ● Use CBC mode with a different random IV for each encryption ● Always authenticate the encryption data (using HMAC): encrypt-then-authenticate● Use HTTPS (SSL/TLS) to protect the communication client/server
  • 35. References October 2011(1) N. Ferguson, B. Schneier, T. Kohno, “Cryptography Engineering”, Wiley Publishing, 2010(2) Serge Vaudenay, “Security Flaws Induced by CBC Padding Applications to SSL, IPSEC, WTLS”, EuroCrypt 2002● Web: ● PHP cryptography extensions ● How to safely store a password ● bcrypt algorithm ● SHA-1 challenge ● Nvidia CUDA ●
  • 36. Thank you! October 2011● Vote this talk: ●● Comments and feedbacks: ●