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TLS/SSL core

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TLS/SSL is a cryptographic communication protocol providing a way of securing protocols such as FTP, HTTP, SMTP, IMAP, POP3, Telnet or Syslog.

TLS and SSL are different versions of the same protocol. TLS 1.0 is a name for what was supposed to be SSL 3.1. When we use the terms "SSL" or "TLS", we generally mean "TLS or SSL".

Tip: Read more about the differences between implicit and explicit modes.

TLS/SSL protocol versions 

Use Parameters.Version property (on client object) to enable or disable specific versions of the TLS/SSL protocol. Supported versions:

  • TLS 1.3
  • TLS 1.2
  • TLS 1.1 (deprecated)
  • TLS 1.0 (deprecated)
  • SSL 3.0 (deprecated)

TLS 1.3 cipher suites 

The following TLS 1.3 ciphers are supported:

  • TLS_AES_128_GCM_SHA256 (AES/GCM with 128-bit key)
  • TLS_AES_256_GCM_SHA384 (AES/GCM with 256-bit key)
  • TLS_CHACHA20_POLY1305_SHA256 (ChaCha20-Poly1305 AEAD cipher)

Use Parameters.SetSymmetricCipherSuites(...) method (on the client object) to specify a list of allowed TLS 1.3 symmetric cipher suites, and Parameters.GetSymmetricCipherSuites() method to retrieve the current setting.

Note: TLS_CHACHA20_POLY1305_SHA256 cipher is not enabled by default. It uses a managed implementation that is slower than AES/GCM alternatives on mainstream Windows platforms.

TLS 1.3 key exchange algorithms 

The following TLS 1.3 key exchange algorithms are supported:

  • secp256r1 (ECDH with NIST P-256 curve)
  • secp384r1 (ECDH with NIST P-384 curve)
  • secp521r1 (ECDH with NIST P-521 curve)
  • brainpoolP256r1tls13 (ECDH with Brainpool P256r1 curve)
  • brainpoolP384r1tls13 (ECDH with Brainpool P384r1 curve)
  • brainpoolP512r1tls13 (ECDH with Brainpool P512r1 curve)
  • x25519 (ECDH with X25519 curve)
  • ffdhe2048 (Diffie-Hellman with 2048-bit modulus)
  • ffdhe3072 (Diffie-Hellman with 3072-bit modulus)
  • ffdhe4096 (Diffie-Hellman with 4096-bit modulus)
  • ffdhe6144 (Diffie-Hellman with 6144-bit modulus)
  • ffdhe8192 (Diffie-Hellman with 8192-bit modulus)
  • X25519MLKEM768 (Post-quantum hybrid ECDH and ML-KEM key agreement)
  • SecP256r1MLKEM768 (Post-quantum hybrid ECDH and ML-KEM key agreement)
  • SecP384r1MLKEM1024 (Post-quantum hybrid ECDH and ML-KEM key agreement)
Note: On Xamarin, Mono and older Windows platforms, ECDSA elliptic curve algorithms are only available with an external plugins. ML-KEM requires a plugin unless running on Windows 11 25H2.

TLS 1.3 signature algorithms 

The following TLS 1.3 signature algorithms are supported:

  • rsa_pss_rsae_sha256 (RSA with RSASSA-PSS scheme and SHA-256)
  • rsa_pss_rsae_sha384 (RSA with RSASSA-PSS scheme and SHA-384)
  • rsa_pss_rsae_sha512 (RSA with RSASSA-PSS scheme and SHA-512)
  • ecdsa_secp256r1_sha256 (ECDSA with NIST P-256 curve and SHA-256)
  • ecdsa_secp384r1_sha384 (ECDSA with NIST P-384 curve and SHA-384)
  • ecdsa_secp521r1_sha512 (ECDSA with NIST P-521 curve and SHA-512)
  • ecdsa_brainpoolP256r1tls13_sha256 (ECDSA with Brainpool P256r1 and SHA-256)
  • ecdsa_brainpoolP384r1tls13_sha384 (ECDSA with Brainpool P384r1 and SHA-384)
  • ecdsa_brainpoolP512r1tls13_sha512 (ECDSA with Brainpool P512r1 and SHA-512)
  • ed25519 (EdDSA with edwards25519 curve and SHA-512)
  • mldsa44 (ML-DSA-44, Level 1)
  • mldsa65 (ML-DSA-65, Level 3)
  • mldsa87 (ML-DSA-87, Level 5)

In TLS 1.3, certificates signed by the following signature algorithms are supported:

  • ECDSA with SHA-256, SHA-384 or SHA-512 (corresponds to TLS 1.3 ecdsa_* algorithms)
  • ECDSA with SHA-1 (legacy certificates; corresponds to TLS 1.3 ecdsa_sha1 algorithm)
  • RSASSA-PSS with SHA-256, SHA-384 or SHA-512 (corresponds to TLS 1.3 rsa_pss_rsae_* algorithms)
  • RSASSA-PKCS1-v1_5 with SHA-256, SHA-384 or SHA-512 (corresponds to TLS 1.3 rsa_pkcs1_* algorithms)
  • RSASSA-PKCS1-v1_5 with SHA-1 (legacy certificates; corresponds to TLS 1.3 rsa_pkcs1_sha1 algorithm)
  • ML-DSA 44/65/87 (experimental, ready for real world testing)
  • EdDSA with edwards25519 curve and SHA-512 (corresponds to TLS 1.3 ed25519; needs a custom certificate verifier)
Note: On Xamarin, Mono and older Windows platforms, ECDSA elliptic curve algorithms are only available with an external plugins. ML-DSA requires a plugin unless running on Windows 11 25H2 or up-to-date Linux with .NET 10 or later.

TLS 1.2/1.1/1.0 cipher suites 

The Parameters.AllowedSuites property of the TlsSocket object makes it possible to specify a combination of following algorithms:

Cipher ID Certificate Key Algorithm Key Exchange Algorithm Encryption Algorithm MAC Alg.
RSA_WITH_​AES_128_GCM_SHA256 RSA RSA AES/GCM AEAD
RSA_WITH_​AES_256_GCM_SHA384 AES/GCM AEAD
RSA_WITH_​AES_128_CBC_SHA256 AES/CBC SHA-256
RSA_WITH_​AES_256_CBC_SHA256 AES/CBC SHA-256
RSA_EXPORT_WITH_​RC4_40_MD5 RC4 MD5
RSA_WITH_​RC4_128_MD5 RC4 MD5
RSA_WITH_​RC4_128_SHA RC4 SHA-1
RSA_EXPORT_WITH_​RC2_CBC_40_MD5 RC2/CBC MD5
RSA_EXPORT_WITH_​DES40_CBC_SHA DES/CBC SHA-1
RSA_WITH_​DES_CBC_SHA DES/CBC SHA-1
RSA_WITH_​3DES_EDE_CBC_SHA 3DES/CBC SHA-1
RSA_EXPORT1024_WITH_​DES_CBC_SHA DES/CBC SHA-1
RSA_EXPORT1024_WITH_​RC4_56_SHA RC4 SHA-1
RSA_WITH_​AES_128_CBC_SHA AES/CBC SHA-1
RSA_WITH_​AES_256_CBC_SHA AES/CBC SHA-1
ECDHE_RSA_WITH_​AES_128_GCM_SHA256 RSA Elliptic Curve Diffie-Hellman AES/GCM AEAD
ECDHE_RSA_WITH_​AES_256_GCM_SHA384 AES/GCM AEAD
ECDHE_RSA_WITH_​CHACHA20_POLY1305_SHA256 ChaCha20-Poly1305 AEAD
ECDHE_RSA_WITH_​AES_128_CBC_SHA256 AES/CBC SHA-256
ECDHE_RSA_WITH_​AES_256_CBC_SHA384 AES/CBC SHA-384
ECDHE_RSA_WITH_​AES_128_CBC_SHA AES/CBC SHA-1
ECDHE_RSA_WITH_​AES_256_CBC_SHA AES/CBC SHA-1
ECDHE_RSA_WITH_​3DES_EDE_CBC_SHA 3DES/CBC SHA-1
ECDHE_RSA_WITH_​RC4_128_SHA RC4 SHA-1
ECDHE_ECDSA_WITH_​AES_128_GCM_SHA256 Elliptic Curve DSA Elliptic Curve Diffie-Hellman AES/GCM AEAD
ECDHE_ECDSA_WITH_​AES_256_GCM_SHA384 AES/GCM AEAD
ECDHE_ECDSA_WITH_​CHACHA20_POLY1305_SHA256 ChaCha20-Poly1305 AEAD
ECDHE_ECDSA_WITH_​AES_128_CBC_SHA256 AES/CBC SHA-256
ECDHE_ECDSA_WITH_​AES_256_CBC_SHA384 AES/CBC SHA-384
ECDHE_ECDSA_WITH_​AES_128_CBC_SHA AES/CBC SHA-1
ECDHE_ECDSA_WITH_​AES_256_CBC_SHA AES/CBC SHA-1
ECDHE_ECDSA_WITH_​3DES_EDE_CBC_SHA 3DES/CBC SHA-1
ECDHE_ECDSA_WITH_​RC4_128_SHA RC4 SHA-1
DHE_RSA_WITH_​AES_128_GCM_SHA256 RSA Diffie-Hellman AES/GCM AEAD
DHE_RSA_WITH_​AES_256_GCM_SHA384 AES/GCM AEAD
DHE_RSA_WITH_​CHACHA20_POLY1305_SHA256 ChaCha20-Poly1305 AEAD
DHE_RSA_WITH_​AES_128_CBC_SHA256 AES/CBC SHA-256
DHE_RSA_WITH_​AES_256_CBC_SHA256 AES/CBC SHA-256
DHE_RSA_EXPORT_WITH_​DES40_CBC_SHA DES/CBC SHA-1
DHE_RSA_WITH_​DES_CBC_SHA DES/CBC SHA-1
DHE_RSA_WITH_​3DES_EDE_CBC_SHA 3DES/CBC SHA-1
DHE_RSA_WITH_​AES_128_CBC_SHA AES/CBC SHA-1
DHE_RSA_WITH_​AES_256_CBC_SHA AES/CBC SHA-1
DHE_DSS_WITH_​AES_128_GCM_SHA256 DSS Diffie-Hellman AES/GCM AEAD
DHE_DSS_WITH_​AES_256_GCM_SHA384 AES/GCM AEAD
DHE_DSS_WITH_​AES_128_CBC_SHA256 AES/CBC SHA-256
DHE_DSS_WITH_​AES_256_CBC_SHA256 AES/CBC SHA-256
DHE_DSS_EXPORT_WITH_​DES40_CBC_SHA DES/CBC SHA-1
DHE_DSS_WITH_​DES_CBC_SHA DES/CBC SHA-1
DHE_DSS_WITH_​3DES_EDE_CBC_SHA 3DES/CBC SHA-1
DHE_DSS_WITH_​AES_128_CBC_SHA AES/CBC SHA-1
DHE_DSS_WITH_​AES_256_CBC_SHA AES/CBC SHA-1
DHE_DSS_EXPORT1024_WITH_​DES_CBC_SHA DES/CBC SHA-1
DHE_DSS_EXPORT1024_WITH_​RC4_56_SHA RC4 SHA-1
DHE_DSS_WITH_​RC4_128_SHA RC4 SHA-1
DH_anon_WITH_​AES_256_CBC_SHA256 no certificate Diffie-Hellman AES/CBC SHA-256
DH_anon_WITH_​AES_128_CBC_SHA256 AES/CBC SHA-256
DH_anon_WITH_​AES_256_CBC_SHA AES/CBC SHA-1
DH_anon_WITH_​AES_128_CBC_SHA AES/CBC SHA-1
DH_anon_WITH_​RC4_128_MD5 RC4 MD5
DH_anon_WITH_​3DES_EDE_CBC_SHA 3DES/CBC SHA-1
DH_anon_WITH_​DES_CBC_SHA DES/CBC SHA-1
Note: On Xamarin, Mono and older Windows platforms, elliptic curve algorithms (ECDH and ECDSA) are only available with an external plugins.
Note: Vulnerable cipher suites are switched off by default. To enable them, set Parameters.AllowVulnerableSuites to true. However, this is strongly discouraged.
Note: CHACHA20_POLY1305 ciphers are not enabled by default. They use a managed implementation that is slower than AES/GCM alternatives on mainstream Windows platforms.
Ciphers based on modular Diffie-Hellman algorithm (those with "DHE_" prefix) are known to be very slow on legacy hardware. To only enable ciphers that are fast and also sufficiently secure, use TlsCipherSuite.Fast enum value.

TLS/SSL extensions 

Rebex TLS supports the following extensions:

TLS/SSL elliptic curves 

The Parameters.AllowedCurves property (on client object) makes it possible to specify a set of enabled elliptic curves used by ECDHE ciphers above. Supported elliptic curves:

Curve ID Curve Name
NistP256* NIST P-256 curve
NistP384* NIST P-384 curve
NistP521* NIST P-521 curve
BrainpoolP256R1** Brainpool P-256 R1 curve
BrainpoolP384R1** Brainpool P-384 R1 curve
BrainpoolP512R1** Brainpool P-512 R1 curve
Curve25519*** X25519 curve

* These curves require a plugin on Xamarin, Mono, and legacy Windows platforms.

** These curves require a plugin on Xamarin, Mono, and Windows earlier than Windows 10 / Windows Server 2016.

*** These curves require a plugin on non-Windows platforms and on Windows earlier than Windows 10 / Windows Server 2016.

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