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ZIP 307: clarify that epk is needed 

The current phrasing implies the 580-byte memo ciphertext suffices for detection. Clarify that the 32-byte ephemeral public key is also needed.

Also added "public" to "ephemeral key" further down.
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Eran Tromer 2021-08-25 19:29:49 -04:00 committed by GitHub
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@ -147,13 +147,14 @@ A recipient detects their transactions by trial-decrypting this ciphertext. On a
that has the entire block chain, the primary cost is computational. For light clients
however, there is an additional bandwidth cost: every ciphertext on the block chain must
be received from the server (or network node) the light client is connected to. This
results in a total of 580 bytes per output that must be streamed to the client.
results in a total of 580 bytes per output that must be streamed to the client (in addition
to the 32-byte ephemeral public key).
However, we don't need all of that just to detect payments. The first 52 bytes of the
ciphertext contain the contents and opening of the note commitment, which is all of the
data needed to spend the note and to verify that the note is spendable. If we ignore the
memo and the authentication tag, we're left with a 32-byte ephemeral key, the 32-byte note
commitment, and only the first 52 bytes of the ciphertext for each output needed to
memo and the authentication tag, we're left with a 32-byte ephemeral public key, the 32-byte
note commitment, and only the first 52 bytes of the ciphertext for each output needed to
decrypt, verify, and spend a note. This totals to 116 bytes per output, for an 80%
reduction in bandwidth use.