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bip85: Say when applications must use BIP85-DRNG - #2333
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Require applications that can need more than the 64 bytes of HMAC-SHA512 output for any derivation path to use BIP85-DRNG for all their paths. Other applications use the HMAC-SHA512 output directly, keeping each application on one entropy-source code path. Existing applications already follow this rule, so their outputs do not change. Bump the specification version to 2.2.0 and record the requirement in the changelog.
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fametrano
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Oct 20, 2026
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Concept ACK. An independent implementation, btclib-wallet's bip85.py, already follows the rule: only DICE and RSA read BIP85-DRNG, and every other application uses the HMAC output directly, so no output changes. Its 65 BIP85 tests, which include every vector the BIP publishes, pass. The RSA section still says the key generator "should" use BIP85-DRNG; a "MUST" there would match the new rule.
jonatack
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@fametrano 4dbd8c3 changes "should" to "MUST" use BIP85-DRNG in the RSA section as you requested. |
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There has been confusion during bip85 app proposals which entropy source to use, lets make their choice explicit in the standard.
What: An application that could need more than the 64 bytes of HMAC-SHA512 output for any of its derivation paths MUST use BIP85-DRNG for all of its derivation paths. All other applications MUST use the HMAC-SHA512 output directly.
How: Two sentences after the paragraph that introduces the HMAC output, a one-line rationale footnote, the version bump and a changelog entry.
Why: This is the per-application rule akarve proposed on #2295 (Sep 19 and Oct 12, 2026). Every existing application already follows it: RSA and DICE use BIP85-DRNG, and BIP39, HD-Seed WIF, XPRV, HEX, PWD BASE64, PWD BASE85 and Nostr never read more than the 64 bytes. No output changes. A new MUST is a backward-compatible change to the specification, so this is a MINOR bump to 2.2.0 per BIP 3.