Chap 81 Electronic mail security

Chap 82 Outline Pretty good privacy S/MIME Recommended web sites

Chap 83 Security Enhancements confidentiality –protection from disclosure authentication –of sender of message message integrity –protection from modification non-repudiation of origin –protection from denial by sender

Chap 84 Pretty Good Privacy (PGP) widely used de facto secure developed by Phil Zimmermann selected best available crypto algs to use integrated into a single program available on Unix, PC, Macintosh and Amiga systems originally free, now have commercial versions available also

Chap 85 Why Is PGP Popular? It is availiable free on a variety of platforms. Based on well known algorithms. Wide range of applicability Not developed or controlled by governmental or standards organizations

Chap 86 Operational Description Consist of five services: Authentication Confidentiality Compression compatibility Segmentation

Chap 87

8 PGP Operation – Authentication 1.sender creates a message 2.SHA-1 used to generate 160-bit hash code of message 3.hash code is encrypted with RSA using the sender's private key, and result is attached to message 4.receiver uses RSA or DSS with sender's public key to decrypt and recover hash code 5.receiver generates new hash code for message and compares with decrypted hash code, if match, message is accepted as authentic

Chap 89 PGP Operation – Confidentiality 1.sender generates message and random 128- bit number to be used as session key for this message only 2.message is encrypted, using CAST-128 / IDEA/3DES with session key 3.session key is encrypted using RSA with recipient's public key, then attached to message 4.receiver uses RSA with its private key to decrypt and recover session key 5.session key is used to decrypt message

Chap 810 PGP Operation – Confidentiality & Authentication uses both services on same message –create signature & attach to message –encrypt both message & signature –attach RSA encrypted session key

Chap 811 PGP Operation – Compression by default PGP compresses message after signing but before encrypting –so can store uncompressed message & signature for later verification –& because compression is non deterministic uses ZIP compression algorithm

Chap 812 PGP Operation – Compatibility when using PGP will have binary data to send (encrypted message etc) however was designed only for text hence PGP must encode raw binary data into printable ASCII characters uses radix-64 algorithm –maps 3 bytes to 4 printable chars –also appends a CRC PGP also segments messages if too big

Chap 813 Compatibility The scheme used is radix-64 conversion (see appendix 5B). The use of radix-64 expands the message by 33%. 24* 4/3=32

Chap 814 Segmentation and Reassembly Often restricted to a maximum message length of 50,000 octets. Longer messages must be broken up into segments. PGP automatically subdivides a message that is to large. The receiver strip of all headers and reassemble the block.

Chap 815 Summary of PGP Services

Chap 816 PGP Session Keys need a session key for each message –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

Chap 817 PGP Public & Private Keys since many public/private keys may be in use, need to identify which is actually used to encrypt session key in a message –could send full public-key with every message –but this is inefficient rather use a key identifier based on key –is least significant 64-bits of the key –will very likely be unique also use key ID in signatures

Chap 818 PGP Key Rings 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

Chap 819 PGP Key Management rather than relying on certificate authorities in PGP every user is own CA –can sign keys for users they know directly forms a “web of trust” –trust keys have signed –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

Chap 820

Chap 821 Format of PGP Message

Chap 822

Chap 823

Chap 824

Chap 825 The Use of Trust One Key legitimacy field: (the extent the PGP will trust this public key; computed by PGP) One or more Signature trust field: (lists of signatures on this certificates; associated with each signature is assigned a signature bit; ; sig trust bit 1 + sig trust bit  key legitmacy bit) Owner trust field : (the degree to which this public key is trusted to sign other certificates ) Associated with each public key in the Key ring

Chap 826

Chap 827

Chap 828 Revoking Public Keys The owner issue a key revocation certificate. Normal signature certificate with a revote indicator. Corresponding private key is used to sign the certificate.

Chap 829 S/MIME (Secure/Multipurpose Internet Mail Extensions) security enhancement to MIME –original Internet RFC822 was text only –MIME provided support for varying content types and multi-part messages –with encoding of binary data to textual form –S/MIME added security enhancements have S/MIME support in various modern mail agents: MS Outlook, Netscape etc

Chap 830 RFC 822 From: "Microsoft Outlook Express Team" To: =?big5?B?t3MgT3V0bG9vayBFeHByZXNzIKjPpc6qzA==?= Subject: =?big5?B?xXeq76jPpc4gT3V0bG9vayBFeHByZXNzIDU=?= Date: Mon, 29 Sep :02: Hello This is a test

Chap 831 Simple Mail Transfer Protocol (SMTP, RFC 822) SMTP Limitations - Can not transmit, or has a problem with: executable files, or other binary files (jpeg image) “ national language ” characters (non-ASCII) messages over a certain size SMTP gateway that translates ASCII to EBCDIC do not use a consistent set of mapping SMTP gateway to X.400 cannot handle nontextual data in X.400 Truncating lines longer than a certain length (72 to 254 characters)

Chap 832 Header fields in MIME MIME-Version: Must be “ 1.0 ” -> RFC 2045, RFC 2046 Content-Type: More types being added by developers (application/word) Content-Transfer-Encoding: How message has been encoded (radix-64) Content-ID: Unique identifying character string. (Optional) Content Description: Needed when content is not readable text (e.g.,mpeg) (Optional)

Chap 833

Chap 834

Chap 835 MIME Example (1) From: "Microsoft Outlook Express Team" To: =?big5?B?t3MgT3V0bG9vayBFeHByZXNzIKjPpc6qzA==?= Subject: =?big5?B?xXeq76jPpc4gT3V0bG9vayBFeHByZXNzIDU=?= Date: Mon, 29 Sep :02: MIME-Version: 1.0 Content-Type: text/html; charset="big5" Content-Transfer-Encoding: quoted-printable X-MimeOLE: Produced By Microsoft MimeOLE V font{font-family:"MingLiu";font-size:9pt;color:#000000} A:hover.defaultA{color:#999900} A:hover.bodyTopLine{color:#0000FF} A:hover {color:#0033FF} :

Chap 836 MIME Example (2) From: "=?big5?B?uOqwVL7Hqvmiug==?=" To: Subject: =?big5?B?pU7C4Lx4qESudrjqOiCw6qXfquqnwKzsp96kar7HuOqkdah0pM6tuL73qHS42w= Date: Mon, 1 Mar :49: MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_NextPart_000_0010_01C3FF83.3B89A160" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express X-MimeOLE: Produced By Microsoft MimeOLE V Return-Path: X-OriginalArrivalTime: 01 Mar :58: (UTC) FILETIME=[63733E80:01C3FF41] This is a multi-part message in MIME format =_NextPart_000_0010_01C3FF83.3B89A160 Content-Type: multipart/alternative; boundary="----=_NextPart_001_0011_01C3FF83.3B8B2800":

Chap 837 MIME Example (2) =_NextPart_001_0011_01C3FF83.3B8B2800 Content-Type: text/plain; charset="big5" Content-Transfer-Encoding: quoted-printable =B3o=ACO=B8=EA=B0T=BE=C7=AA=F9=AA=BA=A5N=C2=E0=B0T=AE=A7=AAA=B0=C8 =A1A=AD= =": =B0=EA=A5=DF=AA=EA=A7=C0=AC=EC=A7=DE=A4j=BE=C7=B8=EA=A4u=A8t=A4=CE= AD=B8=BE= =F7=A8t=B8=DB=BCx=B1=D0=AEv=A1A=B8=D4=A8=A3=AA=FE=A5=F3=A1C =_NextPart_001_0011_01C3FF83.3B8B2800 Content-Type: text/html; charset="big5" Content-Transfer-Encoding: quoted-printable <HTML xmlns=3D" xmlns:o =3D=20 "urn:schemas-microsoft-com:office:office" xmlns:w =3D=20

Chap 838 S/MIME Functions enveloped data –encrypted content and associated keys signed data –(message + signed digest)  base64 encoding clear-signed data –cleartext message + encoded signed digest signed & enveloped data –nesting of signed & encrypted entities

Chap 839 S/MIME Cryptographic Algorithms hash functions: SHA-1 & MD5 digital signatures: DSS & RSA session key encryption: ElGamal & RSA message encryption: Triple-DES, RC2/40 and others have a procedure to decide which algorithms to use

Chap 840 S/MIME Certificate Processing 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

Chap 841 Certificate Authorities have several well-known CA’s Verisign one of most widely used Verisign issues several types of Digital IDs with increasing levels of checks & hence trust ClassIdentity ChecksUsage 1name/ checkweb browsing/ 2+enroll/addr check , subs, s/w validate 3+ID documentse-banking/service access

Chap 842

Chap 843 User Agent Role S/MIME uses Public-Key Certificates - X.509 version 3 signed by Certification Authority Functions: Key Generation - Diffie-Hellman, DSS, and RSA key-pairs. Registration - Public keys must be registered with X.509 CA. Certificate Storage - Local (as in browser application) for different services. Signed and Enveloped Data - Various orderings for encrypting and signing.

Chap 844 User Agent Role Example: Verisign ( Class-1: Buyer ’ s address confirmed by ing vital info. Class-2: Postal address is confirmed as well, and data checked against directories. Class-3: Buyer must appear in person, or send notarized documents.

Chap 845