**Goal of Encryption of Internet Traffic**

- conveys confidentiality to messages while in transit
- changes readable text messages into something that cannot be read
- discourages anyone from reading or copying the messages

**Related Problem**

- if header information is not encrypted, traffic analysis is possible
- traffic analysis - the analysis of header information in order to derive useful information from the headers

**Encryption Components**

- an algorithm
- a key

**Encryption Algorithms**

- a series of steps that mathematically transforms plain-text or other readable information into unintelligible cipher text.
- Cipher text - Data that has been encrypted. Cipher text is unreadable until it has been converted into plain text (decrypted) with a key.

**Decryption**

- The inverse mathematical transformation, which transforms the encrypted cipher text back into something readable, is called decryption.

**Encryption Algorithm - Input and Output**

- a key and plain text are input into an encryption algorithm
- cipher text is output from the encryption algorithm

**Encryption Keys**

- a bit string consisting of
**x**number of bits. A 40 bit key is a string consisting of 40 bits - an encryption algorithm can use one of a large number of possible keys
- the number of possible keys each algorithm can support depends on the number of bits in the key. The longer the key, the more the possible number of keys

**Encryption Key Example**

- example - if the key length is 40, then 2 to the
, where n is the number of bits in the key, results in 1,000,000,000,000 possible key combinations, with each different key causing the algorithm to produce slightly d ifferent cipher output*n*

**Security and Encryption**

- encryption algorithms are considered secure if the security depends on only one factor - key length
- security does not depend on secrecy, inaccessibility, or anything else, only on the key length
- if this factor is true, then the only possible attack against the algorithm is a brute force attack

**Brute Force Attacks and Security**

- all key combinations must be tried in order to find the correct key
- the length of the key determines the possible number of keys available for selection
- the longer the key length the longer it takes to discover which key will actually decrypt
- specifying a long enough key length makes a brute-force attack non-feasible

**Symmetric Encryption**

- identical keys are used to encrypt and decrypt the message
- a message encrypted by one specific symmetric key can only be decrypted by using the same key, it can be decrypted with a different key

**Symmetric Keys**

- a random bit string,
bits long*n* - most often generated on the source computer

**Advantages of Using Symmetric Encryption**

- the encryption process is simple
- each trading partner can use the same publicly known encryption algorithm - no need to develop and exchange secret algorithms
- security is dependent on the length of the key

**Drawbacks of Using Symmetric Encryption**

- a shared secret key must be agreed upon by both parties
- if a user has
trading partners, then*n*secret keys must be maintained, one for each trading partner*n* - authenticity of origin or receipt cannot be proved because the secret key is shared
- management of the symmetric keys becomes problematic

**Problems with Management of Symmetric Keys**

- trading partners must always use the exact same key to decrypt the encrypted message
- key exchange is difficult because the exchange itself must be secure with no intervening compromise of the key
- management of keys is difficult as numbers of trading partners increases, especially when multiple keys exist for each trading partner

**Public Key Cryptography as a Solution for Managing Symmetric Keys**

- public key cryptography simplifies the management of symmetric keys to the point whereby a symmetric key can be used not only for each trading partner, but for each exchange between trading partners
- additionally, public key cryptography can be used to unambiguously establish non-repudiation of origin and receipt

**Asymmetric Encryption - (Public Key Cryptography)**

- based on the concept of a key pair
- each half of the pair (one key) can encrypt information that only the other half (one key) can decrypt
- the key pair is designated and associated to one, and only one, trading partner

**Asymmetric Key Pairs**

- consists of two keys - one private and one public
- private key is secret and only known by the designated trading partner it belongs to
- public key is published widely but still associated only with the designated trading partner

**Asymmetric Key Uses**

- confidentiality
- digital signatures
- both uses depend on the association of a key pair with one, and only one owner of the keys
- both uses depend on one of the keys in the key pair being secret from everyone but the owner of the key

**Confidentiality Using Asymmetric Key Pairs (Encryption)**

- Trading Partner A desires to send a confidential message to Trading Partner B
- Trading Partner A retrieves Trading Partner B's public key and encrypts the message with it

**Confidentiality Using Asymmetric Key Pairs (Decryption)**

- Trading Partner B receives the message and decrypts the message with the secretly held, private key
- The only key that can possibly decrypt a message that is encrypted with Trading Partner B's public key is Trading Partner B's private key

**Digital Signatures Using Asymmetric Key Pairs (Encryption)**

- Trading Partner A desires to send a digitally signed message to Trading Partner B
- Trading Partner A uses their own private key to encrypt a part of the message
- Trading Partner A sends the encrypted part of the message to B

**Digital Signatures Using Asymmetric Key Pairs (Decryption)**

- Trading Partner B receives Trading Partner A's message and obtains A's public key
- Trading Partner B tries to decrypt the encrypted portion of Trading Partner A's message
- If it decrypts, Then Trading Partner B knows it has to be from A because the only thing A's public key will decrypt is something encrypted with A's private key and only A has access to that private key

**Real World Usage of Asymmetric Encryption**

- public key encryption algorithms are considerably slower than symmetric key algorithms
- rarely used as encryption methodology for bulk messages or parts of messages
- normally used in conjunction with a Message Integrity Check (MIC) or to encrypt a symmetric key, where the MIC or symmetric key is what is encrypted using public key encryption algorithms

**Speed Comparison - Symmetric vs Asymmetric**

- software encryption using DES (symmetric key algorithm) is 100 times faster than software encryption using RSA (asymmetric key algorithm) - estimate provided by RSA Data Securities
- hardware encryption using DES (symmetric key algorithm) is anywhere from 1,000 to 10,000 times faster than hardware encryption using RSA (asymmetric key algorithm)

**Encryption Needs for Confidential Commercial Exchanges**

- for interoperability between two trading partners
- standard encryption algorithm(s)
- standard key length(s)
- agreed upon beforehand or within an individual transaction

**Issues**

- how secure is the algorithm?
- how fast are current implementations of the algorithm?
- availability of APIs and/or tools to implement the algorithm
- frequency of use of algorithm with other trading partners
- sufficient key length to discourage brute force attacks

**Common Symmetric Key Algorithms**

- Data Encryption Standard - DES
- Triple DES
- RC2 and RC5
- IDEA

**Block Ciphers vs Stream Ciphers**

- block ciphers - take a set number of bits, typically 64 bits, and encrypts the them as a single block
- stream ciphers - take and encrypt one bit at a time
- Most ciphers belong to the block cipher class.

**Data Encryption Standard - DES**

- most widely used commercial encryption algorithm
- in the public domain, available to all
- a U. S. government encryption standard
- security is known and is dependent solely on the key length
- data sequenced into 64 bit blocks prior to encryption, each block encrypted

**Cipher Block Chaining (CBC)**

- recommended mode for using DES
- each 64 bit block of data is exclusively OR'd with the previous block before encryption
- gives added protection by making each cipher-text block depend on each other
- changes in the cipher text can be detected

**Brute Force Attacks against DES**

- DES specifies a 56 bit key, so there are 2 to the 56th possible keys
- brute force attack means trying every single key (10,000,000,000,000,000) to decrypt 8 bytes of known cipher text into the corresponding plain text

**Resources Required to Break DES Key**

- $1 million dollar hardware based, brute-force attack on DES takes approximately 3.6 hours to recover the DES key
- $1 million dollar software based, brute force attack on DES takes approximately 3 years to recover the DES key
- above figures attributed to B. Schneier, "E-Mail Security", John Wiley & Sons, 1995

**Triple DES**

- variant on DES which encrypts message 3 times with 2 independent 56 bit keys
- effective key length is 112 bits
- brute force attack on Triple DES is not feasible

**RC2 and RC5**

- RSA owned proprietary symmetric key algorithms
- variable key length makes security configurable
- RC2 is a block cipher (similar to DES) and should be used in CBC mode, RC5 is also a block cipher and should be used in CVC Pad mode
- Both use 128 bit key but support key masking for configuration of key length

**International Data Encryption Algorithm (IDEA)**

- a block cipher, in the mold of DES
- uses a 64-bit block size and a 128-bit key
- IDEA in CBC mode is the bulk encryption algorithm used by Pretty Good Privacy (PGP) which makes it the most widely used encryption algorithm for

**Key Lengths and Secure Transactions**

- Algorithms that make a brute force attack not feasible
- Triple DES with 2 56 bit keys
- RC2 and RC5 with 128 bit keys
- IDEA with 128 bit key

**Recommendations on Key Lengths**

- Transactions of minimal or small value - 40 bit RC2 or 56 bit DES
- Most commercial applications need a key length of 75 bits
- High value transactions Triple-DES, IDEA or 128 bit RC2 or RC5

**Conclusions**

- Encryption is the correct method to implement confidentiality for Internet traffic
- Symmetric key algorithms should be chosen for encryption of confidential data
- The more bits in the symmetric key, the less probable the compromise of the encrypted data

## No comments:

## Post a Comment