1. 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 …

2. 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.

3. 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

4. DateBlock Size Digest Size Compute Steps in HMD5 1990 512 128 4 rounds of 16SHA-1 1995 512 160 4 rounds of 20RIPEMD-1601996 512 160 4 paired rounds of 16SHA-512 1995 1024 512 4 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

5. (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 00110110 repeated to block length. opad is 01011100 repeated to block length.