Electronic Mail Electronic Mail SecuritySecurity

Email SecurityEmail Security

email is one of the most widely used and email is one of the most widely used and regarded network services regarded network services

currently message contents are not secure currently message contents are not secure may be inspected either in transit may be inspected either in transit or by suitably privileged users on destination or by suitably privileged users on destination

systemsystem

Email Security EnhancementsEmail Security Enhancements

confidentialityconfidentiality protection from disclosureprotection from disclosure

authenticationauthentication of sender of messageof sender of message

message integritymessage integrity protection from modification protection from modification

non-repudiation of originnon-repudiation of origin protection from denial by senderprotection from denial by sender

Pretty Good Privacy (PGP)Pretty Good Privacy (PGP)

widely used de facto secure emailwidely used de facto secure email developed by Phil Zimmermanndeveloped by Phil Zimmermann selected best available crypto algs to useselected best available crypto algs to use integrated into a single programintegrated into a single program on Unix, PC, Macintosh and other systems on Unix, PC, Macintosh and other systems originally free, now also have commercial originally free, now also have commercial

versions availableversions available

PGP Operation – PGP Operation – AuthenticationAuthentication

1.1. sender creates messagesender creates message2.2. use SHA-1 to generate 160-bit hash of use SHA-1 to generate 160-bit hash of

message message 3.3. signed hash with RSA using sender's signed hash with RSA using sender's

private key, and is attached to messageprivate key, and is attached to message4.4. receiver uses RSA with sender's public receiver uses RSA with sender's public

key to decrypt and recover hash codekey to decrypt and recover hash code5.5. receiver verifies received message using receiver verifies received message using

hash of it and compares with decrypted hash of it and compares with decrypted hash codehash code

PGP Operation – PGP Operation – ConfidentialityConfidentiality

1.1. sender generates message and 128-bit sender generates message and 128-bit random number as session key for itrandom number as session key for it

2.2. encrypt message using CAST-128 / IDEA / encrypt message using CAST-128 / IDEA / 3DES in CBC mode with session key3DES in CBC mode with session key

3.3. session key encrypted using RSA with session key encrypted using RSA with recipient's public key, & attached to msgrecipient's public key, & attached to msg

4.4. receiver uses RSA with private key to receiver uses RSA with private key to decrypt and recover session keydecrypt and recover session key

5.5. session key is used to decrypt messagesession key is used to decrypt message

PGP Operation – Confidentiality PGP Operation – Confidentiality & Authentication & Authentication

can use both services on same messagecan use both services on same message create signature & attach to messagecreate signature & attach to message encrypt both message & signatureencrypt both message & signature attach RSA/ElGamal encrypted session keyattach RSA/ElGamal encrypted session key

PGP Operation – PGP Operation – CompressionCompression

by default PGP compresses message by default PGP compresses message after signing but before encryptingafter signing but before encrypting so can store uncompressed message & so can store uncompressed message &

signature for later verificationsignature for later verification & because compression is non deterministic& because compression is non deterministic

uses ZIP compression algorithmuses ZIP compression algorithm

PGP Operation – Email PGP Operation – Email CompatibilityCompatibility

when using PGP will have binary data to send when using PGP will have binary data to send (encrypted message etc)(encrypted message etc)

however email was designed only for texthowever email was designed only for text hence PGP must encode raw binary data into hence PGP must encode raw binary data into

printable ASCII charactersprintable ASCII characters uses radix-64 algorithmuses radix-64 algorithm

maps 3 bytes to 4 printable charsmaps 3 bytes to 4 printable chars also appends a CRCalso appends a CRC

PGP also segments messages if too bigPGP also segments messages if too big

PGP Operation – SummaryPGP Operation – Summary

PGP Session KeysPGP Session Keys

need a session key for each messageneed a session key for each message of varying sizes: 56-bit DES, 128-bit CAST or of varying sizes: 56-bit DES, 128-bit CAST or

IDEA, 168-bit Triple-DESIDEA, 168-bit Triple-DES generated using ANSI X12.17 modegenerated using ANSI X12.17 mode uses random inputs taken from previous uses random inputs taken from previous

uses and from keystroke timing of useruses and from keystroke timing of user

PGP Public & Private KeysPGP Public & Private Keys

since many public/private keys may be in use, since many public/private keys may be in use, need to identify which is actually used to encrypt need to identify which is actually used to encrypt session key in a messagesession key in a message could send full public-key with every messagecould send full public-key with every message but this is inefficientbut this is inefficient

rather use a key identifier based on keyrather use a key identifier based on key is least significant 64-bits of the keyis least significant 64-bits of the key will very likely be uniquewill very likely be unique

also use key ID in signaturesalso use key ID in signatures

PGP Message FormatPGP Message Format

PGP Key RingsPGP Key Rings

each PGP user has a pair of keyrings:each PGP user has a pair of keyrings: public-key ring contains all the public-keys of public-key ring contains all the public-keys of

other PGP users known to this user, indexed other PGP users known to this user, indexed by key IDby key ID

private-key ring contains the public/private private-key ring contains the public/private key pair(s) for this user, indexed by key ID & key pair(s) for this user, indexed by key ID & encrypted keyed from a hashed passphraseencrypted keyed from a hashed passphrase

security of private keys thus depends on security of private keys thus depends on the pass-phrase securitythe pass-phrase security

PGP Message GenerationPGP Message Generation

PGP Message ReceptionPGP Message Reception

PGP Key ManagementPGP Key Management

rather than relying on certificate authoritiesrather than relying on certificate authorities in PGP every user is own CAin PGP every user is own CA

can sign keys for users they know directlycan sign keys for users they know directly

forms a “web of trust”forms a “web of trust” trust keys have signedtrust keys have signed can trust keys others have signed if have a chain of can trust keys others have signed if have a chain of

signatures to themsignatures to them

key ring includes trust indicatorskey ring includes trust indicators users can also revoke their keysusers can also revoke their keys

S/MIME (Secure/Multipurpose S/MIME (Secure/Multipurpose Internet Mail Extensions)Internet Mail Extensions)

security enhancement to MIME emailsecurity enhancement to MIME email original Internet RFC822 email was text onlyoriginal Internet RFC822 email was text only MIME provided support for varying content MIME provided support for varying content

types and multi-part messagestypes and multi-part messages with encoding of binary data to textual formwith encoding of binary data to textual form S/MIME added security enhancementsS/MIME added security enhancements

have S/MIME support in many mail agentshave S/MIME support in many mail agents eg MS Outlook, Mozilla, Mac Mail etceg MS Outlook, Mozilla, Mac Mail etc

S/MIME FunctionsS/MIME Functions

enveloped dataenveloped data encrypted content and associated keysencrypted content and associated keys

signed datasigned data encoded message + signed digestencoded message + signed digest

clear-signed dataclear-signed data cleartext message + encoded signed digestcleartext message + encoded signed digest

signed & enveloped datasigned & enveloped data nesting of signed & encrypted entitiesnesting of signed & encrypted entities

S/MIME Cryptographic S/MIME Cryptographic AlgorithmsAlgorithms

digital signatures: DSS & RSAdigital signatures: DSS & RSA hash functions: SHA-1 & MD5hash functions: SHA-1 & MD5 session key encryption: ElGamal & RSAsession key encryption: ElGamal & RSA message encryption: AES, Triple-DES, message encryption: AES, Triple-DES,

RC2/40 and othersRC2/40 and others MAC: HMAC with SHA-1MAC: HMAC with SHA-1 have process to decide which algs to usehave process to decide which algs to use

S/MIME MessagesS/MIME Messages

S/MIME secures S/MIME secures a MIME entity with a a MIME entity with a signature, encryption, or bothsignature, encryption, or both

forming a MIME wrapped PKCS objectforming a MIME wrapped PKCS object have a range of content-types:have a range of content-types:

enveloped dataenveloped data signed datasigned data clear-signed dataclear-signed data registration requestregistration request certificate only messagecertificate only message

S/MIME Certificate S/MIME Certificate ProcessingProcessing

S/MIME uses X.509 v3 certificatesS/MIME uses X.509 v3 certificates managed using a hybrid of a strict X.509 managed using a hybrid of a strict X.509

CA hierarchy & PGP’s web of trustCA hierarchy & PGP’s web of trust each client has a list of trusted CA’s certseach client has a list of trusted CA’s certs and own public/private key pairs & certsand own public/private key pairs & certs certificates must be signed by trusted CA’scertificates must be signed by trusted CA’s

Certificate AuthoritiesCertificate Authorities

have several well-known CA’shave several well-known CA’s Verisign one of most widely usedVerisign one of most widely used Verisign issues several types of Digital IDsVerisign issues several types of Digital IDs increasing levels of checks & hence trustincreasing levels of checks & hence trust

ClassClass Identity ChecksIdentity Checks UsageUsage

11 name/email checkname/email check web browsing/emailweb browsing/email

22 + enroll/addr check+ enroll/addr check email, subs, s/w email, subs, s/w validatevalidate

33 + ID documents+ ID documents e-banking/service e-banking/service accessaccess

SummarySummary

have considered:have considered: secure emailsecure email PGPPGP S/MIMES/MIME