fix(keychain): pad DH shared secret to fixed length on Linux#548
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The Secret Service session encryption mode (dh-ietf1024-sha256-aes128-cbc-pkcs7) derived the HKDF AES key from sharedSecret.Bytes(), which returns the minimal big-endian encoding and strips leading zero bytes. When the Diffie-Hellman shared secret had one or more leading zero bytes (~1/256 of sessions per byte), our HKDF input was shorter than the 128-byte zero-padded value the keyring peer uses, so the two sides derived different AES keys. The keyring then rejected CreateItem with "the secret was transferred or encrypted in an invalid way", surfacing as intermittent CI failures on the Linux keychain tests. Encode the shared secret into a fixed-length buffer sized to the group prime (128 bytes for the 1024-bit Second Oakley Group) with big.Int.FillBytes so both sides agree on the HKDF input. Also surface the decrypt error in GetSecret instead of silently returning (nil, nil), and add a deterministic regression test that pins the zero-padded encoding. Fixes #547 Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Assessment: 🟢 APPROVE
The fix is correct and well-implemented. big.Int.FillBytes precisely addresses the intermittent DH shared-secret mismatch by left-padding to the group prime size (128 bytes). The regression test with myPrivate=1 is a clean, deterministic vector that proves the fix and documents the failure mode. Surfacing the decrypt error in GetSecret is also a straightforward improvement over silently swallowing it.
joe0BAB
approved these changes
Jun 3, 2026
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The Secret Service session encryption mode
(dh-ietf1024-sha256-aes128-cbc-pkcs7) derived the HKDF AES key from sharedSecret.Bytes(), which returns the minimal big-endian encoding and strips leading zero bytes. When the Diffie-Hellman shared secret had one or more leading zero bytes (~1/256 of sessions per byte), our HKDF input was shorter than the 128-byte zero-padded value the keyring peer uses, so the two sides derived different AES keys. The keyring then rejected CreateItem with "the secret was transferred or encrypted in an invalid way", surfacing as intermittent CI failures on the Linux keychain tests.
Encode the shared secret into a fixed-length buffer sized to the group prime (128 bytes for the 1024-bit Second Oakley Group) with big.Int.FillBytes so both sides agree on the HKDF input. Also surface the decrypt error in GetSecret instead of silently returning (nil, nil), and add a deterministic regression test that pins the zero-padded encoding.
Fixes #547