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Email is one of the most widely used and regarded network services currently message contents are not secure may be inspected either in transit or by suitably privileged users on destination system
26/11/2017 Lecturer: Nguyễn Thị Thanh Vân – FIT - HCMUTE Introduction Pretty Good Privacy S/MIME DomainKeys Identified Mail 26/11/2017 email is one of the most widely used and regarded network services currently message contents are not secure may be inspected either in transit or by suitably privileged users on destination system confidentiality protection from disclosure authentication of sender of message message integrity protection from modification non-repudiation of origin protection from denial by sender 26/11/2017 is an encryption strategy for (de)encrypting and signing data in general and email/messages in specific developed by Phil Zimmermann provides a confidentiality and authentication service selected best available crypto algothirms to use integrated into a single program on Unix, PC, Macintosh and other systems originally free, now also have commercial versions available The actual operation of PGP consists of four services: o Authentication: using Digital signature • DSS/SHA or • RSA/SHA o Confidentiality: • • • • CAST or IDEA or Three-key Triple DES with Diffie-Hellman (key exchange algorithm) RSA o Compression • ZIP o e-mail compatibility: • Radix-64 conversion 26/11/2017 The digital signature service provided by PGP The sender creates a message SHA-1 is used to generate a 160-bit hash code of the message Encrypt H with RSA using PR a, and the result is prepended to the message Uses RSA with the PU a to decrypt and recover the hash code The receiver generates a new hash code for the message and compares it with the decrypted hash code If the two match, the message is accepted as authentic - PGP support the use of DSS signatures It can be useful in: - to maintain a separate signature log of all messages sent or received; or on an executable program to detect subsequent virus infection, or w hen more than one party must sign a document 26/11/2017 Confidentiality is provided by encrypting messages to be transmitted or to be stored locally as files: generates a message and a session key (random 128-bit number): one-time key – use only once) encrypts message using CAST-128 (or IDEA or 3DES) with session key attaches session keyencrypted with RSA using the recipient’s public key receiver decrypts & recovers session key session key is used to decrypt message using RSA with its private key Recent PGP versions also support the use of ElGamal (a Diffie-Hellman variant) for session-key exchange can use both services on same message o the sender signs the message with its own private key, att to M o then encrypts the message with a session key using CAST-128 (or IDEA or 3DES) o and then encrypts the session key with the recipient's public key using RSA (or ElGamal) 10 26/11/2017 11 by default, PGP compresses message after signing so can store uncompressed message & signature for later verification o & because compression is non deterministic o signing but PGP compresses before encrypting: o to strengthen cryptographic security compress o compressed message has less redundancy than the original plaintext, o cryptanalysis is more difficult encrypting uses ZIP compression algorithm 12 26/11/2017 when using PGP will have binary data to send (encrypted) however email was designed only for text hence PGP must encode raw binary data into printable ASCII characters uses radix-64 algorithm o maps bytes to printable chars o also appends a CRC PGP also segments messages if too big Text ASCII Bit Index Base 64 M 77 (0x4d) 1 19 T 1 22 W a 97 (0x61) 0 0 F n 110 (0x6e) 1 46 u 13 14 26/11/2017 PGP makes use of four types of keys: o one-time session symmetric keys, o public keys, o private keys, and o passphrase-based symmetric keys need a session key for each message, using a symmetric encryption algorithm o of varying sizes: 56-bit DES, 128-bit CAST or IDEA, 168-bit Triple-DES generated using ANSI X12.17 mode uses random inputs taken from previous uses and from keystroke timing of user 15 since many public/private keys may be in use, need to identify which is actually used to encrypt session key in a message o could send full public-key with every message o but this is inefficient rather use a key identifier based on key o is least significant 64-bits of the key o will very likely be unique also use key ID in signatures 16 26/11/2017 the message component: includes the actual data, filename and o a timestamp o o a signature (optional): timestamp, encrypted SHA-1, the Key ID a session key component (optional): the session key and the identifier of the recipient's public key 17 each PGP user has a pair of keyrings: public-key ring contains all the public-keys of other PGP users known to this user, indexed by key ID private-key ring contains the public/private key pair(s) for this user, indexed by key ID & encrypted keyed from a hashed passphrase security of private keys thus depends on the passphrase security 18 26/11/2017 19 20 10 26/11/2017 21 rather than relying on certificate authorities in PGP every user is own CA o can sign keys for users they know directly forms a “web of trust” o trust keys have signed o can trust keys others have signed if have a chain of signatures to them key ring includes trust indicators users can also revoke their keys 22 11 26/11/2017 23 e-mail format standards: o Traditional - RFC 822: text only o Internet Message Format - RFC 5322 o MIME (Multipurpose Internet Mail Extension) - RFC 2045-2049 o S/MIME E-mail Format includes: header and the body Ex, 24 12 26/11/2017 MIME: o an extension to the RFC 5322 framework o solves some of the problems and limitations of the use of SMTP • cannot transmit executable files or other binary objects • cannot transmit text data that includes national language characters • reject mail message over a certain size … The MIME specification includes the following elements o Five new message header fields (information about the body) • MIME-Version, Content-Type, Content-Transfer-Encoding, Content-ID, Content-Description o A number of content formats are defined • Text, image, video… o Transfer encodings are defined that enable the conversion of any content format into a form that is protected from alteration by the mail system • 7bit, 8bit, and binary, base64 25 S/MIME: o security enhancement to MIME email o have S/MIME support in many mail agents • eg MS Outlook, Mozilla, Mac Mail etc o Provide many functions o Use many cryptographic algorithms 26 13 26/11/2017 enveloped data o encrypted content and associated keys signed data o encoded message + signed digest clear-signed data o cleartext message + encoded signed digest signed & enveloped data o nesting of signed & encrypted entities 27 digital signatures: o DSS & RSA hash functions: o SHA-1 & MD5 session key encryption: o ElGamal & RSA message encryption: o AES, Triple-DES, RC2/40 and others MAC: o HMAC with SHA-1 have process to decide which algs to use 28 14 26/11/2017 S/MIME secures a MIME entity with a signature, encryption, or both forming a MIME wrapped PKCS object have a range of content-types: enveloped data: An encrypted S/MIME entity signed data: A signed S/MIME entity clear-signed data registration request certificate only message 29 S/MIME uses X.509 v3 certificates managed using a hybrid of a strict X.509 CA hierarchy & PGP’s web of trust each client has a list of trusted CA’s certs and own public/private key pairs & certs certificates must be signed by trusted CA’s 30 15 26/11/2017 have several well-known CA’s Verisign one of most widely used Verisign issues several types of Digital IDs increasing levels of checks & hence trust Class Identity Checks Usage name/email check web browsing/email + enroll/addr check email, subs, s/w validate + ID documents e-banking/service access 31 proposed enhanced security services: o signed receipts: • to provide proof of delivery to the originator of a message • allow s the originator to demonstrate to a third party that the recipient received the message o security labels: • Is a set of security information of the content that is protected by S/MIME encapsulation • may be used for access control, w hich users are permitted access o secure mailing lists: • The user can not use of each recipient's public key by employing the services of an S/MIME Mail List Agent (MLA) • An MLA can take a single incoming message, perform recipient-specific encryption for each recipient, and forw ard the message • The originator of a message need only send the message to the MLA, w ith encryption performed using the MLA's public key 32 16 26/11/2017 33 see RFC 4684- Analysis of Threats Motivating DomainKeys Identified Mail describes the problem space in terms of: o range: low end, spammers, fraudsters o capabilities in terms of where submitted, signed, volume, routing naming etc o outside located attackers 34 17 26/11/2017 a specification for cryptographically signing email messages so signing domain claims responsibility recipients / agents can verify signature proposed Internet Standard RFC 4871 has been widely adopted 35 to provide an email authentication technique transparent to user o MSA sign o MDA verify for pragmatic reasons 36 18 26/11/2017 37 processes: signing Administrative Management Domain (ADMD) is performed by an authorized module w ithin the signing ADMD and uses private information o verifying ADMD is performed by an authorized module w ithin the verifying ADMD and uses public information from the Key Store o • If the signature passes, reputation inf ormation is used to assess the signer and that inf ormation is passed to the message f iltering sy stem • If the signature f ails or there is no signature using the author's domain, inf ormation about signing practices related to the author can be retriev ed remotely and/or locally, and that inf ormation is passed to the message f iltering sy stem 38 19 26/11/2017 have considered: o secure email o PGP o S/MIME o domain-keys identified email Practice: o Setup mail server (on linux OS) o Configure and add some tools to prevent from spams and establish security policies for mail server 39 Cryptography and Network Security, Principles and Practice, William Stallings, Prentice Hall, Sixth Edition, 2013 o Chapter 18 o Others 40 20 ... 26/11/2017 23 e -mail format standards: o Traditional - RFC 82 2: text only o Internet Message Format - RFC 5322 o MIME (Multipurpose Internet Mail Extension) - RFC 204 5-2 049 o S/MIME E -mail Format... each message, using a symmetric encryption algorithm o of varying sizes: 56-bit DES, 1 2 8- bit CAST or IDEA, 1 6 8- bit Triple-DES generated using ANSI X12.17 mode uses random inputs taken from previous... alteration by the mail system • 7bit, 8bit, and binary, base64 25 S/MIME: o security enhancement to MIME email o have S/MIME support in many mail agents • eg MS Outlook, Mozilla, Mac Mail etc o Provide