# Digital Signature, Digital Certificate

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Digital Signature, Digital Certificate
CSC1720 – Introduction to Internet Essential Materials

Outline Introduction Cryptography Digital Signature
Secret-key algorithms Public-key algorithms Message-Digest algorithms Digital Signature Digital Certificate Public Key Infrastructure (PKI) Secure Electronic Transaction (SET) Summary CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Introduction Cryptography and digital certificates are first appeared in closed commercial, financial network and military systems. We can send/receive secure , connect to secure website to purchase goods or obtain services. Problem: How do we implement them in this global, open network, Internet? To what level of encryption is sufficient to provide safe and trust services on the Net? CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Cryptography 3 cryptographic algorithms: Message-digest algorithms
Map variable-length plaintext to fixed-length ciphertext. Secret-key algorithms Use one single key to encrypt and decrypt. Public-key algorithms Use 2 different keys – public key and private key. CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Keys It is a variable value that is used by cryptographic algorithms to produce encrypted text, or decrypt encrypted text. The length of the key reflects the difficulty to decrypt from the encrypted message. Encryption Decryption Plaintext Ciphertext Key CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Key length It is the number of bits (bytes) in the key.
A 2-bit key has four values 00, 01, 10, 11 in its key space A key of length “n” has a key space of 2^n distinct values. E.g. the key is 128 bits There are 2^128 combinations CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Secret-key Encryption
Use a secret key to encrypt a message into ciphertext. Use the same key to decrypt the ciphertext to the original message. Also called “Symmetric cryptography”. Encryption Decryption Plaintext Ciphertext Secret Key CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Secret Key How to? Original Text Secret key Encrypted Text = +
Encryption Secret key Original Text Encrypted Text = + Decryption CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Secret-Key Problem? All keys need to be replaced, if one key is compromised. Not practical for the Internet environment. On the other hand, the encryption speed is fast. Suitable to encrypt your personal data. CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Secret-Key algorithms
Algorithm Name Key Length (bits) Blowfish Up to 448 DES 56 IDEA 128 RC2 Up to 2048 RC4 RC5 Triple DES 192 References: Blowfish DES IDEA RC2 RC4 RC5 DES-3 CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Public-key Encryption
Involves 2 distinct keys – public, private. The private key is kept secret and never be divulged, and it is password protected (Passphase). The public key is not secret and can be freely distributed, shared with anyone. It is also called “asymmetric cryptography”. Two keys are mathematically related, it is infeasible to derive the private key from the public key. 100 to 1000 times slower than secret-key algorithms. Encryption Decryption Plaintext Ciphertext Public Key Private Key CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

How to use 2 different keys?
Just an example: Public Key = 4, Private Key = 1/4, message M = 5 Encryption: Ciphertext C = M * Public Key 5 * 4 = 20 Decryption: Plaintext M = C * Private Key 20 * ¼ = 5 CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Public-Private Encryption
First, create public and private key Public key Private key Public Key Directory Public Key Public key stored in the directory Private key Private key stored in your personal computer CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Message Encryption (User A sends message to User B)
Public Key Directory User B’s Public Key Encrypted Text Text User A Encryption CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Message Encryption Original Message Encrypted Message
CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Transfer Encrypted Data
User B User A Insecure Channel Encrypted Text Encrypted Text CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

User B’s Private key Private key stored in your personal computer Encrypted Text User B Decryption Original Text CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Asymmetric algorithms
Algorithm Name Key Length (bits) DSA Up to 448 El Gamal 56 RSA 128 Diffie-Hellman Up to 2048 References: DSA El Gamal RSA Diffie-Hellman CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

How difficult to crack a key?
Attacker Computer Resources Keys / Second Individual attacker One high-performance desktop machine & Software 2^17 – 2^24 Small group 16 high-end machines & Software 2^21 – 2^24 Academic Network 256 high-end machines & Software 2^25 – 2^28 Large company \$1,000,000 hardware budget 2^43 Military Intelligence agency \$1,000,000 hardware budget + advanced technology 2^55 Key Length Individual Attacker Small Group Academic Network Large Company Military Inteligence Agency 40 Weeks Days Hours Milliseconds Microseconds 56 Centuries Decades Years Seconds 64 Millennia Minutes 80 Infeasible 128 CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Crack DES-3 (Secret-key)
Distributed.net connects 100,000 PCs on the Net, to get a record-breaking 22 hr 15 min to crack the DES algorithm. Speed: 245 billion keys/s Win \$10,000 CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Message-Digest Algorithms
It maps a variable-length input message to a fixed-length output digest. It is not feasible to determine the original message based on its digest. It is impossible to find an arbitrary message that has a desired digest. It is infeasible to find two messages that have the same digest. CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Message-Digest How to A hash function is a math equation that create a message digest from message. A message digest is used to create a unique digital signature from a particular document. MD5 example Original Message (Document, ) Hash Function Digest CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Message Digest Demo CSC1720 – Introduction to Internet
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Message-Digest Message-Digest Algorithm Digest Length (bits) MD2 128
Secure Hash Algorithm (SHA) 160 References: MD2 MD4 MD5 SHA CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Digital Signature Digital signature can be used in all electronic communications Web, , e-commerce It is an electronic stamp or seal that append to the document. Ensure the document being unchanged during transmission. CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

How digital Signature works?
User A Transmit via the Internet Use A’s private key to sign the document User B received the document with signature attached Verify the signature by A’s public key stored at the directory User B CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Digital Signature Generation and Verification
Message Sender Message Receiver Message Message Hash function Hash function Public Key Digest Private Key Encryption Decryption Signature Expected Digest Digest CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Digital Signature CSC1720 – Introduction to Internet
Reference CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Key Management Private key are password-protected.
If someone want your private key: They need the file contains the key They need the passphrase for that key If you have never written down your passphrase or told anyone Very hard to crack Brute-force attack won’t work CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Digital Certificates Digital Certificate is a data with digital signature from one trusted Certification Authority (CA). This data contains: Who owns this certificate Who signed this certificate The expired date User name & address CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Digital Certificate Reference CSC1720 – Introduction to Internet
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Elements of Digital Cert.
A Digital ID typically contains the following information: Your public key, Your name and address Expiration date of the public key, Name of the CA who issued your Digital ID CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Certification Authority (CA)
A trusted agent who certifies public keys for general use (Corporation or Bank). User has to decide which CAs can be trusted. The model for key certification based on friends and friends of friends is called “Web of Trust”. The public key is passing from friend to friend. Works well in small or high connected worlds. What if you receive a public key from someone you don’t know? CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

CA model (Trust model) Root Certificate CA Certificate CA Certificate
Browser Cert. Server Cert. CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Web of Trust model Bob A B Alice D C
CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Public Key Infrastructure (PKI)
PKI is a system that uses public-key encryption and digital certificates to achieve secure Internet services. There are 4 major parts in PKI. Certification Authority (CA) A directory Service Services, Banks, Web servers Business Users CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Digital 21 . gov .hk Reference: An official homepage
which provides lot of PKI, e-commerce information CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

PKI Structure Certification Authority Directory services
Public/Private Keys Services, Banks, Webservers User CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

4 key services Authentication – Digital Certificate
To identify a user who claim who he/she is, in order to access the resource. Non-repudiation – Digital Signature To make the user becomes unable to deny that he/she has sent the message, signed the document or participated in a transaction. Confidentiality - Encryption To make the transaction secure, no one else is able to read/retrieve the ongoing transaction unless the communicating parties. Integrity - Encryption To ensure the information has not been tampered during transmission. CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Certificate Signers CSC1720 – Introduction to Internet
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Certificate Enrollment and Distribution
CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Secure Web Communication
Server authentication is necessary for a web client to identify the web site it is communicating with. To use SSL, a special type of digital certificate – “Server certificate” is used. Get a server certificate from a CA. E.g. Install a server certificate at the Web server. Enable SSL on the Web site. Client authentication – Client certificates CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Strong and Weak Encryption
Strong encryption Encryption methods that cannot be cracked by brute-force (in a reasonable period of time). The world fastest computer needs thousands of years to compute a key. Weak encryption A code that can be broken in a practical time frame. 56-bit encryption was cracked in 1999. 64-bit will be cracked in 2011. 128-bit will be cracked in 2107. CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Pretty Good Privacy (PGP)
Release in June 1991 by Philip Zimmerman (PRZ) PGP is a hybrid cryptosystem that allows user to encrypt and decrypt. Use session key “a random generated number from the mouse movement or keystrokes” Demo & Tutorial CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

PGP Public Key CSC1720 – Introduction to Internet
Philip R Zimmermann's Public Keys Current DSS/Diffie-Hellman Key: Key fingerprint: 055F C78F C4F 37AF C B2D7 795E -----BEGIN PGP PUBLIC KEY BLOCK----- Version: PGP 7.0.3 mQGiBDpU6CcRBADCT/tGpBu0EHpjd3G11QtkTWYnihZDBdenjYV2EvotgRZAj5h4ewprq1u/zqzGBYpiYL/9j+5XDFcoWF24bzsUmHXsbD Siv+XEyQND1GUdx4wVcEY5rNjkArX06XuZzObvXFXOvqRj6LskePtw3xLf5uj8jPN0Nf6YKnhfGIHRWQCg/0UAr3hMK6zcA/egvWRGsm9d JecD/18XWekzt5JJeK3febJO/3Mwe43O6VNOxmMpGWOYTrhivyOb/ZLgLedqX+MeXHGdGroARZ+kxYq/a9y5jNcivD+EyN+IiNDPD64rl00 FNZksx7dijD89PbIULDCtUpps2J0gk5inR+yzinf+jDyFnn5UEHI2rPFLUbXWHJXJcp0UBACBkzDdesPjEVXZdTRTLk0sfiWEdcBM/5GpNsw MlK4A7A6iqJoSNJ4pO5Qq6PYOwDFqGir19WEfoTyHW0kxipnVbvq4q2vAhSIKOqNEJGxg4DTEKecf3xCdJ0kW8dVSogHDH/c+Q4+RFQ q/31aev3HDy20YayxAE94BWIsKkhaMyokAYQQfEQIAIQUCOlTwWwIHABcMgBE/xzIEHSPp6mbdtQCcnbwh33TcYQAKCRDHRjY5std5Xl e4AKCh1dqtFxD/BiZMqdP1eZYG8AZgTACfU7VX8NpIaGmdyzVdrSDUo49AJae0IlBoaWxpcCBSLiBaaW1tZXJtYW5uIDxwcnpAbWl0LmV kdT6JAFUEEBECABUFAjpU6CcFCwkIBwMCGQEFGwMAAAAACgkQx0Y2ObLXeV5WUQCfWWfTDHzSezrDawgN2Z4Qb7dHKooAoJyV nm61utdRsdLr2e6QnV5Z0yjjiQBGBBARAgAGBQI6VOkSAAoJEGPLaR3669X8JPcAnim4+Hc0oteQZrNUeuMSuirNVUr7AKC1WXJI7gwM q0Agz07hQs++POJBMokARgQQEQIABgUCOlcobQAKCRDXjLzlZqdLMVBtAKDa5VPcb6NVH6tVeEDJUv+tBjp6oACeLoNtfbs2rvJkgKDH WEIDmJdgy2GJAD8DBRA6WP4Y8CBzV/QUlSsRAkmdAKC3TfkSSeh+poPFnMfW+/Y/+AAEEpGSUYAAQEAAAEAAQAA/9sAQwAKBwcI BwYKCAgICwoKCw4YEA4NDQ4dFRYRGCMfJSQiHyIhJis3LyYpNCkhIjBBMTQ5Oz4+PiUuRElDPEg3PT47///EALUQAAIBAwMCBAMFB ………………………………………………………………….. QQEAAABfQECAwAEEQUSITFBBhNRYQcicRQygZk5SVlpeYmZqio6Slpqeoqaqys7S1tre4ubrCw8TFxsfIycrS09TV1tfY2drh4uPk5ebn6On q8fLz9PX29/j5+v/EAB8BAAMBAQEBAQEBAQEAAAAAAAABAgMEBQYHCAkKC//EALURAAIBAgQEAwQHBQQEAAECdwABAgMRBAU hMQYSQVEHYXETIjKBCBRCkaGxwQkjM1LwFWJy0QoWJDThJfEXGBkaJicoKSo1Njc4OTpDREVGR0hJSlNUVVZXWFlaY2RlZmdoaWp zdHV2d3h5eoKDhIWGh4iJipKTlJWWl5iZmqKjpKWmp6ipqrKztLW2t7i5usLDxMXGx8jJytLT1NXW19jZ2uLj5OXm5+jp6vLz9PX29/j5+v/aAA wDAQACEQMRAD8A9mooooAKKKKACsjW/Eum6FGTdS7pcfLEv3j/AIfjWV428XHQrf7HY4e/lHXIxEvqfevH7y8lupXmmuJppWOZJC+AD9 aly7GkIX1Z3OpfE3Up3K2EUVumcdN7fy/pWLL4415wPM1GWPJyNpK/0Fc5btG/Pktkfx7yTVhYAGLsAxbryf5c5rNvzNlG3Q6yz8ZaxEyudQ kcZ+7JtYH867PRfG9nfIsd7/o8p/iI+U/4V5EI/IGV+XUGfnHy9iUsiGSa6q6Jew1XpTDJvAAICDACNUV4K2PS6h574Z3NaBsIQe5jkVO48MS ohjC6s29CjPhlU79cQIYWmBpuNfwroZ6zltyz6Y2Fm65V0IfvVicR7zvFFCOhahMuk1cr+Qp936OMEq9sLZGxTjClgwrHGS7YpMSZrEC7bp OmERjo4F/n5YmCHJCH8QzCOc9+80gjVEsHiJVABrC8yykjKL5x1V/PSArE4QtMLbkBPGmQYOw8bx6jCHoO43QjUzbqRfBMHZqWVJyoII ZCp+n13XM4+NO/cDVsZ8bjch0LIOyMrT85n24yfXRlP0s7BFjLm59Jjhf4djuJWikJawWETlypAy86OYRRuwCbIyNauBeTKy+avZvF2oLvpw H4UnudpC06/O0jkj2lQpn9EEUw11RwO6sq9zYTwAUyKerN00cbCfyiZl01CIo0btcTO6hQK3c67PaloJ9lVH8/mH7LuqkMLDH5ugkpzmed/8 SorfqVkakne6b4mRySFCBXaVZoKmDHzcH2oSSMhM9exyh6dzi1bGu6JAEwEGBECAAwFAjpU6CcFGwwAAAAACgkQx0Y2ObLXeV7lb QCg+N+fI3bzqF9+fB50J5sFHVHM7hYAn0+9AfDl5ncnr4D7 ReMDlYoIZwRR =Bgy+ -----END PGP PUBLIC KEY BLOCK----- CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

PGP encryption Reference CSC1720 – Introduction to Internet
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PGP decryption Reference CSC1720 – Introduction to Internet
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Secure SHell (SSH) Provide an encrypted secure channel between client and server. Replacement for telnet and ftp. Reference: SSH CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

Summary Make sure you understand the relationship between
Encryption Digital Signature Digital Certificate Certificate Authority Understand which Public/Private key should be used to encrypt/decrypt message to/from you? Discuss PGP, SET, SSH, encrypted . CSC1720 – Introduction to Internet All copyrights reserved by C.C. Cheung 2003.

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