cryptography 47.0.0: Disabling OpenSSL legacy provider via CRYPTOGRAPHY_OPENSSL_NO_LEGACY runtime environment variable causes legacy cipher operations to raise UnsupportedAlgorithm

## cryptography OpenSSL Legacy Provider Configuration

The initial search for prior solutions returned sample `sha256:b98c885c2365fa2de56dab1eea1087f3137ec69c260717f69d7ade5d27bee868`, which addresses Rust native extension compilation requirements for source builds on musl toolchains.

### Behavior and Runtime Impact

Developers and language models often assume that `CRYPTOGRAPHY_OPENSSL_NO_LEGACY` is a build-time compiler flag evaluated during package installation, or that setting environment variables inside Python script execution will dynamically toggle OpenSSL provider availability at any point.

In reality, `CRYPTOGRAPHY_OPENSSL_NO_LEGACY` is a runtime environment variable inspected when `cryptography.hazmat.bindings.openssl.binding` initializes OpenSSL binding locks.

- **Pre-initialization setting**: When `os.environ["CRYPTOGRAPHY_OPENSSL_NO_LEGACY"] = "1"` is set before OpenSSL bindings initialize, `cryptography` skips loading OpenSSL 3's `legacy` provider (`_legacy_provider_loaded` is `False`). Attempting to instantiate encryptor or decryptor contexts for legacy algorithms such as `Blowfish`, `CAST5`, `IDEA`, `SEED`, or `ARC4` fails loudly with `cryptography.exceptions.UnsupportedAlgorithm`.
- **Post-initialization setting**: Setting `os.environ["CRYPTOGRAPHY_OPENSSL_NO_LEGACY"] = "1"` after `cryptography.hazmat.bindings.openssl.binding` has already loaded has no effect on currently initialized bindings because legacy provider loading occurs once at binding import time (`_legacy_provider_loaded` remains `True`).

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cffi==2.0.0
cryptography==47.0.0
pycparser==3.00

cffi==2.0.0
cryptography==47.0.0
pycparser==3.00

import os
import sys
import subprocess
from cryptography.exceptions import UnsupportedAlgorithm
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
import cryptography.hazmat.bindings._rust as _rust

# 1. Default behavior (without CRYPTOGRAPHY_OPENSSL_NO_LEGACY set):
# OpenSSL legacy provider is loaded and legacy ciphers (Blowfish) work.
assert _rust.openssl._legacy_provider_loaded is True

key = b"0123456789abcdef"
cipher = Cipher(algorithms.Blowfish(key), modes.ECB())
enc = cipher.encryptor()
ct = enc.update(b"12345678") + enc.finalize()
dec = cipher.decryptor()
pt = dec.update(ct) + dec.finalize()
assert pt == b"12345678"

# 2. Setting CRYPTOGRAPHY_OPENSSL_NO_LEGACY=1 prior to binding initialization
# disables loading the OpenSSL legacy provider and causes Blowfish encryptor
# creation to raise UnsupportedAlgorithm.
no_legacy_code = """
import os
import sys
os.environ["CRYPTOGRAPHY_OPENSSL_NO_LEGACY"] = "1"
import cryptography.hazmat.bindings._rust as _rust
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.exceptions import UnsupportedAlgorithm

assert _rust.openssl._legacy_provider_loaded is False
c = Cipher(algorithms.Blowfish(b"0123456789abcdef"), modes.ECB())
try:
    c.encryptor()
    sys.exit(1)
except UnsupportedAlgorithm:
    sys.exit(0)
"""

res = subprocess.run(
    [sys.executable, "-c", no_legacy_code],
    capture_output=True,
    text=True,
)
assert res.returncode == 0, f"Subprocess failed with stderr: {res.stderr}"

# 3. Setting CRYPTOGRAPHY_OPENSSL_NO_LEGACY=1 AFTER binding initialization
# has no effect because legacy provider loading occurs at import time.
post_init_code = """
import os
import sys
import cryptography.hazmat.bindings.openssl.binding
os.environ["CRYPTOGRAPHY_OPENSSL_NO_LEGACY"] = "1"
import cryptography.hazmat.bindings._rust as _rust
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes

assert _rust.openssl._legacy_provider_loaded is True
c = Cipher(algorithms.Blowfish(b"0123456789abcdef"), modes.ECB())
enc = c.encryptor()
assert enc is not None
"""

res_post = subprocess.run(
    [sys.executable, "-c", post_init_code],
    capture_output=True,
    text=True,
)
assert res_post.returncode == 0, f"Post-init subprocess failed with stderr: {res_post.stderr}"