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Pretty Good Privacy “To PGP or not to PGP? “ Phil Zimmermann What are we going to do ? Background & Concept – Why Is PGP Popular? – PGP’s algorithms Operational Description – Inside look on operations Key Management – The problem & Solution Web Of Trust Pretty Good Privacy First released in 1991, developed by Phil Zimmerman, provoked export control and patent infringement controversy. PGP provides a confidentiality and authentication service - can be used for electronic mail and file storage applications. Available as plug-in for popular e-mail clients, can also be used as stand-alone software. - microsoft exchange - outlook Why Is PGP Popular? Based on well known algorithms - “The main idea” – These algorithm have survived extensive public review and are considered extremely secure. – Integrated these algorithms into a general-purpose application It is availiable free on a variety of platforms (Windows, UNIX, Macintosh, etc.) – Open and free code. Wide range of applicability from corporations that wish to select and enforce a standerized secure to individuals Independent – meaning Not developed or controlled by governmental or standards organizations - Based on mutual trust between clients Operational Description Actual operations of PGP consist of five services: • Authentication – DSS/SHA or RSA/SHA • Confidentiality – CAST or IDEA or RSA or 3DES • Compression – A message may be compressed, for storage or transmission using ZIP • E-mail compatibility – To provide transparency for e-mail applications, an encrypted message may be converted to an ASCII using Radix-64 • Segmentation – To accommodate maximum message size limitations. Authentication/Digital Signature Sender creates a message Sender generates a hash code of the message - uses SHA-1 algorithm in order to generates 160-bit hash code Hash code encrypted with RSA (sender’s private key) - the result is prepended to the message Receiver recover the hash code - uses RSA with the sender’s public key Receiver generates new hash code of the message and compares the two codes. If the two match, the message is accepted as authentic. Note: - PGP only encryptes the hash-code of the message: more efficient in running time and in transfer time Authentication/Digital signature Source A Destination B Private key Public key KRa KRb DP Message H EP || ZIP UNZIP M Message Compare M H PGP Signed Message -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 This is simply the text of the message. It has not been encrypted, simply signed -----BEGIN PGP SIGNATURE----- Version: PGPfreeware 6.5.3 for non-commercial use <http://www.pgp.com> iEYEARECAAYFAj5Ha6AACgkQ99/KQPj2cRNHsQCffKf64L wWQMfRIiKUfs6QrokB7twAnR5gDobzGapPgyLKQ0gL klj1WIIp=gXad -----END PGP SIGNATURE----- Confidentiality/Encryption Sender generates message and also a session key - The session key is a random 128-bit number to be used as a session key for this message only Sender encryptes the message - Uses CAST-128 (IDEA or 3DES) algorithm with the session key Sender encryptes the Session key with RSA and prepended to the message Receiver decrypt the session key - uses RSA with its private key Receiver decrypt the message with the Session key Note: - PGP does not simply using RSA to encrypt the message directly. - Using CAST128 force us to share a key - using public-key algorithm solves the session key distrinution problem. - Given “Store-and-forward” nature of e-mail, the use of handshaking to assure that both sides have the same session key is not practical. - The use of on-time conventional keys strengthens what is already a strong conventional encryption approach. only a small amount of plaintext is encrypted with each key and there is no relationship among keys. Confidentiality/Encryption Source A Destination B Public key KUb Private key KRb Session key Ks EP DP Message ZIP EC || Session key M Message Ks Message M M DC UNZIP Confidentiality & Authentication Source A Destination B Public key KUb Private Public key key KRb KRb Session key Ks EP DP Private key DP KRa Session key Compare Ks M H EP || ZIP EC || M M DC H UNZIP •PGP first signs the message and then encrypts it: - more convenient to store a signature with a plaintext version of a message - for purposes of third party verification Compression Saving space both for e-mail transmission and for file storage PGP uses ZIP to compress the message PGP compress the message after applying the signature but before message encryption: Signature Zip Encryption • One can store only the uncompressed message with the signature for future verification. In case the order was opposite: - it would be necessary either to store a compressed version of the message or to recompress the message each time when verification is required • Compression algorithms are different – the algorithm is not deterministic. - sign after compress will would constrain all PGP implementations to the same compression algorithm • Encryption is applied after compression to strengthen cryptographic security - compressed message has less redundancy than original plaintext Example of ZIP (LZ77) Scheme The brown fox jumped over the brown foxy jumping frog 13 5 26 27 The brown fox jumped over 0b26d13d y 0b27d 5d ing frog •The main assumption is that words and phrases within a text stream (image patterns I the case of GIF) are likely to be repeated • When a repetition occurs, the repeated sequence can be replaced by a short one • Over time, codes are reused to capture new sequences E-mail Compatibility When PGP is used, At least part of the block to be transmitted is encrypted – The resulting block will consist of a stream of arbitraty 8-bit octets – Many electronic mail systems only permit the use of blocks consisting of ASCII text To provide transparency for e-mail applications, an encrypted message may be converted to an ASCII string using radix 64 conversion The use of Radix-64 expands a message by 33% - In fact, the compression should be more than enough to compensate for the radix-64 expansion Encoding Binary Data into Radix-64 Format The scheme used is radix-64 conversion, which expands the message by 33%. Radix-64 blindly converts the input stream to radix-64 format regardless of content, even if the input happens to be ASCII text. - certain level of confidentiality - if the message is signed but not encrypted, the output will be unreadable to the casual observer Segmentation and Reassembly E-mail facilities are often restricted to a maximum message length - for example 50,000 octets. Longer messages must be broken up into segments, which will be mailed separately. PGP automatically subdivides a message that is too large into segments that are small enough to send via e-mail. The segmentation is done after all of the other processing, including the Raidx-64 conversion. - thus, the session key component and signature component appear only once The receiver strips off all e-mail headers and reassemble the block. Key Requirements PGP makes use of four types of keys: - one-time session conventional keys, public keys, private keys , passphrase-based conventional keys Three seperate requirements: A means of generating unpredictable session keys is needed Any user may have multiple public-key/private-key pairs - may wish to change his key pair from time to time - in order to interact with different groups - simply to enhance security by limiting tha anount material encrypted with any one key some means is needed for identifying particular keys Each PGP entity must maintain data base of: - a file of its own key pairs - a file of public keys of correspondents Session Key Generation The Problem : generating unpredictable session keys Session keys are generated using CAST-128 itself: – This is a PGP specific random number generation technique – getting as input: two 64-bit blocks that are treated as plaintext to be encrypted. - based on keystroke stream generated by the user 128-bit key - random input that also combined with previous session key output from CAST-128. The result, scrambling of CAST-128, is to produce a sequence of session keys that is effectively unpredictable Key Identifiers The Problem: user may have multiple public-key/private- key pairs One simple solution would be to transmit the public key with the message. - Would work but an RSA key may be three hundreds of decimal digits in length (1024 bits) PGP solution associate a short identifier with each public key that is unique. then only the much shorter key ID would need to be transmitted. The key ID associated with each public key consists of its least significant 64 bits That is the ID of KU is (KU mod 264) Format of PGP Message Session Key Key Id of Recipients Public Key Component Session Key EKUb Timestamp Key Id of Senders Public Key Signature Leading Two Octets of Message Digest Message Digest EKRa Eks R64 Filename ZIP Time Stamp Message Data PGP Key Rings The problem: must maintain a database in order to supports multiple public/private keys. The Solution : Keys stored locally in a PGP Key Ring – essentially a database of keys. Two rings: - Private-key ring: stores the public/private key pairs owned by that node - Public-key ring: stores the public keys of other users known at this node Private keys stored in encrypted form; decryption key determined by user-entered passphrase. Key Rings Private-Key Ring Timestamp Key ID* Public Key Encrypted User ID* Private Key • • • • • • • • • • • • • • • Ti KUi mod 264 KUi EH(Pi)[KRi] User i • • • • • • • • • • • • • • • Key Rings Public-Key Ring Timestamp Key Public Owner User Key Signature(s) Signature ID* Key Trust ID* Legitimacy Trust(s) • • • • • • • • • • • • • • • • • • • • • • • • Ti KUi KUi User mod i 264 • • • • • • • • • • • • • • • • • • • • • • • • Message Generation Public-Key ring Passphase H Private-Key ring Select IDb IDa Select Encrypted Private key DC Key ID Public key Private key KRb KRa RNG Message digest Session key H EP || Ks Message Signature EP || Output M Message + message Encrypted EC Signature + message Reception Passphase H Public-Key ring Private-Key ring Select Select Encrypted Private key DC Private key Public key KRb KRb Receiver's key ID DP Encrypted Sender’s Session key Key ID Session key Ks Encrypte Encrypted d DP Message + Digest Signature DC Compare Message H Public Key Management Problem The Problem: A’s key ring contains a public key attributed to B but that the key is, in fact, owned by C Two threats now exist: • C can send messages to A and fake B’s signature, so that A will accept the message as coming from B! • Any encrypted message from A to B can be read by C ! Public Key Management Problem (cont.) Possible solutions: • Physically get the key from B • Verify a key by telephone • Obtain B’s public key from a mutual trusted individual • Obtains B’s public key from a trusted certifying authority That would violate PGP’s spirit as an E- mail security scheme for the masses: • It should be possible for people to exchange keys electronically with others whom they have never met and may not even know • Every one who uses this scheme trusts the central authority PGP Key Management PGP Solution: adopts a different trust model – the “web of trust” No centralised authority like a root of trust ! The concept of the web of trust: • The concept: Individuals sign one another’s public keys and create an interconnected community of public-key users. • These “certificates” are stored along with keys in key rings - A signature testifies that the User ID associated with this public key is valid - A signature is formed using the private key of the signer • PGP computes a trust level for each public key in key ring. • Users take part in the assignment of the trust level Trust in Public Key Ring Each user collects signed keys and stores these in the public- key ring. Each entry in the ring has: - Key legitimacy field Measures the degree to which this PGP user trusts that the key is valid for its user. The higher the level of trust, the stronger is the binding of this user ID to this key - Signature trust field Measures how far the PGP user trusts the signer to certify public keys. (The key legitimacy field for an entry derives from the signature trust fields.) - Owner trust field Indicates the degree to which this PGP user trusts the key's owner to sign other public- key certificates. PGP doesn't compute this level of trust; the PGP user assigns it. You can think of a signature trust field as a cached copy of the owner trust field from another entry. Trust in Public Key Ring Key Legitimacy Field (computed by PGP) Signature Trust Field (copies of OTF) Owner Trust Field (assigned by the user) Public-Key Ring Timestamp Key Public Owner User Key Signature(s) Signature ID* Key Trust ID* Legitimacy Trust(s) • • • • • • • • • • • • • • • • • • • • • • • • Ti KUi KUi Trust User i Trust mod flagi flagi 264 • • • • • • • • • • • • • • • • • • • • • • • • Adding a new public key to your public-key ring: Owner trust field: (signed other keys) - If you own the key - ultimate trust is automatically assigned. - If you don’t own the key - PGP asks the user: unknown, untrusted, marginally trusted, or completely trusted Signature trust field: (trusts the signer) PGP searches the public-key ring to see if the author of this signature is among the known public-key owners. - If so, the owner trust value for this owner is assigned to the signature trust field for this signature. OWNERTRUST SIGTRUST - If not, an unknown-user value is assigned. key-legitimacy: (the key is valid for its user) On the basis of the signature trust fields present in this entry. - If at least one signature has a value of ultimate trust, then the key legitimacy value is set to complete - Otherwise, PGP computes a weighted sum of the trust values. 1/X is given to signatures that are always trusted 1/Y is given to signatures that are usually trusted X and Y are user-configurable parameters. PGP Trust Model Example Revoking Public Keys When exposure suspects or simply avoiding the use of the same key for an extended period The owner issue a key revocation certificate – Signed by the owner , with the corresponding private key – Same form of normal signature certificate but includes an indicator that the purpose of this certificate is to revoke the use of this public key The owner should disseminate this certificate as widely and as quickly as possible opponent NOTE: An opponent who has compromised the private-key of an owner can also issue such a certificate. However, this would deny the opponent as well as the legitimate owner the use of the public Key – seems much less likely threat. Next: S/MIME… Radix-64 Conversion Table Radix-64 Encoding 6-bit Character 6-bit Character 6-bit Character 6-bit Character Value Encoding Value Encoding Value Encoding Value encoding 0 A 16 Q 32 g 48 w 1 B 17 R 33 h 49 x 2 C 18 S 34 i 50 y 3 D 19 T 35 j 51 z 4 E 20 U 36 k 52 0 5 F 21 V 37 l 53 1 6 G 22 W 38 m 54 2 7 H 23 X 39 n 55 3 8 I 24 Y 40 o 56 4 9 J 25 Z 41 p 57 5 10 K 26 a 42 q 58 6 11 L 27 b 43 r 59 7 12 M 28 c 44 s 60 8 13 N 29 d 45 t 61 9 14 O 30 e 46 u 62 + 15 P 31 f 47 v 63 / (pad) =
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