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bip-138: clarify CONTENT is a single TLV and forbid all-zero nonce

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pythcoiner committed 2026-06-18 03:02:17 -04:00
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@@ -167,6 +167,9 @@ and integrity.
* let $nonce$ = random(96 bits) * let $nonce$ = random(96 bits)
* let $ciphertext$ = encrypt($payload$, $secret$, $nonce$) * let $ciphertext$ = encrypt($payload$, $secret$, $nonce$)
The $nonce$ MUST NOT be all zero. If the random source yields an all-zero nonce,
the encoder MUST generate a new one.
### Decryption ### Decryption
In order to decrypt the payload of a backup, the owner of a certain public key p In order to decrypt the payload of a backup, the owner of a certain public key p
@@ -309,6 +312,10 @@ it follows this format:
`TYPE` (`LENGTH`) `DATA` `TYPE` (`LENGTH`) `DATA`
`CONTENT` is a single `TYPE (LENGTH) DATA` triple, one blob describing `PLAINTEXT`.
It is not a sequence of entries; it is immediately followed by the payload `LENGTH`
and `PLAINTEXT`.
`TYPE`: 1-byte unsigned integer identifying how to interpret `DATA`. `TYPE`: 1-byte unsigned integer identifying how to interpret `DATA`.
| Value | Definition | | Value | Definition |
@@ -332,16 +339,17 @@ For `TYPE` values defined above:
For all `TYPE` values except `0x01`, parsers MUST reject `CONTENT` if `LENGTH` exceeds For all `TYPE` values except `0x01`, parsers MUST reject `CONTENT` if `LENGTH` exceeds
the remaining payload bytes. the remaining payload bytes.
Parsers MUST skip unknown `TYPE` values less than `0x80`, by consuming `LENGTH` bytes For an unknown `TYPE` less than `0x80`, parsers MUST consume its `LENGTH` bytes of
of `DATA`. `DATA`, treat the content type as unknown, and continue with the payload `LENGTH`
and `PLAINTEXT`.
For unknown `TYPE` values greater than or equal to `0x80`, parsers MUST stop parsing For an unknown `TYPE` greater than or equal to `0x80`, parsers MUST reject the
`CONTENT`.[^type-upgrade] payload.[^type-upgrade]
[^type-upgrade]: **Why the 0x80 threshold?** [^type-upgrade]: **Why the 0x80 threshold?**
The `TYPE >= 0x80` rule means we're not stuck with the current TLV encoding. The `TYPE >= 0x80` rule keeps the encoding upgradeable. A future version can
It has a nice upgrade property: you can still encode backward compatible stuff define a new content type at or above `0x80` that older parsers reject rather
at the start. than misread, while types below `0x80` stay backward compatible.
#### Encrypted Payload #### Encrypted Payload
@@ -349,7 +357,7 @@ For unknown `TYPE` values greater than or equal to `0x80`, parsers MUST stop par
`NONCE` `LENGTH` `CIPHERTEXT` `NONCE` `LENGTH` `CIPHERTEXT`
`NONCE`: 12-byte (96-bit) nonce. `NONCE`: 12-byte (96-bit) nonce. Parsers MUST reject a backup whose `NONCE` is all zero.
`LENGTH`: variable-length integer representing ciphertext length. `LENGTH`: variable-length integer representing ciphertext length.
`CIPHERTEXT`: variable-length ciphertext. `CIPHERTEXT`: variable-length ciphertext.