Network Security: Email Security, PKI

Tuomas AuraT-110.5240 Network security

Aalto University, Nov-Dec 2011

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Outline1. Email security2. Pretty Good Privacy (PGP)3. Certificates4. PGP web of trust5. X.509 public-key infrastructure (PKI)

Only a quick look at advanced aspects.Students are assumed to be familiar with X.509.

6. Key-oriented PKI

Email security

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Security requirementsWhat kind of security is needed for email?

Confidentiality?Authentication?Non-repudiation? Mandatory access control, DRM?Spam control?Phishing prevention? Anonymity?

We use email security as the first example because it is a fairly straightforward application of crypto and allows us to introduce many basic concepts

Crypto does not solve all email security problems

PGP, S/MIME

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Internet email architecture

Alice sends mail to bob@contoso.comWhere are the security vulnerabilities?

Serverexchange.example.com

Servermail.contoso.com

Internet

Receiver bob@contoso.com

Sender alice@example.com

SMTP or proprietary

protocol

IMAP, POP3, webmail, or proprietary

protocolSMTP

DNSQuery for

MX record of contoso.com /

Response: mail.contoso.com

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Order of signing, compression and encryption

Opinions?Observations:

Signing without seeing content is dangerous → sign the plaintextSigning an encrypted message does not prove that the signer knows the content → sign the plaintextEncryption only protects secrecy; in theory, ciphertext might decrypt to multiple different plaintexts → sign the plaintextSignature or MAC might reveal something about message contents → encrypt also the signature or MACCiphertext does not compress → compress before encryptionDecompression might not guarantee unambiguous output in the presence of a malicious influence → sign the uncompressed plaintextForwarding email → encrypt outside signing Receiver might want to decompress or recompress the signed data for storage; authentication of compressed messages prevents that → compress email after authentication

Typical order: sign, compress, encryptExceptions common but need a good justification

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Sign, compress, encrypt

Sender and receiver need to know each other’s public keysOptions to encrypt only or to sign only:

Possible to sign without knowing receiver’s public key, or when sending to a mailing listPossible to encrypt without identifying sender

Hash

M

Alice’s private

key PK-1A

Insecurenetwork

Sender Alice Receiver Bob

Sign

ESK(Z(M, SignA(M))), EB(SK)Encrypt

(symm.)

Encrypt (asymm.)

Bob’spublic

Key PKB

h(M)

SignA(M) EB(SK)

Fresh random session key SK

Authentication Encryption

Hash

Alice’s public

key PKA

Verify

Decrypt (symm.)

Decrypt (asymm.)

Bob’sprivate

Key PK-1B

SK

AuthenticationDecryption

EB(SK)

h(M)

M

SignA(M)

|| || split split MCompress De-compress

Ok?

Pretty Good Privacy (PGP)

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Example: PGP-encrypted message

“Meet me in the park at 6 PM.”

-----BEGIN PGP MESSAGE----- Version: GnuPG v1.4.8 Comment: Encrypted secret message.

hQEOA1e+1x6YuUMCEAQAoST1l/obnXOB6fhIhmLnGVLhuxmsksKD+Efyk7ja9gOx U5X98/25ZVDQz0EiOkRjW2LChuZt9Kesh1DSIRwB/llXCm3pbNX/V+ajkL4Fzxlw jWCCedv527SUNTUP70lhLbh4O2kHHxMdEn41zVo9TPUgtQ1BIo32k/xP2RYtPCEE AJDhcyp+COLaI4idibfSrDDtYcT+hVVFVveIteTIcznoUoS1yVyipE4mBwa380c6 TiwImq63hOhs62c9BOQv7G9cnaqEZNg0nLiVZD+K/JeN00zILm+TzdWZxrW019nA +tsMwznUZ2V/kQZjS9xkPWjn7ZzPTyW6gLhjWQNlr93S0lcBT0CJy285ixFz9UrJ qjK2azsBdXRcVuXFdh84LW1E/8/8DwdLgSK9X/jPNv3/WGLA4Ez2xTFIUorVi5Xe M9dpriEQ0Jg2msnz2bjqRGZliXXo6m8ye/A= =YWDi -----END PGP MESSAGE-----

Random block (IV)+ 2 repeating bytes

Signing time

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Typical PGP message

Recipient key idSession key SK

2 bytes of hash

Data

PK-encryptedsession key EB(SK)

(may repeat for multiple recipients)

ESKZip

EB

Radix-64

=ojUK-----END PGP MESSAGE-----

-----BEGIN PGP MESSAGE-----Version: GnuPG v1.4.8

EncryptedMessage

ESK(…)

Signature type

Signer key id

RSA/DSA signature

For quick checkof the hash

For quick check of successful decryption

Hash &SignA

ASCII armor headersfor sending in text email

ASCII armor checksum

Unix time (seconds since 1 Jan 1970 UTC)

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Radix-64 encodingUse safe ASCII characters to represent values 0..63:ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvxyz0123456789+/

Encode each 3 bytes as 4 characters:+--first octet--+-second octet--+--third octet--+ |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0| +-----------+---+-------+-------+---+-----------+ |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0| +--1.char---+--2.char---+--3.cahr---+--4.char---+

If the data length is not divisible by 3, pad with one or two = characters to indicate actual length

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S/MIMEPGP is mainly used by private persons and academiaS/MIME is a similar standard used primarily by enterprises, e.g. OutlookMessage structure based on the MIME standards Envelopes and signatures are new MIME types→ Base64 encoding

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Non-repudiationProof of authenticity to third parties

Email sender cannot later deny sending the message (i.e. cannot repudiate the message)Third party, such as a judge, is needed to make the decisionThe public key must be somehow registered to bind it to the person signing

Uses:Accountability for sent messagesContract signing

Questions:Does the sender of an email want to go do extra work in order to be accountable for the emails he sends? → Little motivation to sign messagesAre business contracts signed using secure email?

Certificates

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Public-key certificatesSignature verifier must know the signer’s public key. For example, how to verify TA, M, SA(TA, M) ?Identity certificate is a signed message issued by a trusted entity, which binds a name and a public key to each other:CertA = A, PKA, Texpiry, Sissuer(A, PKA, Texpiry)To verify message TA, M, SA(TA, M), CertA

: Verify signature SA with PKA

Check freshness of the timestamp TA

Verify the certificate, and check TA < Texpiry

Security protocols often assume that the protocol participants have certificates, but who issues them?

PGP web of trust

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PGP key distributionPGP users need to know each other’s public keys. But how to verify they are authentic?

Need to verify only the key fingerprint (hash value)Personal verification: ask the person, print on business cards, etc.PGP key ring signed by someone you trust

PGP key ring contains public key, trust level, user id or name and one or more signatures. Each signature includes assurance level

Meaning: signers say that the public key belongs to the user

Trust levels: none, partial trust, complete trustMeaning: level of belief that entity tells the truth when it signs key rings Signer can use this parameter to recommend the key owner as a person of high integrity to sign keys for others

Assurance levels: unspecified, no, casual, heavy-dutyMeaning: how carefully did signer check that the key belongs to the user

Idea: assurance of key-person binding and trusting that person to tell the truth (sign keys of others) are separate issues

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PGP web of trust

Which keys should I use for sending confidential mail, for authenticating received mail, for contract signing?

Me, PKme

Alice, PKA

Gab, PKG

David, PKD

Bob, PKB

Eve, PKE

Foo, PKF

Henry, PKH

Iris, PKI

Strong assurance, complete trust

Strong assurance, parial trust

Strong assurance, no trust

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RevocationCertificate revocation:

Anyone who signed a certificate can revoke itSimilar to a certificate but assurance level “revocation”

Key revocation:Key can revoke itself (private key needed for this)Used when private key compromisedRecommendation: sign a key revocation message for your key and store it in a safe place just in case

PGP key servers are email and ftp-based repositories for key rings, including revocationsCertificates may have a validity period, after which revocations can be discarded

Unfortunately, infinite validity is common PGP practice → need to store revocations foreverCommon practice to revoke PGP keys when they are replaced with a new ones → many unnecessary revocations

X.509 public-key infrastructure (PKI)

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Certificate: Data: Version: 3 (0x2) Serial Number: d1:32:5b:f8:d7:09:02:37:50:57:93:55:84:c9:b2:4c Signature Algorithm: sha1WithRSAEncryption Issuer: C=FI, O=Sonera, CN=Sonera Class2 CA Validity Not Before: Nov 19 12:02:09 2009 GMT Not After : Nov 19 12:02:09 2010 GMT Subject: C=FI, O=TKK, OU=Computing Centre, CN=wwwlogin.tkk.fi/emailAddress=webmaster@tkk.fi Subject Public Key Info: Public Key Algorithm: rsaEncryption RSA Public Key: (1024 bit) Modulus (1024 bit): 00:c7:94:9b:49:29:6f:2d:6d:32:70:97:73:39:1e: 04:20:89:ea:05:89:02:01:1a:d7:2d:ad:86:f6:99: 69:7e:13:19:f2:09:d0:e6:05:ca:93:13:a7:e2:7b: 3b:b6:68:e7:49:c7:3b:53:fd:b5:c1:bc:64:65:6c: 4d:89:37:ab:b5:6b:2a:38:2b:45:82:f6:99:97:21: 57:fc:ac:26:9b:04:3b:ad:13:26:8e:85:ff:44:ba: 4f:1e:27:cc:f2:fd:c1:47:c4:de:b6:d2:6c:2c:48: 6e:a3:cc:cd:0c:ed:75:4b:a2:c7:f0:c2:e1:9b:e9: d3:0c:1b:90:35:c8:ee:e7:01 Exponent: 65537 (0x10001) X509v3 extensions: X509v3 Authority Key Identifier: keyid:4A:A0:AA:58:84:D3:5E:3C X509v3 Certificate Policies: Policy: 1.3.6.1.4.1.271.2.3.1.1.2 X509v3 CRL Distribution Points:

URI:ldap://194.252.124.241:389/cn=Sonera%20Class2%20CA,o=Sonera,c=FI?certificaterevocationlist;binary

X509v3 Key Usage: Digital Signature, Key Encipherment

X509v3 Extended Key Usage: TLS Web Server Authentication, TLS Web Client Authentication X509v3 Subject Key Identifier: 86:4C:D0:93:1A:A4:C4:7C:94:A0:28:04:F3:DA:17:12:18:FF:23:D7 Signature Algorithm: sha1WithRSAEncryption 50:c3:94:71:b3:d2:1d:7f:be:71:5e:fe:ff:ec:09:50:68:f0: 27:54:cd:e8:f2:17:90:3e:ea:6c:e2:81:12:bf:e2:73:72:9e: 02:d3:b4:03:88:2a:6a:b1:00:ca:70:24:1b:3f:da:d6:30:46: 0e:db:99:af:65:7d:d9:58:dc:ed:47:36:04:63:c4:f8:c1:22: e6:ab:f6:63:b8:d4:bf:68:a4:13:73:82:4c:94:b1:9c:1f:fb: e8:87:c5:f1:12:e1:73:e0:ef:71:af:8b:85:cf:e3:9e:6e:dd: d1:0b:11:29:ca:ec:24:37:eb:23:df:63:35:f7:9d:b8:13:47: bf:4a:93:c1:a5:0c:b7:1d:8d:1a:61:22:06:56:17:07:f4:f8: 7d:41:92:04:83:f7:6c:f9:9b:94:27:35:11:c5:d2:e6:95:1d: 71:8a:01:bb:f5:2b:c0:21:e3:7c:5a:0a:44:66:98:38:de:b7: 2f:b7:9d:1f:35:95:05:21:03:a2:d3:ba:7d:e2:4e:87:0f:ad: e1:68:cd:b9:c9:d2:21:67:85:32:6f:0b:d0:26:b2:2d:dd:10: 4e:88:30:e3:d7:52:73:f6:eb:ca:4b:28:56:9f:06:25:a8:91: 45:b5:26:93:01:ab:8b:aa:7d:2e:bd:96:5f:39:50:cc:2a:11: f0:69:4d:ff

X.509 certificate exampleSave certificate into a file and pretty print:% openssl x509 -in cert.pem -noout -text

Subject name

Subject public key

Issuer infoValidity dates

Key usage

CA signature…

Revocation list URL

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X.500 namesISO X.500 standard defines hierarchical directory

More advanced than DNS but not widely usedHierarchical names used in X.509 certificates

X.500 names: C = country, S = state, L = locality, O = organization, OU = organization unit, CN = common name

Names used in practice:CN = Tuomas Aura, O = Microsoft Corporation, L = Redmond, S = Washington, C = USCN = Tuomas Aura, OU = UserAccounts, DC = europe, DC = microsoft, DC = comCN = www.bankofamerica.com, OU = DMZUNIXAPPS, O = Bank of America Corporation, L = Charlotte, S = North Carolina, C = US

Hierarchical naming should ensure a 1-to-1 mapping between names and principals (unlike in PGP web of trust); such names are called distinguished names

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ASN.1ASN.1 standard for defining protocol messages

Abstract notation for data structures, protocol messagesBER/DER encoding rules → standardized binary encoding with recursive TLV (type tag, length, value) structureUnambiguous parsing of binary messagesASN.1 specification of protocol messages is directly compiled into C-code for encoding and decoding themEncoded data unreadable to humansMost Internet standards defined in RFCs use more light-weight bit-field or text-based syntax and manually encoded parsers

X.509 certificates are encoded in ANS.1 DER

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ASN.1 exampleASN.1 (from RFC 3280)

PersonalName ::= SET { surname [0] IMPLICIT PrintableString (SIZE (1..ub-surname-length)), given-name [1] IMPLICIT PrintableString (SIZE (1..ub-given-name-length)) OPTIONAL, initials [2] IMPLICIT PrintableString (SIZE (1..ub-initials-length)) OPTIONAL, generation-qualifier [3] IMPLICIT PrintableString (SIZE (1..ub-generation-qualifier-length)) OPTIONAL }

Compare with RFC-style packet diagrams: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |ST | 0 | TYPE | Reserved | n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PrefixLength | Prefix byte 1 | Prefix byte 2 | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | PrefixLength | Prefix byte 1 | Prefix byte 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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OIDOne ASN.1 type is object identifier (OID)

Globally unique identifiers (similar to const or enum but on global scale)Variable length; each organization can get its own prefix

Examples:1.3.6.1.5.5.7.3.1 = TLS server authenticationiso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) keyPurpose(3)1. 2.840.113549.1.1 = RSA algorithm (PKCS#1)iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)

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Commercial CAs and web sitesWeb browsers and OSs have a pre-configured list of root CAs

Multiple roots!Being on the list enables business of selling certificates

Some commercial CAs certify customers’ CAs, some only end entities

Business reasonsSecurity issues of unconstrained delegation

Wildcard names allow multiple servers to share one certificate

E.g. *.contoso.com for www.contoso.com, mail.contoso.comCompromise for cost reasonsNot standard, supported by browsers

VeriSign Class 3 Public Primary CA

VeriSign International Server CA - Class 3

www.bankofamerica.com PKA

GTE CyberTrust Global Root

Microsoft Internet Authority

www.update.microsoft.com

Microsoft Secure Server Authority

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Constrained delegationImportant concept but not widely usedName constraints:

Constrain the authority of a sub-CA to specific subtrees of the name hierarchyExamples: “.microsoft.com” = all MS hosts, “microsoft.com” = one host or all email addresses on that hostPermitted and excluded subtreesName constraints apply to Subject and SubjectAltName

Path length constraints limit the depth of the CA hierarchyPolicy constraints control policies of sub-CAsImportant idea but no agreement on which constraints to implement

GTE CyberTrust Global Root

Microsoft Internet Authority

www.update.microsoft.com

Microsoft Secure Server Authority

Name and identityWith certificates, it is possible to authenticate the name or identifier of an entity

e.g. person, computer, web server, email address

What is the right name anyway?wwwlogin.tkk.fi, security.tkk.fi, leakybox.cse.tkk.fiGeorge Bush, George W. Bush, George H. W. Bushtuomas.aura@aalto.fi, aura@cs.hut.fi, aaura@hut.fi, taura@cse.tkk.fi, aura@cse.tkk.fi

Who decides who owns the name?aalto.firecent case: Ville Valo on Facebook

Identity proofing is the process with which CA verifies the name of the subject before issuing the certificate

Email to an address from the WHOIS databaseExtended validation certificatesStrong identity

Does knowing the name imply trust?Should I order a second-hand camera from buycam.fi?Should they post the camera to Tuomas Aura?

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Key-oriented PKIs

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Observations about PKIsIn communication, principals are always represented by their public keys→ let’s redefine: principal = public signature keyX.509 puts too much emphasis on names. What does a name mean anyway? What matters is access controlOrange Book definition: access control = authentication + authorization. But authentication of public keys is easy → focus on authorization No global X.509 CA hierarchy exists must live with local CAs → names have only local meaningSince the local authority is a principal, which is a key, all names are relative to a key

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Key-oriented PKIsInfluential research ideas, but no standards yetSimple distributed security infrastructure (SDSI):

Linked local name spaces: my doctor’s secretary, PKB’s AliceName certificates: PK says PK’s doctor is PKD , PKB says PKB’s Alice is PKA Algebra of names and name certificates

Simple public-key infrastructure (SPKI):Focus on delegation of access rights between public keysAlgebra of authorization certificates (5-tuples of <issuer, subject, authority, validity, delegation>)ACL in a server is the root of all authority: PK is allowed to read fileAAuthorization certificate: PK delegates to PK2 the right to read fileA

PolicyMaker:Certificates are programs. They are evaluated by letting them run with the access request and each other as inputLater version called KeyNote is more constrained and similar to SPKI

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Example of SPKI authorizationACL on a web page: <me, PKsales, read/write, “always”, delegate=yes>

Sales department delegates to Alice:<PKsales, PKA, read, “this week”, delegate=yes>

Alice delegates Bob:<PKA, PKB, read/write, “this week”, delegate=no>

Who can access the web page and how?Delegation path me→PKsales→PKA→PKB Constrains accumulate along the path→ PKA (Alice’s key) can read the page this week, and can delegate further→ PKB (Bob’s key) can read this week but not delegate further

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Example of SDSI name resolutionACL in file server: SalesDept’s Salesman can read fileA

Local policy in server: my SalesDept is Contoso’s SalesDeptmy Contoso is PKContoso

Head office issues a name certificate: PKContoso’s SalesDept is PKsales

(signed by PKContoso)

Sales department issues name certificates:PKsales’s Salesman is PKA

(signed by PKsales)PKsales’s Salesman is PKB

(any name can be a group!)

Bob requests fileA from the server. He signs the request with PKB.Server resolves the name in the ACL: SalesDept’s Salesman = Contoso’s SalesDept’s Salesman = PKContoso’s SalesDept’s Salesman = PKsales’s Salesman = {PKA, PKB}→ access allowed

SPKI and SDSI were merged into one system that is not so simple any more

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Alternatives to PKINot all authentication is based on a PKI. Other “trust roots”:

Manual key distribution, e.g. for permanent IPsec tunnel or RADIUSPassword authentication of human usersOnline authentication servers, e.g. KerberosPseudonymity — create new id created for each service and authenticate returning usersLeap of faith -– assume there is no attacker on the first time, e.g. SSHSelf-certifying identifiers — public key as identifier

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Puzzle of the dayNeedham-Schroeder key distribution protocol (1978):Trusted server T shares a secret master key with each user. T distributes a fresh session using the master keys:

1. A → T: A, B, NA1 “Hi, I’m A and would like to talk with B.”2. T → A: ETA(NA1, B, Kses, ticketAB)3. A → B: ticketAB, Eses(NA2)

4. B → A: Eses(NA2-1, NB)5. A → B: Eses(NB-1)

A, B, T = entity namesKTA, KTB = A’s and B’s master keys shared with T NA1, NA2, NB = A’s and B’s nonces i.e. fresh random bit string generated by A and BKses = session key selected by T (fresh random bit string)ETA , ETA , Eses = encryption with the master or session key ticketAB = ETB(Kses,A) “Here is a session key between you and A.”

Encryption is assumed to also protect integrity. Nowadays, we would use standard MACs but they did not exist in the 70s. For example:

EK(M) = AES-CBCK(M, HMAC-SHA-1K(M))

Goal: A and B agree on a session key and authenticate each otherCan you see anything wrong with this protocol? All the messages are sent through the insecure network.

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Related readingWilliam Stallings, Network security essentials: applications and standards, 3rd ed.: chapters 4.2-4.3, 5William Stallings, Cryptography and Network Security, 4th ed.: chapters 14.2-14.3, 15Kaufmann, Perlman, Speciner, Network security, 2nd ed.: chapters 15, 20-22

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ExercisesHow to prevent SMTP spoofing without end-to-end cryptography? What can be filtered at SMTP servers and what cannot? Does signing of emails help spam control?Install an OpenPGP implementation (e.g. GPG). How do you check that the binary or source code has not been tampered with? Would you use PGP itself to verify the signature or fingerprint on the installation package? Set up your own CA e.g. using OpenSSL and issue certificates to your own web server or some other service that uses TLS/SSL authentication. What decisions did you have to make on the way? What open questions do you have after the experience?Consider setting up a PKI in a place where you have worked/studied. How would you distribute the root key and organize certificate enrolment?How would you implement access control in a distributed file system with SPKI (with SDSI)?