Intro Public Key Cryptography¶
links: AC2 TOC - Intro Public Key Encryption - Index
Limitation of Symmetric-Key Encryption¶
For a full intro to symmetric-key encryption visit Private Key Encryption.
Key Distribution Problem¶
One of the biggest problems with symmetric key encryption is the key distribution problem. There are only a few possible secure channels such as
- Personal contact
- Trusted postal service (e.g. sealed or registered letters)
- Codebook (like in World War II)
- Courier (like the "red phone" between Moscow and Washington during the Cold War)
In general, secure channels are expensive and inefficient.
Key Management Problem¶
In addition to the key distribution problem, there is also the key management problem.
In symmetric key encryption, pairwise communication between \(N\) parties requires the following number of different keys:
\[keys(N) = \frac{N(N-1)}{2}\]
Examples: \(keys(6) = \frac{6\cdot 5}{2}\), \(keys(1000) \approx 500000\)
Each of the \(N\) parties needs to store \(N-1\) keys, and the keys should be renewed periodically.
Non-Repudiation¶
- In any communication based on symmetric key encryption, the key is known to at least two parties.
- After sending the ciphertext \(c\) to Bob, Alice could deny being the author of \(m\) because Bob could have created it himself.
- Using only symmetric key cryptography (encryption, MAC), non-repudiation can't be archived.
- Non-repudiation is essential for digital contracts.