Confidentiality Confidentiality is maintained so long as private keys are secure. Authenticity is possible via public-key encryption by encrypting messages.

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Presentation transcript:

Confidentiality Confidentiality is maintained so long as private keys are secure. Authenticity is possible via public-key encryption by encrypting messages twice: E receiverPubKey ( E senderPrivateKey ( message ) ) …more on this later. Authenticity Integrity What prohibits alteration of message parts? At the very least such alterations should be obviously detected. Integrity is addressed via …

Calculate a hash value / checksum / message digest based upon the message. plaintext Message Encoding Message Decoding ciphertext plaintext MD decryption algorithm ciphertext encryption algorithm plaintext MD hash algorithm hash algorithm compare The hash algorithm is a one-way function.

plaintext This H function typically consists of multiple rounds of bitwise logic, parity calculation, word-sized addition, and bit string partitioning/concatenation. block 1 block 2 block 3... block N initial value H chain value H chain value H chain value chain value H message digest H

DateBlock Size Digest Size Compute Steps in HMD rounds of 16SHA rounds of 20RIPEMD paired rounds of 16SHA rounds of 20 Note that the probability of randomly “guessing” a hash digest of N bits is 12N12N e.g., the odds of calculating a hash match for a 128-bit checksum are 1 in 2 64 ≈ 2 * 10 19

(Hashed Message Authentication Code) The HMAC algorithm turns any keyless hashing algorithm into a keyed hash. The security of the HMAC algorithm depends upon the chosen hashing algorithm. plaintext message block 1 block 2 block 3... block N 00…0 key hash algorithm 00…0 MD block size ipad + opad + hash algorithm MDMD The HMAC digest Note that HMAC is more secure than hashing, because it requires a private key. ipad is repeated to block length. opad is repeated to block length.