[Feature] Import legacy AES-256 encrypted seed backups via QR code

[berlinxray]

Summary

Scan a QR code containing an OpenSSL-compatible AES-256-CBC encrypted seed backup, decrypt it with a user-provided passphrase, validate as BIP-39 or Electrum mnemonic, and import as a seed.

Note: This PR is focused exclusively on encrypted seed QR import. The Seed XOR rebuild flow and Electrum XOR support are in a separate PR (#884).

Workflow

1. Scan QR code (auto-detects U2FsdGVkX1 / Salted__ prefix)
2. Prompt for decryption passphrase
3. Decrypt, validate as BIP-39 or Electrum mnemonic
4. Import as a seed

Activated via Settings > Advanced > Encrypted seeds (SETTING__ENCRYPTED_SEEDS, disabled by default).

Changes (10 files)

Core (src/seedsigner/helpers/aes_decrypt.py) — New

• Pure-Python AES-256-CBC decryptor (no pyaes dependency)
• Precomputed InvMixColumns lookup tables for Pi Zero performance
• PBKDF2 key derivation with auto-detection of 10,000 and 100,000 iterations
• decrypt_openssl_aes256cbc() and DecryptionError exception

QR Detection (src/seedsigner/models/decode_qr.py, qr_type.py)

• Add QRType.SEED__ENCRYPTED constant
• Add EncryptedSeedQrDecoder for base64-encoded encrypted seed QRs
• Auto-detect encrypted QRs by U2FsdGVkX1 prefix

Settings (src/seedsigner/models/settings_definition.py)

• Add SETTING__ENCRYPTED_SEEDS toggle (advanced, disabled by default)

Views (src/seedsigner/views/seed_views.py)

• EncryptedSeedPassphraseView — Passphrase entry screen
• EncryptedSeedDecryptView — Decryption + BIP-39/Electrum validation
• EncryptedSeedDecryptionFailedView — Retry/discard on failure

Scan (src/seedsigner/views/scan_views.py)

• Route encrypted QR scans to decryption flow
• Check SETTING__ENCRYPTED_SEEDS toggle
• Add back button support for scan screen

GUI (src/seedsigner/gui/screens/seed_screens.py)

• Fix SeedAddPassphraseScreen to preserve custom title ("Decryption Passphrase")

Tests

• tests/test_aes_decrypt.py — AES-256-CBC unit tests
• tests/test_encrypted_seed_qr.py — Encrypted QR integration tests
• tests/conftest.py — Shared test fixtures

Legacy Workflow (reference)

# Encrypt
openssl enc -aes-256-cbc -pbkdf2 -iter 100000 -in seed.txt -out seed.enc
qrencode -r seed.enc -o seedenc.png

# Decrypt (verify)
cat seed.enc | openssl aes-256-cbc -a -d -pbkdf2

Checklist

• pytest passes
• Tested on Raspberry Pi OS (manual build)
• Tested on SeedSigner OS (Pi0)

Test plan

• All 11 tests pass (AES unit + encrypted QR integration)
• Manual test: Scan encrypted seed QR → enter passphrase → verify seed imports correctly
• Manual test: Wrong passphrase → "Decryption Failed" → retry works
• Manual test: Feature disabled → OptionDisabledView shown

[none34829]

Checked out this branch and ran the tests locally. Went through the AES implementation- verified _inv_cipher_block against the NIST FIPS 197 Appendix C.3 known-answer test (key 000102...1f, ciphertext 8ea2b7ca..., plaintext 00112233...) and it produces the correct output, so the core cipher looks solid.

One thing I noticed with the tests though: every success-path test in test_aes_decrypt.py generates its expected vectors at runtime by shelling out to openssl. If openssl isn't on the machine, those all get pytest.skip'd and the AES never actually gets verified against a correct decryption. The error-path tests are hardcoded but they only cover failure modes.

For a hand-rolled AES on a signing device I think there should be at least one hardcoded vector that works without any external tools. Something like:

def test_decrypt_hardcoded_vector():
    """Pre-computed OpenSSL vector- no openssl binary needed at test time."""
    encrypted = "U2FsdGVkX1+5ER21G7dz8fbBzXbp4p3LiMqJuwXOJN29f+1djh0nhJovkaPluB7Vcm9yr9g5Ss65pFvt+UQTsiYiEwJSiL9D08dn5AXmI5UmPR2WJuBQD14tEMrX/eXXVNLtG3Gy7qbiJkidJepX0w=="
    result = decrypt_openssl_aes256cbc(encrypted, "TestVector2026")
    assert result == "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about"

I generated that with:

echo -n "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about" | openssl enc -aes-256-cbc -pbkdf2 -iter 10000 -base64 -A -pass pass:TestVector2026

Ran it against the branch and it passes. Just means the happy path is always covered regardless of the test environment.