1. K E Y Plain text Cipher text Encryption Decryption
Encryption algorithm
Encryption
Decryption

2. K E Y K E Y Recipients public key Original message Encrypted message
Encryption algorithm
Recipients secret key K E Y
Original message
Decryption algorithm

3. Sender’s
Private key
Message
Message
Digital Signature
Sender’s
Public key
Message
Digital Signature

4. Alpha Certification Authority Inc.
Name: Kamran Ahmad
Public Key:
Date of Issue:January 15, 2003
Date of Expiry: January 15, 2005
Serial Number:
We certify that the above information is true.
Alpha Certification Authority Inc.

5. Root Certificate Authority
User
CA 1
CA 2
User
CA 3
CA 4

6. f(r,d) = r.ed (mod m)

7. char XORhash( char *key, int len){
char hash;
int i;
for (hash=0, i=0; i<len; ++i) hash=hash^key[i];
return (hash%101); /* 101 is prime */ }

8. 1967…. Public key receiver Together, we shall make Pakistan strong ….
Private key sender

9. Block Statement Ends Here
Confidential message
a7u/b34+…
Block Statement Starts Here
3uk7b/…
Block Statement Ends Here
Digital Signature

10. Private Key Receiver
a7u/b34+…
3uk7b/…
Public Key Sender

11. Together, we shall make
Pakistan strong ….
1967….

12. The Process of Sending Messages Using Public Key Cryptography
How Digital Signature Technology Works?
The Process of Sending Messages Using Public Key Cryptography
Hash
Public Key receiver
Private Key receiver
Hash
Sender
Receiver
Original
Message
Internet
Original
Message
Scrambled
Message
Scrambled+Signed
Message
Message Digest + Private Key of sender
Public Key sender to reveal Message Digest

13. Steps Involved in Digital Signature Process
1. Hash function is applied to the original message in order to find the message digest
2. Public Key of the receiver is used to encrypt the message
3. A digital signature is attached to the scrambled message by signing the message digest with Private Key of the sender
4. The encrypted message, the digital signature and the hash function are sent to the receiver
5. Public Key of the sender is used by the receiver to reveal the message digest and, thus, to confirm identity/authenticity of the sender
6. Receiver uses his/her Private Key to decrypt the message
7. Receiver applies hash function to the received original message and computes the message digest – if this message digest matches with the one received from the sender, it confirms that the message has not been altered during transmission

14. The Process of Sending Messages Using Public Key Cryptography
How Digital Signature Technology Works?
The Process of Sending Messages Using Public Key Cryptography
Hash
Symmetric Key
Symmetric Key
Hash
Sender
Receiver
Original
Message
Internet
Original
Message
Scrambled
Message
Scrambled+Signed
Message
Message Digest + Private Key of sender
Public Key sender to reveal Message Digest

15. Imran + Private Key of sender
Symmetric Key
Symmetric Key
Original
Message
Internet
Scrambled
Message
Scrambled+Signed
Message
Imran + Private Key of sender
Public Key sender to reveal the word Imran

16. Public Key Infrastructure (PKI)

17. Key Length
If a key is 1 bit in length it means two possible keys, that is, 0 and 1. If a key is 2 bits in length it means four possible key values, 00,01,10 and 11
Key having 3 bits length can have values -000,001,010,011,100,101,110,111
Number of keys = 2(number f bits)