Transport Layer Security
|Internet||IP ( IPv4 , IPv6 )|
Transport Layer Security ( TLS , English for Transport Layer Security ), wide known under the previous name Secure Sockets Layer ( SSL ), is a hybrid encryption protocol for secure data transmission in the Internet . The last version of the SSL protocol was version 3.0; then it was further developed and standardized under the new name TLS, starting with version 1.0.
TLS in practice
TLS encryption is currently used primarily with HTTPS . Most current web browsers and web servers prefer TLS 1.3 and TLS 1.2, mostly TLS 1.1 and TLS 1.0 are also supported, but this leads to a security warning. SSLv3 and SSLv2 are deactivated in current browsers, as this protocol version has a number of security gaps, including the Poodle attack . The further development TLS 1.3 is supported by Google Chrome , TLS 1.2 is used in the standard configuration of Internet Explorer, Firefox, Google Chrome, Opera and Apple iOS Safari (status 02/2014).
For some time now, more and more website operators have been using Extended Validation TLS certificates (EV-TLS certificates). A field is also displayed in the address line of the browser in which the certificate and domain holder alternates with the certification authority . In addition, depending on the browser and / or add-on used, the address line is (partially) colored green. Internet users should be able to recognize even faster whether the website they are visiting is genuine and be better protected against phishing attempts . From a technical point of view, EV-TLS certificates do not offer any extended protection; the encryption and its strength are identical. Only the owner is verified better and more elaborately.
Since January 2017, the Chrome web browser has been marking websites that collect information without using HTTPS as unsafe. This is expected to lead to a significant increase in the use of HTTPS. In February 2017, HTTPS was activated in 2.57% of all registered German Internet domains as well as in 3.70% of Austrian domains and 9.71% of Swiss domains. An examination of around 40,000 websites of small and medium-sized companies in Baden-Württemberg by the State Commissioner for Data Protection and Freedom of Information in Baden-Württemberg has shown that around 7% of the websites examined are offered via HTTPS. For those websites that are offered via HTTPS, server-side support for TLS 1.0 is still very widespread (99%).
Without a certificate-based authentication, TLS is susceptible to man-in-the-middle attacks : If the man-in-the-middle is active before the key is handed over, it can fool both sides of its keys and thus record all data traffic in plain text and manipulate unnoticed. Because of the lack of trustworthiness of some certification authorities, the security of TLS has been questioned since the beginning of 2010. However, this risk can largely be eliminated by deactivating questionable certification authorities in your own browser.
In connection with a virtual server , for example with HTTP ( for example with the Apache HTTP server via the VHost mechanism), it is fundamentally a disadvantage that only one certificate can be used per combination of IP address and port , since the actual user data of the protocol above (and thus the name of the VHost) were not yet transmitted at the time of the TLS handshake. This problem was corrected with the TLS extension Server Name Indication (SNI) in June 2003 by the RFC 3546 . The required server name is sent when the connection is established. The original extension was described for TLS 1.0, due to the compatibility of the individual TLS versions with one another, SNI is also implemented for TLS 1.1 and TLS 1.2 in accordance with the recommendation.
In the Tor network , TLS certificates are of particular importance for connections to the Internet, since an unencrypted connection using a man-in-the-middle attack there by the computer establishing the connection to the Internet (referred to as " exit node ”) would be very easy. However, since a connection between two endpoints in the Tor network is encrypted, the encrypted transmission of data within the network can be considered of secondary importance, provided one trusts the routing of the connections. Here the main feature of TLS encryption is the authenticity of the other side.
- POP3S for POP3
- SMTPS for SMTP
- NNTPS for NNTP
- SIPS for SIP
- IMAPS for IMAP
- XMPPS for XMPP
- IRCS for IRC
- LDAPS for LDAP
- MBS / IP -TLS
- FTPS for FTP
- EAP -TLS
- TN3270 -TLS
|Older version; no longer supported: SSL 1.0||1994|
|Older version; no longer supported: SSL 2.0||1995||Use not permitted since March 2011.|
|Older version; no longer supported: SSL 3.0||1996||Use not permitted since June 2015.|
|Older version; still supported: TLS 1.0||1999||Support is running out. No longer complies with the Payment Card Industry Data Security Standard (PCI DSS) in payment transactions as of June 30, 2018 .|
|Older version; still supported: TLS 1.1||2006||Support is running out. Since TLS 1.1 uses the non- collision-resistant hash function SHA-1 for creating the signature, the BSI advises against its use.|
|Older version; still supported: TLS 1.2||2008|
|Current version: TLS 1.3||2018||RFC 8446 , also contains new requirements for TLS 1.2|
|Legend:||Older version; no longer supported||Older version; still supported||Current version||Current preliminary version||Future version|
- In 1994, nine months after the first release of Mosaic , the first popular web browser , Netscape Communications completed the first version of SSL (1.0).
- Five months later the next version SSL 2.0 was published together with a new edition of the Netscape Navigator.
- In late 1995, Microsoft released the first version of its Internet Explorer web browser . Shortly thereafter, the first version of its SSL counterpart, PCT 1.0 (Private Communication Technology), also became known. PCT had several advantages over SSL 2.0 that were later incorporated into SSL 3.0.
- When SSL was set as the standard by the IETF in RFC 2246 , it was renamed Transport Layer Security (TLS) in January 1999 . The differences between SSL 3.0 and TLS 1.0 are only minor. But the renaming resulted in version confusion. TLS 1.0 reports in the header as version SSL 3.1.
- TLS was later extended by further RFCs :
- RFC 2712 - Addition of Kerberos Cipher Suites to Transport Layer Security (TLS).
- RFC 2817 - Upgrading to TLS Within HTTP / 1.1 explains the use of the upgrade mechanism in HTTP / 1.1 to initialize Transport Layer Security (TLS) over an existing TCP connection. This makes it possible to use the same “well-known” TCP ports (80 and 443) for both insecure and secure HTTP traffic.
- RFC 2818 - HTTP Over TLS separates secure from unsecure traffic through separate server TCP ports.
- RFC 3268 - Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS) uses the extensibility of TLS and adds the symmetrical encryption algorithms ( RC2 , RC4 , International Data Encryption Algorithm (IDEA), Data Encryption Standard (DES) and Triple DES ) the Advanced Encryption Standard (AES).
- RFC 3546 - Transport Layer Security (TLS) Extensions introduces the concept of extensions, which means that optional data fields or headers can be transmitted especially during the initial negotiation. One of these extensions is Server Name Indication .
- In April 2006, version 1.1 of TLS was standardized in RFC 4346, making RFC 2246 obsolete. In TLS 1.1, minor security improvements have been made and ambiguities have been eliminated.
- In August 2008, version 1.2 of TLS appeared with RFC 5246 , which made RFC 4346 obsolete. The definition of MD5 / SHA-1 in the pseudo random function (PRF) and signed elements has been replaced by more flexible solutions in which the hash algorithms can be specified.
- In February 2015, RFC 7465 was published banning RC4 for encryption.
- In May 2015, recommendations for the secure use of TLS and DTLS were published with RFC 7525 . Accordingly, SSLv2, SSLv3, RC4 and other encryption algorithms that are restricted to a key length of less than 112 bits due to export restrictions should not be used. The use of 3DES for encryption and RSA for key exchange with static parameters is not recommended. We recommend cipher suites that use Ephemeral Diffie-Hellman combined with RSA for key exchange , which offers forward secrecy (against subsequent subsequent decryption), for encryption AES in Galois / Counter mode with 128 or 256 bit key length and the hash function SHA-256 or SHA -384 for the pseudo-random function of TLS.
- In August 2018, TLS version 1.3 was published in RFC 8446 , which has been developed since 2014.
- In October 2018, the manufacturers of the Firefox, Chrome, Edge and Safari browsers stated that they would no longer support the outdated TLS 1.0 and 1.1 protocols starting in March 2020. In Google Chrome 84, support for TLS 1.0 and 1.1 has therefore been removed.
The client establishes a connection to the server. The server authenticates itself to the client with a certificate . The client checks the trustworthiness of the X.509 certificate and whether the server name matches the certificate. Optionally, the client can also authenticate itself to the server with its own certificate. Then either the client sends the server a secret random number encrypted with the server's public key , or the two parties use the Diffie-Hellman key exchange to calculate a shared secret. A cryptographic key is then derived from the secret. This key is then used to encrypt all messages on the connection with a symmetrical encryption method and to protect the message integrity and authenticity with a message authentication code .
TLS protocols in the protocol stack
In the OSI model , TLS is arranged in layer 5 (the session layer). In the TCP / IP model, TLS is located above the transport layer (for example TCP ) and below application protocols such as HTTP or SMTP . In the specifications this is then referred to as "HTTP over TLS", for example. However, if both protocols are to be considered together, an “S” for Secure is usually appended to the protocol of the application layer (for example HTTP S ). TLS works transparently so that it can easily be used to provide secure connections to protocols without their own security mechanisms. It can also be expanded to ensure flexibility and future security for the encryption technology used.
The TLS protocol consists of two layers:
|TLS handshake protocol||TLS Change Cipher Spec. Protocol||TLS Alert Protocol||TLS Application Data Protocol|
|TLS Record Protocol|
TLS Record Protocol
The TLS Record Protocol is the lower of the two layers and is used to secure the connection. It is based directly on the transport layer and offers two different services that can be used individually or jointly:
- End-to-end encryption using symmetrical algorithms . The key used is negotiated in advance via a further protocol (for example the TLS handshake protocol) and can only be used once for the respective connection. TLS support for symmetric encryption , among other DES , Triple DES and AES
- Ensuring the message integrity and authenticity by means of a Message Authentication Code , usually HMAC .
In addition, the data to be backed up is fragmented in blocks of a maximum of 16,384 (2 14 ) bytes and reassembled at the recipient. The standard stipulates that the block size does not exceed this value, unless the block is compressed or encrypted - then the block size may be 1024 bytes (with compression) or 2048 bytes (with encryption) larger. The data can also be compressed before the encryption and before the calculation of the cryptographic checksum. The compression procedure, like the cryptographic key, is negotiated using the TLS handshake protocol.
The structure of a TLS record message is as follows: Content Type (1 byte: Change Cipher Spec = 20, Alert = 21, Handshake = 22, Application Data = 23) | Protocol version major (1 byte) | Protocol version minor (1 byte) | Length (1 short or two bytes)
TLS handshake protocol
The TLS Handshake Protocol is based on the TLS Record Protocol and fulfills the following functions even before the first bits of the application data stream have been exchanged:
- Identification and authentication of communication partners on the basis of asymmetric encryption methods and public key cryptography . This step is optionally a two-way authentication (in this case mutual TLS is sometimes used ), but usually only the server authenticates itself to the client .
- Negotiating cryptographic algorithms and keys to be used. TLS also supports unencrypted transmission.
The handshake itself can be divided into four phases :
- The client sends a client_hello to the server, and the server replies to the client with a server_hello . The parameters of the messages are:
- the version (the highest TLS protocol version supported by the client)
- a 32 byte long random information (4 byte timestamp + 28 byte long random number) which is later used to create the pre-master secret (it protects against replay attacks )
- a session ID
- the cipher suite to be used (algorithms for key exchange, encryption and authentication)
- Optionally, the desired FQDN for Server Name Indication support
- The server identifies itself to the client. The X.509v 3 certificate is also transmitted to the client here, which the client then verifies. If this is unsuccessful, the client breaks the connection. In addition, the server can optionally request a certificate for client authentication via CertificateRequest . This phase can only be omitted if an anonymous cipher suite is used without authentication.
- Here the client uses a certificate to identify itself to the server, provided that the server has sent a CertificateRequest . If the client does not have a certificate, it replies with an empty certificate list. The server certificate received previously contains the public key of the server. If a cipher suite with RSA key exchange is used (see figure), the pre-master secret generated by the client is encrypted with this public key and can be decrypted again by the server with the private key known only to it. Alternatively, the Diffie-Hellman key exchange can also be used here to generate a common pre-master secret . If the Diffie-Hellman secrets of the server and client are freshly and randomly negotiated during the handshake, the requirements for Perfect Forward Secrecy are met.
- This phase completes the handshake. From the existing secret pre-master that can master secret can be derived that a one-time session key ( English session key ) represents. Keys, which are used to encrypt and decrypt the data and for the integrity check, are derived from the master secret . The messages that the communication partners now send to each other are only transmitted in encrypted form.
TLS Change Cipher Spec Protocol
The Change Cipher Spec Protocol only consists of a single message. This message is one byte in size and has the content 1. With this message, the sender informs the recipient that he is switching to the cipher suite negotiated in the handshake protocol in the active session . A major reason for defining a separate protocol for this message is that TLS implementations can combine several messages of a protocol in one record (i.e. a TLS data unit). This is undesirable for the “Change Cipher Spec” message. Because records from different protocols cannot be combined, the problem is solved by defining a separate protocol.
TLS Alert Protocol
The Alert Protocol distinguishes around two dozen different messages. One of them announces the end of the session (close_notify). Others relate, for example, to the protocol syntax or the validity of the certificates used. A distinction is made between warnings and errors, with the latter terminating the connection immediately.
The structure of an error message is as follows: AlertLevel (1 byte: Warning = 1, Fatal = 2) | AlertDescription (1 byte: close_notify = 0, […], no_renegotiation = 100).
The following serious error types are defined in the specification of TLS:
|unexpected_message||Inappropriate message was received.|
|bad_record_mac||An incorrect MAC was received.|
|decompression_failure||Decompression algorithm received incorrect data.|
|handshake_failure||Sender was unable to edit an acceptable set of security parameters.|
|illegal_parameter||A field in the handshake message was out of range or conflicted with other fields.|
The following warnings are defined in the specification of TLS:
|close_notify||Informs recipients that the sender will not send any further messages on this connection. Must be sent as the last message by every partner in a connection.|
|no_certificate||Can be sent in response to a certificate request if the appropriate certificate is not available. (Removed in TLS 1.0)|
|bad_certificate||Received certificate was incomplete or incorrect.|
|unsupported_certificate||The type of the receiving certificate is not supported.|
|certificate_revoked||Certificate was recalled by the signer.|
|certificate_expired||Certificate has expired.|
|certificate_unknown||Other, unspecified reasons occurred while the certificate was being edited that resulted in the certificate being marked as invalid.|
The following warnings have been added to the specification of TLS 1.0:
|unknown_ca||Unknown or untrustworthy CA.|
|protocol_version||Outdated version of TLS / SSL.|
|user_canceled||Cancellation by user.|
TLS Application Data Protocol
The application data is transported via the Record Protocol, broken down into parts, compressed and, depending on the current status of the session, also encrypted. In terms of content, they are not interpreted in more detail by TLS.
Calculation of the master secret
In earlier protocol versions , the master secret is calculated from the pre-master secret with the help of the hash functions SHA-1 and MD5 , in TLS 1.2 with the help of a pseudo-random function specified by a cipher suite . The random numbers from phase 1 of the handshake are also included in this calculation. Using both hash functions should ensure that the master secret is still protected in the event that one of the functions is deemed to have been compromised. In TLS 1.2 this approach is replaced by the flexible interchangeability of the function.
A number of attacks are known to undermine security guarantees on both SSL and TLS. The following list represents some of the known attacks.
Padding Oracle attacks
The cryptologist Serge Vaudenay discovered in 2002 that a man-in-the-middle attacker can obtain information from the padding of a message encrypted with the Cipher Block Chaining Mode (CBC) that can be used to decrypt the message. Through targeted manipulation of an encrypted message, the attacker learns whether the server is reporting a valid padding and thus part of the plaintext has been correctly guessed.
As a protective measure, the server should discard invalid messages without revealing whether the padding or message authenticity was invalid. However, an attacker can also derive this information by analyzing the response times (timing attack). SSL, TLS up to version 1.2 and DTLS are affected, provided a cipher suite with CBC is used. Cipher suites with authenticated encryption are not affected.
In October 2014, security researchers demonstrated the POODLE attack ( Padding Oracle On Downgraded Legacy Encryption ), with which an attacker forces a version downgrade of a TLS connection in order to carry out a Padding Oracle attack against SSL 3.0. For compatibility reasons, SSL 3.0 was still supported by web browsers and other implementations, despite security weaknesses known at the time. Subsequently, the Internet Engineering Task Force marked SSL 3.0 as outdated and specified a method to protect against downgrade attacks on TLS.
SSL 3.0 and TLS 1.0 use a predictable initialization vector in CBC mode. An attacker can use a chosen plaintext attack to determine unknown parts of the plaintext. One attack scenario is the stealing of HTTP cookies , which are transmitted in encrypted form. To do this, the attacker must lure the attack victim to a malicious website that repeatedly triggers HTTP requests to a foreign domain, with the web browser automatically sending the HTTP cookies set for the domain. The attacker can guess the cookie character by character due to the partly self-selected content of the HTTP requests and by listening to the encrypted TLS messages.
The basics of the attack were described in 2004 and first demonstrated in practice in 2011 under the name BEAST ( Browser Exploit Against SSL / TLS ). TLS version 1.1 and higher are not affected as each message is encrypted with a pseudo-random initialization vector.
The use of the optional compression of user data opens up a class of attacks that allow parts of the plaintext to be guessed. The attack scenario is similar to that of the BEAST attack : the attacker carries out a chosen plain text attack and observes the encrypted TLS messages in the network. The compression method removes redundancies from the user data, so that the plain text to be encrypted and thus also the ciphertext becomes shorter. If the attacker has guessed part of the unknown plaintext, for example a character from an HTTP cookie, he learns this from the difference in length of an encrypted TLS message.
The attack was published in 2012 by the creators of the BEAST attack under the name CRIME ( Compression Ratio Info-leak Made Easy ). In addition to SSL and TLS, the SPDY protocol is also affected. The use of compression is not recommended as a protective measure. TLS 1.3 or higher no longer supports compression. The SPDY successor HTTP / 2 uses a simplified compression format ( HPACK ), which compresses less efficiently than Deflate , but is more difficult to attack.
TIME and BREACH are improved variants of the attack. TIME ( Timing Info-leak Made Easy ) derives the size of an encrypted TLS message from the response time without having to listen to the network traffic. Both attacks allow guessing of TLS-encrypted content if TLS compression is switched off and HTTP compression is used instead . Since TLS cannot fundamentally prevent compression attacks, application-specific protective measures must be used, for example not using compression at all.
Downgrade to export encryption
As a result of the export restrictions on cryptography from the United States , numerous exportable cipher suites that only use short keys are specified in TLS. Despite known security weaknesses, some of these were or are still supported by implementations. The TLS handshake is actually intended to prevent a man-in-the-middle attacker from forcing a downgrade to an unsolicited cipher suite by authenticating the handshake messages. The security of the authentication also depends on the negotiated cipher suite, so that the attacker can break the key.
In the FREAK attack ( Factoring RSA Export Keys ) published in 2015, a downgrade to RSA-based cipher suites with 512-bit export keys took place. The attack assumes an implementation error in which the client uses the 512-bit key instead of the longer key from the server certificate. The bug related to OpenSSL and SecureTransport (Apple), among others.
Shortly thereafter, a team of researchers published the Logjam attack, which enables the Diffie-Hellman key exchange to be downgraded to 512-bit residual class groups. The reason is the support of exportable cipher suites with ephemeral Diffie-Hellman. In contrast to FREAK, there is a weakness in the protocol in TLS that can be exploited without implementation errors. The Logjam attack can be carried out with high performance in practice, since a large part of the computing work to break the key can be carried out before the connection is established. The computational effort required during the actual key exchange takes about 70 seconds. As a protective measure, servers should switch off support for exportable cipher suites and use groups of at least 2048 bits. Clients should discard groups that are less than 1024 bits.
In addition to security weaknesses in the protocol, TLS implementations are affected by security-relevant implementation errors on a recurring basis. One of the most serious bugs was the Heartbleed bug discovered in OpenSSL in 2014 .
Public and deliberate breach of encryption
A social attack on the TLS standard was started via the ETSI , in which a version of the standard that can be encrypted and therefore has to be regarded as broken is to find its way into general communication processes.
The attackers derive a justification for their attack on the encryption from the fact that there must be opportunities in business, especially in the financial industry, as well as in authorities, to gain a higher-level insight into encrypted communication without those involved being aware of it. Many professionals and organizations such as B. the EFF warn very clearly against the use of this procedure due to the possible collateral damage.
The attempt to introduce this defective encryption into the TLS family as “eTLS” (“Enterprise TLS”) was fought off by naming rights to TLS. which is why the process will be renamed ETS.
Since ETS / eTLS is recognized as a CVE by TLS, ETS / eTLS can also be described as (deliberately) incorrect implementation of TLS.
As a result, the Technical Committee CYBER of ETSI 2019 received the negative BigBrotherAward :
"For his efforts to define the 'Enterprise Transport Security' protocol (ETS) as part of the new technical standard for encryption on the Internet and thus to equip secure connections with a predetermined breaking point"
Advantages and disadvantages
The advantage of the TLS protocol is the ability to implement any higher protocol based on the TLS protocol. This guarantees independence from applications and systems.
The disadvantage of TLS-encrypted transmission is that the connection setup on the server side is computationally intensive and therefore slower. The encryption itself requires little computing time , depending on the algorithm used .
TLS only encrypts communication between two stations. Scenarios are conceivable in service-oriented architectures in which a message is sent via several stations. If each station is only allowed to read part of the message, TLS is not sufficient because each station can decrypt all of the data in the message. This creates security gaps at every station that cannot decrypt data intended for it.
The best-known program libraries that implement Transport Layer Security include:
- Network Security Services
- mbed TLS , formerly PolarSSL
- SChannel (Microsoft)
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