Symmetric Cryptography, Asymmetric Cryptography, and Digital Signatures.

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Presentation transcript:

Symmetric Cryptography, Asymmetric Cryptography, and Digital Signatures

2 Symmetric Cryptography One key encrypts and decrypts data Cleartext with Key makes Ciphertext Ciphertext with Key makes Cleartext Winning Lotto Winning Lotto #s:

3 Symmetric Cryptography Algorithms Symmetric algorithms have one key that encrypts and decrypts data Advantages –Symmetric algorithms are fast –They are difficult to break if a large key size is used –Only one key needed

4 Symmetric Cryptography Algorithms Disadvantages –Symmetric keys must remain secret –Difficult to deliver keys (key distribution) –Symmetric algorithms don’t support authenticity or nonrepudiation You can’t know for sure who sent the message, since two people have the same key

5 Symmetric Algorithms (Private-key) NameKey sizeNotes DES56 bitsInsecure 3DES168 bitsBeing replaced by AES AES128,192, or 256 US Govt classified info IDEA128 bitsUsed in PGP, very secure Blowfish32 to 448 Public domain RC5Up to 2040 Secure for 72-bits or more

6 Asymmetric Cryptography Algorithms Use two keys that are mathematically related –Data encrypted with one key can be decrypted only with the other key Another name for asymmetric key cryptography is public key cryptography –Public key: known by the public –Private key: known only by owner

7 Asymmetric Cryptography Cleartext with Public Key makes Ciphertext Ciphertext with Private Key makes Cleartext Winning Lotto Winning Lotto #s:

8 Asymmetric Cryptography Provides message authenticity and nonrepudiation –Authenticity validates the sender of a message –Nonrepudiation means a user cannot deny sending a message

9 Asymmetric Cryptography Asymmetric algorithms are more scalable but slower than symmetric algorithms –Scalable: can adapt to larger networks –Each person needs only one key pair Everyone can use the same public key to send you data Each person signs messages with their own private key

10 RSA Developed in 1977 by Ronald L. Rivest, Adi Shamir, and Leonard M. Adleman The algorithm is based on the difficulty of factoring large numbers The Secure Socket Layer (SSL) protocol uses the RSA algorithm

11 Diffie-Hellman Developed by Whitfield Diffie and Martin Hellman Does not provide encryption but is used for key exchange –Two parties agree on a key without ever sending it directly over the network –The numbers transmitted can be used to compute the key, but only by the parties holding secret private numbers Prevents sniffing attacks

12 Elliptic Curve Cryptosystems (ECC) It is an efficient algorithm requiring few resources –Memory –Disk space –Bandwidth ECC is used for encryption as well as digital signatures and key distribution

13 Elgamal Public key algorithm used to –Encrypt data –Create digital signature –Exchange secret keys Written by Taher Elgamal in 1985 The algorithm uses discrete logarithm problems –Solving a discrete logarithm problem can take many years and require CPU-intensive operations

14 Digital Signatures A hash value ensures that the message was not altered in transit (integrity) Provide message integrity, authenticity and nonrepudiation

15 From Wikipedia

16 Digital Signature Standard (DSS) Established by the NIST in 1991 –Ensures that digital signatures rather than written signatures can be verified Federal government requirements –RSA and Digital Signature Algorithm (DSA) must be used for all digital signatures –Hashing algorithm must be used to ensure the integrity of the message NIST required that the Secure Hash Algorithm (SHA) be used

17 Hashing Algorithms NameNotes MD2Written for 8-bit machines, no longer secure MD4No longer secure MD5Security is questionable now SHA-1The successor to MD5, Used in: TLS, SSL, PGP, SSH, S/MIME, IPsec No longer completely secure SHA-2Not yet broken, but no longer recommended. NIST is now developing a new algorithm to replace SHA.