Question about the ECDH

[iSmartcard]

Hello everyone, when I use the bouncycastle encryption library, I get a question about ECDH. With ECDH, the length of the value is 32 bytes and the length of the javacard is 20 bytes. What happened? Does anyone know why?

[AlexLiu]

32 bytes corresponding to ECC256, ECDH protocol based on ecc curve 256 bit,
20 bytes corresponding to ECC160, ECDH protocol based on ecc curve 160 bit

[scplatform]

According the API specification of jc304 or above, the result will be hash with SHA1.

ALG_EC_SVDP_DH
public static final byte ALG_EC_SVDP_DH
Elliptic curve secret value derivation primitive, Diffie-Hellman version, as per [IEEE P1363].
Note:

This algorithm computes the SHA-1 message digest of the output of the derivation primitive to yield a 20 byte result.
See Also:
Constant Field Values

[happy123654]

there will have SHA_1 calculation in the last step of ECDH and the result of SHA_1 is 20 bytes