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2.1 Basic Cryptography Concepts Z Major Security Aspects ENEE739B Fall 2005 Outline: Basic Security/Crypto Concepts Part 2 Secure Media Communications z Typical scenarios and attacks on secure communications z Kerckhoff principle 2.1 Basic Cryptography Concepts z Major security aspects z Symmetric vs. Asymmetric encryption Min Wu Electrical and Computer Engineering University of Maryland, College Park UMCP ENEE739B Slides (created by M. Wu ©UMCP ENEE739B Slides 2005) ) http://umd.blackboard.com (select ENEE739B); [email protected] Part of the slides are used or revised from material courtesy of Prof. Y. Sun of Univ. Rhode Island. M. Wu: ENEE739B Multimedia Security and Forensics (F'05) M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [2] Crypto Terminologies Basic Secure Communication Scenario z Cryptography: the art of secret writing Encryption Key Decryption Key – The art of mangling information into apparent unintelligibility ciphertext in a manner that allows a secret method of unmangling. Alice Encrypt Decrypt Bob plaintext z Three related terminologies plaintext – Cryptology: The study of communication over non-secure channels, and related problems – Plaintext: the message in its original form – Cryptography: The process of designing systems that – Ciphertext: the mangled information achieve secure communications. – Cryptanalysis: Breaking such systems. –Encryption: the process of produce ciphertext from plaintext Revised from Y. Sun’s Slides @ URI for UMD EE739B F’05 for UMD Y. Sun’s Slides @ URI from Revised (The techniques used to recover the secret information EE739B F’05 for UMD Y. Sun’s Slides @ URI from Revised – Decryption: the process reversing the encryption hidden in cryptographic systems) Encryption and Decryption involve some Algorithm and secret They are often used interchangeably. values (keys) M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [3] M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [5] Adversaries Attack Methods Encryption Key Decryption Key z Ciphertext only UMD EE739B F’05 UMD EE739B F’05 ciphertext – Eve has only a copy of ciphertext Alice Encrypt Decrypt Bob z Known Plaintext plaintext plaintext – Eve has ciphertext and the corresponding plaintext, and tries @ OregonState for @ OregonState for the deduce the key. Eve Mallory Oscar Eve’s Goal z Chosen Plaintext * 1. Read the message Passive – Eve has ciphertext corresponding to some plaintext selected by observer 2. Figure out the key Alice is using and read all the her, believing it useful to deduce the key. “Olive” messages encrypted with that key z Chosen Ciphertext * 3. Modify the content of the message in such a way that Bob will think Alice sent the altered message. Active – Eve has a copy plaintext corresponding to a copy of ciphertext adversary selected by her, believing it is useful to deduce the key. Revised from E.Savas’ ECE575 Material Revised from E.Savas’ ECE575 Material 4. Impersonate Alice and communicate with Bob who “Mallory” thinks he is communicating with Alice. * Possible when Eve gains temporary access to encrypter / decrypter M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [6] M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [7] Kerckhoff’s Principle Major Cryptographic Objectives z Relying on secrecy of the crypto algorithm? z Confidentiality or UMD EE739B F’05 – Hard to quantify the security strength or UMD EE739B F’05 – Hide the contents of a message from unauthorized observer – Main tools: encryption / decryption some thinking process of people may be alike security by may have to abandon the entire system when obscurity z Data integrity compromised al @ OregonState f al @ OregonState f – Ensuring the message sent has not been altered => Should always assume an adversary knows the crypto – Main tools: hash functions to detect tampering algorithm used when assessing a crytosystem’s strength z Authentication: entity identification & data-origin authentication – Correctly verify a user’s identity: through password protocol z The security of a crypo system should be based on – Verify the origin of a message (creator, creation time, etc) – the quality/strength of the algorithm but not its obscurity z Non-repudiation – secrecy of the key over a sufficiently large key space (or key Revised from E.Savas’ ECE575 Materi Revised from E.Savas’ ECE575 Materi – A sender cannot deny a transmitted message or transaction length) M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [8] M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [9] Data Confidentiality Data Integrity z Eve should not be able to read Alice's’ message to Bob. z Bob wants be sure that Alice’s message has not been altered. z The oldest and best known aspect of data security. – Transmission errors may occur – The main tools are encryption / decryption – An adversary might intercept the transmission and alter it. des @ URI for UMD EE739B F’05 for UMD des @ URI EE739B F’05 for UMD des @ URI Revised from Y. Sun’s Sli Revised from Y. Sun’s Sli M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [10] M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [11] User Authentication Data Origin Authentication z Password protocol z Authenticate the information about the origin of the data, – When you log on to a computer, the computer need to such as the creator and time of creation. identify your identity. – Bob wants to make sure that the message is really from Alice, and the message was not a replay of previous z Verify communication partner messages from Alice. – Verify that we are communicating with the right person. Revised from Y. Sun’s Slides @ URI for UMD EE739B F’05 for UMD Y. Sun’s Slides @ URI from Revised EE739B F’05 for UMD Y. Sun’s Slides @ URI from Revised M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [12] M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [13] Non-repudiation Symmetric Key Cryptography z Alice cannot claim that she did not send the message. z Alice and Bob know both the encryption key and the – Suppose Bob takes orders from his customer through e- decryption key. mails. Alice made a order through email and later denied – Encryption key and decryption keys are the same; this purchase. Bob needs to show that Alice did send the email. – The encryption key is shared and the decryption key is easy to be calculated from the encryption key. z Data-origin authentication des @ URI for UMD EE739B F’05 for UMD des @ URI EE739B F’05 for UMD des @ URI vs. non-repudiation z Symmetric cryptosystems: – In a paper and pencil world, – All of the classical (pre-1970) systems non-repudiation is provided – DES and AES by a manual signature z Challenge: Alice and Bob need to agree upon a key. Revised from Y. Sun’s Sli – Hard to show non-repudiation Revised from Y. Sun’s Sli in symmetric-key crypto – Public-key crypto can do both M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [14] M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [15] Public Key Cryptography Public Key Cryptography What if Alice and Bob cannot hold a common key ? z Each user has a public key and a secret key. z A nonmathematical way z When Alice encrypts her message with Bob's public key – Bob: send Alice a box and an unlocked padlock – Since Bob is the only one who has access to the secret key, – Alice: put her message in the box, lock it using Bob’s lock, Bob is the only one who can decrypt the message and read and send the box back to Bob. the contents. – Bob can open the box and read the message – Potential attacks: man-in-the-middle z Public key cryptography (asymmetric cryptography) – The encryption key is made public. Revised from Y. Sun’s Slides @ URI for UMD EE739B F’05 for UMD Y. Sun’s Slides @ URI from Revised – The decryption key is only known by Bob. EE739B F’05 for UMD Y. Sun’s Slides @ URI from Revised – It is computationally infeasible to find the decryption key without information known only to Bob. M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [16] M. Wu: ENEE739B Multimedia Security and Forensics (F'05) 2.1 Crypto Basics [17] Symmetric versus Asymmetric Encryption Key Length z Pubic key cryptography is several orders of magnitude z Brute force attack: try every possible key and see which more expensive than symmetric one one yields meaning decryption. – On a Pentium PC, DES: 15 Mbit/s encryption rate –DES: 256 ≈ 7.2 x 1016 possibilities AES: 6 Kbits/s encryption rate z Longer keys are advantageous, but not guaranteed to – DES is typically 1000 times faster than the RSA-scheme make an adversary’s task difficult. des @ URI for UMD EE739B F’05 for UMD des @ URI EE739B F’05 for UMD des @ URI z Public-key systems provide significant benefits in terms of – Not all 128-bit algorithms are equally secure key management: – Guessing the keys is often only one of many ways to – Assume n users want to securely communicate to each other. break/attack the system. Symmetric: n(n-1)/2 keys; Asymmetric: 2n keys – Public-key crypto usually requires longer keys Revised from Y. Sun’s Sli Revised from Y. Sun’s Sli owing to the cipher structure that allows for asymmetry z Hybrid system – Using a public-key system for distributing secret “session key” – A symmetric cipher for the bulk encryption of the data M.
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