0e89c4baa6
- Move CardBackend, CardTransation traits to card-backend - Break SmartcardErrors out from openpgp-card to card-backend - CardCaps are (mostly) openpgp-card specific, move them (mostly) to openpgp-card - Rename pcsc and scdc backend crates: card-backend-pcsc, card-backend-scdc
38 lines
1.7 KiB
Markdown
38 lines
1.7 KiB
Markdown
<!--
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SPDX-FileCopyrightText: 2021-2023 Heiko Schaefer <heiko@schaefer.name>
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SPDX-License-Identifier: MIT OR Apache-2.0
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-->
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# scdaemon based backend (e.g., for the openpgp-card library)
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This crate provides `ScdBackend`/`ScdTransaction`, which is an implementation
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of the `CardBackend`/`CardTransaction` traits that uses an instance of GnuPG's
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[scdaemon](https://www.gnupg.org/documentation/manuals/gnupg/Invoking-SCDAEMON.html)
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to access OpenPGP cards.
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Note that (unlike `card-backend-pcsc`), this backend doesn't implement
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transaction guarantees.
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## Known limitations
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- Uploading RSA 4096 keys via `scdaemon` doesn't work with cards that don't
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support Command Chaining (e.g. the "Floss Shop OpenPGP Smart Card").
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This is caused by a size limitation for client requests via the
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[Assuan](https://www.gnupg.org/documentation/manuals/assuan/) protocol.
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Assuan "Client requests" are limited to 1000 chars. Commands are sent as
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ASCII encoded hex, so APDU commands are limited to around 480 bytes. This
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is insufficient for importing RSA 4096 keys to the card (all other
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OpenPGP card operations fit into this constraint).
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- When using `scdaemon` via pcsc (by configuring `scdaemon` with
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`disable-ccid`), choosing a specific card of multiple plugged-in OpenPGP
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cards seems to be broken.
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So you probably want to plug in only one OpenPGP card at a time when using
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`openpgp-card-scdc` combined with `disable-ccid`.
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- When using `scdaemon` via its default `ccid` driver, choosing a
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specific one of multiple plugged-in OpenPGP cards seems to only work up
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to 4 plugged-in cards.
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So you probably want to plug in at most four OpenPGP cards at a time when
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using `card-backend-scdc` with its ccid driver.
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(This limit has been raised in GnuPG 2.3.x)
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