[RFC17] fix: secret key conversion: must use crypto_box_seed_keypair

rfc/0017_transaction_comment_encryption.md

@@ -2,8 +2,8 @@
 
 This RFC defines a format for encrypting transaction comments for the case of transactions involving a single issuer key and a single receiver key.
 
-Let `Si` the ed25519 secret key of the issuer and `Pi` its associated ed25519 public key.
-Let `Sr` the ed25519 secret key of the receiver and `Pr` its associated ed25519 public key.
+Let `Si` the seed of the issuer and `Pi` its associated ed25519 public key.
+Let `Sr` the seed of the receiver and `Pr` its associated ed25519 public key.
 
 ## Convert Ed25519 keys to x25519 keys
 
@@ -13,9 +13,9 @@ The first step is to convert the Ed25519 keys into x25519 keys (also called curv
 
 Use the first 32 bytes of the extended secret key.
 
-The extended secret key is derived from the seed by the NaCl function `crypto_sign_ed25519_seed_keypair`.
+The extended secret key is derived from the seed by the NaCl function `crypto_box_seed_keypair`.
 
-Let `Si' = Si[..32]`;
+Let `Si' = crypto_box_seed_keypair(Si)[..32]`;
 
 ### Convert Ed25519 public key to x25519 public key