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FA2_CLEAR_SIGNING.md

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FA2 transfer clear-signing (Ledger app)

The operation parser can recognize a subset of FA2 transfer calls to a KT1 contract listed in the registry and show Transfer tokens to (receiver address) and Token Amount instead of a single opaque Parameter field. The Destination field (FA2 contract) is omitted from the review flow for those contracts; the Token name field is not shown.

When clear-signing applies

  • Destination is a KT1 contract whose 20-byte hash is listed in the in-app FA2 token registry (FA2_TOKEN_REGISTRY in app/src/parser/fa2_tokens.c).
  • Entrypoint name is exactly transfer (string match after read).
  • The Micheline parameter matches the single-transfer shape parsed by tz_step_read_fa2_transfer: outer list/Pair of from_, txs list with one Pair of to_ and inner list(pair(token_id, amount)) (or the equivalent direct Pair encoding for a single inner item).
  • token_id must be zero (single-byte Zarith 0x00). Non-zero IDs fall back to generic parameter display (often Parameter with complex/binary presentation).
  • No extra bytes after the parsed amount: the parser must consume the whole parameter expression; otherwise it falls back to binary display.

When the app falls back

  • Multiple transfers in the txs list, non-zero token_id, negative or non-standard amount encoding, or any Micheline shape that does not match the parser: the field is shown as Parameter with is_field_complex set (see fa2_fallback_to_binary).
  • Unknown tokens (contract not in the registry): the amount is still shown as a decimal string, but without symbol/decimal scaling from a registry entry. Transfer tokens to still appears for successful FA2-shaped parameters; Destination stays visible if the contract is not in the registry.

Token metadata

Decimals and symbols come only from the static registry compiled into the app. There is no runtime fetch from chain or third-party APIs.

Tests

  • C unit tests in tests/unit/ctest/tests_parser.c (FA2-related cases).
  • Hex blobs for those tests can be regenerated with scripts/gen_fa2_ctest_hex.py (requires pip install base58).