Andrew Dunn
8e3d6dc535
ecc_pkey_to_curve_x_y() marshalled the affine point coordinates with
BN_num_bytes() + BN_bn2bin(), which produce the minimal big-endian
encoding and strip any leading zero bytes. The affine coordinates are
elements of the curve's underlying field, though, and consumers expect
them at the fixed field width.
About 1 in 256 keys per coordinate has a most significant zero byte on
the curves whose field width is a whole number of bytes, and about 1 in 2
on P-521, where only the low bit of the leading byte is ever set. The
coordinate then comes out one or more bytes short of the field width. When
that happens tpm2_tpm2b_public_from_openssl_pkey() builds a TPM2B_PUBLIC
whose unique.ecc.x or .y is undersized. The TPM 2.0 reference
implementation requires each coordinate size to equal the key size in
bytes, so TPM2_LoadExternal() rejects the public area with TPM_RC_KEY.
The name a TPM derives for the object also stops matching the name
systemd computes in software, because the name hashes the marshalled
public area.
For a signed PCR policy the key is only round-tripped through the TPM at
unlock time, so an enrolled key with a leading-zero coordinate is
accepted by systemd-cryptenroll and systemd-measure and only fails when
the volume is first unlocked, with:
Esys_LoadExternal() Esys Finish ErrorCode (0x000002dc)
Failed to unseal secret using TPM2: State not recoverable
Derive the curve's field width from the EC group degree and zero-pad both
coordinates to it with BN_bn2binpad(). Padding a full-width coordinate is
a no-op, so only keys that a reference-conformant TPM rejects today change
their marshalling; no enrollment that currently unlocks changes behavior.
Every caller of this converter inherits the fix, including the
systemd-creds public-key binding.
Because the width now comes from the group rather than from the BIGNUM
being written, BN_bn2binpad() can fail where BN_bn2bin() could not, so
both calls return -EIO rather than asserting. It reports that through its
return value without touching the OpenSSL error queue, so these use
log_debug_errno() rather than log_openssl_errors(). EC_GROUP_get_degree
is file-local, matching the other libcrypto symbols used only here.
The tests pin the full expected coordinates for a P-384 key and a P-521
key that each have a leading zero byte. test-tpm2 gains a case pinning
the TPM object name for the P-384 key, because the name is derived from
the marshalled public area and this change alters it for exactly those
keys a reference-conformant TPM rejects today.
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