openssl-psk 0.1.1

TLS-PSK socket wrapper for Python 3.10+ — no compiled extensions required

openssl-psk

TLS-PSK (pre-shared key) socket wrapper for Python 3.10+.
No compiled extensions — pure Python using the standard ssl module and ctypes.

pip install openssl-psk

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Why this exists

The standard ssl module has no PSK support below Python 3.13.

openssl-psk works around this limitation by:

• Calling libssl's SSL_CTX_set_psk_*_callback directly via ctypes on Python 3.10–3.12
• Using the native SSLContext.set_psk_*_callback() API transparently on Python 3.13+

No C extension to compile. Just pip install.

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Quick start

Client

import socket
from openssl_psk import wrap_socket_client

tcp = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
tcp.connect(("192.168.1.1", 4116))

# simplest form — just the key
ssl_sock = wrap_socket_client(tcp, psk=b"84155C93FDB08FB384B75C44A7D3B625")

ssl_sock.sendall(b"GET /resource HTTP/1.1\r\nHost: 192.168.1.1\r\n\r\n")
print(ssl_sock.recv(4096))
ssl_sock.close()

Server

import socket
from openssl_psk import wrap_socket_server

PSK_DB = {
    b"device-001": b"84155C93FDB08FB384B75C44A7D3B625",
    b"device-002": b"0AE9DF505E3D3B9A5305409550D42C96",
}

tcp = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
tcp.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
tcp.bind(("0.0.0.0", 4116))
tcp.listen(5)

while True:
    conn, addr = tcp.accept()
    ssl_sock = wrap_socket_server(
        conn,
        psk=lambda identity: PSK_DB[identity],
        hint="my-server",                   # optional identity hint sent to clients
    )
    data = ssl_sock.recv(4096)
    ssl_sock.sendall(b"HTTP/1.1 200 OK\r\n\r\n")
    ssl_sock.close()

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API

wrap_socket_client(sock, psk, ciphers=..., server_hostname=None)

Parameter	Type	Description
sock	socket.socket	Connected TCP socket to wrap
psk	bytes | tuple | callable	Pre-shared key — see accepted shapes below
ciphers	str	OpenSSL cipher string (default: PSK-AES128-CBC-SHA256)
server_hostname	str | None	SNI hostname passed to wrap_socket

Accepted psk shapes:

# 1. Just the key — identity sent to server is b''
wrap_socket_client(sock, psk=b"mypsk")

# 2. Key + client identity
wrap_socket_client(sock, psk=(b"mypsk", b"myidentity"))

# 3. Callable: server hint → key
PSK_FOR = {b"server1": b"abcdef", b"server2": b"123456"}
wrap_socket_client(sock, psk=lambda hint: PSK_FOR[hint])

Returns an ssl.SSLSocket after completing the handshake.

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wrap_socket_server(sock, psk, hint=None, ciphers=...)

Parameter	Type	Description
sock	socket.socket	Accepted TCP socket to wrap (from socket.accept())
psk	bytes | callable	Pre-shared key — see accepted shapes below
hint	str | None	PSK identity hint sent to clients (optional)
ciphers	str	OpenSSL cipher string (default: PSK-AES128-CBC-SHA256)

Accepted psk shapes:

# 1. Just the key — same key accepted for every client identity
wrap_socket_server(sock, psk=b"mypsk")

# 2. Callable: client identity → key (raise to reject)
PSK_FOR = {b"clientA": b"abcdef", b"clientB": b"123456"}
wrap_socket_server(sock, psk=lambda identity: PSK_FOR[identity])

Returns an ssl.SSLSocket after completing the handshake.
When using a callable, raise any exception to reject the client.

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Compatibility

Python	Mechanism	CPython only?
3.10	ctypes → libssl	Yes
3.11	ctypes → libssl	Yes
3.12	ctypes → libssl	Yes
3.13+	ssl.SSLContext.set_psk_*_callback()	No

The ctypes path reads the SSL_CTX* pointer out of CPython's PySSLContext struct at a fixed offset that has been stable across CPython 3.x. It will raise a RuntimeError with a clear message if the pointer is NULL, so you will know immediately if anything changes in a future CPython release.

Requires OpenSSL ≥ 1.0.1 (for PSK cipher support).

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Running the tests

pip install pytest
pytest

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License

MIT — see LICENSE.