Abstract
This module is a rather thin wrapper around (a subset of) the OpenSSL library. With thin wrapper I mean that a lot of the object methods do nothing more than calling a corresponding function in the OpenSSL library.
The reason pyOpenSSL was created is that the SSL support in the socket module in Python 2.1 (the contemporary version of Python when the pyOpenSSL project was begun) was severely limited. Other OpenSSL wrappers for Python at the time were also limited, though in different ways. Unfortunately, Python’s standard library SSL support has remained weak, although other packages (such as M2Crypto1) have made great advances and now equal or exceed pyOpenSSL’s functionality.
The reason pyOpenSSL continues to be maintained is that there is a significant user community around it, as well as a large amount of software which depends on it. It is a great benefit to many people for pyOpenSSL to continue to exist and advance.
These instructions can also be found in the file INSTALL.
I have tested this on Debian Linux systems (woody and sid), Solaris 2.6 and 2.7. Others have successfully compiled it on Windows and NT.
pyOpenSSL uses distutils, so there really shouldn’t be any problems. To build the library:
If your OpenSSL header files aren’t in /usr/include, you may need to supply the -I flag to let the setup script know where to look. The same goes for the libraries of course, use the -L flag. Note that build won’t accept these flags, so you have to run first build_ext and then build! Example:
Now you should have a directory called OpenSSL that contains e.g. SSL.so and __init__.py somewhere in the build dicrectory, so just:
If you, for some arcane reason, don’t want the module to appear in the site-packages directory, use the --prefix option.
You can, of course, do
to find out more about how to use the script.
Big thanks to Itamar Shtull-Trauring and Oleg Orlov for their help with Windows build instructions. Same as for Unix systems, we have to separate the build_ext and the build.
Building the library:
Where ...\openssl is of course the location of your OpenSSL installation.
Installation is the same as for Unix systems:
And similarily, you can do
to get more information.
This package provides a high-level interface to the functions in the OpenSSL library. The following modules are defined:
Generic cryptographic module. Note that if anything is incomplete, this module is!
An interface to the OpenSSL pseudo random number generator.
An interface to the SSL-specific parts of OpenSSL.
See X509.
A class representing X.509 certificates.
See X509Name.
A class representing X.509 Distinguished Names.
This constructor creates a copy of x509name which should be an instance of X509Name.
See X509Req.
A class representing X.509 certificate requests.
A Python type object representing the X509Store object type.
See PKey.
A class representing DSA or RSA keys.
A Python type object representing the PKCS7 object type.
A Python type object representing the PKCS12 object type.
See X509Extension.
A class representing an X.509 v3 certificate extensions. See http://openssl.org/docs/apps/x509v3_config.html#STANDARD_EXTENSIONS for typename strings and their options. Optional parameters subject and issuer must be X509 objects.
See NetscapeSPKI.
A class representing Netscape SPKI objects.
If the enc argument is present, it should be a base64-encoded string representing a NetscapeSPKI object, as returned by the b64_encode method.
A class representing Certifcate Revocation List objects.
A class representing Revocation objects of CRL.
File type constants.
Key type constants.
Generic exception used in the crypto module.
Dump the certificate cert into a buffer string encoded with the type type.
Dump the certificate request req into a buffer string encoded with the type type.
Dump the private key pkey into a buffer string encoded with the type type, optionally (if type is FILETYPE_PEM) encrypting it using cipher and passphrase.
passphrase must be either a string or a callback for providing the pass phrase.
Load a certificate (X509) from the string buffer encoded with the type type.
Load a certificate request (X509Req) from the string buffer encoded with the type type.
Load a private key (PKey) from the string buffer encoded with the type type (must be one of FILETYPE_PEM and FILETYPE_ASN1).
passphrase must be either a string or a callback for providing the pass phrase.
Load Certificate Revocation List (CRL) data from a string buffer. buffer encoded with the type type. The type type must either FILETYPE_PEM or FILETYPE_ASN1).
Load pkcs7 data from the string buffer encoded with the type type.
Load pkcs12 data from the string buffer. If the pkcs12 structure is encrypted, a passphrase must be included. The MAC is always checked and thus required.
See also the man page for the C function PKCS12_parse.
Sign a data string using the given key and message digest.
key is a PKey instance. data is a str instance. digest is a str naming a supported message digest type, for example “sha1”. New in version 0.11.
Verify the signature for a data string.
certificate is a X509 instance corresponding to the private key which generated the signature. signature is a str instance giving the signature itself. data is a str instance giving the data to which the signature applies. digest is a str instance naming the message digest type of the signature, for example “sha1”. New in version 0.11.
X509 objects have the following methods:
Return an X509Name object representing the issuer of the certificate.
Return a PKey object representing the public key of the certificate.
Return the certificate serial number.
Return the signature algorithm used in the certificate. If the algorithm is undefined, raise ValueError.
Return an X509Name object representing the subject of the certificate.
Return the certificate version.
Return a string giving the time before which the certificate is not valid. The string is formatted as an ASN1 GENERALIZEDTIME:
If no value exists for this field, None is returned.
Return a string giving the time after which the certificate is not valid. The string is formatted as an ASN1 GENERALIZEDTIME:
If no value exists for this field, None is returned.
Change the time before which the certificate is not valid. when is a string formatted as an ASN1 GENERALIZEDTIME:
Change the time after which the certificate is not valid. when is a string formatted as an ASN1 GENERALIZEDTIME:
Adjust the timestamp (in GMT) when the certificate starts being valid.
Adjust the timestamp (in GMT) when the certificate stops being valid.
Checks the certificate’s time stamp against current time. Returns true if the certificate has expired and false otherwise.
Set the issuer of the certificate to issuer.
Set the public key of the certificate to pkey.
Set the serial number of the certificate to serialno.
Set the subject of the certificate to subject.
Set the certificate version to version.
Sign the certificate, using the key pkey and the message digest algorithm identified by the string digest.
Return the hash of the certificate subject.
Return a digest of the certificate, using the digest_name method. digest_name must be a string describing a digest algorithm supported by OpenSSL (by EVP_get_digestbyname, specifically). For example, "md5" or "sha1".
Add the extensions in the sequence extensions to the certificate.
Return the number of extensions on this certificate. New in version 0.12.
Retrieve the extension on this certificate at the given index.
Extensions on a certificate are kept in order. The index parameter selects which extension will be returned. The returned object will be an X509Extension instance. New in version 0.12.
X509Name objects have the following methods:
Return an integer giving the first four bytes of the MD5 digest of the DER representation of the name.
Return a string giving the DER representation of the name.
Return a list of two-tuples of strings giving the components of the name.
X509Name objects have the following members:
The country of the entity. C may be used as an alias for countryName.
The state or province of the entity. ST may be used as an alias for stateOrProvinceNameů
The locality of the entity. L may be used as an alias for localityName.
The organization name of the entity. O may be used as an alias for organizationName.
The organizational unit of the entity. OU may be used as an alias for organizationalUnitName.
The common name of the entity. CN may be used as an alias for commonName.
The e-mail address of the entity.
X509Req objects have the following methods:
Return a PKey object representing the public key of the certificate request.
Return an X509Name object representing the subject of the certificate.
Set the public key of the certificate request to pkey.
Sign the certificate request, using the key pkey and the message digest algorithm identified by the string digest.
Verify a certificate request using the public key pkey.
Set the version (RFC 2459, 4.1.2.1) of the certificate request to version.
Get the version (RFC 2459, 4.1.2.1) of the certificate request.
The X509Store object has currently just one method:
Add the certificate cert to the certificate store.
The PKey object has the following methods:
Return the number of bits of the key.
Generate a public/private key pair of the type type (one of TYPE_RSA and TYPE_DSA) with the size bits.
Return the type of the key.
Check the consistency of this key, returning True if it is consistent and raising an exception otherwise. This is only valid for RSA keys. See the OpenSSL RSA_check_key man page for further limitations.
PKCS7 objects have the following methods:
FIXME
FIXME
FIXME
FIXME
Get the type name of the PKCS7.
PKCS12 objects have the following methods:
Returns a PKCS12 object as a string.
The optional passphrase must be a string not a callback.
See also the man page for the C function PKCS12_create.
Return CA certificates within the PKCS12 object as a tuple. Returns None if no CA certificates are present.
Return certificate portion of the PKCS12 structure.
Return friendlyName portion of the PKCS12 structure.
Return private key portion of the PKCS12 structure
Replace or set the CA certificates within the PKCS12 object with the sequence cacerts.
Set cacerts to None to remove all CA certificates.
Replace or set the certificate portion of the PKCS12 structure.
Replace or set the friendlyName portion of the PKCS12 structure.
Replace or set private key portion of the PKCS12 structure
X509Extension objects have several methods:
Return the critical field of the extension object.
Retrieve the short descriptive name for this extension.
The result is a byte string like “basicConstraints”. New in version 0.12.
Retrieve the data for this extension.
The result is the ASN.1 encoded form of the extension data as a byte string. New in version 0.12.
NetscapeSPKI objects have the following methods:
Return a base64-encoded string representation of the object.
Return the public key of object.
Set the public key of the object to key.
Sign the NetscapeSPKI object using the given key and digest_name. digest_name must be a string describing a digest algorithm supported by OpenSSL (by EVP_get_digestbyname, specifically). For example, "md5" or "sha1".
Verify the NetscapeSPKI object using the given key.
CRL objects have the following methods:
Add a Revoked object to the CRL, by value not reference.
Use cert and key to sign the CRL and return the CRL as a string. days is the number of days before the next CRL is due.
Return a tuple of Revoked objects, by value not reference.
Revoked objects have the following methods:
Return a list of all supported reasons.
Return the revocation reason as a str. Can be None, which differs from "Unspecified".
Return the revocation date as a str. The string is formatted as an ASN1 GENERALIZEDTIME.
Return a str containing a hex number of the serial of the revoked certificate.
Set the revocation reason. reason must be None or a string, but the values are limited. Spaces and case are ignored. See all_reasons.
Set the revocation date. The string is formatted as an ASN1 GENERALIZEDTIME.
serial is a string containing a hex number of the serial of the revoked certificate.
This module handles the OpenSSL pseudo random number generator (PRNG) and declares the following:
Mix bytes from string into the PRNG state. The entropy argument is (the lower bound of) an estimate of how much randomness is contained in string, measured in bytes. For more information, see e.g. RFC 1750.
Get some random bytes from the PRNG as a string.
This is a wrapper for the C function RAND_bytes.
Erase the memory used by the PRNG.
This is a wrapper for the C function RAND_cleanup.
Query the Entropy Gathering Daemon2 on socket path for bytes bytes of random data and and uses add to seed the PRNG. The default value of bytes is 255.
Read bytes bytes (or all of it, if bytes is negative) of data from the file path to seed the PRNG. The default value of bytes is -1.
Add the current contents of the screen to the PRNG state. Availability: Windows.
This is equivalent to calling add with entropy as the length of the string.
Returns true if the PRNG has been seeded with enough data, and false otherwise.
Write a number of random bytes (currently 1024) to the file path. This file can then be used with load_file to seed the PRNG again.
If the current RAND method supports any errors, this is raised when needed. The default method does not raise this when the entropy pool is depleted.
Whenever this exception is raised directly, it has a list of error messages from the OpenSSL error queue, where each item is a tuple (lib, function, reason). Here lib, function and reason are all strings, describing where and what the problem is. See err(3) for more information.
This module handles things specific to SSL. There are two objects defined: Context, Connection.
These constants represent the different SSL methods to use when creating a context object.
These constants represent the verification mode used by the Context object’s set_verify method.
File type constants used with the use_certificate_file and use_privatekey_file methods of Context objects.
Constants used with set_options of Context objects. OP_SINGLE_DH_USE means to always create a new key when using ephemeral Diffie-Hellman. OP_EPHEMERAL_RSA means to always use ephemeral RSA keys when doing RSA operations. OP_NO_SSLv2, OP_NO_SSLv3 and OP_NO_TLSv1 means to disable those specific protocols. This is interesting if you’re using e.g. SSLv23_METHOD to get an SSLv2-compatible handshake, but don’t want to use SSLv2.
Constants used with SSLeay_version to specify what OpenSSL version information to retrieve. See the man page for the SSLeay_version C API for details.
An integer giving the version number of the OpenSSL library used to build this version of pyOpenSSL. See the man page for the SSLeay_version C API for details.
Retrieve a string describing some aspect of the underlying OpenSSL version. The type passed in should be one of the SSLEAY_* constants defined in this module.
See Context.
A class representing SSL contexts. Contexts define the parameters of one or more SSL connections.
method should be SSLv2_METHOD, SSLv3_METHOD, SSLv23_METHOD or TLSv1_METHOD.
See Connection.
A class representing SSL connections.
context should be an instance of Context and socket should be a socket 3 object. socket may be None; in this case, the Connection is created with a memory BIO: see the bio_read, bio_write, and bio_shutdown methods.
This exception is used as a base class for the other SSL-related exceptions, but may also be raised directly.
Whenever this exception is raised directly, it has a list of error messages from the OpenSSL error queue, where each item is a tuple (lib, function, reason). Here lib, function and reason are all strings, describing where and what the problem is. See err(3) for more information.
This exception matches the error return code SSL_ERROR_ZERO_RETURN, and is raised when the SSL Connection has been closed. In SSL 3.0 and TLS 1.0, this only occurs if a closure alert has occurred in the protocol, i.e. the connection has been closed cleanly. Note that this does not necessarily mean that the transport layer (e.g. a socket) has been closed.
It may seem a little strange that this is an exception, but it does match an SSL_ERROR code, and is very convenient.
The operation did not complete; the same I/O method should be called again later, with the same arguments. Any I/O method can lead to this since new handshakes can occur at any time.
The wanted read is for dirty data sent over the network, not the clean data inside the tunnel. For a socket based SSL connection, read means data coming at us over the network. Until that read succeeds, the attempted OpenSSL.SSL.Connection.recv, OpenSSL.SSL.Connection.send, or OpenSSL.SSL.Connection.do_handshake is prevented or incomplete. You probably want to select() on the socket before trying again.
See WantReadError. The socket send buffer may be too full to write more data.
The operation did not complete because an application callback has asked to be called again. The I/O method should be called again later, with the same arguments. Note: This won’t occur in this version, as there are no such callbacks in this version.
The SysCallError occurs when there’s an I/O error and OpenSSL’s error queue does not contain any information. This can mean two things: An error in the transport protocol, or an end of file that violates the protocol. The parameter to the exception is always a pair (errnum, errstr).
Context objects have the following methods:
Check if the private key (loaded with use_privatekey[_file]) matches the certificate (loaded with use_certificate[_file]). Returns None if they match, raises Error otherwise.
Retrieve application data as set by set_app_data.
Retrieve the certificate store (a X509Store object) that the context uses. This can be used to add "trusted" certificates without using the. load_verify_locations() method.
Retrieve session timeout, as set by set_timeout. The default is 300 seconds.
Retrieve the Context object’s verify depth, as set by set_verify_depth.
Retrieve the Context object’s verify mode, as set by set_verify.
Read a file with PEM-formatted certificates that will be sent to the client when requesting a client certificate.
Replace the current list of preferred certificate signers that would be sent to the client when requesting a client certificate with the certificate_authorities sequence of OpenSSL.crypto.X509Names.
New in version 0.10.
Extract a OpenSSL.crypto.X509Name from the certificate_authority OpenSSL.crypto.X509 certificate and add it to the list of preferred certificate signers sent to the client when requesting a client certificate.
New in version 0.10.
Specify where CA certificates for verification purposes are located. These are trusted certificates. Note that the certificates have to be in PEM format. If capath is passed, it must be a directory prepared using the c_rehash tool included with OpenSSL. Either, but not both, of pemfile or capath may be None.
Specify that the platform provided CA certificates are to be used for verification purposes. This method may not work properly on OS X.
Load parameters for Ephemeral Diffie-Hellman from dhfile.
Associate data with this Context object. data can be retrieved later using the get_app_data method.
Set the list of ciphers to be used in this context. See the OpenSSL manual for more information (e.g. ciphers(1))
Set the information callback to callback. This function will be called from time to time during SSL handshakes. callback should take three arguments: a Connection object and two integers. The first integer specifies where in the SSL handshake the function was called, and the other the return code from a (possibly failed) internal function call.
Add SSL options. Options you have set before are not cleared! This method should be used with the OP_* constants.
Set the passphrase callback to callback. This function will be called when a private key with a passphrase is loaded. callback must accept three positional arguments. First, an integer giving the maximum length of the passphrase it may return. If the returned passphrase is longer than this, it will be truncated. Second, a boolean value which will be true if the user should be prompted for the passphrase twice and the callback should verify that the two values supplied are equal. Third, the value given as the userdata parameter to set_passwd_cb. If an error occurs, callback should return a false value (e.g. an empty string).
Set the context name within which a session can be reused for this Context object. This is needed when doing session resumption, because there is no way for a stored session to know which Context object it is associated with. name may be any binary data.
Set the timeout for newly created sessions for this Context object to timeout. timeout must be given in (whole) seconds. The default value is 300 seconds. See the OpenSSL manual for more information (e.g. SSL_CTX_set_timeout(3)).
Set the verification flags for this Context object to mode and specify that callback should be used for verification callbacks. mode should be one of VERIFY_NONE and VERIFY_PEER. If VERIFY_PEER is used, mode can be OR:ed with VERIFY_FAIL_IF_NO_PEER_CERT and VERIFY_CLIENT_ONCE to further control the behaviour. callback should take five arguments: A Connection object, an X509 object, and three integer variables, which are in turn potential error number, error depth and return code. callback should return true if verification passes and false otherwise.
Set the maximum depth for the certificate chain verification that shall be allowed for this Context object.
Use the certificate cert which has to be a X509 object.
Adds the certificate cert, which has to be a X509 object, to the certificate chain presented together with the certificate.
Load a certificate chain from file which must be PEM encoded.
Use the private key pkey which has to be a PKey object.
Load the first certificate found in file. The certificate must be in the format specified by format, which is either FILETYPE_PEM or FILETYPE_ASN1. The default is FILETYPE_PEM.
Load the first private key found in file. The private key must be in the format specified by format, which is either FILETYPE_PEM or FILETYPE_ASN1. The default is FILETYPE_PEM.
Specify a one-argument callable to use as the TLS extension server name callback. When a connection using the server name extension is made using this context, the callback will be invoked with the Connection instance. New in version 0.13.
Connection objects have the following methods:
Call the accept method of the underlying socket and set up SSL on the returned socket, using the Context object supplied to this Connection object at creation. Returns a pair (conn, address). where conn is the new Connection object created, and address is as returned by the socket’s accept.
Call the bind method of the underlying socket.
Call the close method of the underlying socket. Note: If you want correct SSL closure, you need to call the shutdown method first.
Call the connect method of the underlying socket and set up SSL on the socket, using the Context object supplied to this Connection object at creation.
Call the connect_ex method of the underlying socket and set up SSL on the socket, using the Context object supplied to this Connection object at creation. Note that if the connect_ex method of the socket doesn’t return 0, SSL won’t be initialized.
Perform an SSL handshake (usually called after renegotiate or one of set_accept_state or set_accept_state). This can raise the same exceptions as send and recv.
Retrieve the file descriptor number for the underlying socket.
Call the listen method of the underlying socket.
Retrieve application data as set by set_app_data.
Retrieve the list of ciphers used by the Connection object. WARNING: This API has changed. It used to take an optional parameter and just return a string, but not it returns the entire list in one go.
Retrieve the list of preferred client certificate issuers sent by the server as OpenSSL.crypto.X509Name objects.
If this is a client Connection, the list will be empty until the connection with the server is established.
If this is a server Connection, return the list of certificate authorities that will be sent or has been sent to the client, as controlled by this Connection’s Context.
New in version 0.10.
Retrieve the Context object associated with this Connection.
Specify a replacement Context object for this Connection.
Retrieve the other side’s certificate (if any)
Retrieve the tuple of the other side’s certificate chain (if any)
Call the getpeername method of the underlying socket.
Call the getsockname method of the underlying socket.
Call the getsockopt method of the underlying socket.
Retrieve the number of bytes that can be safely read from the SSL buffer (not the underlying transport buffer).
Receive data from the Connection. The return value is a string representing the data received. The maximum amount of data to be received at once, is specified by bufsize.
If the Connection was created with a memory BIO, this method can be used to add bytes to the read end of that memory BIO. The Connection can then read the bytes (for example, in response to a call to recv).
Renegotiate the SSL session. Call this if you wish to change cipher suites or anything like that.
Send the string data to the Connection.
If the Connection was created with a memory BIO, this method can be used to read bytes from the write end of that memory BIO. Many Connection methods will add bytes which must be read in this manner or the buffer will eventually fill up and the Connection will be able to take no further actions.
Send all of the string data to the Connection. This calls send repeatedly until all data is sent. If an error occurs, it’s impossible to tell how much data has been sent.
Set the connection to work in server mode. The handshake will be handled automatically by read/write.
Associate data with this Connection object. data can be retrieved later using the get_app_data method.
Set the connection to work in client mode. The handshake will be handled automatically by read/write.
Call the setblocking method of the underlying socket.
Call the setsockopt method of the underlying socket.
Send the shutdown message to the Connection. Returns true if the shutdown message exchange is completed and false otherwise (in which case you call recv() or send() when the connection becomes readable/writeable.
Get the shutdown state of the Connection. Returns a bitvector of either or both of SENT_SHUTDOWN and RECEIVED_SHUTDOWN.
Set the shutdown state of the Connection. state is a bitvector of either or both of SENT_SHUTDOWN and RECEIVED_SHUTDOWN.
Call the shutdown method of the underlying socket.
If the Connection was created with a memory BIO, this method can be used to indicate that “end of file” has been reached on the read end of that memory BIO.
Retrieve a verbose string detailing the state of the Connection.
Retrieve the random value used with the client hello message.
Retrieve the random value used with the server hello message.
Retrieve the value of the master key for this session.
Checks if more data has to be read from the transport layer to complete an operation.
Checks if there is data to write to the transport layer to complete an operation.
Specify the byte string to send as the server name in the client hello message. New in version 0.13.
Get the value of the server name received in the client hello message. New in version 0.13.
We ran into three main problems developing this: Exceptions, callbacks and accessing socket methods. This is what this chapter is about.
We realized early that most of the exceptions would be raised by the I/O functions of OpenSSL, so it felt natural to mimic OpenSSL’s error code system, translating them into Python exceptions. This naturally gives us the exceptions SSL.ZeroReturnError, SSL.WantReadError, SSL.WantWriteError, SSL.WantX509LookupError and SSL.SysCallError.
For more information about this, see section 3.3.
There are a number of problems with callbacks. First of all, OpenSSL is written as a C library, it’s not meant to have Python callbacks, so a way around that is needed. Another problem is thread support. A lot of the OpenSSL I/O functions can block if the socket is in blocking mode, and then you want other Python threads to be able to do other things. The real trouble is if you’ve released the global CPython interpreter lock to do a potentially blocking operation, and the operation calls a callback. Then we must take the GIL back, since calling Python APIs without holding it is not allowed.
There are two solutions to the first problem, both of which are necessary. The first solution to use is if the C callback allows ”userdata” to be passed to it (an arbitrary pointer normally). This is great! We can set our Python function object as the real userdata and emulate userdata for the Python function in another way. The other solution can be used if an object with an ”app_data” system always is passed to the callback. For example, the SSL object in OpenSSL has app_data functions and in e.g. the verification callbacks, you can retrieve the related SSL object. What we do is to set our wrapper Connection object as app_data for the SSL object, and we can easily find the Python callback.
The other problem is solved using thread local variables. Whenever the GIL is released before calling into an OpenSSL API, the PyThreadState pointer returned by PyEval_SaveState is stored in a global thread local variable (using Python’s own TLS API, PyThread_set_key_value). When it is necessary to re-acquire the GIL, either after the OpenSSL API returns or in a C callback invoked by that OpenSSL API, the value of the thread local variable is retrieved (PyThread_get_key_value) and used to re-acquire the GIL. This allows Python threads to execute while OpenSSL APIs are running and allows use of any particular pyOpenSSL object from any Python thread, since there is no per-thread state associated with any of these objects and since OpenSSL is threadsafe (as long as properly initialized, as pyOpenSSL initializes it).
We quickly saw the benefit of wrapping socket methods in the SSL.Connection class, for an easy transition into using SSL. The problem here is that the socket module lacks a C API, and all the methods are declared static. One approach would be to have OpenSSL as a submodule to the socket module, placing all the code in ‘socketmodule.c’, but this is obviously not a good solution, since you might not want to import tonnes of extra stuff you’re not going to use when importing the socket module. The other approach is to somehow get a pointer to the method to be called, either the C function, or a callable Python object. This is not really a good solution either, since there’s a lot of lookups involved.
The way it works is that you have to supply a “socket-like” transport object to the SSL.Connection. The only requirement of this object is that it has a fileno() method that returns a file descriptor that’s valid at the C level (i.e. you can use the system calls read and write). If you want to use the connect() or accept() methods of the SSL.Connection object, the transport object has to supply such methods too. Apart from them, any method lookups in the SSL.Connection object that fail are passed on to the underlying transport object.
Future changes might be to allow Python-level transport objects, that instead of having fileno() methods, have read() and write() methods, so more advanced features of Python can be used. This would probably entail some sort of OpenSSL “BIOs”, but converting Python strings back and forth is expensive, so this shouldn’t be used unless necessary. Other nice things would be to be able to pass in different transport objects for reading and writing, but then the fileno() method of SSL.Connection becomes virtually useless. Also, should the method resolution be used on the read-transport or the write-transport?