Package com.chilkatsoft.chilkat2

Class Dh

java.lang.Object
com.chilkatsoft.chilkat2.ChilkatObject
com.chilkatsoft.chilkat2.Dh
All Implemented Interfaces:
AutoCloseable
public class Dh extends ChilkatObject
<section class="dh-summary">

Perform Diffie-Hellman shared-secret key exchange.

Chilkat.Dh provides the core operations needed for Diffie-Hellman shared-secret exchange. It allows two parties to agree on a shared secret over an insecure channel by using common Diffie-Hellman parameters, generating public exchange values, and computing the same shared secret from the other party's public value.

Choose known parameters

Use UseKnownPrime when the application wants to use a predefined Diffie-Hellman prime group.

Generate parameters

Use GenPG to generate Diffie-Hellman parameters when custom prime and generator values are required.

Set custom P and G

Use SetPG when the prime and generator are already known or have been received from another party or protocol.

Create the exchange value

Use CreateE to create the public Diffie-Hellman value that is sent to the other party.

Compute the shared secret

Use FindK with the other party's public exchange value to compute the shared secret.

Use the result carefully

The shared secret is typically used as input to a key derivation step before being used for encryption or authentication.

Common pattern: Select or generate the Diffie-Hellman parameters, create your public exchange value with CreateE, exchange public values with the other party, then call FindK to compute the shared secret. The primary methods are UseKnownPrime, GenPG, SetPG, CreateE, and FindK.

For an extended overview, see Dh Class Overview.

</section>

Instances hold a native Chilkat object: close them with ChilkatObject.close() (try-with-resources) when done; an unclosed instance is freed when it becomes unreachable.

See Also:
Online reference

  • Constructor Details

    • Dh

      public Dh()
      Creates a new Dh object.

  • Method Details

    • getG

      public int getG()
      Returns the current Diffie-Hellman generator, g. The generator is the small public integer used with the prime modulus p in the modular exponentiation operations that create the public exchange value and shared secret.

      For this class, g is expected to be 2 or 5. Its value is established by UseKnownPrime, GenPG, or SetPG.

      Public parameter: The generator is not secret. Both parties must use exactly the same p and g values for the exchange.

      Returns:
      the value of the G property (read-only)

    • getP

      public String getP()
      Returns the current Diffie-Hellman prime modulus, p, as a hex-encoded SSH1-format bignum. The value is established by UseKnownPrime, GenPG, or SetPG.

      The modulus is a safe prime: p = 2q + 1, where q is also prime. Safe-prime groups are commonly used for finite-field Diffie-Hellman because their subgroup structure is well understood.

      Encoding note: The returned string is the hexadecimal encoding of an SSH1 bignum, which includes the SSH1 bit-length prefix. It is not simply the conventional hexadecimal representation of the integer p.

      Public parameter: The prime modulus is not secret and may be transmitted to the other party. Both parties must use the same p and g.

      Returns:
      the value of the P property (read-only)

    • createE

      public String createE​(int numBits)
      Creates this party's public Diffie-Hellman exchange value, conventionally written as E = gx mod p. Before calling this method, initialize the common Diffie-Hellman parameters by calling UseKnownPrime, GenPG, or SetPG.

      numBits (numBits) controls the size of the randomly generated private exponent retained internally by this Dh object. Chilkat recommends setting it to twice the desired strength of the symmetric key that will ultimately be derived. For example, use 256 when the protocol will derive a 128-bit key.

      The returned E value is a hex-encoded SSH1-format bignum. It is public and is sent to the other party. The size of the eventual shared-secret integer is determined primarily by p; numBits does not make the returned shared secret a fixed-length symmetric key.

      Object state: Call FindK on this same Dh object after receiving the other party's exchange value. CreateE stores the corresponding private value internally, and FindK needs it to compute the shared secret. Use a separate Dh object for each party and generate a fresh exchange value for each session.

      Authentication required: Basic Diffie-Hellman does not authenticate either party and is vulnerable to a man-in-the-middle attack. The surrounding protocol must authenticate the exchanged parameters and public values.

      For language bindings where a string return cannot directly communicate failure, check LastMethodSuccess. If it is FALSE, inspect LastErrorText.

      Parameters:
      numBits - the numBits argument
      Returns:
      the result (never null)
      Throws:
      ChilkatException - if the operation fails; ChilkatException.getLastErrorText() says why

    • findK

      public String findK​(String E)
      Computes the Diffie-Hellman shared secret from the other party's public exchange value. E is the peer's E value, encoded as the hexadecimal representation of an SSH1-format bignum.

      This method must be called on the same Dh object that previously called CreateE. Both parties must also be using the same p and g parameters. Internally, the method computes K = Ex mod p, where x is the private value generated by CreateE.

      The result is the shared-secret integer K, returned as a hex-encoded SSH1-format bignum. When the exchange is performed correctly, both parties compute the same value.

      Do not use the returned text directly as an encryption key: It is an encoded bignum, not a uniformly formatted symmetric key. Feed the shared-secret value into the key-derivation procedure required by the protocol, together with any required transcript, salt, labels, or context. Do not invent an ad hoc conversion when interoperability or cryptographic security matters.

      Key agreement is not authentication: Matching shared secrets do not by themselves prove the identity of the peer. Authenticate the exchange to prevent man-in-the-middle attacks.

      For language bindings where a string return cannot directly communicate failure, check LastMethodSuccess. If it is FALSE, inspect LastErrorText.

      Parameters:
      E - the E argument (not null)
      Returns:
      the result (never null)
      Throws:
      ChilkatException - if the operation fails; ChilkatException.getLastErrorText() says why

    • genPG

      public void genPG​(int numBits, int G)
      Generates new finite-field Diffie-Hellman parameters and stores them in this object. numBits specifies the size of the safe prime p in bits, and G specifies the generator g, which should be 2 or 5.

      On success, the generated values are available through the P and G properties. The prime generation and validation process is computationally expensive, especially for large groups.

      Prefer standardized groups: Most applications should use UseKnownPrime or parameters mandated by the surrounding protocol instead of generating a new group. Standardized groups improve interoperability and avoid the cost and risk of custom parameter generation.

      Modern key sizes: The legacy 768-, 1024-, and 1536-bit MODP groups are not appropriate for new designs. Use at least a 2048-bit standardized group when compatibility requires finite-field DH; a 3072-bit group is commonly selected when approximately 128-bit classical security is required.

      Returns TRUE if the parameters were generated successfully; otherwise returns FALSE. Check LastErrorText for failure details.

      Parameters:
      numBits - the numBits argument
      G - the G argument
      Throws:
      ChilkatException - if the operation fails; ChilkatException.getLastErrorText() says why

    • setPG

      public void setPG​(String p, int g)
      Sets explicit finite-field Diffie-Hellman parameters for this object. p is the prime modulus p, supplied as a hex-encoded SSH1-format bignum, and g is the generator g, normally 2 or 5.

      The method validates that the supplied values satisfy the requirements expected by this Diffie-Hellman implementation. It returns TRUE if the parameters are accepted; otherwise it returns FALSE.

      Encoding requirement: p is not ordinary integer hex. It must contain the hexadecimal encoding of the SSH1 bignum representation, including its bit-length prefix. The P property of another Chilkat Dh object is already in the required form.

      Interoperability: Both parties must use exactly the same p and g. These values are public, but untrusted or nonstandard parameters should not be accepted unless the protocol explicitly permits them and their origin is authenticated.

      Parameters:
      p - the p argument (not null)
      g - the g argument
      Throws:
      ChilkatException - if the operation fails; ChilkatException.getLastErrorText() says why

    • useKnownPrime

      public void useKnownPrime​(int index)
      Selects one of eight built-in safe-prime MODP groups and sets the object's P and G properties. index is a Chilkat selector from 1 through 8; it is not the IKE group number.

      index MODP group Prime bits Definition G

      1 Group 1 768 RFC 2409, section 6.1 2

      2 Group 2 1024 RFC 2409, section 6.2 2

      3 Group 5 1536 RFC 3526, section 2 2

      4 Group 14 2048 RFC 3526, section 3 2

      5 Group 15 3072 RFC 3526, section 4 2

      6 Group 16 4096 RFC 3526, section 5 2

      7 Group 17 6144 RFC 3526, section 6 2

      8 Group 18 8192 RFC 3526, section 7 2

      Security guidance: Selectors 1, 2, and 3 are legacy groups and should not be chosen for new systems. Selector 4 is the 2048-bit group 14; selector 5 is the 3072-bit group 15 and is commonly used when approximately 128-bit classical security is required. Always use the group required by the protocol or the communicating peer.

      Performance: Larger groups require more CPU time and produce larger public values. Choose a group based on the required security level and interoperability requirements, rather than automatically selecting the largest group.

      Parameters:
      index - the index argument