Chapter Summary

Chapter Summary

Key Points

• 1.

  Secure delivery model (Sengupta-Tandon-Clancy 2015): external eavesdropper observes the broadcast $X$. Goal is $I(\mathcal{W}; X) = 0$ — perfect info-theoretic secrecy of the library.

• 2.

  Minimum memory requirement: $M \geq 1$. One file's worth of random key material per user is necessary (cut-set/OTP lower bound).

• 3.

  STC achievable rate $R_\text{sec} = K(1 - (M-1)/N) / (1 + K(M-1)/N)$ — MAN rate with effective memory $M - 1$.

• 4.

  Two-tier placement underpins STC: tier 1 stores 1-file key material (via Shamir sharing); tier 2 stores $(M-1)$-file MAN subfiles. Delivery XORs MAN messages with fresh keys.

• 5.

  Secrecy is not free. Unlike demand privacy (Wan-Caire, Ch 12), which costs nothing, secrecy costs 1 file of memory. At scale ($N \gg 1$), the rate penalty is small.

• 6.

  Shamir secret sharing achieves the $M \geq 1$ bound exactly by compactly storing shares such that $(t+1)$ users can reconstruct keys, $t$ users learn nothing.

• 7.

  Joint privacy + secrecy schemes combine Wan-Caire masking with STC XOR keys — full protection against both insiders and outsiders, at MAN-with-effective-memory rate.