Information and Network Security:31�Public-Key Cryptography

Prof Neeraj Bhargava

Vaibhav Khanna

Department of Computer Science

School of Engineering and Systems Sciences

Maharshi Dayanand Saraswati University Ajmer

Private-Key Cryptography

• traditional private/secret/single key cryptography uses one key
• shared by both sender and receiver
• if this key is disclosed communications are compromised
• also is symmetric, parties are equal
• hence does not protect sender from receiver forging a message & claiming is sent by sender

• The development of public-key cryptography is the greatest and perhaps the only true revolution in the entire history of cryptography.
• From its earliest beginnings to modern times, virtually all cryptographic systems have been based on the elementary tools of substitution and permutation, and can be classed as private/secret/single key (symmetric) systems.
• All classical, and modern block and stream ciphers are of this form.

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Public-Key Cryptography

• probably most significant advance in the 3000 year history of cryptography
• uses two keys – a public & a private key
• asymmetric since parties are not equal
• uses clever application of number theoretic concepts to function
• complements rather than replaces private key crypto

Public-Key Cryptography

• Will now discuss the radically different public key systems, in which two keys are used. Public-key cryptography provides a radical departure from all that has gone before.
• The development of public-key cryptography is the greatest and perhaps the only true revolution in the entire history of cryptography.
• It is asymmetric, involving the use of two separate keys, in contrast to symmetric encryption, that uses only one key.
• Anyone knowing the public key can encrypt messages or verify signatures, but cannot decrypt messages or create signatures, counter-intuitive though this may seem.

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Public-Key Cryptography

• The use of two keys has profound consequences in the areas of confidentiality, key distribution, and authentication.
• It works by the clever use of number theory problems that are easy one way but hard the other.
• Note that public key schemes are neither more nor less secure than private key (security depends on the key size for both), nor do they replace private key schemes (they are too slow to do so), rather they complement them.
• Both also have issues with key distribution, requiring the use of some suitable protocol.

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Why Public-Key Cryptography?

• developed to address two key issues:
• key distribution – how to have secure communications in general without having to trust a KDC with your key
• digital signatures – how to verify a message comes intact from the claimed sender
• public invention due to Whitfield Diffie & Martin Hellman at Stanford Uni in 1976
• known earlier in classified community

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Why Public-Key Cryptography?

• The concept of public-key cryptography evolved from an attempt to attack two of the most difficult problems associated with symmetric encryption: key distribution and digital signatures.
• The first problem is that of key distribution, which under symmetric encryption requires either (1) that two communicants already share a key, which somehow has been distributed to them; or (2) the use of a key distribution center.
• This seemed to negated the very essence of cryptography: the ability to maintain total secrecy over your own communication.
• The second was that of "digital signatures." If the use of cryptography was to become widespread, not just in military situations but for commercial and private purposes, then electronic messages and documents would need the equivalent of signatures used in paper documents.

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Why Public-Key Cryptography?

• The idea of public key schemes, and the first practical scheme, which was for key distribution only, was published in 1976 by Diffie & Hellman.
• The concept had been previously described in a classified report in 1970 by James Ellis (UK CESG) - and subsequently declassified [ELLI99].
• Its interesting to note that they discovered RSA first, then Diffie-Hellman, opposite to the order of public discovery! There is also a claim that the NSA knew of the concept in the mid-60’s [SIMM93].

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Public-Key Cryptography

• public-key/two-key/asymmetric cryptography involves the use of two keys:
• a public-key, which may be known by anybody, and can be used to encrypt messages, and verify signatures
• a related private-key, known only to the recipient, used to decrypt messages, and sign (create) signatures
• infeasible to determine private key from public
• is asymmetric because
• those who encrypt messages or verify signatures cannot decrypt messages or create signatures

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• Asymmetric algorithms rely on one key for encryption and a different but related key for decryption. These algorithms have the following important characteristic:
• It is computationally infeasible to determine the decryption key given only knowledge of the cryptographic algorithm and the encryption key.
• In addition, some algorithms, such as RSA, also exhibit the following characteristic:
• Either of the two related keys can be used for encryption, with the other used for decryption.
• Anyone knowing the public key can encrypt messages or verify signatures, but cannot decrypt messages or create signatures, thanks to some clever use of number theory.

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Public-Key Cryptography

Public-Key Cryptography

• Stallings Figure “Public-Key Cryptography”, shows that a public-key encryption scheme has six ingredients:
• • Plaintext: the readable message /data fed into the algorithm as input.
• • Encryption algorithm: performs various transformations on the plaintext.
• • Public and private keys: a pair of keys selected so that if one is used for encryption, the other is used for decryption. The exact transformations performed by the algorithm depend on the public or private key that is provided as input.

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Public-Key Cryptography

• • Ciphertext: the scrambled message produced as output. It depends on the plaintext and the key. For a given message, two different keys will produce two different ciphertexts.
• • Decryption algorithm: accepts the ciphertext and matching key and produces the original plaintext.

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• Consider the following analogy using padlocked boxes: traditional schemes involve the sender putting a message in a box and locking it, sending that to the receiver, and somehow securely also sending them the key to unlock the box.
• The radical advance in public key schemes was to turn this around, the receiver sends an unlocked box (their public key) to the sender, who puts the message in the box and locks it (easy - and having locked it cannot get at the message), and sends the locked box to the receiver who can unlock it (also easy), having the (private) key.
• An attacker would have to pick the lock on the box (hard).

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Assignment

• Explain the working of Public-Key Cryptography