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    Key Management

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    SubhiAlkourde
    Cryptography and network security focuses on key management and generation as well as key exchange protocols. There are several methods for distributing public keys including public announcement, publicly available directories, public key authorities, and public key certificates. Public key certificates allow key exchange without real-time access to an authority by binding identity to a public key with a digital signature. Diffie-Hellman key exchange is a method to securely establish a shared secret over an insecure channel and relies on the difficulty of computing discrete logarithms.

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    Key Management

    Uploaded by

    SubhiAlkourde
    148 views24 pages
    Cryptography and network security focuses on key management and generation as well as key exchange protocols. There are several methods for distributing public keys including public announcement, publicly available directories, public key authorities, and public key certificates. Public key certificates allow key exchange without real-time access to an authority by binding identity to a public key with a digital signature. Diffie-Hellman key exchange is a method to securely establish a shared secret over an insecure channel and relies on the difficulty of computing discrete logarithms.

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    Key Management

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    Cryptography and network security focuses on key management and generation as well as key exchange protocols. There are several methods for distributing public keys including public announcement, publicly available directories, public key authorities, and public key certificates. Public key certificates allow key exchange without real-time access to an authority by binding identity to a public key with a digital signature. Diffie-Hellman key exchange is a method to securely establish a shared secret over an insecure channel and relies on the difficulty of computing discrete logarithms.

    Copyright:

    © All Rights Reserved
    Download as PPT, PDF, TXT or read online from Scribd
    Download as ppt, pdf, or txt
    148 views24 pages

    Key Management

    Uploaded by

    SubhiAlkourde
    Cryptography and network security focuses on key management and generation as well as key exchange protocols. There are several methods for distributing public keys including public announcement, publicly available directories, public key authorities, and public key certificates. Public key certificates allow key exchange without real-time access to an authority by binding identity to a public key with a digital signature. Diffie-Hellman key exchange is a method to securely establish a shared secret over an insecure channel and relies on the difficulty of computing discrete logarithms.

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    © All Rights Reserved
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    of 24
    At a glance
    Powered by AI
    The key takeaways are about public key cryptography and different methods for key distribution and exchange.

    The different methods for distributing public keys discussed are public announcement, publicly available directory, public-key authority, and public-key certificates.

    The Diffie-Hellman key exchange protocol allows two parties to jointly establish a shared secret key over an insecure communication channel without any prior secrets.

    Cryptography and Network

    Key Management and generation

    Cryptography and Network Security

    Key Exchange
    Public key systems are much slower than

    private key system

    Public key system is then often for short data


    Signature, key distribution

    Key distribution

    One party chooses the key and transmits it to other user

    Key agreement

    Protocol such two parties jointly establish secret key over


    public communication channel
    Key is the function of inputs of two users

    Cryptography and Network Security

    Distribution of Public Keys


    can be considered as using one of:
    Public

    announcement
    Publicly available directory
    Public-key authority
    Public-key certificates

    Cryptography and Network Security

    Public Key Management


    Simple one: publish the public key
    Such

    as newsgroups, yellow-book, etc.


    But it is not secure, although it is convenient
    Anyone can forge such a announcement
    Ex: user B pretends to be A, and publish a key for A
    Then all messages sent to A, readable by B!

    Let trusted authority maintain the keys

    Need to verify the identity, when register keys


    User can replace old keys, or void old keys

    Cryptography and Network Security

    Possible Attacks
    Observe all messages over the channel
    So

    assume that all plaintext messages are available to

    all
    Save messages for reuse later

    So have to avoid replay attack

    Masquerade various users in the network

    So have to be able to verify the source of the message

    Cryptography and Network Security

    Public Announcement
    users distribute public keys to recipients

    or broadcast to community at large

    eg. append PGP (pretty good privacy) keys to email


    messages or post to news groups or email list

    major weakness is forgery

    anyone can create a key claiming to be someone else


    and broadcast it
    until forgery is discovered can masquerade as claimed
    user

    Cryptography and Network Security

    Publicly Available Directory


    can obtain greater security by registering

    keys with a public directory


    directory must be trusted with properties:
    contains {name,public-key} entries
    participants register securely with directory
    participants can replace key at any time
    directory is periodically published
    directory can be accessed electronically

    still vulnerable to tampering or forgery


    Cryptography and Network Security

    Public-Key Authority
    improve security by tightening control over

    distribution of keys from directory


    has properties of directory
    and requires users to know public key for
    the directory
    then users interact with directory to
    obtain any desired public key securely
    does

    require real-time access to directory when keys are


    needed
    Cryptography and Network Security

    Public-Key Authority

    Cryptography and Network Security

    Cont.
    More advanced distribution
    A

    sends request-for-key(B) to authority with timestamp, that is, Ida|Idb|Time


    Authority replies with key(B) (encrypted by its private
    key), that is EKTta(KUb| Ida|Idb|Time)
    A initiates a message to B, including a random number
    Na, its IDA
    B then ask authority to get key(A)
    B sends A (encrypted by As public key) Na and Nb
    A then replies B Nb encrypted by Bs public key
    Cryptography and Network Security

    10

    Cont.
    In above scheme, the authority is

    bottleneck
    New approach: certificate

    Any user can read certificate, determine name and


    public key of the certificates owner
    Any user can verify the authority of certificate
    Only the authority can create and update certificate
    Any user can verify the time-stamp of certificate

    The certificate is
    CA=EKRauth[T,IDA, KUA], where the EKRauth is the private key
    used by the authority.

    Time-stamp is to avoid reuse of voided key

    Cryptography and Network Security

    11

    Public-Key Certificates

    Cryptography and Network Security

    12

    Public-Key Certificates
    certificates allow key exchange without real-time access to

    public-key authority
    a certificate binds identity to public key

    usually with other info such as period of validity, rights of use etc

    with all contents signed by a trusted Public-Key or

    Certificate Authority (CA)


    can be verified by anyone who knows the public-key
    authorities public-key
    To validate the certificate, we need another certificate, one
    that matches the Issuer (of CA) in the first certificate.
    Then we take the RSA public key from the second (CA)
    certificate, use it to decode the signature on the first
    certificate to obtain an MD5 hash, which must match an
    actual MD5 hash computed over the rest of the certificate.

    Cryptography and Network Security

    13

    X.509
    The structure of a X.509 v3 digital certificate is as follows:
    Certificate
    Version
    Serial Number
    Algorithm ID
    Issuer
    Validity
    Not Before
    Not After

    Subject
    Subject Public Key Info

    Public Key Algorithm


    Subject Public Key

    Issuer Unique Identifier (Optional)


    Subject Unique Identifier (Optional)
    Extensions (Optional)
    ...

    Certificate Signature Algorithm


    Certificate Signature
    Cryptography and Network Security

    14

    Sample Certificate

    Certificate:
    Data: Version: 1 (0x0)
    Serial Number: 7829 (0x1e95)
    Signature Algorithm: md5WithRSAEncryption
    Issuer: C=ZA, ST=Western Cape, L=Cape Town, O=Thawte Consulting cc, OU=Certification Services
    Division, CN=Thawte Server CA/emailAddress=server-certs@thawte.com
    Validity

    Not Before: Jul 9 16:04:02 1998 GMT


    Not After : Jul 9 16:04:02 1999 GMT

    Subject: C=US, ST=Maryland, L=Pasadena, O=Brent Baccala, OU=FreeSoft,


    CN=www.freesoft.org/emailAddress=baccala@freesoft.org
    Subject Public Key Info: Public Key Algorithm: rsaEncryption
    RSA Public Key: (1024 bit)
    Modulus (1024 bit): 00:b4:31:98:0a:c4:bc:62:c1:88:aa:dc:b0:c8:bb:
    33:35:19:d5:0c:64:b9:3d:41:b2:96:fc:f3:31:e1: 66:36:d0:8e:56:12:44:ba:75:eb:e8:1c:9c:5b:66:
    70:33:52:14:c9:ec:4f:91:51:70:39:de:53:85:17: 16:94:6e:ee:f4:d5:6f:d5:ca:b3:47:5e:1b:0c:7b:
    c5:cc:2b:6b:c1:90:c3:16:31:0d:bf:7a:c7:47:77: 8f:a0:21:c7:4c:d0:16:65:00:c1:0f:d7:b8:80:e3:
    d2:75:6b:c1:ea:9e:5c:5c:ea:7d:c1:a1:10:bc:b8: e8:35:1c:9e:27:52:7e:41:8f
    Exponent: 65537 (0x10001)
    Signature Algorithm: md5WithRSAEncryption
    93:5f:8f:5f:c5:af:bf:0a:ab:a5:6d:fb:24:5f:b6:59:5d:9d:
    92:2e:4a:1b:8b:ac:7d:99:17:5d:cd:19:f6:ad:ef:63:2f:92:
    ab:2f:4b:cf:0a:13:90:ee:2c:0e:43:03:be:f6:ea:8e:9c:67:
    d0:a2:40:03:f7:ef:6a:15:09:79:a9:46:ed:b7:16:1b:41:72:
    0d:19:aa:ad:dd:9a:df:ab:97:50:65:f5:5e:85:a6:ef:19:d1:
    5a:de:9d:ea:63:cd:cb:cc:6d:5d:01:85:b5:6d:c8:f3:d9:f7:
    8f:0e:fc:ba:1f:34:e9:96:6e:6c:cf:f2:ef:9b:bf:de:b5:22: 68:9f

    Cryptography and Network Security

    15

    Public-Key Distribution of Secret


    Keys
    use previous methods to obtain public-key

    can use for secrecy or authentication


    but public-key algorithms are slow
    so usually want to use private-key

    encryption to protect message contents


    hence need a session key
    have several alternatives for negotiating a
    suitable session

    Cryptography and Network Security

    16

    Simple Secret Key Distribution


    proposed by Merkle in 1979
    A

    generates a new temporary public key pair


    A sends B the public key and their identity
    B generates a session key K sends it to A encrypted
    using the supplied public key
    A decrypts the session key and both use
    problem is that an opponent can intercept

    and impersonate both halves of protocol

    Cryptography and Network Security

    17

    Secret key Distribution


    Simple secret key distribution
    A

    generates KUA and KRA, sends KUA to B


    B generates a secret key ks
    B sends ks to A using As public key KUA
    A decrypts the message to get the secret key ks

    To get more security, the public/private

    keys can be regenerated when needed


    But vulnerable to the active attack!

    Attacker E can compromise the communication


    between A and B as follows
    Cryptography and Network Security

    18

    Cont.
    Attacking
    A

    generates KUA and KRA, sends IDA, KUA to B


    E intercepts the message, transmits IDA, KUE to B
    B generates a secret key ks
    B sends ks to A using As public key KUE
    E intercepts the message, decrypt it and get ks
    E sends A the message Ks, encrypted by KUA
    A decrypts the message to get the secret key ks

    Now E knows Ks, but A, B are unaware of it

    Cryptography and Network Security

    19

    Secret Key Distribution


    So need confidentiality and authentication
    A

    and B need to use a secure method to exchange their


    public keys

    Schemes

    A initiates a message to B, EKUB(Na,IDa)


    B replies it with EKUA(Na,Nb)
    A then replies it with EKUB(Nb)
    A sends B the message EKUB (EKRA(Ks))

    Security

    The first 3 steps are used to assure that A is A, B is B


    Cryptography and Network Security

    20

    Public-Key Distribution of Secret


    Keys
    if have securely exchanged public-keys:

    Cryptography and Network Security

    21

    Diffie-Hellman Key Predist.


    Note:Please check your book and your class notes for this
    algorithm.

    Computationally secure

    if discrete logarithm is intractable

    Scheme

    Assume prime number p public and an integer c public


    Each user u has secret component au
    User u computes bu=c au mod p
    TA certifies it by computing

    (ID(u), bu, sigTA(ID(u), bu))


    The

    common key of two users u and v is

    K=c au av mod p
    Cryptography and Network Security

    22

    Diffie-Hellman Key Exchange


    Computationally secure
    if

    discrete logarithm is intractable

    Scheme

    Assume prime number p public and an integer c public


    Each user u chooses a secret component au (new!)
    User u computes bu=c au mod p
    User v computes bv=c av mod p
    The common key of two users u and v is

    K=c au av mod p

    Cryptography and Network Security

    23

    Diffie-Hellman Problem
    Diffie-Hellman problem definition
    Given bu=gau

    mod p, bv=gav mod p, how to compute


    gavau mod p? Here g is a primitive element of mod p
    The problem is not harder than the discrete logarithmetic problem, because the later one can always be
    used to solve it
    It can be proved that it has the same difficulty as the
    ElGamal encryption system

    Cryptography and Network Security

    24

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