Secure Transactions Chapter 17

The user's machine No control over security of user's machine –Might be in very insecure: library, school, &c. Users disable some features: cookies, Java, JavaScript Might not have 128-bit encryption –Until recently 128-bit encryption could not be legally exported from U.S. Might not be dealilng with a browser Store as little information as possible on user's machine (for a variety of reasons) No control over security of user's machine –Might be in very insecure: library, school, &c. Users disable some features: cookies, Java, JavaScript Might not have 128-bit encryption –Until recently 128-bit encryption could not be legally exported from U.S. Might not be dealilng with a browser Store as little information as possible on user's machine (for a variety of reasons)

The Internet The Internet is inherently insecure Your options: Transmit info anyhow Digitally sign to avoid tampering Encrypt to keep private and avoid tampering Find another way to distribute information The Internet is inherently insecure Your options: Transmit info anyhow Digitally sign to avoid tampering Encrypt to keep private and avoid tampering Find another way to distribute information

The Internet "It is difficult to be certain whether the person you are dealing with is who he claims to be." Repudiation : can you prove to a court that someone took part in a transaction? "It is difficult to be certain whether the person you are dealing with is who he claims to be." Repudiation : can you prove to a court that someone took part in a transaction?

The Internet Ways to address privacy and repudiation issues: S ecure S ockets L ayer ( SSL ) –"is readily available and widely used" S ecure H yper T ext T ransfer P rotocol ( S-HTTP ) –"has not really taken off" Ways to address privacy and repudiation issues: S ecure S ockets L ayer ( SSL ) –"is readily available and widely used" S ecure H yper T ext T ransfer P rotocol ( S-HTTP ) –"has not really taken off"

Your system Keep up to date on warnings and patches for third-party software What do your scripts do (or not do)? –Use SSL to protect privacy –Use registered digital certificate –Check user-entered data carefully –Store information securely Keep up to date on warnings and patches for third-party software What do your scripts do (or not do)? –Use SSL to protect privacy –Use registered digital certificate –Check user-entered data carefully –Store information securely

Using SSL Originally designed by Netscape Now is "unofficial standard" for secure communication between browsers and servers Standardized in TLS –Transport Layer Security –Based on SSL –Not yet widely supported Originally designed by Netscape Now is "unofficial standard" for secure communication between browsers and servers Standardized in TLS –Transport Layer Security –Based on SSL –Not yet widely supported

TCP/UDP IP Various HTTPFTPSMTP... Application layer Transport layer Network layer Host to Network layer Network protocol stack

TCP/UDP IP Various... Application layer Transport layer Network layer Host to Network layer SSL Record Protocol SSL layer SSL Hand- shake Protocol HTTP SSL Change Cipher SSL Alert Protocol The SSL layer The SSL layer is transparent: –Same interface as the underlying transport layer Deals with handshaking, encryption, and decryption The SSL layer is transparent: –Same interface as the underlying transport layer Deals with handshaking, encryption, and decryption

SSL handshake Browser Server Browser connects, asks for certificate

SSL handshake Browser Server Browser connects, asks for certificate Server sends certificate Certificate Server distinguished name Server public key Period of validity Issuer distinguished name Issuer Signature Other information CA's public key public private

SSL handshake Browser Server Browser connects, asks for certificate Server sends certificate Browser sends list of cyphers RC4 with 40-bit key DES with 40-bit key DES with 56-bit key

SSL handshake Browser Server Browser connects, asks for certificate Server sends certificate Browser sends list of cyphers Server selects strongest common cypher RC4 with 40-bit key DES with 40-bit key DES with 56-bit key RC4 with 40-bit key RC2 with 40-bit key 3DES with 168-bit key Idea (128 bit key) RC4 with 40-bit key from browser:server supports:

SSL handshake Browser Server Browser connects, asks for certificate Server sends certificate Browser sends list of cyphers Server selects strongest common cypher Browser sends encrypted random number public private

SSL handshake Browser Server Browser connects, asks for certificate Server sends certificate Browser sends list of cyphers Server selects strongest common cypher Browser sends encrypted random numberServer sends plain text random number Why is this sent in plain text?

SSL handshake Browser Server Browser connects, asks for certificate Server sends certificate Browser sends list of cyphers Server selects strongest common cypher Browser sends encrypted random numberServer sends plain text random number Hash random #s to get session key session

Sending data <head> My Page</ti My Page... Packetize Break up the data into manageable packets.

Sending data <head> My Page</ti My Page... Compress Packetize Each packet is (optionally) compressed.

Sending data <head> My Page</ti My Page... Calculate MAC Compress Packetize Use hash to calculate Message Authentication Code for each packet.

Sending data <head> My Page</ti My Page... Calculate MAC Encrypt Compress Packetize The MAC and compressed data are combined and encrypted using the session key. session

Sending data <head> My Page</ti My Page... Calculate MAC Encrypt Compress Packetize TCP header Combine with header and send

Screening user input Use addslashes() before putting data in database. Use stripslashes() when retrieving data. Magic quotes –Add and strip slashes automatically Use addslashes() before putting data in database. Use stripslashes() when retrieving data. Magic quotes –Add and strip slashes automatically

Screening user input escapeshellcmd() –used to pass data to system(), exec() or execute with backticks Avoid executing shell commands wherever possible, especially with user input. strip_tags() remove HTML tags htmlspecialchars() escape HTML chars –e.g., change < to < escapeshellcmd() –used to pass data to system(), exec() or execute with backticks Avoid executing shell commands wherever possible, especially with user input. strip_tags() remove HTML tags htmlspecialchars() escape HTML chars –e.g., change < to <

Providing secure storage "The most dangerous type of data we store is executable content." Don't allow write access to scripts and directories in document tree. Intruder could write malicious script and execute it by loading through the web server. Scripts that are supposed to write files should write outside web tree. "The most dangerous type of data we store is executable content." Don't allow write access to scripts and directories in document tree. Intruder could write malicious script and execute it by loading through the web server. Scripts that are supposed to write files should write outside web tree.

Providing secure storage Encrypting data probably won't help unless key and decrypting software are on a different machine. Intruder who can get to encrypted file can probably also get to key and decrypting software if they are on the same machine. Encrypting data probably won't help unless key and decrypting software are on a different machine. Intruder who can get to encrypted file can probably also get to key and decrypting software if they are on the same machine.

Protect passwords in scripts Scripts with.php will always be interpreted. Scripts with other extensions (like.inc) could be served as is. Limited protection from other users on same server –Don't use same password for login and database. Back up database. –Set group to apache (must be root) and don't allow public read. –CGI is more secure in this respect. Scripts with.php will always be interpreted. Scripts with other extensions (like.inc) could be served as is. Limited protection from other users on same server –Don't use same password for login and database. Back up database. –Set group to apache (must be root) and don't allow public read. –CGI is more secure in this respect.

Why are you storing credit card numbers? One time transactions: Send card number to transaction processor and don't store Periodic charges: –Don't store on web server –Check up-to-date security info –Be paranoid One time transactions: Send card number to transaction processor and don't store Periodic charges: –Don't store on web server –Check up-to-date security info –Be paranoid